Encyclopedia of Adaptations in the Natural World

ENCYCLOPEDIA OF ADAPTATIONS IN THE NATURAL WORLD Adam Simmons

GREENWOOD PRESS

An Imprint of ABC-CLIO, LLC

Copyright 2010 by Adam Simmons All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, except for the inclusion of brief quotations in a review, without prior permission in writing from the publisher. Library of Congress Cataloging-in-Publication Data Simmons, Adam. Encyclopedia of adaptations in the natural world / Adam Simmons. p. cm. Includes bibliographical references and index. ISBN 978–0–313–35556–1 (hardcover : alk. paper) — ISBN 978–0–313–35557–8 (ebook) 1. Adaptation (Biology) I. Title. QH546.S576 2010 578.4—dc22 2009042504 14 13 12 11 10

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This book is also available on the World Wide Web as an eBook. Visit www.abc-clio.com for details. ABC-CLIO, LLC 130 Cremona Drive, P.O. Box 1911 Santa Barbara, California 93116-1911 This book is printed on acid-free paper Manufactured in the United States of America

For Madge, Stan, Jack, and Josie my ancestors

CONTENTS

Introduction About This Book

1 Making and Using Energy Making and Using Energy—Human Invention Photosynthesis Chemosynthesis Catabolism Ectothermy Chemical Defense—Bombardier Beetle Chemical Defense—Devil’s Garden Ant Electricity Bioluminescence

2 Surviving the Elements Surviving the Elements—Human Invention Thermophilic Bacteria—Surviving Extreme Heat Blood Antifreeze—Surviving Extreme Cold Mammal Fur—Surviving Extreme Cold Mangroves—Surviving Extreme Salt Lungfish—Surviving without Oxygen Melanin—Surviving Radiation Parasitism—Surviving Host Defenses Antibiotics—Surviving Disease Psychrophiles—Surviving Extreme Cold Anhydrobiosis—Surviving without Water Prions—Surviving Everything

xi xiii

1 1 3 6 8 12 14 16 19 21

25 25 28 31 33 35 38 40 42 45 47 49 51

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3 Locomotion Locomotion—Human Invention Bird Flight Insect Flight Running Swimming—Bluefin Tuna Jet Propulsion Parasitic Locomotion Pollination Bacterial Flagellum

4 Materials Materials—Human Invention Silk Bone Chitin Feathers Skin Bio-ceramics Mucus Natural Glues Gecko Feet Resilin

5 Building Structures Building Structures—Human Invention Termite Towers Trees Bird Nests Beaver Lodges Bee Nests Paper Nests Coral Reefs Luminous Gnat Traps Naked Mole Rat Burrows Diatoms Webs

6 Sensing the Environment Sensing the Environment—Human Invention Vertebrate Eyes Insect Eyes Echolocation Electrosense Fire and Smoke Detectors Infrared Vision Jacobson’s Organ of Smell Odorous Genes—The Major Histocompatibility Complex Magnetic Sense Insect Antennae

55 55 58 60 63 66 68 70 72 75

79 79 82 84 86 89 92 94 96 98 100 103

107 107 109 112 115 117 120 122 124 127 128 131 134

137 137 140 142 144 147 149 151 153 155 158 160

CONTENTS Specialized Eyes Mantis Shrimp Eyes

162 165

7 Communication

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Communication—Human Invention Human Brain Human Language Plant Communication Bee Dance Language Bacterial Conjugation Culture DNA

169 171 174 177 180 182 184 187

Further Reading

191

Index

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INTRODUCTION

The ingenuity of the human race seemingly knows no bounds. From humble beginnings we have built staggering monuments, made breathtaking art, split the atom, and conquered space. On the cosmic scale, all this has been created in a blink of an eye—over a mere few thousand years in the planet’s 4.6 billion-year history. Now in the twenty-first century we continue to push the boundaries, make new discoveries, and build more remarkable machines. We are rightly proud of our achievements. Our inventions have made our lives easier and more comfortable. Many diseases, once deadly, have now been overcome with vaccines, antibiotics, and improved hygiene. The raw energies of our universe have been tamed to provide us with controllable power, and we protect ourselves from the extremes of the environment with intricately engineered cocoons. We have come a long way since our first recognizable human ancestors walked the earth some 2.5 million years ago. We sometimes forget, though, that we share our planet with many millions of other creatures—other animals, plants, bacteria, fungi, protozoa, viruses, and even tiny, single, self-replicating strands of protein. Many of these creatures are small and extremely hard to spot, but they are there and they have survived for much longer than we have. Their success lies in their own remarkable adaptations to this planet—perhaps not always so noticeable as our own adaptations, but ingenious adaptations nonetheless. All life on earth is programmed to survive. And after some 3 billion years of evolution it has become quite good at it—it has had to. On all sides, each and every organism on earth is faced with danger. From the elements. From its

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predators. Even from its own kind. After all this, it must then find food, water, and, ultimately, a mate if it is to successfully survive and pass on its genes. As we delve into the huge array of life on earth we find truly ingenious, astounding, and downright weird adaptations that have allowed life to flourish in the most inhospitable of environments. Adaptations that allow organisms to perceive the world in more detail than our most sophisticated microscopes, telescopes, and scanners. Adaptations that allow organisms to move with such speed and agility that they make our advanced modes of transport look sluggish. Adaptations that produce materials of such strength and flexibility that we have not come close to replicating them. Over million of years of evolution, life has adapted well to the harsh conditions of our planet in exactly the same way we have. Human engineers tinker away at a rough plan, making tiny adjustments to hone their design to perfection. Nature does the same. The evolutionary biologist Richard Dawkins described nature as a blind watchmaker. It may not see and plan the tiny changes it makes to its design, but it knows when something is working. This is how evolution works. Physical and behavioral traits are coded for on the genetic material, DNA, held within every cell of every living organism. Small mistakes and alterations in this genetic code that occur as organisms reproduce lead to tiny changes in the traits we see. Some of these changes can be harmful and result in the new individual dying earlier or producing fewer offspring than if the change had not happened at all. These deleterious changes in the gene pool will eventually be lost. Other changes, though, will be beneficial. They will code for improved eyesight, greater physical strength, or a new behavior that gives an individual a competitive advantage over others of its species. These changes will tend to be preserved in the gene pool. Over time, these changes can be honed by natural selection to produce the specialized adaptations we see in nature today. Many adaptations have evolved that match or even surpass our own achievements. Why else would today’s scientists be so keen in borrowing so much of what has been created in nature? This book seeks to capture the most fundamental of these adaptations, some of which may be familiar to you from distant (or perhaps not so distant) school days. Others will be completely new. I hope that you will look on those you knew in a new light, and those you didn’t with a desire to know more. The parallels of these adaptations with our own drive for survival are close. After all, humans are organisms like any other in this planet. We have much more in common with the other creatures who live on this planet than we might think. Viruses, bacteria, single-cell protozoa, plants, and animals (ourselves included) are all driven to survive. It is from this race for survival that many remarkable wild designs have emerged.

ABOUT THIS BOOK

As humans, we are aware of the great challenges to our own survival. Being an animal like the many thousands of others on this planet, it should not come as a surprise that the challenges we face are those faced every day by every other species on earth. This book is divided into seven chapters that group these challenges together: Making and Using Energy Surviving the Elements Locomotion Materials Building Structures Sensing the Environment Communication Each chapter begins with a short history of the human achievements and inventions in that field that have led to our success. As the rest of the chapter will show, though, evolution has created similar adaptations that match and surpass our own. Each of these adaptations is set out in its own entry, with the aim that the reader can dip into the book according to his or her interest —either for research or curiosity. At the end of every adaptation entry is a short section called ‘‘Borrowing from Nature’’ that shows how human engineers, scientists, and inventors are directly taking what they see in nature and developing it for the advancement of human endeavor. Evolution and biology, like all the sciences, have their own jargon that can, to the lay person and expert alike, be baffling at times. I have therefore

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attempted to keep jargon to a minimum and to explain those terms that I do use. Where possible, I use the English names for species, but I also include the Latin name as well should the reader wish to research the subject further. For some adaptations there is no English name for the species that exhibit them, so in those instances I have had to use the scientific nomenclature. Perhaps unusually for a science reference book I have not included references for each entry, as they would have broken the flow of the narrative. Instead, I offer the interested reader a recommended reading list at the end of the encyclopedia to further research the field. This list includes several of the key papers and references I have used in writing this book. I have tried to give as many readily available references as possible. For each entry I have tried to describe the evolutionary advantage each adaptation conveys rather than simply list and describe a series of biological curiosities, fascinating though this would be in its own right. I hope that you find adaptations both interesting and awe-inspiring. Even after many years of study, evolution has a habit of always throwing up something that is truly remarkable. I would like to thank David Paige of ABC-CLIO/Greenwood Publishing for his help, enthusiasm, and ideas during writing. I am also very grateful to Kevin Downing of Greenwood for his help in getting the project off the ground. This book would never have happened at all without the encouragement and friendship of Ross Piper, not to mention the useful discussions and sharing of ideas—thank you. Finally, and as ever, thank you to Maggie, Roy, and Beth for all their love and support.

1 MAKING AND USING ENERGY

MAKING AND USING ENERGY—HUMAN INVENTION Without energy there is no life. Nature has come up with a myriad of ingenious ways of harnessing energy, which it uses to drive the various reactions critical for life. Controlling energy has been important to the success of humans, too. It has helped our ancestors survive inhospitable environments and has been the foundation of the many inventions that make our lives easier today. Although humankind may not have always understood the processes that underlie the reactions that have produced useful energy, there is no doubting the skill in which energy has been harnessed and used to do work. At its simplest, humans have utilized the energy to be found in the elements, such as wind and water, and channelled it to our own ends through mills for driving machinery. There are many human inventions that have led to the successful and comfortable lives we lead today. The most fundamental of these has been the use of fire. Since human ancestors first used fire some 1.5 million years ago, it has provided warmth and protection that allowed humans to move out from the warmer tropics and begin to exploit colder climates. Fire has even played a key part in the human diet. It has been used to make food sources safer, more palatable, and more easily digested. Starch (an important source of carbohydrate) found in plants is much easier to digest if it has been cooked and broken down into the simple sugar, glucose. Today, of course, another form of energy has taken over as the most important in our lives. It plays a role in nearly everything we do, and yet we rarely notice its presence. Electricity provides energy for nearly all the tools, machines, and gadgets that make our lives easier and more enjoyable.

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Most electricity is provided by our power stations, which rely on a fuel of one sort or another to create heat to boil water, the steam from which drives turbines that can generate electricity. Traditionally, fossil fuels are burned, but nuclear power stations work in the same way. Radioactive materials like uranium decay and give off heat, which drives the power station. Although expensive, nuclear power stations are very effective, reaching efficiencies of over 90 percent compared with an average efficiency of 30–40 percent for fossil fuel power stations. Heat-driven power stations are not the only way of generating electricity. More recently, solar panels have been developed that convert light energy to electrical energy. They are made from a group a materials called semiconductors, such as silicon embedded with tiny amounts of other materials, such as phosphorus and boron. Semiconductors are able to absorb light from the sun and store the energy contained within it. When most objects absorb light, the energy that is absorbed is simply given off as heat. (This is why objects get hot in the sun.) Semiconductors are different in that they can absorb energy from the sun and release it as an electrical current. The ability of solar panels to convert sunlight into electrical energy varies greatly. A basic solar panel that can be obtained from a hardware store can convert about 15 percent of the light energy falling on it into useful electricity. The very best solar panels can convert 42.8 percent of the sun’s energy into electrical energy. We are still decades away from creating a solar panel that can convert 100 percent of solar energy into electrical energy, but if we do our energy problems could well be over. We can only speculate about the future, but the answer probably lies in a famous equation that nearly everyone knows but few understand—Einstein’s E = mc2. This simple looking equation shows that the energy (E) stored in an object is equal to its mass (m) times the speed of light (c) squared. The speed of light is a massive number, which means that a lump of any substance, even a tiny amount, contains a huge amount of energy. Einstein’s equation made us realize that a huge reserve of energy is at our fingertips in the very atoms that make up every single thing. The atomic bombs dropped on Hiroshima and Nagasaki were the first real demonstration of the power of the atom. From only one-half of 1 percent of one pound of uranium so much energy was released that a city was flattened and many of its inhabitants were killed or injured. The energy contained in the atom is awesome, but the difficulty comes in releasing it in a controlled way. If we could control these reactions in fusion reactors, one day we could provide the whole of the world’s energy demands from seawater. The energy contained in 150 gallons of running water a minute would be enough to produce all the energy that is used in the United States today. There is no doubt that the mastery of different energies has been the key to the success of the human species. But nature, too, has made use of the physical

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properties of energy to its own ends. As we delve into the many and varied adaptations found in nature we see that not only has evolution helped tame the powerful energies found in this universe, but it has gone further and put them to such fascinating, intricate, and elegant uses.

PHOTOSYNTHESIS By harnessing the energy of the sun, plants, algae, and even some bacteria can make the basic nutrients that are essential for life. Not only does this remarkable process benefit these living power stations themselves, but it is also the basis for much of the energy needed for life on earth to thrive. Thanks to the vast swathes of green plants across the world, on land and in water, we can all eat and breathe. The ingredients for photosynthesis are common enough. Water and carbon dioxide are combined to produce Photosynthesis in the chloroplasts of plants glucose and oxygen. Glucose is a univer- converts water (H 2 O) and carbon dioxide sal source of energy that powers all the (CO2) into carbohydrate (C6H12O6), which is processes that keep a plant alive (as well used by the plant as an energy source, and oxyas animals such as humans, which is why gen (O2). [BSIP / Photo Researchers, Inc.] we eat plants). The challenge lies in getting the energy to drive the process of converting the ingredients into the end product. The energy source for photosynthesis is light emitted by the sun. It is not a straightforward process, though, and requires an impressive feat of natural engineering that is so successful that photosynthesis has become one of the most widespread adaptations on the planet. There are two key stages in photosynthesis. The first stage harnesses the sun’s energy into a usable form. The second stage then uses this energy to make the energy-packed glucose molecules. Both of these stages occur at the molecular level, but really we can think of the molecules involved as miniscule rechargeable batteries. Photosynthesis is about moving energy from one of these microscopic batteries to another—one battery is used to charge up the next, and so on. The first stage in photosynthesis is therefore about topping up one set of tiny molecular batteries with energy from the sun. The ‘‘batteries’’ that absorb the sun’s energy are light-absorbing molecules in the plant’s leaves called chlorophyll, which is a green pigment that gives the leaves their characteristic color. Rather than simply getting hotter when it absorbs sunlight, chlorophyll is able to absorb the energy and store it until it can be used later. The energy absorbed by chlorophyll is put to

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two important uses. First, it is used to break down water, one of the two ingredients in photosynthesis. Water is made up of hydrogen and oxygen, but for photosynthesis the plant is only interested in the hydrogen part. The plant doesn’t need the oxygen part, so when water is broken down the oxygen is simply expelled by the plant. This is why oxygen is produced as a by-product of photosynthesis—it is by this lucky accident that we are able to breathe! Once hydrogen is made from breaking down water it is then put to good use. Energy stored in the chlorophyll is used to pump the hydrogen through a special type of molecule that works much like a water mill. In a water mill, water drives a wheel that turns a machine for grinding flour. In the hydrogen ‘‘mill,’’ hydrogen drives the molecular mill that is used to charge up another molecular battery with energy. This particular molecular battery is a very important molecule that is used in a number of reactions inside the cells of living organisms. It is called Adenosine Tri-Phosphate (ATP). So far, the plant has absorbed energy from the sun and used it to charge up a critical molecular battery called ATP. This then is used to drive the second stage of photosynthesis—using the energy stored in ATP to make glucose. Glucose is packed with energy—that it is why it is so useful to the plant. It can be used as an energy source, and it can be stored easily to keep energy in reserve. Glucose is produced from carbon dioxide, the second ingredient of photosynthesis. Carbon dioxide has very little energy—it is a little like a flat battery. Through a series of chemical reactions, this flat battery is gradually topped up with energy, provided by ATP, until it is fully charged in the form of glucose. The process of converting carbon dioxide to glucose is called the Calvin Cycle. Having now made the energy-packed glucose the plant can now put it to good use. Glucose can be used to power all the processes that go on in plants to allow them to stay alive—processes like reproduction, defense, and growth, to name but a few. Glucose itself is a simple molecule—one of the small molecules we typically think of as sugar. In plants (and indeed in animals) it is often converted to carbohydrate, which are long chains of these simple sugar molecules. Root tubers like potatoes and carrots are huge stores of carbohydrate kept in reserve for when a plant may need to convert them back into useful sugars. From this amazing adaptation, then, the plant’s entire life can be carried out without the need to consume anything else. All its energy is available from just water, carbon dioxide, and a healthy amount of sunlight. This process of photosynthesis that creates useful energy from the sun has been central to the flourishing of nearly all life on earth, both in the food that all organisms eat (carbohydrate) and in the air that they breathe. Of course, it is not for the good of other organisms that photosynthesis has evolved. Photosynthesis is found in plants, algae, bacteria, and lichens and has allowed them to conquer nearly every corner of the earth. By making their own energy and not having to rely on other organisms to provide them with a tasty snack,

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plants, algae, and bacteria have become hugely successful. As a result, our planet is carpeted with plants, lichens, algae, and bacteria, all of which rely on photosynthesis and all of which make for a truly green earth. Although we normally think of plants when we think of photosynthesis, it was actually the humble bacteria that first evolved the ability to make food from the sun. These pioneering bacteria did well and flourished, as they do today, until a very strange and unique partnership was formed. Once life emerged on this planet some 3.7 billion years ago, organisms were made only of one single cell, much like the amoeba of today. Some of these cells— bacteria—could make their own food from the sun’s energy in much the same way we have described above. So how did plants evolve this same ability as bacteria to capture energy from the sun’s radiation? There is no definitive answer, but the most compelling evidence points to what is known as the ‘‘endosymbiotic theory.’’ In essence, this theory describes how photosynthetic bacteria were swallowed up by early plant ancestors and formed with them a mutually beneficial (symbiotic) relationship. Like plants today, photosynthesizing bacteria would have been prey to other organisms and would have often been eaten. The theory states that at some point in time, a bacteria was swallowed by another single-celled organism but not digested—it remained inside its would-be attacker unharmed. By chance, both the bacteria and the predator profited from this unexpected union. The bacteria was protected by living inside another organism, and the predator could make use of the food the bacteria could produce from the sun. Over time both organisms evolved into one living organism, producing the first ancestor of today’s plants. As these single-celled plant ancestors evolved into the multi-cellular plants we see today, they retained the photosynthesizing bacteria that have evolved into specialized structures called chloroplasts. Thus, plants never directly evolved photosynthesis for themselves, but rather acquired the ability from swallowing a bacteria many thousands of years ago—an amazing example of how opportunistic nature can be! Thanks to this happy cohabitation of a bacterium inside a plant it’s probably fair to say that photosynthesis is now one of the most important adaptations to have evolved on this planet. Borrowing from Nature

Plants have long been a source of inspiration for engineers. Solar panels are improving all the time and are already outstripping the efficiency of plants at converting the sun’s energy. A basic solar panel can convert about 15 percent of the light energy falling on it into useful electricity. The very best solar panels can convert 42.8 percent of the sun’s energy into electrical energy. In contrast, the maximum conversion rate of plants is around 25 percent in commercial crop species such as wheat, but most plants only convert around 3 percent of the sun’s energy into food.

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This may suggest that human engineering has outstripped nature, but manmade solar panels are expensive and currently require more energy to make than they can generate. Plants, on the other hand, are entirely self sufficient. The energy they make allows them to grow and reproduce in an ongoing and indefinite cycle of life. Scientists are therefore spending time in developing artificial photosynthesis systems that can produce an energy supply from water and carbon dioxide. In early 2009, researchers from the U.S. Department of Energy’s Berkeley Laboratory took a significant step forward in this goal. They discovered that microscopic crystals of cobalt oxide can control the water-splitting reaction central to photosynthesis. This completes only half the puzzle, of course, but if the rest can be achieved we may have access to an inexhaustible supply of energy.

CHEMOSYNTHESIS Until recently, it was thought that all life on earth is dependent on the food produced by plants, bacteria, and algae through photosynthesis. In the last 30 years, though, as we have explored our planet more thoroughly, we have discovered self-sustaining oases of life where there is no light whatsoever where photosynthesis could not possibly take place. In seemingly uninhabitable environments like the deep ocean or deep within the rocks that make up our planet, whole communities of species flourish and thrive. How is this feat achieved without light to supply the energy? In plants, carbon dioxide and water are broken down using energy from the sun and reformed to make carbohydrate—the body fuel for all life on earth. The key to this reaction is that some energy (from the sun) is needed to make the reaction happen. This is much like what happens when something burns. A candle wick does not catch fire spontaneously, but rather an external supply of heat energy (like a lighted match) is required to start the process off. Some reactions, however, do occur spontaneously. Hydrogen and oxygen, for example, will always react with each other to make water without any extra energy to kick things off. Some ingenious bacteria Deep ocean tube worms (Vestimentifera) have are able to control spontaneous reactions evolved a symbiotic relationship with bacteria like these to make carbohydrate in a prothat can convert carbon dioxide and the hydro- cess called chemosynthesis. With the right ingredients, certain gen sulfide emitted by hydrothermal vents into carbohydrate. [Dr. Ken Macdonald / Photo bacteria can make carbohydrate with Researchers, Inc.] very little effort at all. The ingredients

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are very unusual, though, and are indeed quite toxic to most organisms. Yet some industrious bacteria are quite at home with these toxic materials and have made a very successful way of life with them. One group of bacteria called Thiobacillus makes use of hydrogen sulfide as its basic food-making ingredient. This is the gas released from geysers and hot springs that smells of very bad eggs. Other bacteria (Nitrosomonas) use ammonia as a food source, and others (such as the Methylococcaceae family of bacteria) use methane. All these bacteria allow either hydrogen sulfide, methane, or ammonia to react with water and carbon dioxide to produce carbohydrate. Hydrogen sulfide, methane, and ammonia are all quite rare. Wherever they are found, though, there will always be a colony of bacteria to exploit it. Often these bacterial fuels are found where light cannot reach, which means these bacteria can exploit an untapped niche free from the competition of other—photosynthetic—bacteria, which is why chemosynthesis is such a successful adaptation. In 1977, a deep ocean submersible called the Alvin discovered a highly diverse ecosystem of life living some 8,000 feet (2,500 m) below sea level. With no life-giving sunlight penetrating this far below the surface, it was a mystery as to how this life was supported. The answer was revealed when it was noticed that the greatest density of life was found clustered around huge natural chimney stacks that emerged from the ocean bed. The stacks (or black-smokers as they were called due to the huge black clouds they spouted) were releasing a steady stream of hot water containing hydrogen sulfide. The same effect is seen on land in geysers or hot springs. Bacteria that are capable of producing food from this abundance of sulfur flourish around these black-smokers. Like plants on land, these independent food factories support a huge variety of life that feeds off them, including shrimp, clams, and tube worms. Indeed, after exploration scientists found that in a tiny area of only 225 square feet (21 m2—about the same size as a small house) 798 species could be found, including 171 families and 14 phyla. This is an extremely diverse array of life for an area thought previously to be just a barren desert. This fantastically diverse ecosystem is principally supported by one type of bacteria. For the most part,Thiobacillus bacteria are found as free-living organisms floating near the seabed. Some species, though, have made fascinating partnerships with other organisms, such as tube worms (Vestimentifera) and clams (Vescomyidae). The tube worm is particularly intriguing. It is an animal, but it has no mouth and no gut—it is able to survive seemingly without eating. Instead of the normal digestive tract we find in animals, this remarkable creature has a simple internal bag (called a trophosome) whose cells are packed with chemosynthetic bacteria. Unlike the free-living bacteria that can make carbohydrate from hydrogen sulfide and carbon dioxide alone, these bacteria need oxygen as well. The tube worm provides the bacteria with the oxygen they need, and the bacteria return the favor by producing the carbohydrate needed to feed the worm. This could be one of the few examples of an animal that doesn’t need to feed on another organism to survive.

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Away from the ocean floor, and if you are unlucky enough to stumble into them, there are some caves on this planet that are covered in a thick mucus, which are filled with a noxious gas and which drip corrosive acid on any visitor. One such cave is the Cueva de Villa Luz (‘‘the cave of the lighted house’’) in southern Mexico. Like in the ocean, hydrogen sulfide leaks from the rocks and into the cave, and, just like in the deep-sea, bacteria exploit this ready source of food. It is in this cave, though, that we can fully see the second part of the chemosynthesis reaction. Whereas plants release oxygen as a waste product of photosynthesis, the mucus-cave bacteria excrete sulfuric acid. This is a very strong acid that eats away at the limestone of the cave and makes it bigger and bigger. This is a bacteria that can directly modify its own environment! So far, the bacteria have cut out 1.2 miles (2 km) of rock to make the cave. Again, a whole ecosystem is supported by the food produced by the sulfurdigesting bacteria. Insects, spiders, and even bats thrive in this lightless environment thanks to the bacteria, and all of these organisms have evolved to withstand the acid that eats away at the walls. The spiders are even able to make webs that survive the acidic conditions, which still mystifies scientists to this day. This is truly a fascinating cave that may yet still hold amazing secrets of which we are not aware. Some scientists believe that if bacteria can survive in these conditions on earth, then similar communities may dwell under the rocky surface of Mars. This is not too far fetched a theory. Geological drilling in Washington State revealed bacteria surviving in solid rock some 5,000 feet (1500 m) underground. Borrowing from Nature

Bacteria that feed from toxic chemicals and convert them into safer waste products are being explored to clean up toxic waste dumps. The first bacteria that was used to treat sewage in the 1990s was Dehalococcoides ethenogenes. The same bacteria is also used to remove chlorine from carcinogenic waste, rendering it safer. Thiobacillus is perhaps the most widely used bacteria, though, being able to digest toxic heavy metals. Other bacteria have been used to clean up oil spills and treat a particularly dangerous class of cancer-causing chemicals called aromatic hydrocarbons.

CATABOLISM Energy is required by all living organisms for them to move, grow, reproduce, repair themselves, and indeed do all the things that are needed to stay alive. Energy is available from the food that an organism eats or makes for itself by photosynthesis or chemosynthesis. The challenge is then to convert this energy into a useful form to power their cells. In all life on earth food is broken down—in a process called catabolism— in a way that creates a particularly useful molecule called Adenosine

MAKING AND USING ENERGY

A scanning electron micrograph of a mitochondrion. These organelles are found inside nearly all living cells and are involved in breaking down carbohydrate to release energy to fuel the cell. [Professors Pietro M. Motta & Tomonori Naguro / Photo Researchers, Inc.]

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Tri-Phosphate (ATP). ATP is what is known as a biological energy carrier—it literally carries energy to where it is needed. This molecule is used to power all the processes that go on inside an organism’s cells. Whereas humans have made use of electricity as the main source of power for our inventions, nature uses ATP. So really, we can think of ATP as the universal battery that is used to power all of life on earth. All organisms break down their food to produce ATP, but there are many weird and wonderful ways that have evolved to achieve it. Many catabolisms have evolved to exploit every environment on the planet. Thanks to these amazing adaptations, many organisms can survive and thrive in some of the harshest conditions on earth. All animals, humans included, get most of their energy from carbohydrate. This is broken down in our cells with oxygen to make ATP, carbon dioxide, and water. This is why we need to breathe oxygen—it is essential for our bodies to get energy from our food. Catabolism that uses oxygen to get energy in this way is called aerobic respiration. This form of respiration takes place in the hundreds of very tiny structures called mitochondria that are found inside our cells. These mitochondria work like miniature power stations, taking fuel from food and converting it to energy in the form of ATP. Mitochondria evolved in much the same way as chloroplasts in the leaves of plants; that is, they evolved from bacteria. These bacteria existed as free-living organisms millions of years ago until a very strange union was forged with another single-celled organism. We are talking about a time when all organisms were made up of just one cell. At this time, along with bacteria there existed single-celled organisms that were the very ancient ancestors of today’s animals and plants. These animal ancestors could eat other organisms by swallowing and digesting them. At some point, though, an animal ancestor swallowed a bacteria and instead of digesting it, kept it alive inside its singlecelled body. When our single-celled ancestors evolved into multi-cellular plants and animals, they retained the bacteria inside their cells, and these evolved into mitochondria. What is fascinating, though, is why this unusual union benefitted both our animal ancestor and the bacteria living inside it. For the bacteria, perhaps it is a little easier to understand. Living inside another organism gives it protection and a constant supply of food and minerals. For the single-celled animal ancestor, the benefits are a little more subtle. When life first emerged on earth some 3 billion years ago there was no oxygen at all. All single-celled life got its energy from the other gases that swirled around the atmosphere—nitrogen, methane, sulfur, and water vapor. Once photosynthesis evolved in singlecelled plants, though, oxygen began to be released into the air. Oxygen is toxic gas. Even to this day our bodies need clever defenses to protect our cells and tissues from the corrosive gas despite its important role in keeping us alive. Early bacteria were the first organisms to evolve to make use of oxygen as a way of making energy. As it happens, aerobic respiration is a

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much more efficient form of catabolism than using any other gas. Bacteria that could use aerobic respiration, therefore, did very well with this new adaptation, gaining a competitive advantage over other bacteria. Of course, any other organism that housed a few of these bacteria could benefit from their new and efficient way of making ATP. What’s more, the bacteria could help protect their host from the toxic effects of the poisonous, oxygen-rich environment. So again we see that a highly efficient adaptation has evolved in a very opportunistic way. Thanks to a union that evolved between two single-celled organisms millions of years ago, all animals and plants are able to convert their food to energy with great efficiency. Aerobic respiration is not the only way of releasing energy from food, though. We are familiar with species that can break down carbohydrates, fats, and proteins for energy, but less well known are the few bacteria that call toxic chemicals food. In principle, anything can be used as a food supply. There are some bacteria that use some unusual chemicals as food. Pseudomonad bacteria, for example, can break down the explosive nitroglycerin and TNT for energy. One group stands alone, though, in its ability to break down toxic waste for food, a group of bacteria called Rhodococcus. There are 12 species of Rhodococcus and they can be found in a wide range of environments, including soil, rock, boreholes, groundwater, ocean sediments, animal dung, insect guts, and in plants, animals, and humans. These bacteria have the unexpected ability to digest a wide variety of highly toxic compounds such as coal, petroleum, steroids, chlorinated phenolics (used in the plastics industry), certain acids, and even the drug heroin. How are they capable of breaking down such a range of toxic chemicals? First, they have evolved an outer membrane that not only is capable of withstanding the toxic nature of their food, but that also helps the bacteria stick to whatever it is attempting to devour. Second, these bacteria have evolved to produce the necessary enzymes to break down their toxic food. (An enzyme is a biological molecule whose job it is to break down other molecules.) What is incredible, though, is that the genes that control the enzyme production can be transferred between these bacteria. This is achieved by the bacteria making a biological bridge from one individual to another and passing key parts of their DNA to each other. Specifically, they transfer the pieces of DNA that are specifically involved with their amazing catabolic abilities. Thanks to this ability to share DNA, each species can carry a whole library of DNA that can produce the right enzyme for the right chemical—an amazing example of teamwork having evolved in one particular group of bacteria. It is easy to see the benefit of such an adaptation. Using food that would harm other organisms opens up a huge niche to be exploited. Borrowing from Nature

The ability of different bacteria to catabolize unusual and often toxic materials is of great use in industry. Industrialization across the globe has led to

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hugely elevated concentrations of metals in soils, which can have catastrophic effects for growing plants. The presence of bacteria known to be digesters of certain toxic metals can be used as indicators that soil quality is deteriorating, which can be an important early warning system particularly for soils used for agriculture. What’s more, as we’ve seen above certain bacteria can break down toxic chemicals, making contaminated areas safer for humans and other wildlife. In addition to bacteria that break down nitroglycerin, bacteria have been used to digest cancer-causing chemicals, tar, ammonia, and even radioactive waste. The remarkable ability of these tiny organisms to withstand normally uninhabitable environments and even use toxic waste as food has allowed humans to clear up spills that would otherwise have had serious effects on local ecosystems.

ECTOTHERMY All organisms have an optimal body temperature. This is because inside every organism, thousands of chemical reactions are firing away to keep it alive. These chemical reactions are very dependent on temperature. If the temperature is too cool, the reactions don’t happen quickly enough and we become sluggish. If the temperature becomes too high, the tiny molecules (enzymes) that control these reactions start to break down and death rapidly follows. It’s all about balance. Most of these reactions have an optimal temperature of around 100°F (38°C), which is why our own bodies are kept at around that temperature. Humans have little problem staying warm. Even in very cold conditions, our body regulates its temperature to a constant 98 degrees Fahrenheit. We have this trait in common with all mammals and birds—we are all warmblooded animals. We are all endotherms. Being able to regulate body temperature provides a great advantage. Endothermic animals can keep their internal reactions ticking over throughout the day regardless of how hot or cold the environment is. This means that they can be active at all times of the day, making them free to forage for food, seek out mates, and care for their young whenever they need to. There is a cost to this freedom, though. To maintain a constant body temperature, warmblooded animals need to generate heat constantly. Likewise, mechanisms to cool down are needed when things get too hot. All this takes energy, and quite a lot of it. In fact, around 80 percent of the food that warmblooded animals eat is used up in maintaining their body temperature. That means that a lot of our waking hours are used to gather food. Some small, active birds like sparrows and finches need to eat almost constantly to stay alive simply because so much energy is used up to keep warm. There is another way, though. Most animals, including reptiles, amphibians, fish, and arthropods (insects, spiders, crustaceans, and the like), are coldblooded, or ectothermic. These animals have less control over their body

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temperature. They need to absorb heat from their surroundings before their bodies are warm enough for their internal reactions to start firing. Thus, they tend to be active only during the day when they can get heat from the sun, although this is by no means exclusive. Most ectothermic animals can therefore be found basking in the sun on hot rocks during the day. Having a good place to warm up is an important commodity. In many species there are fierce fights for the best sunbathing spots. Male side-blotched lizards from California will fight over the hottest rocks in the desert. Similarly, male speckled-wood butterflies from Europe fight over the sunniest spots in otherwise shady woodland. In both of these species, and in many others, the victors not only get the warmest location in which to bask, but also the pick of the females who are attracted to the best sunbathing spots. This may sound like a primitive solution to the problem of getting enough energy to move around, but that is far from the truth. The cold-blooded way of life has been extremely successful. Of known animal species, 99 percent are cold-blooded and only 1 percent is warmblooded. What’s more, coldblooded animals show just as complex and fascinating behaviors as their warmblooded cousins. As none of their food is wasted on simply keeping warm, ectotherms can focus on much more important behaviors, such as finding a mate and caring for their young. Living with cold blood does not mean simply sitting in the sun and warming up before setting out to perform the day’s activities. Many cold-blooded animals are very efficient at converting the sun’s energy into warmer body temperatures. One of the best is the leatherback turtle (Dermochelys coriacea), a huge marine turtle that ranges through every ocean of the planet. You might think that such a far-ranging animal would be at the mercy of the large variations in temperature from tropical to arctic oceans, especially when you consider that it is capable of diving down into near-freezing waters some 1,000 feet (300 m) below the surface of the water. The fact is, though, that the leatherback can maintain a steady body temperature of 77°F (25°C) or higher wherever it swims. This is some 31.5°F higher than the average sea temperature of 45.5°F (7.5°C). The leatherback achieves this remarkable feat through a series of adaptations that are designed to generate and maintain body temperature. Like most ectotherms, the leatherback can warm up by basking. Its huge carapace is black in color, which absorbs heat energy, so a few hours in the sun can rapidly get its body to optimum temperature. But what about the chilling effect of the sea? Apart from when it needs to lay its eggs, the leatherback never leaves the water. For most animals, ourselves included, this would lead to significant cooling down. The leatherback gets around this problem with some neat adaptations. The trick is to keep heat in the body. Much heat is lost in animals when the heat in warm blood that is pumped to the extremities (flippers in the case of the leatherback) is lost to the cooler surrounding ocean. The leatherback avoids

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this by making sure warm blood does not arrive at its flippers in the first place. It does this by a clever arrangement of its blood vessels at the base of its flippers. Warm blood from the body is carried in arteries that run parallel to the veins carrying cool blood back from the flippers. Because the arteries and veins run next to each other, the warm blood heats up the cool blood. In effect, the precious warmth of the turtle is recycled inside the body—it never gets to a place where it can be lost to the elements. This is quite a common adaptation. It is through this ingenious method of internal heat engineering that many animals stay warm—it is why penguins do not lose all their heat through their feet perched on the frozen ground. It is a simple evolutionary solution to the problem of fighting the cold, but it is one that has allowed a huge diversity of animals to colonize cold climates. The arrangement of blood vessels helps, but it will not stop the leatherback from cooling down indefinitely. Unusually for a reptile, the leatherback has a thick (7 cm) layer of fat just under its skin that helps keep the heat in. This fat is a useful insulator that prevents heat being lost from the body, but it has another hidden property. It is actually a specific type of fat called brown fat. This type of fat has the remarkable ability to store heat that is either absorbed from basking or generated by the powerful swimming muscles. The heat store can then give it out again when the body starts to cool. It is believed that it is the presence of brown fat that allows the leatherback to maintain such a high temperature without needing to metabolize food. The leatherback, therefore, has all the benefits of both the warmblooded and coldblooded ways of life without the drawbacks of either. It truly is the perfect ectotherm. Borrowing from Nature

Although it has not directly been influenced by nature, the counter-current heat exchange mechanism used to preserve heat in leatherback turtles is widely used in engineering to preserve heat or control temperature. Man-made counter-current heat exchanger work in exactly the same way as in nature. Pipes containing cold and hot fluids are placed next to each other, but with the fluid flowing in the opposite direction. Heat is transferred from one fluid to the other. This is a perfect example of evolution and human invention arriving at the same adaptation independently.

CHEMICAL DEFENSE—BOMBARDIER BEETLE When it comes to protection from predators, sometimes attack is the best form of defense. This is a trait employed by many animals, plants, and bacteria that otherwise look very vulnerable to predation. One of the most effective forms of defense is to use toxic chemicals that debilitate potential predators. We are very familiar with the stings of bees and wasps, but these are just the

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tip of the iceberg when it comes to venoms and poisons that have evolved in the longstanding chemical wars in nature. Some of the chemicals that have evolved and which are deployed against potential attackers can cause pain, swelling, blistering, numbness, vomiting, and even seriously raising or lowering blood pressure. One species, though, goes to the extreme of mixing a cocktail of reactive chemicals in its own body before A bombardier beetle, Brachinus sp. releasing it on an unsuspecting predator. [Richard Parker / Photo Researchers, Inc.] Bombardier beetles (Brachinus) are small and unassuming looking insects being only about 1 cm long. Although they can fly, bombardier beetles, like other ground-dwelling beetles, take a little while to prepare themselves for flight. Anyone who has seen ladybugs take off will know that they first have to open out their hard wing casings (called elytra) and then unfurl their wings beneath. Under attack, a beetle cannot afford this delay, so bombardier beetles have evolved another defense mechanism that allows them time to escape. Beneath the bombardier beetle’s plain exterior lies a powerful chemical weapon to strongly discourage potential predators from taking too close an interest. Within its abdomen are two glands that produce two highly noxious chemicals—hydroquinone and hydrogen peroxide. They are unpleasant enough chemicals on their own (anyone who has bleached their hair blonde will attest to the eye-watering properties of hydrogen peroxide), but this isn’t enough for the bombardier beetle. When the beetle is threatened, both chemicals will flow from their glands into a main reservoir, which in turn feeds into a reaction chamber. The reaction chamber is covered with specialized cells that produce and release enzymes (the molecules that control biological reactions) that break down the hydroquinone and hydrogen peroxide into smaller molecules. These reactions produce a huge amount of heat energy (more than 210°F [100°C]) that vaporizes the chemicals, adding to their potency. The pressure increases until the hot and noxious chemicals are violently forced from an opening on the tip of the bombardier beetle’s abdomen with an audible ‘‘pop’’ and into the face of a predator. This jet of chemicals can have a devastating effect on potential predators. It can certainly cause serious pain on mammal and insect predators, and some small predators can be fatally wounded by the defense. Even curious collectors can have their skin badly burned if they don’t handle the insect with care. Given the potent weapon housed within the bombardier beetle’s abdomen, there is an understandable number of safeguards to ensure the beetle doesn’t kill itself with a misfiring explosion. The valve between the reservoir and the reaction chamber is one way, so there is no chance of the reacting chemicals

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flowing back into the beetle’s body; the only way is out and into the face of an attacker. For this reason, the beetle must empty the reaction chamber before topping it up with new chemicals—the valve will only open when the reaction chamber is clear. The reaction chamber itself is thickly walled to prevent the chemicals from rupturing the beetle’s own body. Despite these necessary precautions, the whole process can be achieved in a fraction of a second. One abdomen full of superheated chemicals can be expelled at an attacker in up to 30 controlled bursts. Each burst can send a spray up to 20 cm. Ground dwelling beetles like the bombardier beetle can come under attack from a number of potential enemies, not least of which are ants that scurry over the ground and will attack more or less anything moving. Ants are small and hard to shift, which is why the bombardier beetle has evolved to be highly accurate with its chemical attack. The tip of the abdomen of these beetles is highly mobile so it can direct a deadly accurate burst to wherever the danger lies. Evidence shows that it can target a predator (like an ant) that has grabbed hold of particular leg. Not only can bombardier beetles target which leg is being attacked, but even which segment. It can even aim at predators attacking its back. This is far from being a crude weapon. What remains a mystery, though, is how the bombardier beetle avoids scorching its own body in these attacks. Whatever the mysteries remain with this remarkable insect, it has found an incredible method for dealing with predators who could kill an otherwise defenseless animal. Borrowing from Nature

Teams from the University of Leeds in the United Kingdom are learning how the bombardier beetle can spray its liquid so far and so accurately. Replicating how the bombardier beetle fires its chemical cocktail, the team has been able to fire pulses of hot water over distances of up to 13 feet (4 m) and has been able to control the size of the droplets in the spray. The technique has applications for inhalers, needle-free injections, fuel-injection systems in engines, and as fire extinguishers. The fire extinguishers could be especially neat. The Leeds team’s system allows for control of the droplet size so the spray to put out the fire can be tailored to the circumstances. Other teams are exploring the bombardier beetle’s ability to produce a hot chemical reaction for use in the aviation industry. This could provide a useful answer to the problem of how to reignite a gas turbine engine that has cut out, not an easy feat when the temperature outside could be as low as minus 58°F (−50°C).

CHEMICAL DEFENSE—DEVIL’S GARDEN ANT For many species, defense means growing some form of physical protection. Nature is full of species with tough shells, sharp spines, or foul-tasting

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chemicals to deter predators. Some species, though, have evolved a mutually beneficial partnership that involves both species protecting each other. Ants are well known for the mutual partnerships that they have evolved with other species. Many ant species will tend and protect certain species of plant in exchange for food and shelter. To achieve this, ants tend to use the energy released by chemical reactions to deter predators of their host plant. Chemicals produced in their bodies can burn and irritate any animal who happens to think about browsing on an ant-protected plant. One species, though, has taken this protection to the extreme, using chemical energy not only to deter predators, but also to take care of their favorite plant’s competitors as well. The devil’s garden ant, Myrmelachista schumanni, is a small ant that lives in the Amazonian rainforest of Peru. Like many ants, it has formed a close relationship with one species of plant—in this case, the tree Duroia hirsuta. So diligent is the devil’s garden ant in looking out for its host tree that it can help cultivate an entire stand of some 600 Duroia trees to the exclusion of any other plant species. This is a remarkable achievement given the extraordinary plant biodiversity in the Amazonian rainforest. Everywhere else in the rainforest this same area of land would be covered by hundreds of different plant species. Indeed, so unusual is this phenomenon of an area being dominated by one plant species that local legend tells of the stand being created by an evil spirit. This is how these crops of Duroia have come to be known as ‘‘devil’s gardens.’’ So how does such a small ant engineer such an otherworldly feat of gardening in the most species-rich environment on the planet? Like many ants that form these mutualisms with their host plants, the devil’s garden ant will protect Duroia from herbivores. They will attack and possibly kill other insects that attempt to eat the leaves of the host plant. The ants will also spray larger herbivores with formic acid, an irritant, to discourage them from taking further bites out of their host tree. Direct defense against herbivores will certainly help Duroia flourish, but it cannot explain why no other species of plant grows in the devil’s gardens. To achieve this, the devil’s garden ant takes a more direct approach with other plant species that happen to be growing nearby. The devil’s garden ant can recognize which plants are Duroia and which are of a different species. When it finds a non-host species, ants will climb up on to the leaves and bite a hole in the tissue. The ant will then insert its abdomen into the hole and squirt a drop of formic acid into it. The formic acid spreads through the veins of the leaf and will, within a few hours, cause necrosis—the death of the tissue. This happens on a massive scale, and soon any plant that is not Duroia will be bereft of leaves and its means for making food. Established trees may take a while to die, but once a Duroia stand is established the devil’s garden ant will keep it clear of other species by killing off any unfamiliar seedlings as soon as they emerge from the soil. Freed from competition from other plants, Duroia trees flourish. Most important for Duroia is that is can grow out of any shade from larger trees.

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This allows it to get more light, which makes it more productive and which, in turn, allows for more rapid and healthy growth. The ants gain from this relationship because it is within specially modified Duroia stems, called domatia, that they live. More trees means more domatia in which to live and rear young. Furthermore, the ants carry out other farming activities inside the domatia. This time, instead of cultivating a crop of trees, they farm livestock, or at least scale insects that live on the sap of the Duroia. The ants look after these scale insects and feed off their honeydew—a sugary, sweet waste product secreted from the scale insect’s abdomen. A mature stand of Duroia can support a very large colony of ants, although this one colony will have been started by just a single female. The devil’s garden ant queen will colonize a single, isolated Duroia tree. After she has reared her first brood of young, her offspring will clear the forest by poisoning rival plant species, and over time more and more Duroia trees will grow in the space left. The ants will spread to colonize the new Duroia trees and so on as the stand expands. The whole stand is maintained by the single colony. Devil’s garden ant colonies have multiple queens, which means the colony and the unique genetic material that is represented in that colony can survive for hundreds of years—and potentially indefinitely. Researchers have found colonies tending Duroia stands that are over 800 years old, and there are others that could be much older. Since this is such a beneficial mutualism to have evolved, why do Duroia stands not dominate the whole rainforest? There is no limit to the size of the ant colony. There are many queens, so there is no definitive range that they can occupy. The answer seems to be that the ants are perhaps too successful in excluding other plant species from their garden. Duroia trees in devil’s gardens grow so vigorously that they attract many more herbivores eager for a meal than they otherwise might. Such a predominance of one species that is so productive leads to a high rate of herbivory—more than the ants can protect against. Once the devil’s garden reaches a certain size, the rate of herbivory seems to be sufficient to prevent significant spread, although it is not clear if this is the whole story. Clearly, though, the adaptation is extremely beneficial to the Duroia tree, the devil’s garden ant, and even the scale insects that are farmed inside the trees domatia. Borrowing from Nature

It is perhaps fair to say that the devil’s garden ant and those like it have not led to highly innovative breakthroughs in human invention. But the formic acid that they produce when under attack does have a use for the people who live in the forest. Locals will put their hands on certain ants’ nests to elicit a defense response from the colony inside. Before long, hundreds, if not thousands, of ants swarm out to defend their home, squirting formic acid on the intruder. Once the ants have been brushed off the hands and arms, the person

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who incited the attack will be well covered in formic acid, which will be used to rub on the skin and act as a strong mosquito repellant—a handy trick in areas where malaria is prevalent.

ELECTRICITY There is little doubt that one of the most significant human advances has been to harness electrical energy. Nearly everything in our lives makes use of the energy held within electrical currents. Even our bodies are similarly reliant on electricity. Through our nerves, electric currents control our movements and form our memories. Other animals have evolved to use electricity to communicate with each other or as a weapon. Some have even evolved to detect electrical currents emitted by their prey, making them highly efficient hunters. Probably the most well known organism that uses electricity is the electric eel (Electrophorus electricus). This inhabitant of the murky river waters of South America is not actually an eel but really a type of knife fish, related to carp and catfish. The ‘‘electric’’ part of its name is spot on, though. Most of its body is taken up with the necessary organs to produce and emit electricity. All its other major organs, from mouth to anus, are crammed in to the front 20 percent of its body. The remaining 80 percent of the electric eel’s body is made up of three separate organs of modified muscle used for generating electricity. Running nearly the length of its body on the underside is the Hunter’s organ. Its dorsal side is then divided into two organs: the Main organ toward the head end of the body and the Sach’s organ toward the tail end. The electrical organs are each made up of highly modified, flattened muscle cells. These disc-shaped cells, called electrocytes or electroplaques, are stacked one in front of the other and run the length of each organ. Each electrocyte is capable of producing a weak electric charge—only 0.10 to 0.15 volts. This is certainly not much and would not be capable of the huge jolts of electricity for which the electric eel is famous. But the electrocytes do not work independently. They are lined up in series, which means that the electric charge flows from one to the other, building in electric potential. There are some 5,000 to 6,000 electrocytes stacked in the three electric organs, so working together they can produce some 500 to 650 volts—enough to kill a man. The same mechanism is seen in man-made, battery-powered items that require more than one standard battery to operate. So how do these electrocytes work? When not emitting an electrical pulse, the inside of each electrocyte is negatively charged by moving tiny, positively charged sodium and potassium ions (atoms that are missing a few electrons) from inside of the cell to the outside. The positive ions are pumped across the cell membrane by specialized ‘‘pump’’ proteins. In this resting state there is no current flowing from one cell to another. All cells are negatively charged on the inside and positively charged on the outside. This is like putting batteries in the remote control the wrong way—the same poles are next to each

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other (positive to positive and negative to negative) so that no current can flow from one cell to the next. To generate an electric pulse, a change must happen in the electrocytes. Electrocytes are like normal muscle cells—they are each attached to a nerve cell. Normally the nerve cell would cause a muscle cell to twitch, but in electrocytes they have a different effect. Each electrocyte is shaped like a flat disc and has two obvious sides (think of the two sides of a frisbee or plate). The nerve is attached to just one of these sides. When a nerve impulse reaches the electrocyte it causes the protein pumps on that side of the cell to open and stop pumping positive ions out of the cell. This allows the positively charged ions to rush inside the cell. This is like holding water at a dam and then opening the sluice gates. Now, the inside of the cell on that side switches from being negatively charged to being positively charged. The neat part is that the other side of the electrocyte—the side that has not been stimulated by a nerve—remains unaffected. The ion pumps are still working, so the non-nerve side remains negatively charged inside the cell and positively charged outside the cell. If we look at the charges in the electrocyte now, on one face of the cell they are negatively charged outside the cell and positively charged inside. On the opposite face of the cell there is a negative charge inside the cell and a positive charge outside. From one side of the electrocyte to the other, the charges now alternate. This means a current can flow from one side of the cell to the other. The electric current must flow from one electrocyte to the next to generate a large overall buildup of electricity. To achieve this, the nerves triggering the reaction in each cell must be timed to perfection to switch the polarity of the cell just in time for the electric charge to be passed on. The nerves must stimulate each subsequent electrocyte a split second after the previous electrocyte was stimulated. After discharging the electrical charge stored in the electrocytes it does take a short time to ‘‘recharge’’ the cells, although this charge can be held for a long time. Electric eels have been known to still be able to discharge a jolt of electricity up to nine hours after their death. Each of the three electric organs produce electricity in the same way, although they differ in their function. The Sach’s organ produces a constant, weak electric field that seems to have a number of functions, including communication. The most important function, though, is as a navigation device—very important for a fish with poor eyesight that lives in murky waters. The electric field emitted by the Sach’s organ is distorted by the presence of objects in the field. These distortions can be detected by specialized sense organs called electroreceptors that are sensitive to electrical energy. If either prey or a predator is detected, the electric eel can produce a much stronger electric charge to stun other fish that it wants to eat or flee from. This 650 volt charge is produced in the Main and Hunter’s organs. Electric eels are not alone in their ability to generate or detect electrical energy. Many fish produce an electric charge for communication and

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navigation. It is not surprising then that other species of fish have evolved to detect these signals as a way to find prey even where light conditions are poor. The duck-billed platypus, a most unusual creature all around, is one of only two mammals known to detect electrical charges (the other being the echidna). Again, it makes use of this remarkable ability to hunt for food. The use of electricity in this way is certainly unusual in nature, but it allows for fish like the electric eel to gain a huge advantage over its prey and competitors in an environment where vision is poor. Borrowing from Nature

All organisms move charged ions around cell membranes as part of their normal cell processes. This is not always done expressly in order to produce a strong electrical current like the electric eel does, but it can have that helpful side effect. To this end bacteria are already being put to good use for the dual function of breaking down chemical waste into less harmful by-products and generating useful electricity at the same time. As discussed in the entry on chemosynthesis, some bacteria can use a whole range of chemicals as a food source. Certain bacteria get their energy from this waste by aerobic respiration. In aerobic respiration, oxygen is needed to mop up electrons released in the process of breaking down the chemical food source. In controlled conditions, these bacteria can be deprived of oxygen and the electrons they produce can be harnessed and used to generate an electric current. Microbial fuel cells are, to date, only found in laboratories and are not commercially viable, even on a small scale. They are capable of producing only a few volts, although their designers are hopeful of being able to produce microbial fuel cells that can be stacked and used in series in the same way as the electrocytes of electric eels. This could be a prize well worth aiming for. Not only does it offer an inexpensive solution to energy production, but it also could be the answer to the problem of disposing of hazardous chemical waste at the same time.

BIOLUMINESCENCE In nature, it is unusual for species to evolve any trait that draws too much attention to themselves. For the most part, if you are easily seen, you are easily eaten. However, sometimes it is very beneficial to be seen, and some organisms can be very striking indeed if it suits their needs. To these ends we see many dazzling displays of color in peacock tails or brilliant flowers that draw attention to themselves to reproduce. In more extreme cases, though, nature has moved beyond simply using color and has evolved the ability to actively produce light. Human engineering has come up with some fairly crude ways of making light, and for the most part we rely on the humble light bulb. Here, electricity

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passes through a filament that glows and produces light. However, as anyone who has touched a light bulb that has been on for a while will know, this form of light also produces a lot of heat. In fact, 90 percent of a light bulb’s energy is wasted as heat. Any organism that wanted to produce light in this way would soon find itself a readymade barbecue for a passing hungry predator. A species of loosejaw dragonfish, PachystoInstead, another method has evolved, mias microdon. The tear-shaped organ below and this method doesn’t produce any heat its eye (the photophore) emits a red light at all. It relies on two chemicals—luciferase which illuminates potential prey. The dragonfish is one of the few fish species that can and luciferin—that when they react prosee red light, allowing it to hunt with stealth. duce a bright, cool glow. This is because the reaction is so efficient that 100 percent [Dante Fenolio / Photo Researchers, Inc.] of the energy released by the reaction goes into light production. No wasted energy is released as heat. Both chemicals can be safely stored by the organism and combined when needed. The production of cold light by organisms is called bioluminescence. Bioluminescence is found in many species both on land and in water. On land, it is found in glow-worms (which is actually a beetle, Lampyris), fireflies, and even in the earthworm. The eerie glow sometimes seen on rotting matter is caused by light-producing bacteria and fungi. But it is in the ocean where bioluminescence is most widespread. Light from the sun cannot penetrate the sea more than around 650 feet (200 m) below the surface. Below this point, approximately 70 percent of all species have some form of bioluminescence, which makes sense with no natural light about. In the sea, bioluminescence is found in species of all sizes. Single-celled plankton called dinoflagellates can produce a light that can be seen from space when they gather in a large enough bloom. Certain fish (such as the angler fish) and jellyfish (the angler jellyfish) can produce light with which to lure prey to their waiting mouth. Even the largest squid in the ocean can produce light. Each of these animals puts their light to their own use—for defense, hunting, mating, and even to disappear completely from view. Because deep under water there is very little light, many deep-sea organisms have very poor eyesight. Indeed, what little eyesight they have is restricted to seeing blue light because red light cannot penetrate far in water. One family of deep-sea fish, however, has evolved a neat trick to exploit this. Species of the Loosejaw dragonfish family (the Malacosteidae) can see red light. To make the most of their keen eyes, they have evolved the ability to produce red light from an organ beneath their eyes. They quite literally have a couple of torches below their eyes with which they can penetrate the gloom of the murky deep ocean and pick out tasty morsels. Because this torchlight is red, which other

MAKING AND USING ENERGY

ocean-bound organisms cannot see, potential prey are not alerted to the loosejaw’s presence. Nor, indeed, are potential predators of the loosejaw drawn to its torches, making these fish the perfect stealth-attacker. There are times, though, when species use light to be seen. Most people are probably familiar with the light that can be produced by fireflies in controlled flashes. There are several species of firefly, nearly all of which use flashing lights to attract potential mates. To avoid confusion, each species has a particular pattern of flashing light so it attracts members of the same species. However, in one instance this species-specific light display has been exploited to a deadly end. Females of the species Photuris mimic the flashing pattern produced by females of the species Photinus. Photinus males are fooled by this trickery and are drawn to the mimicking Photuris females. Instead of finding a willing mate, these unwitting males encounter a hungry femme fatale and are quickly devoured. Interestingly enough, the story doesn’t end there. Recently, scientists discovered that the femme fatale fireflies gain an additional benefit from eating the Photinus males. In eating the males of a different species they absorb a defensive poison called lucibufagin, which they are unable to make themselves. Fireflies that have this poison repel the spider predators that feed on fireflies—in particular, Phidippus jumping spiders. So not only do these femme-fatale fireflies get a tasty snack, but they also gain valuable protection from predators. Perhaps the most thorough use of light is found back in the ocean. All squid are capable of producing light. Certain species, though, like the large squid Taningia danae, have taken it to the extreme and exploit light production in three separate ways. Squid have light emitting organs (called photophores) located all over their body. On the main trunk of the body the large squid can produce a light that varies in color depending where they are in the sea. Much like a chameleon effect, the large squid can produce a light that causes it to blend in with its surroundings, protecting it from predators and remaining hidden from unwary prey. Safely obscured from view, large squid can hunt for their own prey, and they even use their light producing powers to help them do that. Photophores on the end of their arms can deliver a bright flash that can both blind its prey and allow the squid to judge the distance to its now bewildered victim before it starts its final assault. This is particularly important in the dark deep ocean where the inability to see prey can give it the chance it needs to escape. When it comes to finding a mate, the large squid again uses its incredible ability to produce light to assist it in its goal. The changing colors of the large squid are used in a courtship display to attract a potential mate. It seems that the complexity of the display is what makes a suitor particularly attractive, although this behavior has not been well studied and there is still much to learn about these fascinating creatures and their amazing ability to produce and use light.

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Borrowing from Nature

We know that all man-made devices for producing light are very inefficient. Consequently, engineers are keen to replicate the perfectly efficient bioluminescence seen in nature. However, it has proved tricky to create a light bulb that uses the luciferase and luciferin reaction because a constant supply of these chemicals is needed to produce a continuous light. In nature, bioluminescent organisms can make more, but that is not possible in man-made devices. Luciferase and luciferin have been used in glow sticks that can be ‘‘cracked’’ to break a connection between two chambers, allowing the chemicals to mix and produce a bright, if short-lived, light. Perhaps more interestingly, though, bioluminescence is being developed in the field of medicine. Bioluminescence has been used in gene and stem-cell research to show where and how certain genes work in the body and how they work differently to cause certain diseases. A gene that codes for bioluminescence is inserted next to the gene that is being studied. When the gene being studied is activated, there is a characteristic bioluminescent glow to signify that the study gene is working. This is used in both in vitro and in vivo gene research. It is thought that this way of targeting certain genes and their products could be a great step forward in the treatment of cancer.

2 SURVIVING THE ELEMENTS

SURVIVING THE ELEMENTS—HUMAN INVENTION Perhaps surprisingly for an animal that has colonized every land mass on earth, humans are not well adapted to the extreme environmental conditions that they encounter. Naked and exposed, humans suffer from heat, cold, high and low pressure, drought, lack of oxygen, and radiation. The human body does have some ability to become acclimatized to extreme conditions, though. At high altitude, for example, muscle cells produce more mitochondria, and blood cells produce more hemoglobin in order to compensate for the lower concentrations of oxygen in the air. And yet, despite this adaptability of the body, humans would die out from many of the places they currently live if it were not for the various inventions they have created to allow them to survive. Human ancestors left Africa some 1 million years ago to colonize the warmer climates of Eurasia, where no particular inventions would have been needed for survival. After that initial migration, there was little human colonization until 100,000 to 50,000 years ago, a period that corresponds with a huge proliferation in human innovation. At this time there was a great increase in tool production and use. It is thought that this Great Leap Forward (to use the phrase coined by evolutionary biologist Jared Diamond) was triggered by a significant evolutionary event in humans that led to greater capacity for thought and problem solving. This proliferation in the use of tools would have allowed early humans to make use of the furs and skins of animals to make protective clothing that, in turn, allowed humans to venture into more extreme habitats. It is likely that since the first human evolved from an ape-like ancestor, humans have been using natural features as shelters. Today, gorillas and

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chimpanzees build nests on the ground or in trees to sleep in, so it is not impossible that early humans used vegetation in a similar way to gain some shelter during the night. We do know, though, that humans were using caves for shelter 50,000 years ago at the time of the Great Leap Forward. These natural shelters would have offered excellent protection from the rain, from cold at night and in winter, and from the extreme heat and radiation of the sun in the middle of the day. At the same time, humans would have had good fire-starting skills that could warm the caves and provide further protection from the cold. Caves, of course, would not have been the only shelters used by humans as they migrated across the continents. Cutting tools would have allowed humans to cut wood to the appropriate size to build a variety of shelters. As we know from indigenous peoples alive today, a range of frames can be built and covered either with vegetation and mud or with skins from animals. Such shelters could be made permanent or temporary according to the need at the time. As with many inventions, the use of natural materials allowed humans to adapt well to the challenges they faced. With clothing made from furs and skins, and shelters built from wood, stone, and even snow, human populations could colonize and survive in more or less any environment on the earth. It wasn’t until around 100 years ago, though, that there were even greater leaps forward. Perhaps the most extreme land environment on earth is the Antarctic. Since the early 1900s humans have pitted their strength, stamina, and fortitude against this freezing, dry habitat. The clothing worn by these early explorers were heavy and designed to keep the wearer warm and shielded from the huge winds that blow through the frozen desert. Again, they were made from animal skins and furs, but they had one major drawback. The clothing was not breathable, which meant that sweat could not dissipate. During periods of intense exertion the men (explorers would have always been men at this time) would sweat, which would soak their clothing. In periods of less activity, the sweatsoaked clothes would draw all-important heat away from the wearer, causing him to get colder and colder. Learning from the lessons of these first Antarctic explorers, modern coldweather clothing relies on man-made fabrics and a system of layering. The base layer close to the skin is thin, soft, and made from a water-repelling (hydrophobic) synthetic material that does not absorb water, therefore moving it away from the body. Above the base layer modern explorers wear insulating layers, which can be removed or added depending on the conditions. A typical insulating layer is made from a polypropylene fleece, a versatile plastic polymer that can be woven into a thick insulating layer and even treated to be waterrepelling. The outer layer gives mainly protection from the wind. It does not have to be waterproof because no rain falls in the Antarctic. In fact, by not being fully waterproof, the clothing can breathe and allow perspiration to escape. There will be some insulation from this layer, especially around the

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neck and cuffs, from synthetic fibers or natural ones. Even today, though, arguably the best insulating material is the down from birds, such as the eider duck. Even with these modern clothes, traveling to the Antarctic still remains a huge challenge to humans. And it is not just the earth’s poles where temperatures can drop to life-threatening temperatures. At the highest points on earth the freezing temperatures can still be extremely dangerous. Furthermore, at that altitude there is less oxygen in the air, which begins to starve muscles of oxygen and makes work difficult. Beyond 23,000 feet (7,000 m) above sea level, the low levels of oxygen indirectly lead to serious brain conditions that can cause death. The body’s response to low levels of oxygen is to send more and more oxygencarrying blood to the brain. Unfortunately, this has the effect of causing the brain to swell, physically damaging it. This is why altitudes above 23,000 feet (7,000 m) are known as the death zone. The human body can only survive a limited time here before it must turn back or die. To counteract the effects of the death zone, mountaineers carry with them independent supplies of oxygen to avoid brain swelling. In fact, these oxygen canisters are filled with a normal mix of some 21 percent oxygen and 79 percent nitrogen—the same composition as the air we normally breathe. It is the same, fairly simple technology that is used to exploit one of the most inhospitable environments on earth for humans—underwater. Although the mix of air and the design of regulators that control the flow of air to a diver have changed over the years, air tanks for scuba divers have remained more or less the same since invented by Jacques Cousteau and Emile Gagnan. The invention of ‘‘portable air’’ has been key to humans’ ability to travel to places where they would otherwise be unable to survive. However, the canisters carried by underwater divers and mountaineers have a very limited supply—after a few hours the explorer must return to the surface or to lower altitudes unless he or she can carry a huge number of spare canisters. However, there are some artificial air supplies that last much longer and have allowed humans to conquer perhaps the most inhospitable environment of all—space. Rather than compressing air into canisters (which is still used to supply a certain amount of air), space stations are fitted with devices that have oxygen stored chemically. Simple chemicals like potassium chlorate (KClO 3) and sodium chlorate (NaClO3) contain oxygen (the O3 part of their chemical formula). This oxygen is not bound particularly strongly in these chemicals and with a little heat it is readily released as a gas (leaving common salt). These chemical oxygen stores are extremely efficient ways of storing a lot of oxygen in little space. Weight for weight a chemical oxygen canister is able to supply 10 to 20 times the oxygen than a regular compressed air canister. A chemical oxygen canister contains a sodium chlorate pellet and an igniter to heat it sufficiently to release the oxygen. Such devices are used in space stations, airplanes, and submarines. Although, as they occasionally have a tendency to cause fires (because of the need for a heat source and due to the fact that they

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release oxygen, a flammable gas), they tend only to be used as emergency backup supplies. Space travel not only presents humans with the problem of no oxygen, but also of radiation. The high energies held within certain wavelengths of the electromagnetic spectrum and the high energies held within radioactive particles—both of which are encountered in space—cause physical damage to human cells and organs. The earth’s atmosphere offers some protection against these dangers, but out in space no such natural protection exists. On spacecraft, metals are used to offer some protection from radiation, such as aluminum. This has a similar effect to lead-lined suits used to protect wearers from radiation leaks on earth. These metals do not offer complete protection, however, and innovative solutions are being explored for long-distance space exploration such as a possible journey to Mars. Exotic solutions such as electrostatic fields and liquid hydrogen shells are among the suggestions being considered. On earth, though, perhaps some of the simpler designs are the ones that have been the most successful and the most useful to us in our daily lives. A particularly hot day in summer can submit us to dangerous levels of ultraviolet (UV) radiation that can increase the risk of cancer. Sunglasses and sunscreen that contain UV filters can reduce the risk by blocking the light waves before they can penetrate our skin and damage our cells. Within sunscreen, two basic types of UV filter can be used. Organic (carbon-containing) chemicals directly absorb UV. Inorganic chemicals (such as zinc oxide) tend more to reflect the UV light away from the skin. Through its remarkable ability for innovation, the human species has been able to exploit all the habitats on the planet. However, some extreme habitats exist where even the most modern technology cannot keep humans alive indefinitely. Although no one organism is more successful that humans in the breadth of habitats it can survive in, there are many species out there that can survive indefinitely in the places where humans can only venture for a short period. Some of these species can even survive in space.

THERMOPHILIC BACTERIA—SURVIVING EXTREME HEAT Survival at very high temperatures is tough. Humans, like all mammals, are able control their body temperature, but these controls are limited and at certain temperatures our bodies will suffer and die. Above about 110°F (45°C) our muscles will become rigid and immobile, and the millions of proteins that drive the reactions inside our cells (enzymes) will break down. Because of our own experience and the experience of most animals and plants, it was thought that no life could survive indefinitely at temperatures much above 120°F (50°C). True, some species had been observed to enter protective states at very high temperatures, but none were seen to thrive and reproduce at such high temperatures.

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That is, until the late 1960s when scientist Thomas Brock discovered a new species of bacteria living in the hot springs of Yellowstone National Park. These springs are made from hot water bubbling through the earth’s crust, having been heated by the molten magma of the earth’s core. It is not surprising, then, that temperatures in these springs reach close to boiling point: nearly 212°F (100°C). Yet within these springs bacteria were found to thrive. The first bacteria to be discovered by Brock was called Thermus aquaticus, referring to the hot water in which the bacterium was found. But A computer simulation of DNA-polymerase since then around 50 thermophilic in the process of replicating DNA (the helical (meaning ‘‘heat loving’’) bacteria have molecule in the center). Bacteria like Thermus been found either in the hot springs on aquaticus have very robust DNA-polymerase the surface of the planet or in the hydro- enzymes that do not break down at high temthermal vents found deep within the peratures, allowing them to survive in conditions of up to 250°F. Resistant DNAocean. Until they were discovered, such bac- polymerase enzymes from these bacteria are used in genetic and forensic research. teria were not expected to exist. Above [Laguna Design / Photo Researchers, Inc.] 120°F (50°C) a number of things happen. The cell membranes that keep an organism’s cells together will break down. DNA will begin to unravel. The proteins and enzymes that drive the cellular reactions that are essential for life lose their shape and become useless. In large, multicellular organisms like humans the body can respond up to a point to protect its cells, DNA, and proteins. But if large animals can’t survive in near-boiling temperatures, what chance does a single-celled bacterium have? Thermophilic bacteria have evolved a number of neat adaptations to help solve the problem. First, their cell membranes are not like the cell membranes of animals. They have a number of molecular ‘‘add-ons’’ that keeps the membrane very rigid and resistant to temperature. There are various add-ons that help, but the most important are molecules of fat that are combined with a molecule of ether (the same compound that, in gas form, was used as an early anesthetic). The extra fat and ether molecules act as a scaffold that supports the cell membrane and give it extra strength. Thus, the bacteria can stay in one piece, but there is still the problem of protecting the DNA, proteins, and enzymes within the cell that keep the bacteria alive. DNA contains all the genes that control the lives of all living organisms. It is made up of two strands, which are tightly twisted together like a rope. DNA must stay coiled up; otherwise it becomes easily damaged. At high

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temperatures, though, this coiling breaks down and the two strands fall apart, which shortly leads to an organism’s death, particularly in a single-celled organism like a bacterium. Thermophilic bacteria such as Thermus aquaticus have a unique ability to coil up their DNA even tighter than usual by twisting it back on itself repeatedly. This is called supercoiling. It does this with a type of enzyme that is not found in any other living organism. By coiling up very tight, the bacteria DNA can combat the damaging effect of extreme heat. Finally, thermophilic bacteria have very rigid and robust cell proteins and enzymes. These are complex biological molecules that control the reactions that go on inside all living organisms’ cells. They are intricately folded molecules, and their shape is the key to their success. At high temperature, proteins and enzymes lose their shape, making them useless—a bit like a shapeless, melted key trying to unlock a door. All enzymes hold their shape through a series of linking molecular bonds that act as supporting struts. In organisms that live in cooler environments these bonds are very weak and easily break down. They are much stronger in thermophilic bacteria, however, allowing the molecules to keep their shape. Thermophilic bacteria all belong to a group of bacteria called the archaebacteria, very ancient bacteria that have been on this earth for as long as life has been. It is thought that they first evolved this ability to thrive and reproduce in extreme high temperatures when the earth’s climate was very hot and very steamy. It seems that some of the species maintained this ability to survive in near-boiling conditions and used it to exploit environments like hot springs and hydrothermal vents. Being rather unique, they can exploit these habitats free from the competition of other bacteria. Species like Thermus aquaticus found in hot springs tend to thrive around 160 to 175°F (70 to 80°C). Hydrothermal vents in the deep ocean tend to be hotter, and species are found there are capable of withstanding temperatures of more than 212°F (100°C). (Because the water in the deep ocean is under such high pressure, it doesn’t boil.) The bacterium Pyrolobus fumarii can still reproduce at 235°F (113°C). The current record holder is a very recently discovered species—simply called Strain 121—that can reproduce at an incredible 121°C or 250°F! Borrowing from Nature

During growth and reproduction all organisms need to replicate cells. All of us started life as a single cell that has been replicated many times during our lives. One of the key steps in the process of cell replication is the replication of the DNA held within it. This in itself is a fairly complicated business, but in essence it involves pulling the two strands of DNA apart and replicating them both to produce two ‘‘daughter’’ copies from the original. The enzyme that controls this DNA replication process is called DNA polymerase. DNA polymerase is found in Thermus aquaticus like any other organism, although, of course, the enzyme in T. aquaticus is resistant to extreme heat.

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This feature of its DNA polymerase has been exploited to allow for one of the most significant techniques used in genetics today—the polymerase chain reaction. Researchers and forensic scientists who want to work with DNA for whatever reason need a lot of it to carry out their tests. Rather than get a huge sample of DNA from the subject (which may not even be possible in police forensic work), a small sample of DNA is taken and replicated over and over again, making use of the DNA polymerase enzyme. The process of replication by the polymerase chain reaction can be carried out automatically by machines in just a few hours. But it requires high temperatures of around 175–195°F (80–90°C) to work. DNA polymerase taken from any other organism would just not work at these temperatures, but the DNA polymerase from T. aquaticus can. As a result, geneticists and forensic scientists can get the amounts of DNA they need to carry out their work.

BLOOD ANTIFREEZE—SURVIVING EXTREME COLD All organisms, from the humblest single-celled bacteria to the largest mammal, need some fluid or other to carry essential nutrients around their body to stay alive. In animals like humans, this life-critical fluid is blood. Blood carries nutrients to organs, and it helps regulate body temperature. Without it, animals die. Blood can be lost if the animal sustains a serious injury, causing it to bleed to death, but that is not the only way. It is much less common, but the same effects of blood loss can occur when an animal’s blood freezes, which is what happens when its body temperature falls to below 29.3°F (−1.5°C). Because of these serious consequences, most animals avoid very cold temperatures. One group of fish, however, calls these environments home. The Notothenoid fish are native to the freezing waters of the Antarctic and make up over 95 percent of the creatures that live there. Here the seas are as cool as 28.6°F (−1.9°C), only fractionally warmer than the freezing temperature of salt water: 28.4°F (−2.0°C). Normally, this would freeze the blood of any fish straying into these waters. The Notothenoids, however, have a secret weapon. They have a biological antifreeze in their blood that stops their bodies from freezing. The Notothenoids can cope with very tiny ice crystals forming in their bodies. Their secret is to stop these tiny ice crystals from growing bigger, and having a damaging effect, by producing a group of proteins called antifreeze proteins. These antifreeze proteins bind, at a molecular level, to the tiny ice crystals in the body and prevent them from growing bigger. This way, the fish can lower the threshold temperature at which their blood freezes to 27.5°F (−2.5°C). This enables them to cope perfectly well in the near-freezing Antarctic waters. It is thought that the Notothenoid antifreeze adaptation evolved some 5 million to 14 million years ago. It has been such a successful adaptation that

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the Antarctic waters are dominated by these fish, not because they have aggressively kept out other species, but simply because they are the only fish that can survive there. Since the early Notothenoids invaded the Antarctic, around 120 species have evolved, each species exploiting a different niche in this freezing ecosystem. The evolution of the antifreeze proteins was a mystery that scientists have only recently understood. Ever since antifreeze proteins were first recorded in the 1970s, it has been assumed that they were produced by the fish’s liver, which is where most blood proteins are synthesized. It was only in 2006 that scientists discovered that the proteins are made in the pancreas and stomach and are secreted into the intestines, where they are absorbed into the blood. It used to be thought that antifreeze proteins evolved from a group of proteins whose use was to prevent the gut from suffering from very cold conditions. This appears not to be the case, though. It is now believed that the ability to make antifreeze proteins evolved from scratch from a mutation in the so-called ‘‘junk’’ DNA of the fish. Junk DNA is apparently useless sections of DNA found between the genes that code for the essential biological molecules that make up the body of an animal. Scientists did not believe that junk DNA could evolve in this way, so there are still some questions to answer before the mystery of how this adaptation has evolved is finally solved. It is fair to say that the ability to make antifreeze proteins has been a remarkable adaptation. Free from competition of less-hardy fish, the Notothenoids have evolved to colonize every part of the ocean. The Threadfin Pithead, Aethotaxis mitopteryx, can be found some 2000 feet below the surface. At the other end of the extreme, the Bald Rock Cod, Pagothenia borchgrevinki, can be found lurking right under the ice that forms over the surface of the sea, and even burrowing right through the ice itself. In fact, the Bald Rock Cod is so well adapted to these freezing climes that it will die in waters warmer than just 42.8°F (6°C). This is the lowest upper threshold temperature known to kill an animal. The Notothenoids truly have conquered the Antarctic seas thanks to this one adaptation. Borrowing from Nature

There are already a number of commercial uses for Notothenoid antifreeze proteins, which is not surprising considering that they are around 300 times more effective than conventional chemical antifreezes of the same concentration. By inserting the antifreeze protein genes in yeast and bacteria, the proteins can be grown and harvested on a large scale. It is hoped that this technique can be adapted and that the genes can be inserted into plants to engineer cold-resistant crops that can resist potentially fatal frosts. Antifreeze proteins harvested from bacteria already have industrial applications, and scientists are exploring their use for cold-storing foods that become

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inedible when frozen. As an extension of this, the fish proteins may find a use in the field of cryogenic storage of body parts. The use will most likely be for transport and storage of organs for transplant rather than for multimillionaires who want their bodies frozen before death and reanimated some time in the future!

MAMMAL FUR—SURVIVING EXTREME COLD As humans, we are well aware of the risks of extreme cold. We are very vulnerable if temperatures drop too low, as we lack a substantive layers of insulation. Being warmblooded animals, we can regulate our internal body temperature reasonably well, but the system is far from perfect. Humans evolved in the tropics and so are suited to warmer climates rather than cooler ones. Naked, a human will start to feel cold as tem- Scanning electron micrograph of the fur of peratures drop below 77°F (25°C). At 54°F the polar bear, Ursus maritimus. The hollow (12°C) humans lose manual dexterity. (Try hairs trap air which help insulate the bear’s tying your shoelaces if your hands are cold body. The hair is also oily to repel water. [Andrew Syred / Photo Researchers, Inc.] and you will know how difficult this can be.) At 46°F (8°C), we lose touch sensitivity. As temperatures continue to drop, the human body will struggle to keep its internal temperature constant. Blood flow will be reduced to the extremities to conserve heat within the bulk of the body, which is why frostbite affects the fingers and toes. As temperatures drop yet further our internal body temperature will drop by a few degrees and the effects of hypothermia will kick in. The affected person will become sluggish and eventually die because the body temperature is too low for metabolism to take place. To get around these problems and to exploit colder climates, humans have taken to using the insulation of other animals. Humans have worn furs for thousands of years to keep warm for the very good reason that fur is an excellent insulator of heat. Mammals are the only animals that are covered in hair. Hair itself is made from a long chain of dead cells filled with the strong protein keratin. It grows from a deep pit in the skin called a hair follicle. Cells grow at the base of the hair within the follicle and are gradually pushed upward as other cells grow behind them. This is how hair grows. This growing point within the follicle is weak, and hairs can be pulled out with relative ease. Hair itself, though, is very strong thanks to its structure of keratin. A single human hair can support a weight of 6.6 pounds (3 kg). Hair follicles are supported by a sebaceous gland that secretes cells filled with a fatty oil that coats the hair to keep it supple, strong, and, to some extent,

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waterproof. Each hair is connected to an arrector pili muscle, which can be contracted to cause the hair to stand on end. This is a very important feature of the hairs that make up an animal’s fur. When the hairs are erect, the fur as a whole becomes ‘‘fluffed up.’’ Air becomes trapped in the fur, which helps insulate the animal’s body. Humans, although essentially hairless, still show this adaptation. When we are cold we get ‘‘goose flesh,’’ which is caused by the arrector muscles pulling to make our hairs stand on end. This adaptation is controlled by the hormone epinephrin (adrenalin), which explains why we get ‘‘goose flesh’’ when we are scared or nervous and our bodies start to produce epinephrin. The simple process of trapping air between a dense layer of hair has proved to be a very successful adaptation. Mammals are able to withstand great extremes of cold thanks to their fur and a layer of blubber. One of the most successful animals in this regard is the polar bear, which has a thick layer of fur that allows it to live quite comfortably in temperatures as low as −34°F (−36°C) and still maintain a steady internal body temperature of 98°F (37°C). Its fur is made from two layers: a layer of long, tough hairs some 5–15 cm long overlays a layer of shorter, softer hairs. The softer fur traps air between the hairs, and the longer fur stops the air from dissipating. The softer fur is white and much like the hair on any other animal. The outer fur, however, is transparent and hollow—the normal core of hair is missing, leaving only the outer sheath. It is this hollow hair that has led to a number of wild speculations about the polar-bear’s ability to keep warm, almost none of which are true. It has been suggested that the hollow, transparent hair might work like a fiber optic cable. Fiber optics are thin, transparent tubes of plastic that refract (bend) light inside them in such a way that it runs down the optic like electricity flows through a wire. It has been suggested that the transparent hair of a polar bear works in the same way and funnels light down the hair directly to the skin, which would be warmed up. This is an interesting idea but just not true. Polar bear fur is effective at keeping heat in, but it is not as high-tech as some people like to think. It is much more likely that hollow fur traps air inside it, giving it further insulating properties. In addition to fulfilling its primary role of insulation, fur has other important secondary functions. Fur can be colored by the molecule melanin produced within the growing cells at the base of a strand of hair. Melanin is actually a whole class of colored molecules of which there are two main types: eumelanin is the dark pigment that is dominant in brown and black hairs, phaeomelanin is a lighter pigment found in blond and red hair. Hairs are therefore restricted to a palate of reds, yellows, browns, and blacks. Mammal furs can come in a wide variety of colors and patterns. Each pelt is perfectly adapted to give its owner a competitive edge, whether it assists with communication, camouflage, or as a warning signal. In some animals, fur even plays a role in getting rid of dangerous poisons. The pen-tailed tree shrews of the Malaysian rainforests feed on the giant flowers of the bertam palm. Thanks to the heat of the rainforest, the sugary nectar

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of these flowers ferments with naturally occurring yeasts and produces alcohol, sometimes in concentrations of up to 4 percent alcohol by volume, about the same as a normal beer. The tiny shrews can consume this without even the slightest sign of drunkenness because their bodies metabolize alcohol quickly and produce a metabolic by-product called ethyl glucuronide. This waste product is safely excreted in the cells that produce the animal’s fur. Thanks to this ingenious adaptation, tree shrews have access to a nutritious food source that many other animals must avoid. Borrowing from Nature

With less and less fur being used for human clothing, there is a great deal of effort being put into finding synthetic materials that match the insulating properties of animal fur. Synthetic materials are coming close to the effectiveness of furs by mimicking the structures. Hollow fibers made from polyester help trap air like polar bear fur. Some artificial furs are layered with a fluffy layer of fibers laid over the top of thinner fibers—again, just like a polar bear. Even the fur of the pen-tailed shrew is being studied, although not for its insulating properties. Instead, research is being undertaken to understand how the tree-shrew’s metabolism works to nullify alcohol by converting it to ethyl glucuronide and excreting it in hair. This could lead to an important drug to help treat alcohol addiction.

MANGROVES—SURVIVING EXTREME SALT Mangrove swamps are unique habitats found on the coasts of the tropics and subtropics. The waterlogged soil is highly saline (salty) and very low in oxygen, making it one of the more harsh environments for plants to grow. The swamps tend to be dominated by mangrove trees of the family Rhizophoraceae, although other species can survive there. These hardy trees have evolved several adaptations to cope with such a poor environment, allowing them to thrive where other species simply cannot survive. With high temperatures causing evaporation from tropical coasts, sea water here has very high concentrations of salt, which causes two problems for plants. The first problem involves the plants’ getting water, which may sound strange because plants growing on the coast are constantly submerged in water. In normal conditions, water will move from the surrounding soil into a plant’s root cells by a process called osmosis. During osmosis, water will move across a cell membrane from a place where salts are very diluted to an area where salts are very concentrated. In normal circumstances, the salt content of water in the soil is very diluted, which means it will pass into the more concentrated fluid within the root’s cells. In mangrove swamps, the reverse happens. The sea water is so salty that it is more concentrated than the fluid in a plant’s root cells. Water therefore would flow out of the root, causing the plant to dry up.

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The highly specialized roots of mangrove plants. Aerial roots allow the plants to draw up oxygen in oxygen-poor waterlogged soils. [F. Stuart Westmorland / Photo Researchers, Inc.]

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The second problem is that too much salt can be toxic to a plant. Salt is made up of sodium and chlorine and will split into these constituent elements when it dissolves in water. If this water is absorbed by the plant, the chlorine will destroy the chlorophyll in a plant’s leaves, depriving it of its ability to derive useful energy from photosynthesis. Salt-afflicted plants often show a characteristic ‘‘scorched’’ look on their leaves where the normally green chlorophyll has been broken down. What makes matters worse on tropical coasts is that tides and variable weather conditions can lead to variable levels of salt in the sea water, making it harder for a plant to adapt to the conditions. One solution to the problem of getting water is to tolerate salt and to make their root sap more salty than the surrounding water by actively absorbing salts. This means that water will be drawn in from the surrounding sea water rather than it being lost from the roots. Of course, this means that the plants must be protected from the high salt levels that they take on board. Species that adopt this method for acquiring water must then secrete the salt that they have taken up. Species like Avicennia, Acanthus, and Aegiceras have specialized salt glands on their leaves that secrete salt. Sodium and chlorine salts are actively transported to these salt glands where the salt is excreted. Water is conserved thanks to a waxy, waterproof coating over the leaves that prevents water from escaping. Salt crystals deposited on the leaves are quickly blown away by the wind or rain. Other mangrove species, like Bruguiera, Lumnizera, and Rhizophora, get water thanks to specially adapted root cells that pump in water and actively filter out the salt. This process is energetically expensive, but it does prevent very high levels of salt from building up the sap. This is not a perfect solution, though, and they must still excrete salt from their leaves as well. Another way in which toxic salts can be excreted is for the plant to transport them to old leaves and stems. These are no longer essential for photosynthesis and can be used as a salt repository. Soon the leaves will fall off, taking the salt with them. These adaptations allow mangrove trees to survive in conditions that would be too salty for many other forms of life to survive, but even they cannot survive extreme salt conditions for long. Mangrove trees must ‘‘flush’’ their cells fairly regularly with fresh water that flows from the rivers and streams that meet the coast. This helps remove salt in the plants’ cells and provide them with much needed salt-free water. Not only must mangrove trees cope with salty conditions, but they are faced with the challenge of growing in very oxygen-poor soils. The mud into which their roots reach is heavily waterlogged—being on the coast—which means there is very little oxygen present. In other plants, oxygen is drawn up by the root cells as well as water. This is possible because most soils will hold water but also have pockets of air within them. This is why gardeners and groundskeepers are so keen to aerate their soils. Mangroves still absorb their essential oxygen through their roots, but they have adapted a unique way of doing it. Given that there is no oxygen to be

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had below water, mangrove roots will grow above the water level as well. These roots are called aerial roots. Aerial roots are found in different guises, but the most common type are the pencil-thin roots called pneumatophores (meaning ‘‘air-carrier’’). Mangrove trees can put out as many as 10,000 of these pneumatophores in order to get enough oxygen to respire. All aerial roots are covered with tiny pores called lenticels that only allow air to pass through; water and salts are kept out. Once air has been taken in through the lenticels it passes into one of the large air spaces within the aerial root called aerenchyma. These air spaces are used to transport air to other parts of the plant, but they also act as a reservoir of air for when even the aerial roots are submerged at high tide. The salt conditions of tropical coasts represent a very harsh environment in which to exist. Mangrove trees are highly specialized to be able to live there, and even these remarkable plants are on the brink of survival. There are very few plants that have adapted to this way of life, which means that mangrove swamps are dominated by only a few species. The reward for these trees, though, is that this environment is their own. Having overcome such inhospitable conditions they can flourish free from competition from other species, making them, evolutionarily speaking, a very successful group of plants. Borrowing from Nature

The ability for mangrove species to survive in highly saline conditions has important implications for developing the crops of the future. Intensive farming relies on the application of chemical fertilizers to the soil. Over time, these farming practices can increase the levels of salt and reduce soil fertility, making it hard to grow conventional crops. Biotechnology companies are already exploring whether it is possible to engineer crops with the ability to cope with salty soils by inserting genes from mangrove trees.

LUNGFISH—SURVIVING WITHOUT OXYGEN

The African lungfish, Protopterus annectens. This species can use both gills and its primitive lungs to gain oxygen. [Tom McHugh / Photo Researchers, Inc.]

It is believed that life began in the earth’s early oceans. Over time life moved from water onto land. This may sound like a fairly straightforward progression, but it is not. Evolutionarily speaking, the move from water to land represents a huge leap. Bathed in water, fish and other aquatic organisms don’t have to worry about conserving water. On land, organisms most certainly do. What’s more, adaptations for moving in water are next to useless on land,

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and of course breathing on land is a completely different prospect to getting oxygen under water. To an organism living in water, dry land represents as severe and hostile an environment as it can get. So how did life move from the seas to land? Surely the need to evolve an entirely different way of life would be too much of a leap. What we must remember, though, is that evolution works by taking small steps, not big ones. Over time, small changes and adaptations build up to allow water-bound organisms to break out and colonize what would have represented an inhospitable desert. Rarely do we see these tiny evolutionary steps take place; we tend just to see the big differences that are the result of thousands and thousands of years of evolution. But there is one group of fish—the lungfish—which still to this day shows adaptations that would have been hugely significant in the move from water to land. The lungfish are a primitive fish from the class Sarcopterygii. They are a solid-looking fish between 5 and 6.5 feet long (1.5–2 m). They can be found in Australia, South America, and Africa in freshwater pools that are prone to cycles of flooding and drought. The wet season represents little problem for lungfish, as they can feed freely in their pools and grow in size. The dry season, though, can cause their pools to shrink dramatically. During these times, the lungfish seek refuge within the mud of the drying pool bed. It can burrow up to 3 feet (1 m) down to seek out moisture. Once buried, they will curl up and excrete a thick mucus that covers their body and prevents them from drying out. The lungfish will also enter a state of torpor where its metabolic rate lowers. It can last up to two years in this state of estervation. As their name suggests, lungfish have evolved air-breathing lungs to cope with these dry seasons, when the pools in which they live will become depleted of oxygen and their gills become useless. When the lungfish burrow into the mud, they will leave a small hole in their mucus cocoon to allow air to get through so they can breathe. The Australian lungfish use gills to breathe during the rainy season when their pools are filled and well oxygenated. They will switch to lung-breathing only in times of drought. The African and South American lungfish, however, have lost the ability to get enough oxygen from water using gills alone and depend on lung-breathing all-year round. The presence of primitive lungs in the lungfish is certainly compelling evidence to suggest how organisms might have evolved the necessary adaptations to colonize land. But there is more to this adaptation than simply being a pair of air sacs capable of gulping down air. Air taken into lungs is not just a mixture of gases, but will hold a certain amount of moisture. Water in the lungs, even tiny amounts, is not a good thing. As the lungs breathe out they will collapse on themselves. When this happens, on every breath, moisture can cause the surfaces of the lungs to stick together. To see the principle at work submerge a plastic bag in water. When you pull it out you will see that it becomes stuck

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to itself because of the binding properties of the water molecules. If this were to happen to lungs, it would make it impossible, or at least very difficult, to breathe in again. To get around this problem, lungfish have evolved an anti-glue (or ‘‘surfacant’’) protein that lubricates the lungs and prevents them from collapsing and sticking together. This surfacant protein (actually a mixture of proteins, fats, and cholesterol) is found in all animals with lungs, including humans. Although our lungs have evolved well beyond the primitive air sacs of the lungfish, we share the same adaptation for keeping our lungs from collapsing and sticking together. Seeing as they are still found in animals separated by some 300 million years of evolution, surfacant proteins are clearly a successful adaptation that have allowed life to flourish on land. Borrowing from Nature

There is little we take from lungfish that has a practical application, although there is currently a great deal of research into understanding the production of lung surfacants as part of the treatment of diseases like cystic fibrosis, which causes the production of a very thick mucus in the lungs. Lungfish also give us a valuable insight into how land-based animals evolved. In addition to the evolution of lungs and their mucus cocoon, lungfish also show other primitive adaptations for surviving dry conditions. Their basic body plan mirrors that of all land-based animals. They have two pairs of fins, which lungfish can use to walk along the bottom of their pool. These correspond to the front and hind limbs of terrestrial vertebrates. Lungfish truly are living fossils that give us a glimpse 300 million years into the past when animals were leaving the water to colonize the land.

MELANIN—SURVIVING RADIATION

A scanning electron micrograph of an eye melanocyte cell, showing the pigment granules which give the cell color. [Steve Gschmeissner / Photo Researchers, Inc.]

The energy of the sun is responsible for so much of life on earth. It is even an important source of vitamin D, which is essential for bone health and immune functions in humans and other mammals. However, the energy from the sun can be damaging as well. Ultraviolet (UV) light from the sun can damage the DNA within our skin cells as well as affect our immune system and eyes. UV radiation destroys DNA at the molecular level, which can cause skin cancer, the most common form being melanoma.

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There are a number of remarkable adaptations that have evolved to protect animals against the sun. For example, DNA has a remarkable ability to repair itself after damage sustained from UV radiation. The first and most important line of defense, however, is melanin, the pigment that gives our skin color and that blocks out much of the sun’s harmful rays. It is our natural sunscreen. There is no doubt of melanin’s ability to protect human skin. Skin cancers are far more common in Caucasians, whose skin is much lighter and lacking in melanin than the skin of African races. But even Caucasian races are able to produce melanin in response to exposure to sun, offering some protection against UV. Sunlight triggers the production of this pigment from the amino acid tyrosin by stimulating the enzyme tyrosinase, which controls the conversion of one molecule into the other. It is this controlling enzyme that is lacking in albinos, preventing them from making any melanin at all. There are two common types of the pigment: red melanin (phaeomelanin) and black melanin (eumelanin). Red melanin is more prevalent in fairskinned people and gives red-haired people their hair color and the reddish freckles on their skin. Red melanin gives some level of protection against UV, but nowhere near as much as black melanin, which can block out nearly all the light falling on it. Black melanin is a dark pigment that is found in a special type of cell under the skin called melanocytes (literally meaning ‘‘black cells’’). The dark pigment directly blocks the sun’s rays. Nearly all light and importantly over 99.9 percent of the harmful UV radiation that falls on melanin is absorbed. The secret to melanin’s amazing ability to shield our cells from the damaging effects of the sun’s rays lies in its molecular structure. Melanin has evolved to achieve what is known as ultrafast internal conversion of energy, which is a remarkable ability of certain molecules found in nature to rapidly absorb a lot of energy and dissipate it very quickly and safely before it can cause any damage. There are two important stages in the process of light absorption that gives melanin its extraordinary protective powers. By absorbing all the UV before it can hit sensitive cells lying beneath the skin, it takes the brunt of the attack from the sun. But anything that absorbs a lot of energy is liable to be badly damaged itself. Think of what happens when a car is given too much gas and the engine over-revs. If it is kept up for too long the engine will shake itself apart. Melanin, however, can go on absorbing the energy from the UV radiation over and over again without damage. This is because it can release the energy as harmless heat before any damage can be done. The whole process of absorbing and releasing energy is extremely quick (as the name ‘‘ultrafast internal conversion’’ implies). The whole process takes less than one picosecond—one million-millionth of a second. This quick absorption and release of energy from the sun occurs much quicker than any man-made product—human engineers have got nowhere

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close to replicating this remarkable feat. There are several other biological molecules that show this ultrafast internal energy conversion. DNA itself can absorb and release energy from UV radiation, for example, which helps protect the genetic material from damage should any UV get past the melanin defense. It is likely that DNA, melanin, and other natural molecules that are involved in combating the very powerful energy from the sun evolved this lifesaving ability to quickly absorb and release energy when life first evolved. When DNA first appeared in the primordial soup millions of years ago, its ability to withstand attack from UV radiation would have helped it survive to make the first formative steps in producing life on this planet. Apart from DNA emerging in the first place, this ability to withstand radiation from the sun may well have been the first adaptation to evolve on this planet, allowing life to thrive. Borrowing from Nature

Man-made sunscreens are effective at filtering between 95 and 97 percent of the harmful UV that can cause skin cancer—just short of melanin’s 99.9 percent absorption. It is not surprising, therefore, that sunscreen manufacturers are turning to nature to boost the efficacy of their product. Melanin used to be a prohibitively expensive product to collect. The most common source was cuttlefish ink. (Melanin gives the ink its dark color.) Now, melanin can be produced on more industrial scales in fermentation tanks and can be used in the mass-produced sunscreens. Currently, melanin is absorbed onto microscopic sponges that can be used in a cream. This technology is still being developed, but it is hoped that it will succeed in making exposure to the sun safer than ever before.

PARASITISM—SURVIVING HOST DEFENSES Animals such as humans must be able to cope with fluctuating environmental conditions and attack from other organisms if they are to survive. Some animals, though, do not have to worry about such conditions. They have evolved to live in an environment where they can escape from unpredictable environmental conditions and where there are few, if any, predators or infectious bacteria and viruses. These animals have taken the evolutionary step of living inside other organisms—they are parasites. Living inside another organism has many benefits, particularly if it is within a mammal that regulates its body to near-constant conditions. Food is easily obtained and there is protection from the harsh outside world. However, parasites do not have it all their own way. Most host organisms are none too keen on having something else living inside them. Parasites cause disease and are a drain on the nutrients that the host needs for itself. Consequently,

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hosts have evolved elaborate defenses against potential parasites, and it is this attack that the parasite must defend itself against if it is to survive. Humans are subject to many parasites, but one of the most common is the roundworm, Ascaris lumbricoides. It is thought that around 1.4 billion people are infected with this species—some 25 percent of the world’s population. Adult worms live inside the small intestine after a complex migration through the body. Eggs live in the soil and get into the body by the human host swallowing infected food. The eggs must first survive the harsh, acidic conditions of A section through the roundworm, Ascaris the human stomach. Thanks to a tough, lumbricoides. The thick cuticle helps the chitinous outer coating that is ridged to give worm to resist the immune response from extra strength, the eggs will survive to pass the human host in which it lives. [Science through to the duodenum. Rather than Source / Photo Researchers, Inc.] growing into adults here, the eggs hatch and the larvae burrow into the muscular gut wall. From here, the larvae are carried in the blood to the heart and then to the lungs. The larvae will emerge from the lungs into the lung cavity and either work their way up to the throat or will be coughed up. At this point, they are reswallowed and must survive the acidic gastric juices of the stomach once more. Only larvae that are fully mature at this point will survive. Once again, they pass through the stomach and reach the intestine, where the final molts occur and the larvae develop into fully grown adults. Adult females grow to just under 1.5 feet (0.5 m) in length, although males are a little smaller. Once in the intestine, life is good for the giant roundworm. It can feed on food particles passing through the intestine, and the warm, moist environment of the gut provides near ideal living conditions. However, there are two things that are not ideal for this intestinal worm. Being an oxygen-breathing organism, humans have comparatively high levels of oxygen coursing through their bodies. Oxygen is a corrosive gas, and it does the roundworm no good at all. Without taking action, a roundworm’s body would be gradually corroded by the oxygen, causing death. To counter this potential problem, Ascaris uses a specialized molecule that mops up the oxygen that is present in the human gut. This is a remarkable adaptation because it allows the worm to create its own ideal environment in which to live. The molecule that Ascaris uses to clear away the corrosive oxygen is in fact the same molecule that all oxygen-breathing animals use to store and transport oxygen around the body—hemoglobin. In humans, oxygen binds to

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hemoglobin just tightly enough so that it can be transported through the body in blood but not so tightly that it can’t be readily released again for respiration. In Ascaris, however, hemoglobin has evolved to bind to oxygen extremely tightly and not release it at all. In fact, Ascaris hemoglobin ‘‘grabs hold’’ of oxygen about 25,000 times more tightly than human hemoglobin, making it an effective way of detoxifying this poisonous gas. In addition to having to make this environmental adjustment, Ascaris faces a more direct attack. Humans, like most mammals, have evolved an effective defense against parasites. The host immune system can detect a foreign body lurking in its intestine and will produce protein-digesting enzymes (proteases) to attack it. These protein-digesting enzymes (such as pepsin and trypsin) eat away at the bodies of potential invaders, causing death. Again, though, Ascaris has a couple of tricks up its sleeve. It has evolved a very specialized, hardened cuticle (skin) that is resistant to these attacks. The cuticle of Ascaris roundworms is very different from those of related, but freeliving (nonparasitic) worms. If you were to look at one under a very powerful microscope you would see that the proteins that make up the cuticle are bound tightly and are wound together in a very strong triple-helix structure, much like the thin but tough ropes that climbers use. A thick skin certainly helps fend off the host’s immune attack, but it would be much better to neutralize the attack at the source. This is precisely what Ascaris does. It can release a substance that binds to the host’s proteindigesting enzymes and render them useless. It is still not clear how the roundworm manages this. The roundworm certainly releases some neutralizing chemicals itself. There is also evidence that the parasite can release another chemical that manipulates the host’s organs, causing them to produce neutralizing chemicals too. It seems that the parasite can use the host’s own body to counteract its own immune defenses! Just as humans have manipulated their environment to increase their own survival, Ascaris has evolved to create a perfect environment inside a human host in which to flourish. This is indeed a remarkable adaptation to exploit a novel environment. Borrowing from Nature

The amazing ability of Ascaris hemoglobin to bind so tightly to oxygen is being exploited by scientists as a potential way to treat cancer. Current cancer treatments are very aggressive forms of treatment. Scientists are therefore working on ways in which cancer cells can be killed off by starving them of oxygen. Like normal cells, cancerous cells need oxygen for respiration, and without it they die. Various attempts have been made, but with little success so far. With the relatively recent discovery of how Ascaris hemoglobin works to bind oxygen, there are hopes that this could prove successful where other ideas have failed.

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ANTIBIOTICS—SURVIVING DISEASE When most people think of antibiotics, they think of penicillin. Even before the drug was discovered, scientists were aware of the antibacterial properties of the fungus, Penicillium for over 100 years. It was recognized that the fungus is able to release a chemical that kill bacteria and other fungi by attacking their outer cell membrane. The research that was triggered by these observations led to the isolation of the key chemical (penicillin) involved, and the first antibiotic medicine was created. Since then, many fungi and bacteria have been found to produce and secrete antibiotic chemicals. Many of these have been exploited by pharmaceutical companies. Antibiotics are not just used by bacteria and fungi, though. All organisms come under attack from bacteria and fungi that can cause serious disease. There is therefore a great selection pres- The fungus Cordyceps is one of the few species sure to develop resistance or a counterat- resistant to the antifungal molecules produced tack to these pathogens. In animals, it is by ants. The fungus affects the ant’s brain those species that live communally causing it to clamp its jaws on a leaf in the for(humans included) where there is great- est canopy before it dies. The fungus then est risk of bacterial and fungal disease. grows from the ant allowing it to disperse. Living close together means infectious [Gregory G. Dimijian, M.D. / Photo Researchers, Inc.] agents can easily be passed from one individual to another. The various species of communal ant are perhaps the most close-living of all communal animals. Living so close together, they run a high risk of an infection taking hold and wiping out an entire colony of millions of individuals in just a few weeks or even days. It is not surprising, then, that certain species of ant have evolved a solution to the problem. The fierce bull ants of Australia are one such species that have evolved a defense against bacterial and fungal attack. Over the millions of years they have lived on this earth, they have evolved glands that secrete antibiotics. Bull ants have two of these antibiotic-secreting glands, called the metapleural glands, each situated just above the back leg. These glands constantly secrete a milky fluid that covers the ant’s body. The fluid is packed with powerful antibiotic chemicals called metapleurins that protect the individual from fungal and bacterial attack that can be so deadly to tiny insects such as these.

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Metapleurins are powerful antibiotics, destroying most bacteria and fungi on contact by breaking down their outer cell membrane. As a result, the surface of the bull ant is almost entirely free from bacteria and fungi. It is much cleaner than most human skin. Not all bull ants have the necessary glands to produce this disinfectant. Unfortunately for the males, it is only the females— the queen and her workers—that are suitably equipped. A male can benefit from the protective fluid produced by females by brushing against them. If a male becomes separated from the colony, though, it will soon become badly infected. So why have males not evolved this useful gland? In the ant colony, males are fairly useless. Once they mate, they soon die. For their short life they are protected by the antiseptic produced by the females, but after mating and leaving the nest they quickly die. Given the huge evolutionary advantage of being able to fight disease, nearly all species of social ant produce at least one type of antibiotic. Not all species produce such a potent chemical as the bull ant, though. Leaf cutter ants produce antibiotics in their metapleural glands, but they are only mildly antifungal—just enough to afford some protection from disease. The reason for this is that their food source is a fungus that they farm in their nest. The leaves that they cut and bring back to their nests are not eaten by the ants but are used to feed the growing fungus fields that the ants cultivate and eat. Given the close relationship ants have with fungus, leaf cutter ants have evolved a very specific array of antifungal chemicals. Each species of leaf cutter ant will farm just one species of fungus to feed the colony. The introduction of a different species of fungus to the nest would be disastrous. Not only would it be wasteful, but an invading fungus could cause disease in the colony. Leaf cutter ants are therefore very careful farmers. Their crop of fungus is like a field of wheat with no weeds, and to achieve this they have a tailored arsenal of weed killers. The saliva of the ants contain antiseptic so that newly cut leaves can be sterilized before they are brought into the nest. If any fungal spores do find their way into the nest, the ants carry with them a broad range of antifungal chemicals that will attack the ‘‘weed’’ species, but leave the ‘‘crop’’ species alone. Although they do produce their own antibiotics, leaf cutter ants get these weed killer antifungal chemicals from bacteria, which they carry with them on their bodies. These bacteria produce and secrete antibiotics specifically adapted to the nest environment to ensure a highly productive crop. The story, however, doesn’t quite end there. As we know all too well from the spread of bacteria that are resistant to antibiotics, nature can evolve in response to these protective chemicals. Ants, therefore, are not fully immune to the attack of bacterial and fungal pathogens. A common disease of ants is the fungus, Cordyceps. This fungus is resistant to the chemicals produced by ants and can quickly infect an unsuspecting ant. Not only does it overwhelm an infected ant by spreading through its internal organs, but it will manipulate the unfortunate individual to perform a bizarre behavior in the last moments of

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its life. The fungus affects the ant’s brain and causes it to climb a nearby plant and bite down hard on its stem. Jaws clamped over the plant, the ant will die in this position. From there the fungus can grow from its body and, from its advantageous position, release its spores, which are dispersed through the air. No matter how sophisticated the defense, evolution always seems to produce something one better. The next step will be for ants to evolve a new chemical to resist the current resistant Coryceps. Perhaps then we will have an answer to our seemingly unstoppable resistant bacteria plaguing our hospitals. Borrowing from Nature

Humans have been manufacturing antibiotics for some 60 years. These antibiotics have helped millions of people survive potentially deadly diseases and can be considered one of the most important discoveries of our history. It will perhaps come as little surprise, then, that pharmaceutical companies are seeking ways to exploit this recently discovered antibiotic resource in ants. At least two of the antibiotic metapleurins from bull ants have been patented and are used as antiseptics in hospitals. Interestingly, however, although bull-ant metapleurins are a recent discovery in Western medicine, their effects have been well known to Australian Aboriginals, who have been using bull ants to treat cuts and scrapes for hundreds of years. The leaf-cutter ants, too, have been exploited in the production of manmade antibiotics. The antibiotic-producing bacteria they carry with them in their bodies are called Streptomyces. It is relatives of these bacteria that are used to produce many of the commercially important antibiotics used today.

PSYCHROPHILES—SURVIVING EXTREME COLD Life rarely thrives in freezing conditions. The biological molecules that keep things ticking over in an organism’s cells slow down and become sluggish as temperatures begin to fall past 32°F (0°C). As a result, the organism itself will become lethargic and will eventually freeze to death. A few hardy creatures, however, can survive below freezing. They are called psychrophiles, meaning ‘‘ice-loving.’’ Large animals like polar bears can survive on the ice thanks to their huge size and warm blood. For smaller, cold-blooded animals, though, life is much more tough. Yet head to the Alaskan glaciers just before dawn and the normally pristine white snow will be covered in billions of tiny, threadlike black worms, each one no bigger than half an inch (1 cm) long. Every one of these worms is from same species—Mesenchytraeus solifugus—which is very closely related to the humble earthworm that burrows through garden soil. Perhaps surprisingly, the ice worm has very few discernible physical adaptations that allow it to cope with life on the ice. Yet it completes its entire life cycle at around 32°F (0°C)—a temperature that would easily kill its earthworm cousin.

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The ice worms of Alaska keep to a very tight temperature range. They die if conditions drop to below 19°F (−7°C) or if they creep above 43°F (6°C). If the environment gets too warm their bodies disintegrate. As temperatures rise, the ice worm’s digestive juices get overactive and begin to attack the body—they will literally melt in warm conditions. By spending their entire life on and in the ice they can stay near their optimal temperature. To avoid getting too cold or warm ice worms will migrate through the ice throughout the day, seeking out the ideal temperatures. This is why they spend the day crawling through tiny cracks in the ice during the day and emerge at night to feed on snow algae. How does the ice worm cope with the cold? All organisms get their energy from the high-energy molecule Adenosine Tri-Phosphate (ATP), which is made by metabolizing food. When temperatures drop, metabolism slows (as determined by the basic laws of physics), which means that less ATP can be produced. Without this energy supply, organisms will become sluggish and will eventually die. This is why animals slow down when things get cold. Ice worms, though, have overcome this response to the cold. They can actually increase the amount of ATP (energy) in their cells as temperatures drop. They apparently can contravene the physical laws of thermodynamics! This phenomenal ability means that they have enough energy to keep moving even as temperatures drop to 43°F (−6°C), the point at which they eventually freeze and die. This last point had researchers stumped. How can an animal increase the rate of ATP production as temperatures drop, seemingly defying physics? It is not clear. The answer is probably that ATP (energy) production stays constant regardless of temperature, and it is the efficiency at which ATP is used that increases at lower temperatures. In other words, the ice worm becomes more efficient at burning energy as temperatures drop. This would explain why energy levels seem to increase when temperatures fall. This is still an incredible feat, though, and no one is quite sure how it is done. The upshot of it is that even at 32°F (0°C) ice worms can move around at the same pace that its cousin the earthworm achieves at 68°F (20°C). They put this mobility to good use in foraging for food. They can move through the snow and ice browsing on algae or they can be found clinging on to the icy banks of glacial streams and ponds, snapping up passing morsels of food. The ice worm is the largest animal to live entirely on the ice at freezing temperatures, although on the glaciers of Alaska there are other species that can pull off the same feat of energy conservation from three other kingdoms of life. As well as the adaptation being found in animals (the ice worm), it is also seen in bacteria, fungi, and algae, suggesting that the adaptation has evolved more than once in response to the stresses of life on the ice. What’s more, it has even evolved thousands of miles away in the Gulf of Mexico right at the bottom of the ocean. Here, another worm, which is also called an ‘‘ice worm’’ (although completely unrelated to the Alaskan ice worm), lives in the frozen methane ice deposits that erupt from the ocean floor.

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All of these creatures are able to increase the amount of energy-rich ATP in their cells as temperatures drop, allowing them to keep moving. From various experiments it seems that this ability is achieved in part by specially adapted biological molecules that are involved both with the production of ATP and with the breaking down of ATP to release energy to fuel an organism’s cells. There are undoubtedly other adaptations the ice worm has evolved to survive in icy glaciers, but these mysteries are as yet unknown. Some secrets are beginning to be exposed—like the discovery of symbiotic bacteria found living in the ice worm’s gut, which may be part of the picture. This adaptation needs to be explored further, but it may hold the key to the ice worm’s survival on the ice in the extreme winter conditions when surface temperatures fall to below 104°F (40°C) and food is very hard to come by. Borrowing from Nature

NASA scientists have recently become very interested in these humble little worms. Understanding how they can survive in such extreme conditions could give valuable insights into what life might be like if ever it were discovered on icy planets such as Europa, one of Jupiter’s moons. More mundane applications of research into ice worms may be in the field of organ transfers. The same adaptations that keep ice worms moving at cold conditions may help in developing better storage of human organs before transplants.

ANHYDROBIOSIS—SURVIVING WITHOUT WATER Water is essential for all life, from the tiniest bacteria to the largest mammal. It is involved in nearly all the reactions that keep an organism’s cells alive, and so it is probably the most important commodity inside any living creature. In humans, water makes up nearly 70 percent of our bodies, and it needs to be constantly replaced because it is lost when we sweat, breathe, and excrete. It is not surprising then that most organisms tend to stay away from places where water is hard to come by. There are several adaptations allowing Scanning electron micrograph of the tardianimals, plants, bacteria, and fungi to cope grade. In times of drought, the tardigrade with drought, but there are some places on folds its body in on itself and adopts a barrel earth that are simply too dry for life to shape (tun formation). It will also replace persist. Except, that is, for a few hardy the water in its body with a sugar, trehalose, creatures that show some ingenious to prevent damage to its cells. [Steve Gschmeissner / Photo Researchers, Inc.] solutions to the problem.

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Probably the driest place of earth is in the McMurdo Valley in Antarctica, where it is so cold that more often than not any available water is frozen solid. Early explorers who dared to enter the frozen wasteland reported that no life persisted here at all. However, deep within the desiccated soils there exists a hardy animal that has a remarkable ability to cope without water. It is a curious creature called a tardigrade, otherwise known as the water bear. This animal has no problem in surviving the tough, near-perpetual winters when water is impossible to come by. The tardigrade is not a very big animal, such are the tough conditions in which it lives. The species living in the Antarctica tend to be no longer than 0.5 mm long. This small size could actually be a problem when it comes to retaining water. Small animals have a larger surface area-to-volume ratio than larger animals, so there is a larger relative area from which water can be lost. As it happens, though, losing water from the body is not really a problem for these creatures. When conditions get too extreme, tardigrades are able to enter what is known as a cryptobiotic state. Literally, cryptobiotic means ‘‘hidden life,’’ and that is a very apt description. You would have to look very hard to find any signs that the animal is not dead when it enters this state. Tardigrades are able to enter a cryptobiotic state in response to a range of conditions, but when there is a lack of water, it is called an anhydrobiotic state, meaning ‘‘life without water.’’ In very dry conditions, the tardigrade has to go through a number of stages before it is ready to see out the tough times ahead. First it curls up into a very tight ball to prevent a fatal loss of water. This is not so easy for the tardigrade, which has a distinct head and eight distinct legs. To manage it, its limbs fold in on themselves so they are tucked inside its body and its body folds inward so it turns from a recognizable animal into an indistinct oval blob. It is a little like a tortoise retreating into its shell, but instead of simply drawing in its head and legs it actually inverts them inside its body. This stage is called tunformation because the end product looks very much like the tun barrels used to store beer. In very dry conditions, the tardigrade can cope with extreme water loss. When they enter the anhydrobiotic state, their bodies have only 1 percent of the water they normally hold. On its own, this would clearly be a problem— their bodies would collapse in on themselves, causing irreparable damage. The tardigrade has a neat solution. It replaces the water with a sugar, trehalose, that protects their organs from the damage normally associated with desiccation. It is a like filling their bodies with a jelly. Of course, once fully tucked up and stuffed with protective trehalose, the tardigrade can’t hunt for food. That is why it must reduce its metabolic rate to next to nothing to avoid wasting energy. During anhydrobiosis, its metabolic rate falls to just 0.01 percent of its normal level. With such low metabolism, these hardy creatures burn practically no energy whatsoever. There have

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been some claims that tardigrades can survive for up to century in this state, although this is likely to be false. There is evidence, though, that they can survive for up to 10 years in an anhydrobiotic state. Perhaps what is even more remarkable is that is only takes a few drops of water for the tardigrades to snap out of their living death and return to their normal state of being in just a few short hours. It is not only tardigrades that can survive in the driest place on earth. They share the soils of the McMurdo dry valleys with three species of nematode worm that can enter the same anhydrobiotic state when there is no water to be found. These worms are slightly larger than the tardigrade, being only about 0.04 inches (1 mm) long. Tun formation is slightly easier for them— having no limbs, they can easily coil up. But they too replace their body water with trehalose sugar and reduce their metabolism to the bare minimum. Tardigrades are indeed a hardy group of animals. They can enter a cryptobiotic state to avoid a range of conditions. They can tolerate very salty conditions, very acid conditions, and extreme temperatures—from close to absolute zero (the coldest possible temperature) to up to 480°F (250°C— hotter than a kitchen oven). Amazingly, as a by-product of their resilience to these natural (if extreme) conditions, they are also able to withstand many other potentially fatal conditions. They can survive bombardment by potentially damaging X-rays, the conditions inside a vacuum, and pressures equivalent to 6,000 atmospheres. Despite their small size and unassuming appearance they are truly one of the most hardy creatures on this planet and have been able to fashion a niche out of exploiting habitats that other creatures simply cannot survive. Borrowing from Nature

So fascinated by tardigrades’ ability to survive, and perhaps spurred on by a desire to find out what, if anything, might kill a tardigrade, on September 14, 2007, scientists sent tardigrades into space as part of the Tardigrades In Space (TARDIS) program launched by the European Space Agency. The aim of the program is to explore the impact of space travel of such animals, possibly with the view of their use in any future projects to colonize other planets.

PRIONS—SURVIVING EVERYTHING To flourish, living organisms must cope with many adverse conditions—the cold, heat, radiation, too much or too little water, immunological defenses, too much or too little salt, poisonous chemicals . . . the list goes on. All these environmental conditions can attack an organism’s body and the genetic material that controls how that body works. Those individuals or species that succumb to these adverse conditions will suffer or even die. The ones that have adaptations to survive these conditions, though, will enjoy a huge competitive

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advantage over those that cannot and will be evolutionarily successful as a result. Many adaptations have evolved to cope with these pressures, some of which have been covered elsewhere in this book. There is one group of organism, though, that has taken the art of survival to the extreme. These creatures are the simplest form of life on earth, and they are pretty much resistant to anything that is thrown at them. They are the prions. The ability of prions to survive is Prions are disease-causing proteins that convert based on simplicity. They have no body, normal proteins into copies of themselves. On the left is a computer simulation of the nor even what we might recognize as genetic material—they have no DNA. normal-shaped protein. On the right is the disease-causing prion. The prion is made of For this reason, some scientists quesmany rigid protein sheets (shown as arrows in tion whether they can be considered to the simulation) which makes it extremely resil- be alive at all. But they can replicate ient. [AP Photo / Professors Stanley Pruisner / themselves, and on this definition we Fred Cohen, University of California–San might consider them to be like any Francisco Medical School] other living organism. Like viruses, prions are infectious organisms. They cause the degenerative diseases called Transmissible Spongiform Encephalitis (TSE) diseases. TSEs include degenerative brain diseases in humans such as Creutzfeldt-Jakob disease (CJD) and fatal familial insomnia. The latter is an inherited disease that affects the thalamus in the brain, causing complete sleeplessness and eventually death. Another well-known TSE disease is Bovine Spongiform Encephalitis (BSE) in cows, otherwise known as ‘‘mad cow disease.’’ So what are prions? Very simply, they are pieces of protein that can make more copies of themselves. This makes them very much like DNA, though unlike DNA they make copies of themselves by converting normal, uninfectious proteins that are already in a host’s body into the infectious prion form. In humans, the ‘‘normal’’ protein is an important antioxidant protein that is found on the surface of our cells. If a human body is infected with a prion, this ‘‘vampire protein’’ will manipulate the shape of any normal antioxidant protein that it comes across and convert it into another prion protein. The newly formed prion protein is then capable of converting other normal antioxidant proteins and so on. As prions get to work, they cluster together to form a protein plaque called an ‘‘amyloid,’’ causing holes to develop in the infected tissue, typically the brain, which gives the tissue its characteristic ‘‘spongy’’ look. After infection, death may be a long time in coming, but it is a certain death as the brain gradually becomes destroyed. Despite being so methodical a killer, prion diseases are

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not common because they have a very poor mode of infecting new hosts. Infection is by ingestion, which entails the rather gruesome (to humans anyway) business of eating infected brain material. It was for this reason that the otherwise very rare degenerative disease Kuru was so common to the indigenous peoples of Papua New Guinea. The tribe members were cannibals who ate the brains (and everything else) of the dead. Methodical though prions are at converting normal proteins, a key part of their success has come through the remarkable resilience to more or less any attack. They are extremely heat resistant. Easily coping with temperatures of 175°F (80°C), prions can even survive, for a short period at least, temperatures of over 212°F (100°C). They are resistant to ultraviolet (UV) radiation and ionizing radiation, both of which are so damaging to DNA. They are also resistant to an animal host’s defenses, which would normally destroy foreign proteins. This means that they can survive well both inside the host body and outside. They are capable of lying dormant, waiting to be consumed by another unwitting host. The secret to this incredibly effective adaptation lies in the structure of the protein. The cells of all living organisms make a group of proteins called enzymes. These are proteins that bind to other molecules in the body and either join them to other molecules or break them down to smaller molecules. Enzymes are critical in both the growth of the body and the breaking down of food. In any of these reactions, it is largely the enzyme’s shape that is important. They must bind to the other molecule in a specific way to change it. Enzyme reactions have therefore been likened to a key fitting inside a lock—get the wrong key and the lock won’t open. Prions are exactly the same. It is their shape that is key to changing the normal proteins into prion proteins. It so happens that the particular shape required to convert the normal protein to the infectious type also conveys a prion’s powerfully resistant properties. All proteins are made up of building blocks (amino acids) that are held together as either loose ‘‘strings’’ or rigid ‘‘sheets.’’ The normal protein that is used as an antioxidant is made up mostly of these loose strings. Once is it transformed into the infectious prion form, however, many of these loose strings are converted to the much stronger sheets, giving the prion great strength that can resist extreme heat, cold, and radiation. So, how did this unusual group of organisms evolve? It is not at all clear. It is likely that a mutation in the host’s DNA that codes for the normal protein created the prion form. Normally, such a mistake would rapidly become extinct through the normal processes of natural selection, but thanks to the prion’s ability to replicate itself and survive for long enough to infect a new host it was suddenly able to survive as an independent organism, no longer simply a product of the host’s genome. This may sound incredibly unlikely, but this, after all, is what evolution and adaptation are all about—chance mutations that turn out to be successful.

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Borrowing from Nature

Prions are a fascinating group of organisms that are not well understood. Scientists are increasingly turning their attention to these simple proteins, however, mostly to understand the risk of transfer of bovine prions to humans (the link between BSE and CJD). An interesting side effect of this research is that by better understanding prions and how they affect the brain, scientists are hoping to learn more about more common degenerative brain diseases like Alzheimer’s disease and Parkinson’s disease, which currently afflict millions of people.

3 LOCOMOTION

LOCOMOTION—HUMAN INVENTION The first recognizable human ancestor appeared on the planet some 2.5 million years ago. From their origins in Africa, our ancestors moved first into Eurasia about 1 million years ago and began colonizing western Europe about 500,000 years later. From here, humans spread to Australasia about 40,000 years ago. East Asia was reached by 20,000 BC, and from there North America was colonized in 12,000 BC. Humans continued their migration and had reached South America by 10,000 BC. Our arrival into the colder climates of Greenland was much more recent—happening at around 2,000 BC. The spread of humans across the planet has been impressive, certainly as a result of human adaptations that have allowed us to use tools to find food and keep warm. It is equally true, though, that certain colonization events could only have taken place thanks to the design and construction of human modes of transport. To colonize certain parts of the globe, humans would have had to cross significant bodies of water, which means that our ancestors would have had to build boats. It is likely that the first of these were simple rafts, little more than humans resting on trunks and branches and drifting across the ocean. These tree trunks could have next been hollowed out to make early canoes. After this, human would have constructed simple coracles, which are more complex canoes made by stretching animal skins over a wooden frame. Even these basic designs were sufficient to exploit much of the land masses on the planet. One of the important steps in human evolution was the move from a huntergatherer way of life to an agricultural one, which led to the gradual domestication of animals. The first to be domesticated was the dog at around 10,000 BC,

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most likely used as a pet and to assist with hunting. However, around 4,000 BC the horse was domesticated and rapidly became a mode of transport rather than simply a source of food. Evidence of humans using the horse for moving around comes from the steppes of the Urals about 3500 BC . Graves from 2500 BC include horses buried with chariots, suggesting that human societies at this time had harnessed the power of the horse to draw carriages. Horses fulfilled the vast majority of personal human transport needs for nearly 6,000 years until 1866 when Nikolaus Otto built the first gas-powered engine. These engines were fitted to bicycles to create the first motorcycles in 1885 and were soon fitted to a truck in 1896. Even at the turn of the twentieth century, automobiles were expensive and not especially high powered, but they have since been redesigned and refined to produce vehicles capable of covering hundreds of miles in a day. The internal combustion engine that lies at the heart of most cars is based on the principles of steam power that had been designed centuries before. The first steam engine was built in 1698 by an Englishman, Thomas Savery, using a fairly simple design. Fuel is burnt to heat water and generate steam in order to drive a turbine, which in turn can be used to drive wheels. This, of course, opened the door to the whole industrial revolution as well as mass transit systems such as trains and ocean liners. Car engines work very much in the same way. Controlled explosions of fuel drive pistons up and down, which turn crank shafts to drive the wheels of the vehicle. It really is the automobile that has taken the next step in giving humans such personal freedom to get from point A to point B. Improvements to the automobile for moving across land continue to be made to the extent that we now impose speed limits on our roads to protect our safety. This moved the quest for speed to the flat salt plains of the desert, where a vehicle can, with moderate safety, accelerate to whatever speeds it is capable of. In 1999 British engineers produced a vehicle that reached speeds of 763 miles per hour. The same team is currently planning a vehicle that can break the 1,000 miles per hour mark—faster than a bullet fired from a gun. This vehicle, called the Bloodhound, is powered by a rocket strapped to a jet engine and requires bodywork capable of withstanding pressures of 1 ton per square foot (12 tons per square meter). The trick with such fast-moving land vehicles is keeping them on the ground, as often they get sucked into the air and flip. This is not a problem for aircraft that actually want to be sucked into the air. Lift is generated by air flowing over an object at a faster speed above the object than below it. This creates higher air pressure below the object than above it, pushing it into the air. The design of airplane wings is based entirely on the process of air flowing over them in this way. In cross-section, an aerofoil wing is flat at the bottom and curved on the top. With air having to travel further over the top of the wing than below it, air above the wing moves faster, creating the required differential in air pressure.

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Once in the air, the speeds achieved on the ground can easily be reached and passed because there is much less drag. The fastest unmanned aircraft currently is the X-43, which travels at speeds of more than 7,000 miles per hour. The fastest manned aircraft is probably the Lockheed’s SR-71 (the Blackbird), which made its maiden flight on December 22, 1964 and has achieved speeds of over 2,000 miles per hour, nearly three times the speed of sound. The key to achieving these tremendous air speeds has been the invention of the jet engine. The thrust produced by a jet engine is really an extension of the simple propeller engines seen on early aircraft and still used in smaller planes today. A fan sucks air into the engine, where is passes through a compressor that condenses the air and puts it under pressure. In the compressed air, fuel is burnt, which drives a turbine that forces the air and fuel exhaust from the engine and drives it forward. The SR-71’s engines were related to this basic jet engine design, but instead of using a fan to suck in the air it used its own forward momentum, giving the engine its name, the ramjet. The engine used by the high-speed X-43 is another variation on the jet engine theme called a scramjet. The speed of manned aircraft has not exceeded those of the SR-71 because fast-moving aircraft and land vehicles are notoriously hard to maneuver. Even the slightest movement away from traveling in a straight line can result in disaster at high speeds. Modern aircraft used for the military need to compromise speed for agility. Probably the most advanced military aircraft is the Eurofighter, which travels at a relatively modest top speed of 1,500 miles per hour. It is incredibly agile, though, thanks to a very un-aerodynamic design. A complicated computer system is needed to keep the plane airborne—without it the craft would drop from the sky. This innate lack of stability, however, gives the aircraft excellent maneuverability. Both air and land vehicles are driven by designs that rely on the principle of turning a wheel. Piston-driven crank shafts drive the wheels of cars, and propellers drive the jet engines of airplanes. Even the helicopter is lifted from the ground by a propeller. Human engineering for water-bound vehicles is no different. The propeller is used to drive boats, submarines, and even torpedoes. Much like the jet engine, the impeller is an invention that compresses water after it is sucked into an aquatic ‘‘engine’’ by a propeller fan so that it is expelled at a greater pressure, generating more thrust. Perhaps the one exception to this reliance on a spinning wheel or propeller to generate thrust is the use of rockets, which involves the expelling of highpressure fuel exhaust from the end of a vehicle to generate thrust. Initially, this form of propulsion was seen only in missiles and fireworks, but it is the main form of propulsion that has driven spacecraft since man first orbited the earth. Rocket propulsion is not especially rapid, though, especially when trying to cover the vast distances between planets in space. NASA is hoping to change this with its variable specific impulse magnetoplasma rocket, or VASIMR.

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The VASIMR engine is a plasma-based propulsion system. Plasma is a unique state of matter different from solid, liquid, or gas. Perhaps it is best described as being an ionized (electrically charged) gas-like substance. The important feature of plasma is that it can be directed by magnetic fields, which is what the VASIMR engine does. Electric fields heat the hydrogen-based plasma like any rocket fuel and the magnetic field directs it as it is ejected from the engine, allowing for a high amount of precisely directed thrust. The benefit of such an engine is that the fuel, hydrogen, is extremely common in space so huge fuel tanks would not be needed. Such an engine may help humans in their desire to step foot on relatively nearby planets such as Mars, but a completely new type of engine would need to be designed to travel further. There is still plenty of engineering required before we can fully realize the dream of traveling huge distances to fully explore the universe.

BIRD FLIGHT Early attempts at flight by humans met with spectacular failure. As Orville Wright, who finally cracked man-made flight, put it: ‘‘We got plenty of flying fever from watching the birds, but we got nothing about their secret of balance.’’ This secret of balance lies in a bird’s wing, and it took many years to understand exactly how it works to generate both lift and thrust. To generate lift, a bird’s wing must have a particular shape, known as an aerofoil. This is important because lift is generated by the way in which air flows over the wing. In cross-section, a bird’s wing looks like a lopsided teardrop—this is the classic aerofoil shape. Thanks to the way that it is curved, air flowing over the top of the wing moves quicker than air flowing underneath. As a result, the wing generates an area of low pressure above it and an area of high pressure below it. This both pushes and drags the wing upward, creating lift. All birds, although they may have slightly different shaped wings, generate lift this way—this is how they can become airborne and fly. By subtly adjusting the position of its wings, a bird can maneuver itself with great agility through the air. When it flaps its The ruby-throated hummingbird, Archilowings down, a bird will hold its wings out chus colubris. Hummingbirds are unique among birds in their ability to beat their flat so as to generate as much lift as poswings in a figure-eight, generating nearly sible. When it brings its wings back up continual lift and allowing them to hover again it draws them closer to its body to and even fly backward. [Millard H. Sharp / reduce the effect of drag though the air. Even by drawing in their wings in the Photo Researchers, Inc.]

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upstroke there is still some drag caused by the turbulent flow of air over the wings when held in the position. This could be disastrous for the bird and could cause it to drop from the skies if it wasn’t for a neat adaptation. In the front of the wing there lies a small, finger-like limb called the alula. Indeed, since wings have evolved from the finger bones of birds’ ancestors, the alula really is a small finger—an evolutionary remnant of the second digit. The alula acts to smooth the turbulent air flowing over the wing during the upstroke. In the downstroke, the limb is neatly tucked into the wing and is concealed from view. On the upstroke, though, it is forced out and away from the wing to create a gap between it and the leading edge of the wing. Some of the turbulent air flows between this gap so that the air flowing over the wing is much smoother and the risk of stalling in midair is reduced. In aerodynamics, this effect is called the slot effect. There is no muscular control of the alula; it simply moves into position because of the air pressure passing over it. This happens at exactly the right time to prevent the bird from stalling and having a nasty accident. The same effect is achieved by spreading the feathers at the tip of the wing. They will open out and create a gap that generates lift and thrust throughout the up and down flap. It is not just the wing that generates lift. The tail plays an important part too by ensuring the right flow of air over the bird to produce lift and thrust. By coordinating the position of their tail and wings, birds can achieve a remarkable range of aerobatic skills. One of the most quick and agile birds is the common swift, a migratory bird that flies each year from sub-Saharan Africa to Scandinavia and Russia and back again, each one-way trip covering 70,000 miles. Covering such huge distances is a major undertaking and is fraught with danger. The swift, however, is well designed for the task. Its body is designed to cut through the air with minimum resistance. It has a compact and streamlined body, and its wings are long and pointed. The swift’s wing bones are thick and strong near its thorax, which means that the energy from the wing muscles can be efficiently transferred to the wing itself. As a result, the swift can beat its wings quickly and with considerable force, allowing it to reach high speeds of up to 60 meters per second or 130 miles per hour. From their stocky base, the wing bones taper out into a long and mobile skeleton extending to the tip of the wing. This gives the swift tremendous control over its wing, which allows for high agility. The swift’s agility is put to good use. It lives nearly its whole life on the wing. It will fly with its beak open to collect small flying insects, and it will swoop down to drink from bodies of water. It will even sleep and mate in the air. The only time it will come in to land is to lay eggs and tend the brood. It can rest while in flight by switching off half its brain at a time, a little like flying on autopilot—another amazing adaptation for this consummate pilot. The birds with the greatest agility, though, are the hummingbirds. At only 2 to 2.5 inches (5–6 cm) long and weighing just 0.004 lbs (1.9 grams), the bee hummingbird is one of the smallest warmblooded animals on the planet.

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Despite its small size, including a tiny heart and lungs, the bee hummingbird is capable of some of the fastest and most agile movements seen among the birds. Not surprising for such an active animal, it has a very high metabolic rate to give energy to its muscles. The fuel of choice for the bee hummingbird is the energy-rich, sugary fluid nectar produced in the flowers of plants. But therein lies a problem. Despite its tiny stature, even the bee hummingbird cannot perch on the delicate flowers to get at their nectar. To overcome this challenge, the bee hummingbird has evolved a complex wing structure that allows it to perform some highly spectacular aerobatics to get at their food. Unlike other birds, hummingbirds are able to hover and maintain their position in the air, giving them time to dip their beak into the flower without needing to alight on it. Bird wings are typically hinged at the shoulder joint, which restricts their movement to a basic up and down ‘‘flap.’’ The bee hummingbird, like all hummingbirds, lack this restrictive hinge and so can beat their wings in a figure of eight. Moving the wings in this way generates near continuous lift, which differs from other birds that generate lift only in the downward flap. What’s more, the bee hummingbird can beat its wings at some 80 to 200 times a second. Thanks to this ability to move their wings unconstrained at high speed, the bee hummingbird can maintain a highly precise control over its flight. It can fly forward, backward, sideways, and even upside down. Consequently, it can hover and maintain a constant position in the air, allowing it to get to its favorite food. Borrowing from Nature

As Orville Wright commented, birds have long been the inspiration for human flight. Understanding how the aerofoil works and the ‘‘slot’’ effect of the alula on the wing have been transferred directly to aviation engineering to give our modern planes lift and to control the effects of turbulent air, which could cause a plane to stall and drop from the skies.

INSECT FLIGHT Insects are one of the most diverse and abundant orders of life that inhabit the planet. It is fair to say that insects owe much of this evolutionary success to their ability to fly. Compared with their flightless cousins, insects that can fly have better opportunities for finding food and mates, evading predators, and colonizing new habitats. As with any group of animals, different insects specialize in different types of flight behaviors. The Monarch butterfly, a fairly fragile and weak-looking animal, is capable of a huge migration across continents for around 3,000 miles (5,000 km). Other insects like the familiar housefly show a speed and agility that put the most advanced jet planes to shame.

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There is no doubting the ability of insects to fly well. Just how they do it, though, has baffled scientists for many decades. Insect flight, at first glance, seems improbable. Insects seem in some cases too heavy for their wings to lift them or too small and weak to beat their wings fast enough, or simply that the way in which they beat their wings shouldn’t generate any lift. In short, judging by conventional aerodynamics, most insects should not be able to get off the ground at all. But insects are not bothered about the theory of conventional aerodynamics. They do things differently from human engineers and even from birds, and the evidence is there for all to see—insects fly extremely well, indeed. The power to generate the necessary lift and thrust comes from an insect’s wings and strong thoracic (chest) muscles. These are very efficient muscles that deliver a lot of power over a long A high-speed photograph showing the liftperiod of time. Hoverflies can hover in generating vortices produced by a butterfly as midair because they can beat their wings it takes off vertically. [Dr. John Brackenbury / 1,000 times a second. The muscles are Photo Researchers, Inc.] also capable of delivering huge bursts of speed. Tropical bees and wasps can travel up to 45 miles per hour thanks to muscle power output that is some 30 times that of a human leg muscle. But what about lift? Bird wings create lift because they are shaped like an aerofoil. Air passing over the wing generates a differential in air pressure on either side of the wing, resulting in upward thrust and keeping the bird in the air. Insect wings are very different. For one, they are flat and not at all shaped like an aerofoil. To generate the right flow of air over their wings they must use them in various ingenious ways. The key to their success comes from exploiting a phenomenon called vortices. A vortex is a swirling pocket of air or liquid. You can see this if you dip your hand in any still body of water (a bath, pool, or sink full of water) and slowly move it in any direction. You should see little whirlpools form on either side of your hand. These are vortices. When you move your hand through water, the whirlpools will tend to move with your hand, almost as if they were stuck to it. If you were to move your hand quicker they could become unstuck and be shed from your hand. The same happens in insect flight. The insect will beat its wing so the vortex that forms on the leading edge (the front edge of the wing) stays put. If you could

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see the vortex in the air (which you can’t unless you blow smoke or something visible over the wing) you would see the little whirlpool of air on the front and top edge of the wing. This in effect creates an aerofoil over the insect’s flat wing. Air has to go around the vortex, so the airflow over the wing is now exactly like that over an airplane’s wing or a bird’s wing. It creates a differential in air pressure that effectively sucks the wing upward, creating lift. Vortices formed at the back of the wing—the trailing edge—do not typically stick to the wing like they do at the leading edge. These vortices that are shed from the wing are put to good use too. Thanks to a complicated way of beating their wings, insects can use the shed vortex to generate lift on the upstroke as well as on the downstroke. (Birds generally only get lift from the downstroke.) By moving its wing through a shed vortex on the upstroke, an insect can gain lift thanks to the way air pressures differ on either side of the wing. It is thought that interacting with shed vortices in this way generates two to three times more lift than flapping alone and gaining lift just from the downstroke. The use of vortices is certainly a great solution for otherwise cumbersome insects to be able to fly. It exploits some complicated physics, and it has only recently been understood how insects manage to do it. In fact, the physics of vortices and how to generate them over the wing in just the right way is an extremely precise art for insects to achieve. Often they must position their wings in such a way that they are always on the cusp of stalling and dropping out of the air. This precision has been fine-tuned over millions of years of evolution, however, and insects no more think about it than you or I do with breathing in and out. Having mastered the physics of flight, insects have evolved several adaptations to conquer the air. It is the design and arrangement of the wings themselves, though, that determine how agile the insect can fly. Insects have one or two sets of wings. Having two pairs of wings, like the dragonfly, is the basic model, although a lot can be done with this simple piece of kit. Dragonflies are agile creatures capable of impressive midair acrobatics. In a tight turn, these miniature air aces can exert a force of 2.5 G, which is impressive for a creature the size of a small pencil. Two wings moving independently is not particularly aerodynamic, though. Some insects have therefore evolved to fly with just one set of wings. Insects like house flies, horseflies, and robber flies are extremely quick and agile thanks to the gradual reduction in size, throughout their evolution, of their second pair of wings. The reduced wings are not entirely useless, though, and indeed they play a critical role in keeping two-winged flies balanced in the air. If you look closely at the hind-wings of a modern house fly you will see the two very tiny wings that have a rounded, clubbed end. These vestigial wings, called halteres, have evolved to be highly efficient balancing organs. When a house fly is airborne, the halteres beat in time the normal forewings, but just out of phase. Thanks to the heavy clubbed end of the organs, the halteres beat in one particular direction, like the pendulum of a clock. If the insect makes a

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sudden change of direction—either intentionally or due to a sudden gust of wind—the stem of the haltere will twist. This twisting is detected by a dense cluster of nerve endings attached to the haltere, and the information is fed to the brain so the fly can take the appropriate action to stay at altitude and on course. The result is a fast flying, highly agile animal that is nigh on impossible to catch. The fossil record reveals just how successful this design for flight has been. It gives evidence for there being winged insects for some 305 millions years. Furthermore, unlike birds, which have had to adapt existing limbs to evolve wings and flight, insects have kept all six legs, which means that they enjoy the benefits of active living both in the air and on the ground. Insect wings are versatile adaptations allowing fast flight, hovering, and breathtaking air acrobatics. It is thanks to these adaptations for flight that have allowed insects to dominate the animal kingdom and spread so prolifically on this planet. Borrowing from Nature

Recent insights into how insects manipulate vortices around their wings is helping engineers produce insect-like micro air vehicles (MAVs). MAV designers are looking to the unconventional way in which insects flap their wings to get their tiny vehicles airborne and mobile. MAVs are very much in their infancy, but they do hold great potential for survey and mapping work as well for the military. Understanding how insects stay in the air is key to unlocking this great potential.

RUNNING Running is about speed and endurance. Some animals are natural sprinters, others are more long-distance runners. For whatever the purpose, running involves a complex series of movements that are very different from walking. Given that running fast or for long periods of time can make the difference between getting a meal or going hungry (or indeed being a meal or staying alive) it is not surprising that there have evolved several useful adaptations in the animal kingdom to hone the art of running. As anyone who has tried to get close to one in the wild can attest, lizards are quick sprinters, yet they cannot run fast for long for the very good reason that they are physically unable to breathe while running. Without the oxygen needed to refuel their cells, a lizard’s muscles will become exhausted after about a minute or two. It can take hours to recover the energy. As a result, running lizards will stop frequently to catch their breath before they become completely exhausted. The problem for lizards lies in their anatomy. Their legs and feet are positioned to the side of their body, which means they run and walk in a sprawling, swaying motion. Each step results in the body twisting left and right.

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The cheetah, Acinonyx jubatus, in full stride. Its flexible spine allows it to take strides of up to 23 feet (7 m). [G. Ronald Austing / Photo Researchers, Inc.]

When a lizard steps with its right foot, its body flexes to the left and squeezes air from its left lung. What’s more, the muscles that are used to move the ribs and inflate the lungs are the same muscles that are used to move the trunk while walking and running, and they can only do one job at a time. When it walks, therefore, a lizard can only breathe with both lungs when its body is straight between steps. At walking pace this is fine, and the lizard can get enough oxygen to its muscles by using only one lung at a time. When it starts running, however, the body flexes so frequently that there is not enough time for either lung to fully inflate. Holding their breath, lizards can run for long enough to escape predators or to catch its prey, but not for much longer. Other reptiles have evolved ways around the problem of running and breathing at the same time. Crocodiles and alligators use part of their pelvis, their pubic bone, to allow the lungs to inflate while running. In addition to inflating the lungs by expanding the rib cage, alligators and crocodiles can do so by pulling the liver toward the tail. This works a little like the diaphragm in humans; by pulling the liver down, pressure in the chest cavity is reduced and causes air to rush in to the lungs. The key here is a hinge joint in the pelvis that allows the pubic bone to swing out of the way to allow the liver to be pulled toward the tail. When the alligator breathes out, the liver is pulled back into place and the pubic bone swings back to its original position. Birds use a

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similar method to breathe, and it has been suggested that dinosaurs may have done so too, allowing them to be active for prolonged periods of time. Certain reptiles, then, have evolved an adaptation that allow them to run and breathe at the same time. The true masters of running, though, are the mammals, some of which have evolved in such a way that their entire bodies are adapted for speed and endurance. Mammals have their limbs below their body and a flexible spine, both of which allow for movements that efficiently transfer power from the muscles into forward momentum. The spine and body movements during running allow the lungs to fully inflate, which provides for sufficient oxygen to sustain the hardworking muscles. The best-known sprinter is the cheetah. It can reach speeds of 70 miles per hour and accelerate to this speed in about three seconds. The power to get to these speeds comes from the muscles, which are packed with ‘‘fast-twitch’’ fibers that contract quickly and powerfully and can continue to work even in anaerobic conditions (when the muscles are starved of oxygen). Fast-twitch fibers are balanced with slow-twitch fibers, which are used for endurance running and walking. Cheetahs have a 20 percent higher concentration of fasttwitch fibers than other fast-running mammals such as the horse or greyhound. For the muscles to be used efficiently, the cheetah has a highly adapted skeleton for running. It is designed to allow a huge stride length, some 23 feet (7 m) at full speed, and it can complete four strides each second. This stride is possible only from having such a flexible spine, which not only gives the cheetah a long stride length but also gives extra energy to the running animal. When it bends, it stores energy, which is released when the spine springs out straight again. Further range in movement is permitted because of the way the hips are allowed to pivot and because the shoulders are not attached to the collarbone. The cheetah is well-adapted to sprinting. In addition to powerful muscles and a flexible skeleton, the cheetah has enlarged nostrils, sinuses, lungs, and heart to get oxygen into the blood and to the muscles. It also has a huge respiratory capacity and is able to take 150 breaths per minute—double what humans are capable of. The animal is extremely light, weighing only 77 pounds (35 kg). In fact, it is so light that this could potentially cause problems with stability when at it is running at top speed and needing to change direction. To counteract this, the cheetah has specialized paws that have only partially retractable claws. (All other cats can fully retract their claws.) These claws act like running spikes, giving extra grip over tricky terrain. Furthermore, the elongated, dog-like pads on the soles of the paws give more traction than the round pads of other cats. Finally, there is the cheetah’s tail, which helps stabilize and steer the cat while in full flight. Although they are excellent sprinters, cheetahs cannot keep up their pace for long. Other animals specialize in long-distance endurance running. Dogs and horses are both excellent long-distance runners, but perhaps the best long-distance runner is Homo sapiens. The skin that is covered in sweat

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glands prevents overheating, but humans also have a wealth of other adaptations that make them excellent endurance athletes. Long tendons in our legs store energy with each step and release it when our feet spring forward. We are well-balanced too. Swinging arms, a twisting torso, and strong muscles that keep our head still all allow for efficient distance running. Even human buttocks, which are much larger in humans than other animals, are designed as a counterbalance for when we run. All this combined with a highly efficient cooling system means that we can keep running for mile after mile. Borrowing from Nature

The study of locomotion in humans has been put to use for athletes who seek to learn more efficient ways of running and jumping to better their performance on the track. Studies of animals provide important information for walking robots that are able to cover tricky terrain in areas where humans or conventional vehicles can’t go, such as over the surface of other planets.

SWIMMING—BLUEFIN TUNA All life found in the seas and oceans moves around in one of two ways. The first is to simply drift along at the mercy of the tides and currents of the ocean. Organisms that have evolved this form The bluefin tuna, Thunnus thynnus. [Richard of locomotion are called plankton, meanEllis / Photo Researchers, Inc.] ing ‘‘drifting.’’ Planktonic organisms include single-celled bacteria, the quirky diatoms and dinoflagellates, and certain small invertebrates. The developing offspring of larger animals are often planktonic. The second way is for an organism to move under its own steam. These animals are sometimes referred to as nekton, meaning ‘‘swimming.’’ The animals that have evolved their own means of locomotion tend to be relatively large vertebrates. Within this group of animals several adaptations have evolved for high-speed movement through water, most of which are seen in one of the fastest swimmers on the planet—the tuna fish. There are several species of tuna, the largest and fastest being the Northern bluefin tuna (Thunnus thynnus). Movement through water is difficult because it is a very dense medium. Much more energy is needed to propel an object through water than through air. To overcome this, tuna are well adapted both to generate a huge amount of thrust to propel them through the water and to reduce the drag on their body as they move.

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Some fish generate thrust through gentle flicks of their various fins. This is fine for meandering through reefs to pick at tiny morsels of food, but it is no good for rapid movement in the open ocean. Faster-moving fish undulate their whole body to drive themselves through the water. In part, this is what the bluefin tuna does. Using densely packed, powerful muscles in its body, it drives its tail from side to side to create thrust. Unlike other fish, though, it keeps the front part of its body absolutely still to reduce drag. Bluefin tuna are essentially made of muscle, which is why they are so sought after by fishermen. Contractions of these strong muscles pull on two tendons, one on each side of the body, which are connected to the tail fin. There are two types of muscle that bluefin tuna employ. The outer layer of muscle is made up of ‘‘slow-twitch’’ muscle fibers. These muscles burn energy with oxygen (aerobic respiration) and are used for long-distance cruising. Below these muscles are the powerful muscles used for quick bursts of speed to make a kill or escape from danger. These muscles are made from ‘‘fast-twitch’’ muscle fibers that burn energy at a much higher rate and in the absence of oxygen (anaerobic respiration). The fast-twitch muscles soon build up high levels of lactic acid and become rapidly fatigued, so they can only be used for short sprints rather than long-distance swimming. Tuna’s fast-contracting muscles burn a huge amount of energy to generate the power required for swift swimming. This means that even at cruising speed, bluefin tuna have a high rate of metabolism. To support this metabolism, the tuna’s muscles need to be enriched with oxygen in the blood. However, because the bluefin tuna has such a rigid head to stop it from moving about when it moves its tail, it cannot actively pump water over its gills like other species of fish. Therefore, to get oxygen out of the water, bluefin tuna use a process called ram ventilation. The very act of swimming quickly through the water forces water over the gills. This means that if the fish stops swimming it will not get the oxygen it needs and it will die. The gills themselves are densely packed with capillaries to extract oxygen from the water. To get the oxygen to the muscles, bluefin tuna have a specialized heart. It is pyramid shaped and highly muscled to pump blood quickly through the body. The muscles provide the power, but this needs to be put to good use. It is no good generating a lot of power with an inefficient propulsion system. The key adaptation for fast swimmers like the bluefin tuna is therefore the specialized design of the tail fin. The muscles in the tail of the fish contract in a wave so water is pushed along the body to the tail fin. The fin must release this water in such a way as to push the fish forward. The bluefin tuna (and other swift swimmers like marlin and sailfish) have a sickle-shaped tail fin. Furthermore, the part of the tail immediately in front of the fin, called the caudal peduncle, is very narrow and flat like a coin standing on its end. Only relatively recently has is been realized that a crescentmoon shaped fin is the most hydrodynamic shape and most efficient at creating thrust. The reason lies in the way that swirling vortices (which are like

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mini-whirlpools) build up and are shed from the tips of the fin. The vortices interact with the fin, moving from side to side to push the fish forward. The same crescent-moon design is seen in the tails and wing shapes of fastmoving birds like swifts. With these adaptations, the bluefin tuna can generate a huge amount of thrust to push it through the water. Much of this energy can be transferred into forward momentum because the bluefin tuna has several adaptations for reducing drag through the water. The bluefin is flattened vertically so it can cut though water. Its deep body, as well as being packed with muscle, acts as a keel for balance. Its head is cone-shaped to punch through the water like a bullet. Their pectoral fins on either side of the body just behind the head are used to generate lift to stop the fish from sinking, but these can be retracted into a groove to give further streamlining at high speeds. Even the eyes are flush to the surface of the body to allow water to flow smoothly over the fish. Thanks to these adaptations to reduce drag, alongside the remarkable power that the bluefin’s muscles and tail generate, bluefin tuna can reach speeds of more than 60 miles per hour (100 kilometers per hour) in short bursts. These fish are perfectly adapted for high-speed locomotion in water, allowing them to catch fast-moving prey and to escape their own predators. Borrowing from Nature

The powerful and perfectly shaped tails of tuna are currently being explored to see whether they can provide inspiration for a new form of marine propeller. It is still in the early stages of development, but the idea completely rewrites conventional wisdom on how to propel a vehicle through water. Rolls Royce, the company leading the research, is hoping the tuna can unlock the secrets of how to get the most propulsion out of tiny amounts of energy.

JET PROPULSION In the 1930s, aviation was revolutionized by the invention of the jet engine. Until then, airplanes were driven forward by simple propellers that forced air backward, generating forward thrust. Jet engines take this general principle and magnify the force of the thrust produced. A propeller is used to suck in air to the engine, where it is compressed and mixed with fuel and then ignited. This burning mixture of high-pressure air and fuel is ejected from the end of the engine, generating a lot more thrust than a propelThe longfin inshore squid, Loligo pealei. As with all squid, this species has a siphon ler would have been capable of on its (shown just below the eye) through which it own. Jet propulsion is now used in aircan force water to propel itself forward. planes, rockets, spacecraft, and even land vehicles. [Richard Ellis / Photo Researchers, Inc.]

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Jet propulsion has been responsible for the fastest speeds recorded by man-made machines. Certainly there is nothing in nature that can match such velocities, although there are no organisms that need to travel so fast. High straight-line speed can be a hindrance when an individual might need to stop or corner at some point, but jet propulsion has indeed evolved in nature and it as proved to be a highly success adaptation. Squid have an built-in jet propulsion system that enables them to move at high speed through water, allowing them to hunt, evade predators, and even dance with each other. Squid are unusual-looking creatures. Different species of squid vary in size from just 12 inches (30 cm) to 65 feet (20 m) in length. Their body is a muscular foot called the mantle, which is the same as the body of other mollusks such as snails and slugs. At the end of the mantle is a small head that consists of a mouth and two very large eyes. Around the mouth are 10 tentacles, two of which are longer than the others and have modified, clubbed tips for striking prey and potential predators. The mantle has two fins that it beats in a ‘‘ripple’’ effect that can propel it forward or backward at low speeds. When a squid needs to travel much faster, though, it can deploy its jet propulsion system housed in the mantle. The mantle is a hollow organ. Water can be drawn into the mantle cavity through an opening by the head. This is done by contracting certain muscles in the elastic mantle wall, which increases the volume of the mantle cavity, much like the expansion of our thoracic cavity when we breathe in. The squid can then quickly contract other mantle wall muscles, squeezing out the water inside the cavity. The opening by the head, through which water was drawn in, is now clamped shut by powerful muscles. As a result, the water being squeezed from the mantle can only escape through a thin funnel called the siphon. The siphon is extremely mobile and can point in any direction, which gives the squid a great deal of control over its jet propulsion and therefore the directions in which it moves. Squids can perform several of these bursts of jet propulsion in quick succession, which is enough to escape prey or strike, at lightening speed, at unwitting prey. In this way, squid can travel up to 25 miles per hour through water and have even been known to hurl themselves from the water and through the air to escape predators. Even when traveling at these speeds, they remain highly mobile. Recently, scientists recorded the giant squid, Taningia danae, moving quickly under jet propulsion and flipping on the spot before shooting off in the opposite direction—not bad for an animal that is over 6.5 feet (2 m) long. Elsewhere in the oceans of the Southern hemisphere lives a completely different type of organism that moves in a very similar way. Marine salps (of the class Thaliacea) look exactly like jellyfish. They range in size from a few millimeters to a few centimeters and have soft, gelatinous, transparent bodies. Despite their appearance, they are not jellyfish, but tunicates, one of the

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invertebrate chordates. Humans are chordates as well, so marine salps show all the basic traits common to humans and related animals, although salps lack a backbone. These creatures are more closely related to us than to the jellyfish that they so closely resemble. Salps move about by jet propulsion. They draw water inside themselves through one aperture and force it out of the other, thanks to contractions of muscles surrounding each opening. In addition to using it for movement, though, these creatures make use of the water passing through them by filtering the water and feeding off the algae left behind. The filter is a mucus bag that is made up of sticky threads that create a fine mesh. It is sufficient to trap tiny algae, but at the same time allows water to pass through to propel themselves through the ocean. Borrowing from Nature

It is fair to say that this is one area in which humans have far surpassed any adaptation seen in nature. The next step for human engineers is to develop propulsion systems in a quest for greater and more efficient thrust. Engineers are already exploring beamed energy propulsion systems (essentially using lasers) and ion propulsion (which to date has only been heard in episodes of Star Trek). This leaves the humble squid somewhat far behind, but for these deep sea animals, their form of jet propulsion serves them fine. It allows them to strike with deadly speed, evade predators when attacked, and even take to the skies, albeit briefly.

PARASITIC LOCOMOTION All organisms undergo some form of locomotion at some point in their lives. Animals, for the most part, move about under their own steam. Many other organisms, though, move by being carried on currents of air or water. By this latter method of locomotion even plants, which spend the vast majority of their lives in one place, move about as seeds, often covering great distances. There is a further group of organisms, though, An electron micrograph of Rhinovirus-14, one that could find it hard to move from of the viruses responsible for the common cold. place to place. Parasites living inside other organisms must move, eventually, [Kenneth Eward / Photo Researchers, Inc.] to another host. Many parasites achieve this by exiting the initial host, often as eggs or larvae, and waiting to be ingested again by another unwitting individual. There are some parasites,

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however, that have evolved the ability to move from host to host by directly manipulating their host’s behavior. We have all suffered from the common cold and are very familiar with the symptoms that come with it. There are more than 100 different common cold viruses, collectively called rhinoviruses (‘‘rhino’’ meaning ‘‘nose’’). When the virus is inhaled into the nose, individual virus cells attach themselves to a particular part of the nasal cells called the ICAM-1 receptor. Once the virus has docked with the receptor on the outer cell membrane it can enter the cell itself. Inside the cell, the virus hijacks the host cell’s own machinery to replicate itself many times over. Eventually, the host cell will be filled with virus particles, at which point it dies and ruptures, releasing the new copies of the virus that will infect other host nasal cells and repeat the process. After the initial infection with the virus, this process of viral replication goes on for about 8 to 12 hours before any symptoms are shown. It is estimated that an infection can be triggered by just 1 to 30 virus particles, but these numbers grow rapidly during the incubation period. Between 12 and 72 hours after infection, symptoms are displayed. Part of the immune response to viral infection is the release of inflammatory mediators (which include histamine, kinins, interleukins, and prostaglandins) to protect against further infection. These inflammatory mediators dilate blood vessels and cause mucus glands to secrete the mucus so familiar to anyone who has had a cold. Inflammatory mediators also trigger coughing and sneezing. What is striking, though, is that the symptoms caused by the inflammatory mediators are not necessary to effect a full recovery from infection. Some 25 percent of people who are infected with the cold virus do not develop symptoms. But the action of coughing and sneezing is extremely helpful for the cold virus. A sneeze sends thousands of tiny droplets of saliva and mucus into the air. Each one is packed with virus particles, the perfect way for the virus to travel from one host to another. The manipulation of the host by the virus gives it an excellent means to move to the next host, but whether the virus is simply benefiting from our human immune response or whether the virus has evolved this adaptation directly it is not clear. It doesn’t matter, though, as evolution has led to rhinoviruses being dispersed by the host sneezing. Other parasites, however, clearly have a much more direct effect on the host, changing its behavior to benefit only the parasite and not the host at all. Like many parasites, the parasitic flatworm Leucochloridium paradoxum has two dependent hosts. It must pass through a bird host and a snail host to complete its life-cycle. It will live in the bird host until, as eggs, the parasite will pass out of the bird in its droppings. The eggs will then hatch into tiny larvae called miracidia, which swim freely in water. From here they can enter the next host when a snail living near the pool or stream ingests the infected water. Once in the snail’s digestive system, the miracidia migrate to the main digestive gland, where they change into the next larval stage called the cercariae.

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These migrate through the snail’s body again and collect in long tubes called sporocysts, which extend into the snail’s eye stalks. Several hundred cercariae can be contained in one sporocyst. The sporocyst in the snail’s eyestalk is visible through the snail’s translucent skin. What’s more, it will pulsate so that the overall effect is very much like a juicy caterpillar crawling about in the vegetation. The effect is so realistic that birds will peck at the infected snail’s eye stalks thinking they are a tasty morsel, resulting in the parasite being ingested once more by its bird host. In addition to changing the appearance of the snail, the fluke also manipulates its behavior. No longer will the snail move around under the cover of vegetation as it would usually, but instead will crawl up the plants that ordinarily provide it shelter and sit on top of the leaves in plain view of bird predators, increasing the chance of the snail’s caterpillar-like eyestalks being spotted by a hungry bird. A similar mind-control manipulation is seen with the rabies virus. The bulletshaped rhabdovirus infects host cells much like any virus; it binds to the cell membrane and then uses the host cells’ DNA replicating machinery to make copies of itself. The virus will replicate in the muscle cells of its host, which includes most mammals, even humans. From there it will migrate to the nerves and eventually to the brain. The final stages of the infection sees the virus migrate to the saliva ducts, tear ducts, sweat glands—anywhere that water is excreted. It is here that the virus will get into the next host. To achieve this, the virus manipulates the host’s behavior. The infected animal will cease to be wary of other animals and will be prone to attack and bite anything that it comes across. If it manages to bite another animal, the infected animal will pass on the virus through its saliva. Thanks to its manipulation of another organism, the virus has a very reliable method for moving from one host to another. Borrowing from Nature

It is unclear exactly how parasites manipulate their host’s behavior. It is likely that a chemical is produced by the parasite itself or causes one to be produced by the host that alters the behavior-controlling part of the brain. It is a truly incredible adaptation to ensure that the parasite can move to the next host. Most of the attention focused on these parasites lies in treating the infection rather than making use of their behavior-controlling adaptation. For the cold virus, there is a great deal of attention on treating the sneezing symptoms that the virus elicits, so understanding how the virus affects humans is leading to better treatments.

POLLINATION For much of their lives, plants find themselves rooted to the ground and unable to move about. This is clearly not a huge problem for them. Plants are enormously successful organisms that dominate the earth, so a sedentary

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life is clearly no hindrance. However, there is a key part of their life-cycle when plants do need to move from one place to another. For those species that reproduce sexually, the pollen from one flower must travel to another, where it can enter the ovary and fuse with an egg to create a seed from which a new plant will grow. Some plants rely on wind to disperse their pollen, but others have evolved a series of ingenious adaptations to engage an animal to carry their pollen for them. The advantage of employing an animal courier to carry pollen from one plant to another is that the plant does not have to invest potentially huge amounts of energy to produce the vast numbers of pollen grains that wind-pollinated plants release. The strategy has its costs, though. To attract an animal to itself the plant must offer some form of reward. The simplest solution to this problem is to offer up some of the pollen itself. Pollen is nutriThe flower of the wild orchid, Ophrys scolopax, tious, and many insects, birds, and mammimics the shape, color, and smell of the female mals will readily eat it. The cyads, which long-horned bee (Eucera longicornis). [Perennou were around at the time of the dinosaurs Nuridsany / Photo Researchers, Inc.] and have changed little since, use this approach. Weevils that are attracted to the pollen spilling from the cyads’ huge cone-shaped, pollen-holding structure will get covered in pollen grains. When they fly to another plant for the next feast pollen on their backs will be transferred to the cyad’s sticky stigma, where it can fertilize an egg. Other species offer pollen but restrict its access to just one courier species. Like all flowering plants, the pink gentian from southern Africa produces its pollen in specialized organs called anthers. However, the gentian hides its pollen inside its anthers, rather than growing it on the outside for any animal to get at. The only way for the pollen to escape is through a tiny hole at the top, and only one group of bee has evolved to get at it. African carpenter bees cling to the anthers and buzz their wings together at a precise frequency. This provides exactly the right vibrations to shake the pollen from the hole in the anthers, which the bee will gather up and store in its specialized carrying baskets on its legs. Again, as a foraging bee travels from plant to plant in search of food, some of the pollen is transferred between plants, allowing fertilization to take place. Of course, a common reward that plants offer their couriers is the sugary liquid nectar. The flowers of many plants are shaped so that to get at the nectar

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hidden deep within the nectaries of the flower, the insect or bird must position their bodies in a certain way. This position allows the animal to get its reward but also requires it to brush past the pollen-heavy anthers and sticky stigma, the female organ of plants. In addition to having a particular shape, many species grow flowers that release attractive scents and that are colored to attract certain insects. Furthermore, as many insects can see ultraviolet (UV) light, flowers invest in UV reflecting patterns. Often these markings act as a ‘‘landing strip,’’ directing the insect where to land and how to get at the nectar. The mountain laurel from North America and the Spanish Iris from northern Spain, among many others, use markings to direct their pollinators. Not all plants offer an honest reward for the services of their pollinators. Sex sells just as well as food, and many plants A male long-horned bee is fooled by the orchid offer insects what they perceive to be a mimic and attempts to mate. As it does so, the orchid deposits a pollen-containing packet willing mate. The orchids are especially called a pollinia on its head. The bee will then well adapted to these seemingly undertransfer the pollen as it attempts to mate with hand tactics. The mirror orchid grows other orchids. [Perennou Nuridsany / Photo in regions west of the Mediterranean in Europe. Its flowers are a metallic Researchers, Inc.] violet-blue color with a yellow border trimmed with red hairs. On either side there are two smaller oval shapes that look like wings, giving an overall effect of a bee. To be more specific, the effect is of a female bee. The illusion is completed by the plant releasing a scent that mimics the pheromones released by sexually receptive females. The whole appearance is enough to fool a male, which will try to grasp the flower as if he were mating with a real female. As he tries to copulate, a curved column at the top of the flower swings down and glues a packet of pollen, called a pollinia, to the male’s head. This curved structure has both male and female organs so that as it swings down it picks up any pollen that has been deposited on the male by a nearby plant. Many orchids use this approach, which has led to the evolution of many ingeniously designed flowers. In some species, such as the yellow bee orchid, hairs on the surface of the flower trick a male pollinator into thinking that the female he is trying to mate with is sitting on her perch with her head

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pointing downward. When he triggers the mechanism to deposit the pollinia, the packet of pollen is deposited on the bee’s abdomen, rather than its head. Orchids are therefore described as being either ‘‘front loaders’’ or ‘‘back-loaders.’’ The result is that two species of orchid can share a pollinator. One species is fertilized by depositing pollen on the abdomen, and the other has its pollen carried on the bee’s head. There are many specialized adaptations that in one way or another coerce an animal, typically an insect, to carry a plant’s pollen for it. Each one is perfectly adapted for the plant in question and means that even stationery organisms can move about, albeit by recruiting the locomotive talents of other species to do so. Borrowing from Nature

The success of many of the crops grown by humans is dependent on their pollination by insects. Disease or adverse weather conditions can devastate populations of pollinators like the honey bee, which in turn can have devastating effects on crop yields. This is why there is a great deal of interest in studying the pollinating behaviors of certain key insects. Agriculturalists are now beginning to realize that suitable habitats for their crop pollinators need to be provided alongside crop fields to ensure that plants are fertilized so seeds and fruits can be produced.

BACTERIAL FLAGELLUM All organisms show some form of locomotion. Animals show the greatest range of movements, but not one species is capable of generating constant thrust, not even the hummingbird. At some point in the movement of a wing or leg there is a reduction in thrust as it returns to its starting point. Some animals come close to generating continuous thrust— the undulating bodies of fish, snakes, and eels, for example—but there are still inefficiencies in these movements. Humans, on the other hand have invented the wheel, which can spin freely and so can produce constant forward thrust, be it in cars or as the propeller of an airplane or boat. It was thought that a free-spinning wheel cannot evolve in animals, as there are too many limiting factors. You can get close by whirling

An electron cryotomography scan of a bacterium flagellum shown from different views. Bacteria flagella are the only wheel-like, free-spinning structures known to have evolved. [Gavin Murphy / Nature / Photo Researchers, Inc.]

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your arm around in a circle, but your shoulder will never spin freely in its socket because the muscles, tendons, and ligaments would soon get twisted. Without being able to evolve a wheel, animals are doomed to roam with imperfect movement. Remarkably, though, the wheel has evolved—not in animals, but in bacteria. This is a truly staggering adaptation and quite possibly the most complex and elegant one on this planet. Until very recently, it was deemed so complex that questions were asked about whether it could have evolved by natural selection at all. As with human designers, bacteria put their wheel to use to move around. Attached to one end of the bacteria’s body is a free-spinning wheel that is in turn linked to a long, tail-like filament called the flagellum. Driven by a tiny motor, the flagellum can spin like a propeller to drive the bacterium through water. There are three elements to the flagellum. The motor that drives the flagellum is called the basal body, which is embedded in the cell wall of the bacterium. Within the basal body is a series of three rings, which are about 20 nanometers in diameter. One ring sits within the inner membrane, one sits within the cell wall itself, and the third ring sits within the outer membrane. Within these rings is a thin rod that is free to spin through 360 degrees. Attached to the rod is the second element to the device, called the hook. It connects the basal body with the third element: the filament, which is a thin, whippy structure made from a protein called flagellin. It is about 5 to 15 micrometers (5,000 to 15,000 nanometers) long. As the rod within the basal body spins, the filament spins too and propels the bacteria through the water. In total, about 40 proteins self assemble to create this sophisticated piece of natural engineering. The typical source of energy in living organisms is the energy-rich molecule called adenosine tri-phosphate (ATP), and so one might assume that this is the fuel that drives this bacterial motor. However, although ATP is used in bacteria to power other functions (including the building of its flagellum), it is not used to power the basal body motor. Instead, the motor is powered by electrically charged sodium ions (in some species) or electrically charged hydrogen ions (in others) flowing across the cell wall. The bacterial flagellum is an electric motor! This is a highly efficient power source. Nearly 100 percent of the energy stored within the sodium and hydrogen ions crossing the cell membrane is transferred to the spinning filament, which allows the filament to spin at up to 100 times a second. This can drive a bacterium forward at a speed of about 25 micrometers (10 bacterial body lengths) a second. In relation to body size, that is about twice as fast as a human can run at full speed. In all bacteria that have a flagellum, the rod (and therefore filament) spins most of the time in a counter-clockwise direction. But the direction can be reversed occasionally, causing the bacterium to do a somersault so it faces the opposite direction. Switching back to a counter-clockwise spin, the bacterium

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can then swim off in a different direction. This behavior, called tumbling, is very important for an individual bacteria to steer. Bacteria have tiny sensing molecules that can detect whether they are moving toward a food supply or something poisonous. Bacteria will therefore adjust their direction so they travel toward food and away from toxins. Their sensors interact directly with the tiny molecular switch that controls the direction the rod rotates. In toxic environments bacteria tumble more frequently to attempt to find a way out of danger. This behavior of detecting and responding to chemical stimuli in the environment is called chemotaxis. A spinning flagellum is an excellent way for a bacterium to get about, although there are times in their lives when bacteria cease swimming and cluster together to form a community of bacteria called a biofilm. Biofilms form and adhere to surfaces (a good example is the bacteria that make up dental plaque on teeth) or float to the surface of water. They are typically very resistant to host defenses such as antibiotics and so they help bacteria survive in harsh environments when individuals living on their own might easily be killed. When clustering in this way, the last thing bacteria would want are spinning flagella, which could break up the biofilm. When bacteria switch from a free-swimming lifestyle to a more sessile way of life they disengage their flagellum. One might expect the flagellum to stop spinning somehow, but this is not the case. Instead, the rod keeps spinning, but the link between the rod and filament is disengaged. Bacteria have evolved a microscopic clutch for their motors! The advantage of having a clutch is that the bacteria can easily return to their free-swimming way of life simply by dropping the clutch and reengaging the filament with the motor. The clutch has been identified as a protein called EpsE. When the bacteria wants to stop swimming it produces this one protein that disengages the rod from the filament. It does this by changing the shape of the basal body. When it wants to swim again, the EpsE protein is broken down, allowing the filament to reengage with the spinning rod. Borrowing from Nature

There is a great deal of interest in designing and building nano-robots driven by microscopic engines like the basal body motor. These can be used for a number of applications, such as delivering drugs and repairing organs from within the body. By studying the bacterial flagellum, which self assembles, nanotechnologists are gaining insights into how they might produce engines on their own tiny, man-made machines.

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MATERIALS—HUMAN INVENTION Many species of animal make use of tools to aid their survival, but humans are the undoubted masters. Manual dexterity and a keen, problem-solving brain have allowed us to design and build a cornucopia of gadgets that have assisted our quest for survival. Our human ancestors are likely to have discovered the first basic tools by chance. Certainly, though, their inquisitive brain would have helped discover the range of uses such tools could be put to. Today’s higher apes show a similar tendency to use tools when confronted with a problem that cannot easily be solved using their hands. It is uncertain when our human ancestors would have taken a more proactive approach to tool making. Our brain is certainly capable of addressing a certain problem and imagining what tool would help solve it. From there, we can design the tool that has formed in our mind. Our ancestors will have had this same ability. There has even been the suggestion made recently that it was the accidental discovery of tools that kick-started the evolution of the human intellect. However the use of tools arose in human history, one of the limiting factors in tool making is the materials that are available. The earliest tools were made from wood, bone, or stone. The first stone tools were made some 2.5 million years ago. Even this far back our ancestors were carefully choosing their materials to make the right tool. The first tools made by Homo habilis were made from stones of volcanic rock worn smooth by the waters of creeks and rivers. These cobbles were struck by other stones to flake off pieces of stone to create a hand-held tool with a sharp edge but a smooth handle. These simple tools could have been used to butcher animals so humans, with weak jaws and teeth, could get to the energy-rich flesh within.

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Stone tools have the benefit of remaining in the archaeological record so we can see the type of materials used by early man, but we assume that other materials were used as well. Wood, bone, and horn have varying toughness, but each can be easily shaped and fashioned into a tool. Leather and furs from the skins of animals could have been used for clothing and housing materials. Sinews from animals would have been used for making tight-binding cordage, likewise, the fibers and bark of plants could have been used for similar purposes. These natural materials used by early humans were, and still are, extremely versatile media to work with. Humans still use them to this day and in some instances they remain superior to man-made materials. Of course, synthetic materials can be made from naturally occurring ‘‘ingredients.’’ Copper occurs naturally as a metal and is readily shaped, being a soft metal. This form of metal was used some 10,000 years ago, but it took a further 3,000 years before copper was extracted from its ore (metal contained in rock). This was quite a leap forward in producing materials. For nearly half a million years prior to this moment fire had been used only for cooking and warmth. Around 3500 BC humans discovered that copper could be made stronger if it were melted together with tin and then allowed to cool, making the first alloy (a mixture of two metals). The resulting bronze was used for making a wide range of tools, including weaponry and farming tools. Further alloys were being discovered, though, and in 900 BC brass was discovered by mixing copper with zinc. Two thousand years after discovering how to make bronze, advanced fire-making technologies using kilns allowed hotter fires to be maintained, which allowed iron to be extracted from its ore. Although it is a weaker metal than bronze, wrought iron was a preferred material because it kept a sharper edge, making it useful for weaponry. Iron can be made stronger, though. It was soon discovered that iron mixed with small amounts of carbon create steel, a very strong and workable metal indeed. It is only in the last 150 years that we have begun to move away from a dependence on metals and natural materials like wood, although these materials are still very important. Plastics are man-made materials that are lightweight, moldable, and noncorrosive. All plastics can be heated and shaped, which means they have a huge range of applications. Typically, they are polymers. This means that their chemical structure is made from a single molecule repeated over and over again in chains or sheets. The way the chains interact with each other gives the particular plastic its physical properties. There are a number of naturally occurring polymers, and indeed the first man-made plastic produced in 1862 (Parkesine—named for its inventor Alexander Parkes) was derived from cellulose that is found in plants. Parkesine displayed the classic plastic trait of being readily moldable when heated. From this point, there followed a whole range of man-made plastics. Celluloid appeared in the nineteenth century, first as a replacement for ivory in billiard balls and then in its more familiar guise as photographic film for motion

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pictures. It wasn’t until the invention of Bakelite in the early twentieth century, though, that plastics really took off. Bakelite was the first entirely synthetic plastic to be produced. It is produced first as a liquid, which molds to whatever container it is poured into and sets hard as a resin. Unlike other plastics, it cannot be melted down again to be reused. In liquid form, bakelite can be added to more or less any material to strengthen it. During the Second World War the U.S. government used it in many instances in armor and weaponry as a replacement for steel. For many years after, it could be found in most household objects because it does not break, fade, crease, or crack under a wide range of extreme conditions. The list of successful plastics is huge and include familiar names such as nylon, cellophane, PVC, teflon (noted for its ability to allow nothing to stick to it), polyethylene, velcro (plastics shaped into complementary loops and hooks), and perspex. Each of these has a fundamental impact on nearly every aspect of our lives and has enabled huge leaps forward in cutting-edge fields like medicine, space exploration, and the military. The family of plastics contains a hugely versatile and varied range of materials, and there is great scope to produce more. Many are carbon-based, but the key to their properties lies in the other elements that hang from the carbon ‘‘skeleton.’’ By tweaking the basic chemical building blocks of carbon a whole range of properties can be generated. Similar to plastics are synthetic materials like kevlar. These are important materials because they have a much greater strength-to-weight ratio than metals like steel, which means they can be used in structures that need to be strong yet light. Kevlar, or poly-paraphenylene terephthalamide to give its chemical name, is made from long molecular polymer chains that bind together very tightly. Some polymers have weak interactions between the long chains, which gives them flexibility and moldability. The tight binding between kevlar polymers gives the material overall strength, albeit with flexibility as well. Kevlar is used in ropes and cables, canoes, tire walls, drum heads, woodwind reeds, electricity generators, fiber optics, in brakes, and for many construction purposes. Probably its most recognized use, though, is in body armor. Sheets of kevlar in body armor and helmets can ‘‘catch’’ a bullet and slow it sufficiently to prevent serious injury in the wearer. It is thought that hundreds of soldiers’ lives are owed to the bullet-stopping properties of this remarkable material. Although humans still rely on many naturally occurring materials, there are many synthetic materials that have improved on what we have harvested from nature. Man-made materials have the benefit of being produced in wellcontrolled environments, which means that their microscopic structures can be put together with minimal flaws and weaknesses. This is crucial for producing steel, say, which is going to be used to build skyscrapers hundreds of feet in the air, as the engineers need to know the exact properties of the material they are working with. New materials are constantly being produced, but much of

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the inspiration comes from materials found in nature that continue to outperform anything produced by humankind.

SILK Humans have developed a myriad of materials to build ever stronger and taller structures. Similarly, the natural world abounds with materials that are put to good use in the construction of impressive structures. Perhaps the most remarkable of these is silk, which can be spun and used by insects in a numerous ways. Silk has been valued by humans A scanning electron micrograph of spider silk showfor centuries because of its unique ing the different type of strands that a spider is properties, but the ability to procapable of producing. [Mona Lisa Production / duce silk is restricted to certain Photo Researchers, Inc.] insects and arachnids. Silk is a protein with a microscopically complex structure, and it is secreted from the glands of silk-producing animals as viscous liquid. This liquid is directed and manipulated with specialized organs located at the opening to the silk gland. For many years it was not understood how this liquid became the solid fibers we are familiar with. Some scientists reasoned that contact with air changed the silk from a liquid into a solid, but it’s now evident that tension causes this change. The simple act of pulling the liquid silk tight hardens it into a solid, flexible material. The microscopic structure of silk varies considerably from species to species, but the basic structure is a long chain of amino acids (the building blocks of proteins) that are arranged into loose spirals and rigid sheets. The sheets are arranged together to form protein crystals, similar in appearance to a section of corrugated iron. These crystalline regions are separated by tangles of loose protein chains, and it is the combined properties of these hard areas and more elastic regions that bestow silk with it extraordinary characteristics. Silk is very strong indeed, much more so than other biological materials such as bone or tendon, and it is twice as elastic as nylon. In terms of tensile strength, spider silk is about half as strong as steel, but it much less dense. The tensile strength-to-density ratio of silk is five times that of steel and is on a par with super strong fibers such as Kevlar. It has been said that a strand of spider silk the same thickness as a pencil could stop a jumbo jet in flight. Bold claims aside, silk is undoubtedly a remarkable material and it has been instrumental in the success of several species that can make and use it.

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Of all the invertebrates that produce and use silk it is the spiders that employ it most widely. They not only produce a bewildering array of webs to catch prey, but they also use it for drag lines to anchor them safely when they are climbing; to fly through the air on long, fine threads that catch the wind; and as wrappings for disabling and storing their prey. Thanks to its molecular structure, it is also waterproof, which means that it is an excellent material for spiders and insects to wrap their eggs and developing young to protect them from the elements, both from getting too wet and from drying out. Although the spiders are the undisputed masters at using silk, some other arthropods are also very accomplished when it comes to producing and using the material. Pseudoscorpions produce silk from their mouthparts, and they use it to produce tiny silken igloos in which to molt, lay eggs, and see out cold weather. The larvae of bees, ants, wasps, and some butterflies and moths secrete a cocoon of silk just before they pupate. The silk produced by bees, ants, and wasps consists of multiple helices wound around each to produce a fiber that is very tough and with quite different properties compared to the silk that is produced commercially from the silk moth (Bombyx mori). The larvae of some fly species also produce silk, and the black fly larva use silk to anchor themselves in fast flowing water. They secrete a small pad of silk onto a suitable rock and attach themselves to this secure anchor using small hooks on their rear end. In many instances, structures built using silk are left exposed to the elements over quite a long period of time. Silk structures can be used for days, weeks, and even years, which exposes the material and the structure to attack from microbes and other pathogens that could be harmful to the insect or spider that constructed the web in the first place. Some spiders live in environments heavy with bacterial and fungal infection. Funnel web spiders that tunnel through the soil are exposed to many pathogens, as are cave spiders that come into contact with thick fungal growth on the walls of their warm, humid caves. To protect themselves from these pathogens, they have evolved an antimicrobial silk that allows them to live in near-sterile conditions. The silk is also very helpful for spinning an antiseptic cocoon for spider eggs, offering great protection against disease when the young are at their most vulnerable. It is not clear how this antimicrobial property is achieved. It does not appear that spiders secrete an antibiotic fluid that coats the silk, but rather that silk itself is resistant to attack by bacteria and fungi thanks to its molecular structure. It seems that this resistance comes from the presence of certain small proteins that make up the silk. Each of silk’s many uses requires it to have different physical properties. The exact molecular structure of the silk protein produced by a particular species determines these properties. Having more loose protein chains in the structure will make silk more flexible and elastic. With a greater proportion

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of stiff protein sheets, the material will be much stronger or more waterproof. There are even molecules that can be added to the silk that can make it sticky for catching prey. What is more remarkable is that one individual can often produce different types of silk at will. There is no doubting the enormous benefit of being able to produce silk. In the species that produce it silk is used for so many aspects of life that it can be considered the single most important adaptation that they have evolved, enabling them to exploit a range of unique evolutionary niches. Borrowing from Nature

The antiseptic properties of silk have been alluded to in literature for several centuries. Even Shakespeare mentions it in A Midsummer’s Night Dream. Scientists are only now beginning to fully explore this folklore, but the hope is that the strength, flexibility, and sterility of silk can support several medical uses, from bandages to futuristic stitches that hold transplanted organs in place, or as scaffolds to allow grafts to knit together. The main area of interest, though, is in making use of silk’s remarkable properties of strength. It can stretch by up to 40 percent of its length and can absorb 100 times more energy than steel before breaking. Research is being carried out on the molecular structure of silk. It seems that silk is made up of distinct regions of alternating flat, rigid sheets of protein and amorphous ‘‘blobs’’ of protein, which can curl up on themselves and stretch out like a spring. The relative frequency of each region gives different silk its properties. Research is looking to recreate these silk regions and to put them together in different ways for different purposes. One of these purposes is to produce bulletproof vests that are lighter and tougher than kevlar.

BONE Almost invariably, stable structures are built around a solid scaffold. For most vertebrates, this scaffold is a skeleton of bone—though some vertebrates, such as sharks, have skeletons of cartilage. Like all good structural materials, bone is strong, hard, and flexible. It grows with the body and can even repair itself if it breaks. What’s more, in several species it has evolved in highly specialized ways, which has allowed these organisms to exploit new niches and a unique way of life. The basic structure of bone is quite simple. It is made from a repeating matrix of collagen fibers, the springy molecule that gives skin its plumpness, which is strengthened by various hard minerals. Collagen, which makes up about 30 to 40 percent of the weight of bone, is itself a strong material. Most of bone’s strength, though, comes from hard minerals that are built up around the collagen matrix. These minerals are typically calcium phosphates, the most common of which is a mineral called hydroxyapatite.

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The collagen matrix and calcium salts work well together to give bone its strength. Collagen has very high tensile strength, which means that it can withstand being pulled apart. The calcium salts, conversely, have very high compressional strength, which means that bone can withstand high weight loading. This latter capability is very important when you consider the weight of animals A light micrograph through a section of bone showlike elephants and the pressure ing Haversian canals and the concentric rings of their own bulk must place on their lamellae around them. Haversian canals carry blood leg bones. Unfortunately, howvessels, lymph vessels, and nerves. Lamellae are ever, bone does have one weakmade from compacted collagen fibers and minerals ness—it has a very low torsional produced by bone-forming cells called osteoblasts. strength. Most fractures are the [Steve Gschmeissner / Photo Researchers, Inc.] result of the bone being twisted. The strength of bone is not due simply to a random mixture of collagen and minerals. It is a highly structured material, and it is this structure that confers its toughness. The outer layer of bone is a thin but tough fibrous layer of tissue called the periosteum, which serves to protect the bone and acts as an attachment point to the muscles and tendons which join to it. Within the periosteum is a thick layer called compact bone. This, as its name suggests, is hard and compact. It is the periosteum that gives bone its rigidity and resistance to bending. Compact bone is highly structured, being made up of a series of concentric layers of the calcified collagen matrix. Bone that supports a limb has two distinct regions that have slightly different structures. The area at each end of the bone (where the bone makes a joint with another bone) is called the epiphysis. The long section between the ends is called the diaphysis. Both are covered with compact bone, although the diaphysis generally has a thicker layer to protect it from direct blows to the side of the limb. Within the compact bone layer of the diaphysis is a cavity filled with yellow bone marrow. The cavity of the epiphysis, however, is filled with a different type of bone called spongy bone. Spongy bone is made up of collagen and calcium salts, as with all bone, but they are arranged as a very loose, air-filled network. This gives the bone in the epiphysis more elasticity, which allows it to cushion the weight that is borne on the joints. The gaps between the strands of spongy bone is filled with red marrow, which is involved in producing red and white blood cells. The yellow marrow that fills the cavity inside the diaphysis part of the bone is involved with fat storage. The fact that bone is not solid all the way through is significant because it saves weight. Bones are strong, but at the same time light. This is

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best seen in birds, whose bones are hollow with very few strengthening supports to keep some rigidity. This weight-saving adaptation has allowed them to conquer the skies. If they had the bone structure of humans or other mammals they simply would not be able to take off under their own weight. It is not just marrow that runs through bone. Bone cells called osteoblasts grow within the hard material, producing more collagen and calcium salts. These cells are supported by blood vessels that run through narrow tubes, called Haversian canals, which permeate even compact bone. Even when an animal has reached its full size, osteoblasts continue to produce new bone, albeit at a much slower rate than when an animal is growing. This is balanced by the activity of cells called osteoclasts, which break down bone material. Bones are constantly being recycled and rebuilt to keep them strong. Bones being able to grow is a crucial adaptation, which allows young animals to develop into adults. This is very different from invertebrates, which need to shed their outer skeleton before they can increase in size. What’s more, the mechanisms that allow for bone to grow also allow bone to heal itself. When a bone is broken, osteoblast cells are produced in great numbers from the periosteum. This allows a collagen matrix to be rapidly produced to bridge the gap between the two broken ends. More slowly, calcium salts are laid down around the collagen framework to rebuild the compact bone. Borrowing from Nature

For such a lightweight, porous material, bone is remarkably strong. Pound for pound it is a stronger material than steel. It is also much more versatile given that it remains strong despite being filled with gaps. The basic structure of bone has been well-known for many years, but still there are discoveries being made. It is now recognized that much of bone’s strength comes from the atomic level. At this miniscule scale, scientists have observed that collagen molecules and tiny hydroxyapatite crystals are stacked on top of each other, which allows them to bond together very tightly. This structure even allows tiny cracks to appear that, rather than weakening the structure, actually make it much stronger. These discoveries are helping nano-engineers to design new strong, but lightweight materials that will take us beyond steel and carbon fiber as our basic, man-made building blocks.

CHITIN Arthropods are the most diverse group of organisms on earth and the most numerous of all the animals. This is a group that contains the insects and the crustaceans, each of which have the same basic design that has proved to be a very successful template for survival. Unlike mammals whose frame is supported by an internal skeleton of bone, arthropods are supported by an external skeleton of a substance called chitin. Although this may sound strange to

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us, there is one important benefit of this design. In addition to providing a rigid structure to allow movement, an external skeleton can form a protective outer layer of armor plating. Arthropods are indeed wellarmored creatures. The thick shells of crabs are a very good example, but many insects are also well protected thanks to a hard exoskeleton. Both the crab’s shell and insects’ exoskeletons are made from a versatile material called chitin. Like bone, chitin can be very strong and provides important protection for the body. Unlike bone, chitin is extremely light and can be very flex- A scanning electron micrograph of a butterfly wing ible, allowing insects to be very fast scale made from chitin. [Cheryl Power/Photo moving. Often these two adapta- Researchers, Inc.] tions—hardness and flexibility— are combined to produce agile yet well-protected individuals. This versatility of chitin is due to its molecular structure. Unlike similar materials like keratin, a protein, chitin is actually a carbohydrate. And unusually for a naturally occurring material, it is a polymer—that is, a long, endlessly repeating chain of one basic molecule. There are many man-made polymers, notably plastics, and these have the property of being firm but flexible. Chitin is no different. The basic polymer structure can be strengthened by the addition of certain other materials, such as calcium carbonate in the case of a crab’s shell. For the most part, though, chitin’s different properties come from how the material is shaped and laid down in a growing organism. Chitin is a very adaptive material. The secret of its success lies in how it can be deposited in layers to achieve whatever properties are required for the job. Depending on how thick it is, the material can be either very rigid or very flexible. Typically, a thick layer covers the body of arthropods in a structure we would recognize as shell. The chitin shells of crabs are very strong and rigid, offering protection from predators and from the high pressures of the deep ocean. This rigidity would be no good for the jointed limbs of crabs or insects, and yet chitin is used for these body parts too. For these organs, then, chitin is laid down in very thin layers that allow the joints to move freely and quickly. The versatility of chitin doesn’t end with it being either hard or flexible. It can form a waterproof layer that is impermeable not only to water but to gases as well. This is a very useful property for the outer armor of an insect

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that, like our skin, needs to keep water out. In other structures, such as in the gills of insect larvae or in crabs, chitin can do the opposite and let water and gases (like oxygen) pass through it. A further important property of chitin is that it can do all these things and yet it remains a very moldable material. This means that arthropods, and insects in particular, have been able to evolve a remarkable range of shapes and sizes without compromising their successful basic body design, which protects the organs and gives support for muscles and other tissues. Insects have therefore been free to evolve unique body shapes that can be useful for additional defense, such as in growing spines or additional armor plating or for adaptations like flight, camouflage, and communication. There is no doubt that chitin is a very important material for arthropods, being used for the body armor, limb joints, and wings. However, it is also the material used for many of the specialized organs in arthropods. In a highly specialized form it makes the lenses that in turn make up the compound eyes of insects. It is also used for other sense organs that are involved in taste and smell. It can also be molded into the fine, iridescent scales that reflect light in such a way as to give insects like butterflies their incredibly vibrant and metallic color. For insects one of the most important uses of chitin is to protect their young when they are just eggs. Chitin allows adult insects to have evolved into many weird and wonderful shapes, and their eggs are no different. The eggs of each species of insect has its own unique design that is perfectly adapted to the environment in which they are laid. For example, the eggs of water bugs, which are laid in water, have a shell with an intricate microstructure called a plastron, which allows oxygen, but not water, to pass through it to enable the developing embryo to breathe but not drown. This is achieved by folding chitin into a series of grooves and chambers that allows air dissolved in the water to accumulate, but not the water itself. Butterflies, on the other hand, lay their eggs on dry land and in the open. Their eggs are therefore toughened by bands of chitin wrapped around them, giving them the appearance of hand grenades. These eggs are therefore very tough and can withstand very dry conditions without desiccating. Borrowing from Nature

Chitin is very similar in structure and properties to another natural carbohydrate polymer, cellulose. Cellulose is a key material used for making plant cell walls, and it is this that gives plants like trees their tremendous strength and flexibility. There is little to distinguish these two polymers on the molecular level. A few alternative atoms hanging off the basic molecular structure here and there are the only differences. Scientists have exploited this principle and experimented with changing a few of these atoms of both chitin and cellulose. The results have been very fruitful. Chemically altered chitin has been used for bandages, burn dressings, food additives, drug capsules, and even cosmetics.

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Chitin is also finding its way into surgical medicine. Because chitin is biodegradable (it breaks down easily in water if not maintained or protected by the wax that typically covers it in nature) it can therefore be used for internal sutures for organs that can simply dissolve weeks or months after an operation.

FEATHERS With between 8,000 and 10,000 species known today, birds represent a very successful class of animal. They all share common features that have allowed such a diverse group to have evolved. Although some species have lost the ability, most birds are able to fly, which has allowed them to exploit habitats and ways of life unavailable to landbound animals. There are several key adaptations that have led to the evolution of flight. Various weight-saving adaptations such as hollow bones mean A scanning electron micrograph of a feather of the that less energy is required for an indi- Eastern bluebird (Sialia sialis). These feathers show vidual to get off the ground. To get off the remarkable microstructure that reflect blue the ground, the evolution of wings from light, giving the feathers a blue color. [Edward hand and finger bones has given birds a Kinsman / Photo Researchers, Inc.] means to generate the lift and thrust essential for flight. But perhaps the most significant adaptation is the evolution of the feather. This versatile material is crucial for many behaviors in birds and has been the key to their evolutionary success. Feathers grow from follicles embedded in a bird’s epidermis, much like hair growing from follicles in humans. They are arranged in rows so that the feathers overlap to provide the whole body with protection and insulation. Like hair, feathers are made up of the fibrous protein keratin, which fills the cells growing at the base of the feather. As more cells grow behind them, the keratin-filled cells will eventually die to leave a tough protein skeleton that constitutes a feather. Exactly how the cells grow and how the keratin is laid down determines what kind of feather is produced. There are several types of feather for different purposes. Quill feathers are long and thin and are found only on the tail and wings. Contour feathers are much shorter and cover the body, filling the gaps between the quill feathers. Down feathers, or filoplumes, lie beneath the contour feathers. These are fluffy and almost hair like and are important for insulation. All feathers have the same basic structure, though. Running along the ‘‘spine’’ of the feather is a stiff, narrow cylinder called the rachis. On each side of the rachis is a row of

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barbs that themselves support even smaller barbules. On one side of each barbule is a row of hooks. These fit into a groove on the neighboring barbule so they sit together tightly. Interlocking barbules are an important feature of a feather. They maintain the integrity of the feather without the need for producing one solid structure, which allows for the feathers over the bird to create a complete, but lightweight, covering that is essential for flight and for warmth. To align the barbules of the feathers, a bird must regularly preen its feathers. If you find a discarded feather you can see this effect. First, ruffle the feather so there are gaps in the vanes on either side of the rachis. Then pinch the rachis and run your fingers gently from the base to the tip. The barbules will realign like a zip, and the feather will be restored to its original shape. There are more barbules on the quill feathers because the feather shape is essential for flight. Contour and down feathers have fewer barbules because there is less need for them to maintain a continuous flat surface. The integrity of feathers gives the wing a light but rigid structure that allows air to flow over it to generate the lift and thrust needed for flight. Feathers are extremely important in restricting heat loss from the body. The down feathers will form a loose network of fibers in which air is trapped. The contour feathers over the top will help trap the air and create a warm, insulating layer. Even a thin layer of feathers offers excellent insulation. Emperor penguins living in the Antarctic have to cope with temperatures as low as −22°F (−30°C) and wind speeds of up to 125 miles (201 km) per hour, and yet they still maintain a body temperature of 100.4°F (38°C). In part this is thanks to a layer of blubber beneath their skin, but it is also down to their feathers which are small and densely packed with a woolly down layer close to the skin. Penguin feathers are also good at shedding water as they leave the ocean after hunting for food. Indeed, all birds’ feathers are waterproof thanks to a covering of oil that is secreted from an oil gland near the bird’s tail. This is spread over the whole body using the beak when the bird preens itself. In addition to being essential for insulation and for flight, feathers play an important role in attracting mates or staying camouflaged thanks to their color. For most birds, the color of their feathers are produced by a mixture of three types of pigment that are embedded in a bird’s feather cells as they grow. Melanins produce yellow, red-brown, dark brown, and black colors. Carotenoids are responsible for brighter yellows, oranges, and reds. Porphyrins are the least common group of pigments and produce greens, reds, and pinks. There is a fourth group of pigments called psittacofulvins that produce the deeps reds and oranges of parrots (the name means ‘‘parrot-pigment’’). Psittacofulvins differ from the more common carotenoids that have to be gained from the bird’s diet. The parrot pigments can be produced by the parrots directly and are not dependent on diet, which means they remain brilliantly colored no matter what they eat.

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Although many birds have blue-colored feathers, no blue pigment exists in birds. The dazzling display of birds like peacocks come from the shape and structure of the feathers themselves. The crystal structure of the feathers reflect and bend light in such a way as to produce a blue color. Greens, yellows, and reds can be produced in the same way. On the edge of the tiny barbules of these feathers there is a crystal lattice of melanin and keratin rods that gives the barbule a unique two-dimensional structure, which reflects and bends light in a certain way to produce color. The color produced depends on the number and spacing of these protein rods. These feather crystals are known as photonic crystals because of their color-producing properties. The colors and patterns of bird feathers are certainly important for mating displays and in settling territorial disputes. The size of the black ‘‘bib’’ on the breast of male sparrows is a signal of how strong and therefore how dominant that male is. Disputes can be settled easily with a quick check of the opponent’s bib size, meaning an argument can be resolved without the need for the risky business of fighting. Recently, a further function of feather color has been discovered. The feathers of young chicks reflect not only visible light, but ultraviolet (UV) light as well. Some species such as great-tits can see UV light, and brooding females use this ability to check on the health of their offspring. UVreflecting feathers require a lot of energy to grow. A healthy UV glow tells the mother that the chick is fit and well. By judging their chicks’ state of health from their glow, parents can adjust the food they give to certain individuals. Borrowing from Nature

The photonic crystal structures of peacock feathers are inspiring nanotechnologists to produce sophisticated, man-made equivalents for use in optical communications. By copying the melanin and keratin rod structure in peacocks and embedding them on sheets of zinc, nano-technologists have produced tiny structures that can vary the color and intensity of light given off. These new biological optics have the benefit of not only producing a range of colors important for communications, but also they are much easier and energy-efficient to produce because they use a production method that has already evolved in nature.

SKIN Covering the entire body and with a surface area of some 22 square feet (2 m2), skin is our largest organ. Its thickness varies, from only 0.02 inches (0.5 mm) on our eyelids, to 0.15 inches (4 mm) or more on the palms of our hands and soles of our feet. Remarkably, it accounts, on average, for about 16 percent of our body weight. Although this may sound heavy and bulky, skin is really a thin, lightweight material. Human engineers have so far failed to make a comparable material that is so thin, yet strong, flexible, waterproof,

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and stain-proof. Thanks to its impressive properties, skin plays a critical role in our survival and has evolved over time to be a highly specialized adaptation. Skin is made up of two main layers— the outer epidermis and the inner dermis. The deepest layer of the epidermis is constantly producing more cells. These new cells are gradually pushed toward the surface of the epidermis by those growing behind it. It takes one to two weeks for a new cell to be pushed up to the skin surface. Eventually, as they get to the outer layer, the cells will become filled with keratin, an extremely tough protein, and die. Keratinized cells are gradually sloughed off with daily wear and tear, which is why they need to be constantly replaced. There are two types of keratin found A section of human skin showing the layers of in the body. Hard keratin is found in tissue. [Gary Carlson / Photo Researchers, Inc.] hair and nails (and horn and hooves in other animals). Soft keratin has a slightly different molecular structure and is much more flexible, which makes it the ideal material for skin. Thanks to soft keratin, skin is tough and waterproof and protects deeper cells from physical damage, infection, and desiccation. The inner dermis is made up of strong collagen and elastic fibers, again combining strength with flexibility. It is collagen that gives skin its firmness and ‘‘plumpness.’’ With age, our skin loses collagen, which is why our skin wrinkles and sags as we get older. Within this tough core there are several types of cells, each of which carries out a unique but crucial role. Blood vessels puncture the dermis to bring nutrients to the growing cells of the epidermis. Perhaps more importantly, though, they play a key role in regulating our body temperature. When we are too hot and need to lose heat, our dermal blood vessels dilate and fill with blood, allowing the heat it holds to dissipate into the air. When we are cold, these blood vessels constrict so our blood, and the heat within it, remains in the core of our bodies and away from our extremities. This is why humans, especially Caucasians, look pale or even blue when we are very cold—there is little blood flowing to our skin to give its usual color. The dermis also contains sensory nerve cells to detect pressure and temperature. It is thanks to these nerve cells that we can control our grip (feedback from pressure nerve cells tell us when we are gripping too hard or too gently) and that we can avoid potentially dangerous environments. Touching very hot or cold surfaces triggers the nerves and elicits a lightening quick reflex in our muscles

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to pull ourselves away from harm. The dermis layer also contains follicle cells that produce hairs. Follicles are typically associated with oil and sweat glands, which keep hair and skin supple and waterproof. It is the sweat glands, though, that are perhaps the most significant feature of human skin. Sweating is another way in which humans keep cool. There are around 2.6 million sweat glands over an average human body, so they can quickly coat the skin in a film of water when the body gets too hot. This water is heated by the heat energy in our bodies, and as it evaporates heat is lost to the air. Being largely without body hair, humans can shed heat much more quickly than other animals such as dogs, which have fur and very few sweat glands at all. This simple trait of having many thousands of sweat glands over our bodies has been a key feature in shaping our entire evolution. Being able to lose heat quickly is an important trait for those animals living in the savannahs of Africa, and this includes our human ancestors. Overheating can damage the cells of the body, which is why many savannah animals are fairly inactive during the day. Early humans may have been different, though. They may have been able to brave the soaring temperatures thanks to their ability to sweat and keep cool. This would have allowed them to adopt a particular way of life. Early humans were most likely scavengers rather than hunters. Not being particularly quick, chasing down living prey would have been difficult, not to mention dangerous. But with keen eyes, humans could have spotted circling vultures and made their way quickly to the carcass below. Running over long distances without overheating would have allowed humans to be successful scavengers—especially at times when other animals would have been inactive. Further on in the history of human evolution, this ability to run long distances would have been used, as it is today in some hunter-gatherer societies, to chase down prey until it collapsed from heat exhaustion. This method, although grueling, incapacitates the prey without the danger of having to tackle it directly. The peoples who still employ this hunting technique will only perform the behavior when temperatures are over 100°F, when they know that animals such as antelope will suffer in the heat. Humans could only have evolved this behavior thanks to their skin and the specialized sweat glands, which make the difference between keeping cool and suffering heat stroke during such vigorous activity in the high temperatures of the day. Borrowing from Nature

Skin plays such an important role in protecting the body that we simply cannot live without it. Burns victims with more than 50 percent damage to their skin used to have little chance of survival. Recently, artificial skins have been developed and successfully used to graft on to burn victims. An artificial dermis can be made from cow collagen and a carbohydrate from shark cartilage.

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This is then covered with a thin silicon wrap, which mimics the tough epidermal layer. When used in surgery, this artificial skin is not rejected by the patient’s body but instead acts as a temporary scaffold to encourage new skin to grow. This artificial skin is not a permanent solution, but it offers critical protection while the body recovers and new skin is grown or skin grafts take hold.

BIO-CERAMICS There are many shelled creatures that walk and crawl the earth. Many of these species are the soft-bodied, unsegmented invertebrates called the mollusks. These animals are found in marine waters, fresh water, and on land. They include snails, slugs, limpets, mussels, oysters, squids, and octopuses and are one of the most successful groups of animals that live A scanning electron micrograph of a snail radula. on this planet. Their body is made The radula is the tongue-like organ found in all up of a muscular foot for moving mollusks. It is studded with rows of mineralabout and a main body (the dorsal encrusted teeth. [Clouds Hill Imaging Ltd. / Photo hump) covered by a piece of skin Researchers, Inc.] called the mantle. It is this mantle that is the organ that secretes the shell that so characterizes so many of these animals. The materials used to build this shell are a form of calcium-based minerals that are very much like the ceramics made by humans. Not only are these materials used for shells, but they are also found in mollusks’ teeth, which are perhaps the hardest that have evolved in any animal. Anyone who has walked along a beach and stopped to pick up a seashell will know that mollusks who live in the ocean make beautiful shells. Mollusks get the materials they need to build these shells by extracting various calcium carbonates from the water in which they live. Typically, these calcium minerals are absorbed in the forms known as calcite and aragonite. The mollusk constructs these minerals into layers of crystals embedded in proteins and other organic compounds, which are then secreted to make the hard shell that protects their soft and vulnerable body. The mineral crystals are arranged as microscopic ‘‘bricks’’ that can be bound together by a mortar of proteins and sugars. Closer inspection reveals that these mineral bricks are stacked on top of each other with proteins to create a layered effect, which is strong, flexible, and light. The net result of this natural engineering is a material that is up to 30 times stronger than calcium carbonate made in the laboratory.

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Mollusk shells are incredibly versatile materials, thanks to their innate strength and the way that they are layered, which affords movement and flexibility. They can withstand the enormous pressures of the deep ocean and the incessant pounding of waves, and they are strong enough to cope with the attacks of predators. Despite this strength, mollusk shell is very lightweight, allowing its inhabitant the freedom to move about. The flexibility of the material also allows the animal inside to grow. The calcium mineral shells of mollusks are well-known successful adaptations that have allowed sufficient protection to conquer both land and sea. What is perhaps less well-known, though, is that a similar ceramic material is also found in the teeth of mollusks to give them extraordinary hardness. The teeth of mollusks are embedded on a mobile organ called the radula, a tongue-like organ that is used to rasp at rocks to scrape off the algae that grow there. To remove their food successfully, mollusks have evolved a radula that is covered with up to 150 rows of tiny, but extremely hard, teeth. These teeth are so hard that some mollusks have even evolved to predate other mollusks, using their teeth to drill through the hard shells to get at the soft bodies within. The radula continues to grow through the mollusk’s life as the tip is worn away from its constant activity. Softer teeth at the back of the radula gradually harden as they get closer to the tip of the tongue. As the teeth mature, they are built up with many layers of minerals. The core of the tooth is made up of iron-containing minerals such as limonite and lepidocrocite as well as the calcium mineral hydroxyapatite, which makes up some 70 percent of bone in mammals. This core is known as an organic scaffold around which the hard outer shell of the tooth can be built. This hard outer layer is made up of a material with much more tightly packed iron and silicon minerals than the core, giving the tooth a strength that can withstand the scraping across tough rock. As the teeth mature, more minerals are added, giving them strength. It has been shown that this maturation is a carefully controlled process so that the minerals are added to the teeth in precise amounts and at exactly the right time. The whole process provides mollusks with a tough set of teeth that can get food from some of the most unlikely and inaccessible of places. Borrowing from Nature

The strong, flexible mollusk shells are being used as a model for developing flexible concrete, which can be used in structures that must endure extreme movement, particularly tall buildings found in places susceptible to earthquakes. They have also been used as a model for new ceramics, including coatings for turbine blades in jet engines that need to withstand extreme stress, heat, and corrosion. The ceramics industry is equally interested in the mollusk’s radula teeth, the structure of which is being copied to be applied to the fields of dredging, drilling, and mineral processing.

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MUCUS Mucus will be familiar to us all—we tend to produce a lot of it when we are infected by the common cold virus. But what exactly is this slimy substance? It is common and is produced by vertebrates, invertebrates, bacteria, and fungi alike. Despite having many functions in nature, the structure is essentially the same. It is made up principally of The pacific hagfish, Eptatretus stoutii. [Tom mucins, which are one form of a special McHugh / Photo Researchers, Inc.] group of proteins called the glycosylated proteins (a protein molecule with a carbohydrate [sugar] molecule attached to it). Other proteins are mixed in with mucins, and this basic mix of molecules gives mucus its property of a sticky, viscous fluid. In humans and other mammals, mucus contains molecules that are part of the immune system. It lines the lungs and airways, the intestines, the genitals, eyes, and auditory systems, offering lubrication and protection from infection. It plays a similar role in other animals, although there are certain animals that have evolved unique uses of this essential material. The hagfish are thin, primitive, worm-like creatures, closely related to lampreys, found mainly in the Pacific Ocean, some 4,000 feet below the surface. They belong to the class of organisms called the Myxini, which appropriately enough means ‘‘mucus.’’ This is especially apt because the hagfish has evolved an entire life based around the slimy material. Hagfish mucus, though, is unlike the stuff produced by other animals. Mixed in with the basic structure of mucins is a unique kind of fibrous protein that is very similar in structure to collagen. These fibers are very long, around which mucins cluster and produce a very cloying, sticky mucus. This sticky mucus swells when it comes into contact with seawater, a property which is exploited by the hagfish when it hunts. Having a very slow metabolism, the hagfish can remain motionless on the ocean floor for months. When an unsuspecting fish passes by, though, it is roused from its torpor and will swim toward its prey. Rather than stopping as it gets to its quarry, the hagfish will swim inside the fish through its mouth or gills. Once there, it will quickly secrete its mucus, which soon overcomes the fish’s gills, suffocating it. Even large fish species have no defense against this unique attack. Once its prey has been killed, the hagfish will then eat it from the inside out, using the body as a temporary shelter. To suffocate its prey, the hagfish has to produce a lot of mucus. But its trick is that 99.996 percent of the mucus it produces is made from seawater. Only a very tiny amount is made from its fibrous protein and mucins, but thanks to their unique molecular structure the mucus still remains sticky and so can suffocate the prey. Despite investing in only small amounts of its own building material, the hagfish can produce as much mucus as it needs. This is the most

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dilute mucus produced by any organism, and it is certainly the most efficient. It is a highly specialized adaptation that allows the hagfish to lead a way of life that no other organism is capable of. This immobilizing mucus is also used by the hagfish as a defense, creating for itself a protective cocoon when under attack. This deters predators, but how does the hagfish itself extract itself from its sticky predicament? Its solution is a remarkable one. Hagfish are the only animal able to tie themselves in a full, overhand knot. This knot starts at the head and works its way back over the body, scraping the sticky mucus away as it goes. With a flash of gymnastic contortion, the hagfish is free and can go about its business once more. Also to be found in the sea is the fat innkeeper (Urechis caupo), which is a type of spoon worm that lives in the mud flats of the Pacific coast. It is a fairly indistinctive creature some 6–8 inches (15–20 cm) long. It lives within the mud itself, making for itself a U-shaped burrow, about 3 feet (1 m) long, in which it sits. The burrow provides excellent protection from predators but does present a problem for getting food—to leave would expose it to the foraging birds that patrol the coastal flats looking for food. To get around this problem it feeds from within its burrow. To do so, it exudes a cone of mucus, about 4 inches (10 cm) long, which fills part of the burrow. As the mucus is produced, the forward part of its body is positioned inside the net. This is then folded back to position the net and to keep it taut and in the correct shape. The structure of this mucus net is such that water can pass through, but tiny food particles cannot. To maximize its catch, the innkeeper worm undulates its body to pump water through its burrow, creating a current flowing though its mucus sieve. The innkeeper worm can pump up to 23,000 liters of seawater though its burrow each day. To get the maximum amount of water flowing through the net, the innkeeper worm contracts its body to about one-quarter of its normal diameter, allowing water to flow past it and into the net. After about 20 minutes of pumping water through its burrow, the innkeeper worm feeds off the particles of food collected. It does this by detaching the net and feeding it, along with the food trapped there, into its mouth using its long proboscis. It can do this very quickly, finishing its meal in under a minute. Thanks to its unique use of mucus, the innkeeper worm can feed almost constantly without expending much energy or risking predation. Away from the ocean, there is another animal that has made full use of this versatile material. Dogs have an amazing ability to sniff out faint smells and to distinguish one from another. It is this trait that makes them such good hunters and such effective companions for narcotics police officers. A dog’s nose has many more chemical sensors than a human nose, but it seems that a great deal of its sensitivity owes much to the mucus which lines it. The inside of the nose is covered with very thin tubes filled with mucus. Thanks to the mucus, these tubes effectively ‘‘pre-sort’’ smells before they get to the more refined sensors, which do the detailed work. As chemicals are absorbed through the mucus at different rates they arrive at the sensors below at different times, albeit a fraction of a second.

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This means that the chemicals making up a smell can be broken down and analyzed by the nasal sensors in great detail. A dog’s tremendous sense of smell has allowed it to hunt efficiently and evolve a complicated array of behaviors to enable living in social groups—all possible thanks to mucus. Borrowing from Nature

The hagfish’s mucus is coming under very close scrutiny in the field of emergency medicine, particularly military emergency medicine. Although it has yet to be replicated it would be extremely valuable as a ‘‘space-filling gel,’’ a viscous fluid that can fill large wounds and help stop bleeding. Mucus in nature also typically carries antibacterial molecules that would help treat infected wounds. Recognizing the important role mucus plays in a dog’s sophisticated organ of smell, engineers are applying a layer of slime to odor-sensing ‘‘electronic noses.’’ These devices are used for a range of purposes, including to detect spoiling food. To date, electronic noses have not been great at distinguishing odors, and they tend to be specialized in detecting one type of smell. By applying a layer of artificial mucus over the sensors, however, they are able to distinguish certain odors by detecting the time it takes for the component chemicals to diffuse through the layer of slime.

NATURAL GLUES Glues and adhesives are extremely useful materials that have been key to many human engineering endeavors. Adhesives have been used on spacecraft, in our homes, in our cars, in businesses, in huge skyscrapers, and even in art and crafts classes in kindergarten. We have developed glues that can stick nearly any two substances together. But man-made glues are not the most versatile of substances. They can take a long time to set, and they are often difficult to apply without also bonding with the applicator (or indeed our fingers). Glues in nature, on the other hand, have a much wider range of uses and properties. Invariably, their adhesiveness is finely tuned to whatever purpose they are required for and they can be delivered and applied quickly and without fuss (or mess). Spiders are truly a living factory of amazing natural materials. Silk is well known to be a versatile, strong, and sticky substance that spiders have used for very many behaviors. Some spiders, though, also produce a variant on silk that is much more like a fast-setting glue. Spitting spiders (of the family Scytodidae) are aptly named. In the blink of an eye, they can fire a sticky fluid from their fangs and trap any unwitting prey within range. Using its mouthparts (its chelicerae), the spitting spider can direct the sticky spray backward and forward in a zigzag pattern to cover as large an area as possible. The glue is squirted by a quick contraction of the fang muscles, and after only 0.14 seconds the prey is entangled and unable to escape.

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The glue also contains paralyzing venom that further subdues the prey, allowing the spider to approach and deliver the final killing blow with its fangs. The speed of this is critical as many of the prey species hunted by spitting spiders are fast moving, such as mosquitoes or jumping spiders. What is remarkable is that this adhesive is so perfectly adapted to its use. It can be stored as a liquid in the spider’s body and yet midair become sticky enough to ensnare a tasty morsel. This is a direct function of the molecular properties of the glue, which has yet to be fully replicated in man-made glues. Spitting spiders are not the only animal to use glue as a weapon. The curious-looking velvet worms (Onychophora), which look like caterpillars withstumpy legs, are furnished with a pair of deadly slime glands on either side of their mouth. When hunting, the velvet The water-resistant glue-like protein threads worm will rear up on its legs and spit at that attach mussels and other bivalves to its prey just like the spitting spider. rocks. [James King-Holmes / Photo Again, this is a fast-moving and rapidly- Researchers, Inc.] setting jet that can trap prey (such as ants) before they can escape. And they have a good range at about two feet (0.6 m). What is remarkable about the velvet worm, though, is how it copes with potentially messy moments when its glue falls on its own body. Unlike humans stuck together with super glue who may take several painful moments to free themselves, the velvet worm has a handy trick that comes to the rescue. Its body is coated with a fast-acting solvent that instantly frees any body parts that may have become stuck by this tremendous adhesive, allowing it to use its deadly glue without any embarrassing moments of self adhesion. One of the most difficult places humans have found to use adhesives is underwater. Yet there are species who do produce an underwater glue, and for a very good reason. Animals like mussels and barnacles live and feed by sticking themselves to rocks, boats, ropes, and even large sea-bound mammals like whales. The glues they use must be very powerful, as they need to work on wet, salty surfaces and against the relentless pounding of the waves. Before settling on a spot, all shellfish larvae are free-floating organisms that drift on the currents of the ocean. To develop into adults, certain species of

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shellfish larvae seek out a specific spot on which to settle. They will smell out places where marine bacteria have attached themselves and are living. These marine bacteria produce a glue to stick themselves to a surface, and the industrious larvae make use of this glue to stick themselves on, too. Over time, and as the larvae develop into adults, they make their own glue with which to stick themselves on. Mussel larvae are able to produce a glue that can stick to all sorts of surfaces: rock, wood, metal, teeth, bone, and even teflon, a substance that was designed to resist any adhesive. Man-made glues produce a very poor bond when mixed underwater, so how does the mussel do it? The mussel’s glue gland comprises two separate compartments. One produces resin-like proteins, and the second produces chemicals that behave like hardeners. On entering the water, the proteins and chemical hardeners mix and, in minutes, solidify into a near unbreakable bond. This seems fairly straightforward, but it turns out that the mussel produces around 10 different proteins that tangle up with each other to produce the hardened glue. This complexity is very difficult to recreate, which is why a very strong, man-made underwater glue is still some way off, although huge leaps forward have been made thanks to the insides provided by the humble mussel. For the mussel, though, evolution has provided it with an adaptation that allows it to live in one of the most physically violent places on earth without being swept away to sea. Borrowing from Nature

The underwater glues of shellfish are perhaps the most interesting for human engineers. Adhesives that work well in seawater can be used to make repairs to the hulls of ships and to the stations of piers and oil rigs. There have also been suggestions of using these glues to repair damaged body organs that have moist surfaces that are not easily stuck together—or even to stick soft tissue to harder tissue like muscle to ligaments or ligaments to bone. Dentists are watching this glue research carefully, too. Reconstructive dental surgery needs a glue that can be applied while still soft, bonds quickly in a wet environment, and adheres strongly to teeth and bones. Adhesives currently used are not great at bonding these kinds of surfaces, especially when wet. Mussel glues, on the other hand, can do exactly that.

GECKO FEET The lizards that make up the gecko family (Gekkonidae) are in a class of their own when it comes to their ability to stick to seemingly unscalable surfaces. Found in forests, they have no problem clambering over trunks and branches in any direction. They can cling upside down and even on sheer surfaces such as glass. There is no doubt that this ability must come from the adhesive properties of its feet, but until recently it has not been clear how that is achieved. Close

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inspection reveals that it is not down to a secretion of some sticky fluid. Nor is it true that geckos’ feet are covered in tiny suction cups, as was previously thought. What has evolved in geckos is a much more ingenious solution. A close look at a gecko’s feet under a high-powered microscope reveals that it is covered in a dense network of millions of tiny hairs made from keratin. These hairs, called setae, A scanning electron micrograph of the underside of a are very tiny indeed. One gecko foot showing the tiny rows of hair-like setae. seta is only as long as the [Andrew Syred / Photo Researchers, Inc.] diameter of a human hair, about 100 millionths of a meter (100 micrometers). They are only 5 millionths of a meter (5 micrometers) wide. So tiny are these setae that there can be up to about 2 million on each gecko foot. And it doesn’t end there. The tips of each of these tiny setae branch out into even finer hairs called spatulae. There are about 1,000 of these spoon-shaped spatulae on each seta. As you might expect, the spatulae are very small indeed. They are only about 200 nanometers (200 billionths of a meter) wide—less than the wavelength of visible light. So how do these millions of tiny setae and spatulae allow geckos to run upside down on the ceiling? At the molecular level there are certain forces that bind various surfaces together. Probably the best known of these molecular forces is the hydrogen bond, a weak attraction between hydrogen atoms. It is this bond that causes water to cling to surfaces and get drawn up thin capillary tubes. But it is another, even weaker molecular bond that the gecko makes use of. It is called the van der Waals force, which is a very weak attraction that occurs between any two surfaces no matter what they are made of. Each spatula exerts this weak van der Waals force and will create a weak attraction to any surface it comes into contact with. On its own, this would not create a particularly strong bond, but considering there are many millions of these spatulae that will be in contact with a surface at any one time, all together they create a very strong bond indeed. The collective force exerted by the spatulae mean that a gecko could support all its weight, hanging upside down, with just one toe. The spatulae of 1 million setae could support a 45-pound (20-kg) child. This is more than enough force to keep a 0.2-pound (100-gram) gecko stuck to the ceiling. The question is, then, how geckos can move their feet at all with such strong forces holding them to a surface. The answer lies in the angle at which the setae and spatulae lay across a surface. When the foot is put down, the setae lay flat. They will

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cease to exert their force when they are angled away from the surface, though. All the gecko has to do is roll its toes upward and its foot will become unstuck. The strong forces exerted are clearly no hindrance to the gecko, as they can move at fairly high speeds, faster than one meter per second even up smooth, vertical surfaces. It is the direct contact of spatulae with a surface that allows a A higher magnification scanning electron micro- gecko to cling on without slipping. graph of the underside of the gecko foot showing Any dirt on the feet would therethe microscopic spatulae, which allow the gecko to fore reduce their adhesiveness. cling to nearly any surface. [Andrew Syred / Photo It is strange, then, that geckos neiResearchers, Inc.] ther groom their feet nor secrete any cleaning fluid that might wash dirt away. Yet gecko feet do not lose their adhesion over time. It seems that the setae and spatulae are self-cleaning. As they walk, any dirt is transferred from their feet to the surface they are walking on rather than sticking to their feet. This seems remarkable given the attractive forces that gecko setae seem, collectively, to impart. Why does the dirt not stick to the setae? Again, the answer lies in the molecular forces imparted by the setae. All materials impart weak, attractive van der Waals forces, but it seems that gecko setae are made of a material that have a weaker force than usual. Overall, this does not matter because the setae work together to produce an overall very strong attraction. What it does mean is that a tiny speck of dirt will be more attracted to the surface over which the gecko is walking rather than to its feet. The activity of walking causes dirt to stick to the surface on which the gecko is scurrying over. After only a few steps, dirty feet are cleaned and the gecko’s feet are once more fully adhesive. We might marvel at the gravity-defying abilities of a gecko, but what are the benefits of such a skill? Lizards provide a tasty snack to many predators, and in the tangled and stony forests where the gecko lives, making a sure-footed and quick escape is the difference between life and death. When the gecko itself is hunting, the ability to scale improbable surfaces allows it to get to resting insects that otherwise would be well out of reach. The adhesive properties of a gecko’s foot is not simply an amazing party trick but a key adaptation for survival. Borrowing from Nature

The problem with wet adhesives like glue is that they tend to be single use. If they can be detached and reapplied, they rapidly lose their adhesiveness as they get clogged with dirt. To recreate a dry adhesive like gecko setae would

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be extremely useful where surfaces need to be joined, unjoined, and rejoined a number of times. Velcro strips are a crude option, but that relies on attaching a specialized strip to each surface. The attraction of a setae-like adhesive is that it can stick to anything (with the exception of Teflon, which was designed specifically to prevent even van der Waals adhesion) without having to treat the surface. Thanks to the simple principle behind gecko feet, synthetic setae are now being designed. So far, engineers have fabricated arrays of plastic pillars, similar to setae, that are little more than two millionths of a meter (2 micrometers) tall, spaced about the same distance apart. So far, only a very small square of this material can be produced because of the difficulties involved with the fabrication process. However, this sample is proving to be very successful. A 0.08 square inch (0.5 cm-square) piece of tape is sufficient to hold an object over 0.2 pounds (100 grams), just as adhesive as a gecko’s foot. There are many potential applications for ‘‘gecko tape.’’ It can be used in a vacuum, so it may have applications in space. It can be used in microsurgery and in handling silicon wafers. Some engineers are trying to use the technology to design tiny robots capable of climbing over walls and ceilings just like live geckos. These gecko-bots can be used to look for survivors in burning and collapsed buildings or could even be used to explore the tricky terrain of Mars. This simple but hugely effective adaptation truly has a tremendous range of applications if it can be replicated on a large scale.

RESILIN For many years, the jump of a flea was regarded as a major zoological problem. This tiny insect of only a few millimeters is capable of jumping over 20 times its own body length. What’s more, it seemingly never gets tired. Should they be so inclined, fleas can jump 600 times an hour for three days without needing to eat any food. The reason why they need to jump so high and so frequently is clear. When they do eat, adult fleas eat only blood, which means climbing onto an animal to feed. One of the adaptations for this life of clambering over other animals is to have no wings, which makes it much easier to push through the thick hair of its host. With no wings, though, the insect needs to rely on its tremendous jump to reach its food supply. This is why the flea needs to jump such great distances, but how does it do it? Fleas have been called insects that fly with their legs. This is because, over the thousands of years of natural selection, they have evolved smaller and smaller wings until only a very tiny remnant has remained. This makes it easier to navigate the tangled fur of its host, but it has another important role. It is the vestigial remains of their wings that are involved in setting themselves up for their almighty leap. More precisely, it is one part of the tiny wing remnant that is important. The pleural arch is the hinge that, in the flea’s ancestors,

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used to connect the wing to the thorax (chest) and the muscles beneath. It is this tiny structure that is the secret of a flea’s prodigious jumping ability. The flea’s pleural arch is horseshoe shaped and can be flexed when compressed. Beneath it, though, is a small pad made from a unique protein called resilin, a highly elastic protein very similar to dense rubber. Like all elastic materials, resilin absorbs energy when it is compressed and releases When jumping, a flea’s hind legs extend it again when it springs back into its original rapidly thanks to the elastic resilin pad shape. Unlike man-made rubber that transfound at the base of the leg. This can be fers little energy when it bounces back into compressed and then released by the flea shape, resilin is highly efficient. It releases to power its jump. [Steve Gschmeissner / 97 percent of the energy stored inside it Photo Researchers, Inc.] during compression. Resilin is undoubtedly a superb material for the flea to have at its disposal. But how does it make the best use of it? When readying itself for a jump a flea will bend and bunch up its hind legs much like we would if preparing to leap from a standing start. This causes the pleural arch to bend and the resilin beneath to compress, storing energy. Where the hind legs meet the body, fleas have a simple locking mechanism that allows them to hold the compressed pleural arch and resilin pad in place without needing to keep the muscles of the leg tensed. It also prevents an unwanted release that could see the flea missing its intended target completely. When it is ready to jump the flea can release the catch, causing the resilin pad and pleural arch to spring back into shape, forcing its legs straight. This sudden release of energy propels the flea high into the air and hopefully onto an unsuspecting animal where the flea can settle in and feed. There is a tremendous amount of energy that can be stored and released in resilin. A flea’s jump reaches an acceleration imparting 140G of force in little more than a millisecond. Muscles would never be able to simulate this feat. They contract too slowly and would rapidly become fatigued. In fleas, the resilin pad simply springs back into shape and it is immediately ready for another leap. It even works just as well at low temperatures, which otherwise reduce the efficiency of muscle. Resilin is such a versatile material that it has evolved to be used in many species of insect. The durability and elasticity of resilin is used in flying insects in the hinges of their wings. Thanks to resilin, bees can flap their wings in almost frictionless motion 500 million times throughout their life. This durability of resilin is important because insects do not continue to produce it as adults, so it is not replaced and repaired. The structure of this remarkable protein is quite simple. It is made up of a number of short chains of amino-acids, the building blocks of proteins.

MATERIALS

One key amino-acid in these chains is proline. Proline has a unique, elbowshaped structure that it causes the protein chain to bend into a U-shape. It is believed that a succession of these U-bends creates a spiral that can compress and stretch exactly like a spring. These protein springs can be repeated over and over to produce as much elastic resilin as is needed. Borrowing from Nature

Until recently, the best elastic material scientists could produce was a synthetic rubber called polybutadiene. This rubber was way short of resilin in efficiency, releasing only 80 percent of energy absorbed after compression. But with such an amazing material available in nature it is little wonder that scientists tried to better their efforts by replicating resilin. Scientists can now successfully isolate the resilin gene (from a fly), insert it into a bacterium, and harvest the protein produced. This harvested resilin seems to show the same elastic and durability properties as that found in nature. Now, the task is to fully test it and apply the material to a range of uses. Engineers are exploring its use in spinal disk implants, microelectronic mechanical devices, drug delivery vehicles and systems, and as synthetic prosthetic veins and arteries.

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BUILDING STRUCTURES—HUMAN INVENTION If humans were instantly wiped from the face of the earth one of the most indelible marks we would leave is the multitude of large structures we have built. These remarkable feats of human engineering cover the earth, from the monuments of the ancient world through to the skyscrapers and huge edifices of our modern cities, and even the immense dams able to hold back and harness the power of the planet’s great rivers. For the vast majority of the 100,000 years or so of human existence, buildings were fairly insignificant and huts were the peak of our architectural abilities. However, around 5,000 years ago in the Fertile Crescent, the arc of land watered by three enormous rivers—the Nile, Euphrates, and Tigris—all this changed as our ancestors turned from a hunter-gatherer existence to settled societies underpinned by agriculture. At this time, humans began to build more elaborate buildings which grew into the first human cities. Although these ancient societies lacked the machines we have today, they were capable of erecting some very impressive structures that still have the ability to impress. Take the pyramids of Egypt, which are huge monuments to the dead. They are colossal tombs that were built as symbols of the power and wealth of the dead kings they were built to house. The great pyramid in Egypt was the tallest man-made structure on earth for 3,800 years following its completion in around 2560 BC. In its day it stood some 480 feet (146 m) high, but the loss of the casing stones and erosion leaves it at 453 feet (138 m) today. Approximately 600,000 stone blocks were needed to build the pyramid, and the whole structure weighs in the region of 5.9 million tons. It was built over a 20 year period in such a precise

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way that it would severely press the building techniques of the modern day if we tried to recreate it. The joins between the huge stone blocks are so perfect that we still have no idea what techniques the builders used to cut and shape them, but the finished, albeit ancient, structure clearly shows they were masters of their art. Apart from the Egyptian pyramids, the world is dotted with other, ancient structures that still attract millions of tourists each year. However, the most impressive human structures have been built following the industrial revolution, when advances in science and engineering made truly astonishing construction possible. Metallurgists were able to produce good quality iron and eventually steel that would form the internal scaffolds of the new age of structures. Stone is still used in buildings to this day, but it was replaced in the really ground-breaking structures by concrete and bricks—cheap building materials that could be tailored to suit various needs. With these new materials at their disposal, engineers continually pushed the boundaries of possibility, a trend that continues to this day. In the 1920s and 1930s, engineers in New York and Chicago competed with each other to build the tallest buildings, and in doing so they produced such famous landmarks as the Chrysler building and the Empire State building. With land prices so excessively high in city centers the world over, the pressure to build upward is as great as ever. The competition to build the tallest skyscraper has never really died, and a visit to any major city in the world will be dominated by views of towering buildings. Taipei 100 in Taiwan was the tallest complete skyscraper, measuring 1,670 feet (509 m) to the top of its tallest spire. However, this was easily surpassed in 2009 when the Burj Dubai was finished. This enormous building reaches 2,684 feet (818 m) into the air, making it the tallest man-made structure on earth. Skyscrapers aside, humans have also constructed other enormous structures, and some of the biggest of these are dams. The Three Gorges Dam over a stretch of the Yangtze River in China is due to be completed by 2011 at a cost of $30 billion. When finished it will be the largest dam ever constructed. The scale of this building project is immense. The dam wall is 7,575 feet (2309 m) long, 607 feet (185 m) high, 377 feet (115 m) thick at the bottom, and 131_feet (40 m) wide at the top. The project has used 960.5 million cubic feet (27.2 million m3) of concrete and 463,000 tons of steel, enough to build more than 60 Eiffel Towers. Many of the earth’s great rivers have been tamed by dams. However, large bodies of water and the barriers they present provide other problems for engineers to solve. One of the most impressive of these solutions is the project which gave the United Kingdom a fixed link to continental Europe—the Channel Tunnel. This feat of engineering enables trains to pass under the English Channel at high speed for a distance of 31 miles (50 km). The project involved tunneling through the seabed at an average depth of 160 feet (48 m) to produce two train tunnels and one service tunnel, two of which were lined

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and fitted out with tracks and other equipment to allow trains to pass through. To make things even more complicated, the tunnel was completed by two separate teams, one working south from the United Kingdom and the other working north from France. Their efforts had to be precisely matched so they met somewhere underneath the channel. Eleven huge tunneling machines were used to make the tunnels and line them with curved, concrete slabs. The project took six years and depended on 15,000 workers. Human ingenuity has enabled the construction of some truly remarkable structures, but how do these compare to the structures we can see in nature? Nature’s structural ingenuity is very different from what humans have produced, but it is nonetheless impressive thanks to life’s ability to adapt to environment and circumstance. Evolution has come up with some incredibly elegant solutions to a bewildering array of problems.

TERMITE TOWERS Some of the most impressive structures in the natural world are those constructed by insects, particularly those that live in complex societies. The most well-ordered insect societies are those formed by ants, bees, wasps, and termites. In terms of sheer engineering achievements, it is the latter group of insects that stands out. The structures that they build in which to live not only offer simple protection from the elements and predators but are carefully designed and engineered to allow them to live in style and comfort. The termites are unrelated to the other social insects—their closest relative is in fact the cockroach. A termite colony can contain hundreds of thousands of individuals, most of which are workers. These workers are not much to look at. They are small, practically blind, and soft-bodied, but it is these seemingly unremarkable animals that are responsible for constructing some of the most impressive structures found in nature. The worker termites do not build from detailed plans. There are no architects and no foremen to manage the endeavors of these tiny insects, yet these unseeing builders are indeed following instructions. Their actions in their nest are influenced ultimately by the egg-laden queen who lives in a special chamber at the center of the mound. Chemicals produced by the queen act like messengers to give each and every termite a certain amount of guidance in its actions. Using these simple cues and the innate abilities they are born with, the worker termites construct nests of incredible size and complexity, and it is only in recent years that we have started to realize how incredible these structures are. Often, these nests are disparagingly referred to as mounds, implying a simple heap of material without any internal structure. In reality, nothing could be further from the truth. Any termite nest, particularly the large structures constructed by some of the Macrotermes species, is a masterpiece of organic architectural design. In terms of dimensions, some African species make towering

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A termite mound. [Georg Gerster / Photo Researchers, Inc.]

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structures up to 30 feet (9 m) high containing a labyrinth of passages and chambers. If we scale these structures up according to the relative heights of the builders, they would dwarf the tallest human-built skyscrapers. The workers of the species responsible for these insect towers are a mere 0.2 inches (5 mm) long, so the tallest mounds are equivalent to humans building a skyscraper more than 1.2 miles (2 km) high. Depending on the species in question, termites use combinations of soil, saliva, and their own excrement to produce a paste that dries in the equatorial sun to a substance with properties similar to very hard brick mortar. All termites protect their nests vigorously, and when the nest is breached, which it inevitably is by predators, the worker termites are quick to repair the damage, and in a few hours huge holes in the nest can be seamlessly repaired. Foraging termites will even build tunnels out of the same material for them to travel through. Being so soft-bodied, they are extremely susceptible to the elements. A mortar tunnel allows them to move about in a controlled environment, protected from predators and the sun. Size and building materials aside, what sets termite nests apart is their internal complexity. There is a very good reason for this complexity. Termite mounds are often situated in the middle of a baking desert, with no shade to offer protection from the relentless sun. Temperatures can easily soar to potentially lethal levels, not only for the termites themselves, but for the fungus (Termitomyces) that they farm for food at the heart of their homes. To ensure the fungus can grow at optimal temperatures, the whole mound structure is designed to operate as a sophisticated air-conditioning unit. A cross section through the structure reveals a central passage in the tower, reminiscent of a chimney. Branching away from this main chimney is a complex series of passages akin to the veins in an arm. The nest, where the termites live, itself is a spherical structure in the ground below the tower. The whole elaborate edifice above the nest is part of a remarkably elegant climate-control system. To ensure the nest stays healthy the structure as a whole ‘‘breathes,’’ powered by the ebb and flow of the wind. The obvious part of the termite mound, the tower, extends into the air where winds are slightly stronger. The wind blowing across the top of the tower pushes and pulls air in and out of the subterranean nest chambers. If the colony grows and the air in the nest gets a little stuffy the workers extend the height of the tower, increasing the rate at which the air is exchanged. Air is drawn up through the mound thanks to the tall chimney, but a key part of the unit is the chambers that are dug deep into the soil. The termites dig deep enough to get to the cool, moist soils that are not heated by the sun. As the air is drawn up through the mound, the cool deep-soil air is sucked through the mound, cooling the whole colony. The result of this simple, but effective design is that the mound is kept at a constant 87°F, the perfect temperature to grow a crop of fungus. What is remarkable is that this temperature is achieved even when the surrounding environment fluctuates from 34°F at

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night to over 100°F at noon. This is a reliable system that comes at no cost, which consistently outperforms many of today’s air-conditioning systems. Borrowing from Nature

Recently, scientists have been trying to unlock the secrets of how these termite structures are made, in the hope of developing buildings for humans that will not require expensive and inefficient systems for heating and ventilation. However, such technology has already been used by humans since the fourth millennium BC. Persian wind-towers (also known as Badgirs) use complicated arrangements of vents and tunnels to cool buildings from the wind alone. In fact, so successful is this technology that ice can be produced in ice wells even in the hot climate of the Middle East. More widespread use of this technology—potentially honed by what we can learn from termites—could result in free (or at least cheap) air conditioning, which would be both a cost saving and an important contribution to cutting our energy output in the quest to fight climate change.

TREES Plants first appeared on earth some 465 million years ago. Even this long ago, these primitive organisms showed many of the characteristics of modern plants. They had a waxy outer cuticle to preserve water, a vascular system for transporting water and nutrients from roots to leaves, and small openings in their leaves (stomata) to draw in carbon dioxide from the atmosphere and to expel oxygen. Eventually, this plant respiration would radically change the makeup of the atmosphere, allowing the evolution of oxygenbreathing life forms, such as humans. The early plants of the Ordovician period would not have been very tall, however. Mostly, they would have reached only a few inches. There are tree-like organisms that have been found in the A cross section of a branch from a hardwood tree. fossil record from this time, but it The concentric rings show successive periods of now has been shown that these growth. The holes show the tube-like xylem and were not trees at all, but giant phloem cells that strengthen the tree and carry water fungi that would have fed off bacand nutrients from the roots to the leaves. [Clouds Hill Imaging Ltd. / Photo Researchers, Inc.] teria, algae, and lichens that would

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have formed a crust over the soil. These fungi are called Prototaxites and grew up to 20 feet (6 m) high and 3 feet (1 m) wide. These early plants would not have colonized the earth to the extent that plant life now does. Some habitats would have been tough for the ill-adapted plants to survive. The evolution of trees, though, changed things completely. These new trees differed from other plants by a key adaptation called secondary growth. This is the process by which a specialized tissue called cambium continuously grows, adding girth to the stem and roots of a plant. This is crucial for the growth of trees because the new cambium cells that are produced each year will become either wood or tissue through which the tree can carry water and nutrients. Thanks to this ability to grow thicker and stronger as well as a better vascular system, trees were able to grow much, much taller than their ancestors. Plants that can grow taller can grow free from competition for light and thus grow yet further. The ability to grow taller than other species gave trees a huge competitive edge, leading to such great success, evolutionarily. The appearance of trees on earth therefore sparked an evolutionary race for size and dominance that eventually resulted in the green planet we see today. Early trees could grow tall, but would not be familiar to us today. They would not have had recognizable leaves, but rather thin, whippy branchlets that would have been capable of photosynthesis. These trees also had very shallow roots. The next significant change in trees was the evolution of fernlike leaves that were better at capturing light. This occurred some 370 million years ago and is seen in the conifer ancestor Archaeopteris. At the same time, Archaeopteris evolved deep roots to get at hard-to-reach water and nutrients, allowing it to colonize previously uninhabitable environments. Again, this would have given these advanced trees a great competitive advantage, allowing them to colonize areas inhospitable to other plants. Following these significant periods of plant evolution, trees are today among the largest living things on earth and represent one of the most sophisticated natural structures. Modern Sequoia trees can stand at over 330 feet (100 m) tall with a 100 foot (30 m) circumference, weighing some 1,800 tons. Trees can grow to such tremendous sizes thanks to a process during secondary growth called lignification. As a tree grows from a seed, the stem is initially green and very weak because there is nothing but cellulose in the cell walls to provide support. As the seedling grows into a sapling it quickly outpaces the structural support provided by cellulose. At this point, lignin, a very complex polymer molecule, begins to get laid down in the cell walls of the growing tree. This lignin forms the bulk of the woody part of the tree and plays an important role in mechanical support, water transport, and disease resistance. A key role for the woody part of the tree is in the support of the delicate water- and nutrient-carrying vessels. In non-woody plants and in young trees, two types of vessels are involved in water transport, xylem and phloem. As trees grow much larger, though, only xylem is involved with the long-distance

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transport of water up the massive trunk. Xylem vessels are dead cells with a large empty space in the middle for water to pass through. Water is dragged up the tree under tension, so the xylem cells are under constant threat of caving inward. It is the thick layers of lignin in the xylem cell walls that gives the cells strength enough to withstand these pressures. Physically, then, trees are able to grow very tall. Deep roots anchor these massive structures as well as draw up essential water and nutrients from the soil. But how do trees draw water up to heights of nearly 100 meters? It was first thought that water, and the nutrients dissolved within it, was drawn up by capillary action. This is the property of the forces that are created between water molecules that sees them being drawn up narrow tubes. You can see it happen in narrow drinking straws. The water inside the straw will be a little higher than the water level in the glass. In very narrow tubes, water can be drawn up much higher, although eventually the forces drawing water up the tube will be cancelled out by gravity. It is now known that capillary action will only draw water up a tube the thickness of a xylem vessel to a height of 3–33 feet (1–10 m) (depending on the type of xylem), still some way short of the tree tops. Therefore, another force that either sucks or pushes water up the xylem vessels must exist. Evidence shows that it is the former of these possibilities. The cells in a tree’s leaves are packed with water. The air surrounding a tree’s leaves, on the other hand, has very little water at all. There is a strong differential in water potential between a leaf and the surrounding air, so water is drawn out of the leaves by the process of evaporation. With water being drawn from the leaves to the air, there will be less water in the leaf cells than in the stem cells behind them. Water is therefore drawn from the stem to the leaf. This effect—called transpiration—occurs all the way down the tree, with water being dragged up from the roots to the leaves. This is why water in the xylem is under tension—it is being sucked upward rather than being pushed from behind. What’s more, the cohesive nature of water (thanks to the forces that produce the capillary action effect) means that it will cling to the xylem walls and will not be dragged back to earth under the full force of gravity. As a result, less force is required to suck water up the huge distances of a tree. Calculations by botanists show that the water pressures in a tree mean that water can be dragged some 380 feet (115 m) upward—exactly the height of the tallest trees. Borrowing from Nature

The wood produced by cambium cells during secondary growth is one of the most important materials used by humans. Different trees produce different types of wood, thanks to their different sizes of secondary xylem cells. Trees like conifers produce versatile softwoods, whereas the huge deciduous trees of the rainforest produce dense hard woods. Even the super-light balsa wood has a number of uses for its insulation and buoyancy properties.

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Engineers are even designing artificial trees based on what they know from real ones. The aim is that they could be used to extract and purify water from even the most arid soil. The process of transpiration where evaporation at the leaf drags water all the way up the tree is being mimicked in these artificial trees. The key has been to find a membrane that can be used at the top that has the same properties as a leaf. Now such a substance has been found, a ‘‘hydrogel’’ that has tiny pores within it just the right size to allow the correct level of evaporation.

BIRD NESTS Perhaps the most familiar structures built by animals are the nests created by birds. Across the globe, birds construct a wide range of nests in which to lay their eggs and rear their young. Although relatively common, there is a great deal of care and craftsmanship that goes into constructing even the simplest nest. Nest-building behavior is an important adaptation that is only part of the huge amount of energy adult birds invest in rearing their young. At their simplest, bird nests are places in which to lay eggs and feed young, but in many cases they are much more than that. The basic nest design has been honed, through evolution, to give rise to some of the most remarkable structures seen in nature. For some species, nest building has even evolved into an art form. There is great diversity in the nesting behaviors of birds. These structures can range from simple scrapes in the ground to extensive, multi-chambered constructions that can be fully weatherproof and which can survive for several years. The simplest nests are little more than a haphazard cluster of vegetation, twigs, and stones. Ducks, geese, swans, penguins, and storks often build only rudimentary nests, which are just enough to raise their eggs from the ground to protect them from predators or natural events like flooding. Simple though they may be, even these nests offer a competitive edge The intricate nest of the Baya weaver bird, to the young that might suffer without Ploceus philippinus. The nest is enclosed with such a structure to grow up in. a tunnel entrance below to protect the eggs One of the more spectacular of these and young from predators. [E. Hanumantha haphazard nests is that made by the Rao / Photo Researchers, Inc.]

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golden eagle (Aquila chrysaetos). This magnificent bird will heap up vegetation to support its eggs and young. The location an adult chooses for this messy nest is right on top of last year’s nest. As these birds can live up to 30 years in the wild, that can be a lot of vegetation piled up. Over time, these structures can reach up to a ton in weight. For this reason, the golden eagle chooses to nest on rocky crags, which are able to take this massive weight as well as offer protection from even the most nimble of predator. Birds of prey such as eagles and hawks have also been known to adorn their nests with green sprigs of foliage. There have been some reports that this is simply for aesthetic purposes, not least because they continue the behavior even when their young have flown the nest. But there seems to be a much more important function of this aspect of nest building. Many trees have evolved the ability to produce insecticide poisons to deter herbivores. Hawks and eagles appear to choose leaves that are particularly high in these poisons. The net result is that potentially disease-carrying insects are repelled from their nests, thus protecting their young. Other bird species line their nests with feathers for a similar reason. Their features have antibacterial properties that helps prevent disease. Some of the more familiar nests are cup-shaped. Birds like the song thrush (Turdus philomelos) and crows (Corvus corone) build this type of nest from twigs, leaves, mud, feathers, moss . . . almost anything that can be found. Cup-shaped nests offer more protection than the more haphazard versions and are more secure. Their walls prevent eggs from rolling out of the nest, which is crucial for those nesting high in trees. Other species take this design further and enclose the cup with a roof, which affords further protection from both predators and the elements. The long-tailed tit (Aegithalos caudatus) builds a very intricate and solid nest that is also well camouflaged by the careful application of lichen to the outside. Perhaps the most impressive builder of domed nests is the Hammerkop (Scopus umbretta). Over 8,000 twigs go into the construction of this nest, which can be as large as 6.5 feet (2 m) high and 6.5 feet (2 m) wide. This nest has a long entrance tunnel to deter predators and is strong enough to withstand the force of a full-grown man walking over it. This may seem like an over-elaborate structure, but again the adaptation of nest building has evolved to ensure that offspring have a high chance of survival and so the adaptation continues to survive. All nests require skilled construction, but the most impressive nest-building skill is shown by the weaver birds of the Icteridae and Ploceidae families. As their name suggests, weaver birds carefully weave their nests from long blades of grass. These intricately crafted nests are built hanging from a branch or twig. This choice of location, coupled with the enclosed design with a drooping entrance tunnel, effectively keep predators away from the precious eggs within. Protection of eggs and young is the primary function of nests, but the weaver bird nests also serve another function. Given that their construction

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takes painstaking skill and precision, they provide an excellent signal of its builder’s abilities. Males will build their nests in communal trees before they have mated. Females will choose who to mate with based on the quality of their nest. The most skillful males will get the pick of the females. These top-quality males will also be able to build more than one nest and so raise more than one family. Nest-building is a difficult skill to master in weaver birds. Young males may not even get beyond building the basic foundations of the nest and will attract no mates because of it. By learning from their neighbors, young birds will improve their skill and will be better placed to compete for the attentions of potential mates in the future. For male weaver birds, success with the opposite sex is entirely dependent upon their ability to provide a wellbuilt home. Other birds use their nest-building skills as a sign of their fitness and quality, even to the extent that the structure they build is not used to house eggs and young at all, but only to attract a mate. The bowerbirds (Ptilonorhynchidae) of New Guinea and Australia build large, ornate structures to catch the eye of a potential female mate. Size, diversity, and color are the key features of these bowers. The basic shape is made from towers of sticks and foliage, which acts as a stage for ‘‘treasures’’ collected by the male. Colorful and shiny objects are carefully placed under the bower, including: feathers (especially blue ones), snail shells, iridescent beetle wing casings and heads, bones, and flowers. Often, foraged man-made objects, such as spoons and car keys, find their way into the display. The largest bower is made by the Vogelkop Gardener Bowerbird (Amblyornis inoratus) from New Guinea. It manages to build a bower over 6.5 feet (2 m) tall and 6.5 feet (2 m) across. Not bad for a bird the size of a song thrush. Borrowing from Nature

Bird nests, while impressive structures, are not a patch on even the simplest huts built by humans, and there seems to be little to learn from them. There is one type of nest, though, that is particularly interesting to some human cultures for a different reason. Swifts of the genus Collocalia build nests entirely from their own saliva. During the breeding season, their salivary glands enlarge to enable them to produce enough of this remarkable fluid, which hardens to produce a strong nest. They even make their nest in the pitch dark of the caves in which they roost, finding their way by echolocation. Unfortunately, the main predator these swifts encounter is man. These nests are an edible delicacy in China and are used to make birds nest soup, often causing huge damage to the local swift populations.

BEAVER LODGES Anyone who has owned a pet guinea pig, rat, or mouse will know that rodents are prolific gnawers of wood, paper, and cardboard. The tools of

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choice from each of these creatures are the two prominent front teeth that can make light work of hard foods like nuts. Other rodents, like squirrels, use their teeth to cut through hard obstacles that are in the way of burrows or runs. However, the most impressive use of rodents’ chisel-like teeth is found in beavers of North America (Castor canadensis), who are able to cut down trees by gnawing around the base in order to make elaborate homes. The first part of any construction is to get hold of the necessary building materials. It is commonly known that beavers cut down trees and branches to this end, but it is less well known how they manage it. Beavers’ teeth, like those of all rodents, are rather unlike the teeth of any other animal. Our human teeth are composed of a strong material called dentine, which is surrounded by an even harder material called enamel. Rodent teeth are made up of the same basic materials, but instead of surrounding the whole core of dentine with enamel, there is only a thick cap of the harder material at the front. At the back of the tooth, the dentine is exposed. Dentine is a hard material, but it is not as hard as enamel. Given the punishment of continual gnawing through hard materials like wood, both enamel and dentine will get worn down. Crucially, the dentine, which is less hard than enamel, will get worn away more. As a result, the tip of a tooth can get sharpened into a chisel-like point, which gives the tooth the ideal shape for cutting wood. With the constant gnawing and wearing down of both dentine and enamel, you might think that it would not be long before the whole tooth would be worn down to a stump. This would indeed be the case if it were not for the

A beaver dam and lodge. [Edward Kinsman / Photo Researchers, Inc.]

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beaver’s second neat adaptation. Its teeth, like the teeth of all rodents, grow continually. They get worn away at the tip, but more dentine and enamel are laid down at the root, so the tooth stays the same length for the animal’s whole life. Beavers (and domestic rabbits, mice, and rats) will instinctively gnaw at hard materials, even if there is no direct need, in order to prevent their teeth from growing too long. So thanks to its ever-growing chisels, the beaver can furnish itself with the necessary materials to build its home. The beaver feeds on leaves and bark trees and on aquatic vegetation, and so it will live in an area with plenty of trees with a stream running through it. It is across the stream that the beaver will build its familiar dam behind which it will build its lodge. The beaver will drag the trunks and branches felled from the nearby trees to the stream and carefully interlace them to build a sturdy fence across the body of water. The gaps between branches are filled with mud, boulders, and sods of earth, packed in using its front feet to form a near-impenetrable barrier against the flow of the stream. Beaver dams are so well constructed that they can be up to 10 feet tall, built solidly to withstand the pressure of water behind them. Once the dam has been constructed, beavers will build themselves a lodge in which to live. This is typically located in the middle of their artificial lake where a small hillock (typically some 10 to 13 feet (3–4 m) wide) has been surrounded by the rising waters. The beaver will dig a tunnel into this mound from below the water line. From this entrance tunnel, the mound is hollowed out to make a snug burrow. For added insulation, the chamber is lined with mud and rushes. Within the lodge there will tend to be two chambers—a wet room for drying off after entering from the water and a dry chamber where the family will live. In the absence of a suitable hill in the lake, lodges can be built from scratch or as part of the dam itself. As with the dam, branches form a scaffold around which boulders, smaller branches, and mud is packed to make a watertight wall within which chambers can be built and insulated. Once the basic lodge is finished, the beaver will build a few extra features. More entrances are added so they can escape quickly if threatened by predators. Further tunnels and vents allow for a complex system of heating, ventilation, and air-conditioning to ensure perfect living conditions inside. The lake and canals constructed by the beaver act as a protective moat, which allows them to reach their feeding places in relative safety. Even when the lake freezes in winter, beavers can survive by feeding on submerged logs and vegetation that have been trapped by the rising waters of the artificial lake. Beavers will even cut leafy branches and stock their lake before winter comes precisely for this purpose. For some parts of winter, then, beavers can survive without leaving their dammed lake. Their lodge provides warmth and shelter, and the lake provides food. Much like humans, beavers have evolved a successful adaptation that alters their environment to their own ends.

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Borrowing from Nature

The beaver’s skills at constructing a home are very similar to many of the early log-cabin homes built by humans, although perhaps a beaver lodge has a few more features to ensure that the air inside is fresh and warm. Human building techniques have moved well beyond this, and there is little to learn from the beaver. Its lake-forming behavior is an important one for humans, though. They are important for renewing and refreshing streams that may have become silted up, and they create an important wetland habitat for many birds and plants. Even when the pond dries up and turns to marshland, then wet meadow, the habitat supports many rare species. Although inadvertently, the beaver is one of nature’s conservationists.

BEE NESTS

A honeybee nest with brood cells layered to help maintain a constant temperature needed for the development of the eggs, larvae, and pupae. [George D. Lepp / Photo Researchers, Inc.]

Bees are most familiar to us when they make their nests in specially prepared hives, allowing bee keepers to harvest their honey. Of course bees do not just live in man-made hives. Although a few bees build freestanding nests, most species will adapt a suitable hollow for their home. This hollow is then adapted using two key building materials made by bees: wax and propolis. Wax is a strong but malleable material that is secreted from glands on the bee’s abdomen. The bees will mix the secreted wax with their own saliva to make it more workable. Propolis, on the other hand, is not produced by bees directly, but has to be collected and created. It is a more sticky substance than wax and is made from the saps and resins collected from plants by foraging bees. It has a very strong antibacterial and antifungal property and is used to help prevent disease spreading in the nest. The nest is built to regulate the temperature inside to as near

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optimal levels as possible. Honey bee colonies are able to keep the temperatures of their nest at 95–97°F (35–36°C) even when surrounding temperatures can fall below freezing or soar to over 113°F (45°C). The consequences of not keeping the nest at the appropriate temperature can be very dangerous. Adults and larvae alike can be killed off by the cold, and fungi can infest the colony if the temperature is allowed to get too high. The ability to regulate nest temperature is therefore a crucial adaptation. Bees are very choosy about where they build their nest. Some honey bees use tree hollows or abandoned animal burrows, which offer not only physical protection but also insulation. As the bees inhabiting the hive generate body heat, further insulation around the hive helps keep this heat trapped inside even in cold conditions. The amount of heat produced by the bees can partly be controlled by the bees themselves. To raise the temperature of the nest they can rapidly contract their wing muscles, generating body heat. So as not to cause damage to the wings by beating them in an enclosed space bees can detach the wing from the muscle beneath. The muscle contracts, but the wing does not beat. Once a suitable site has been found the nest itself must be built. In many stingless bees there are four key layers to the nest. First, the outer wall of the nest is lined with batumen (meaning ‘‘wall’’), a mixture of wax and propolis. In addition to protecting it from bacterial and fungal attacks (thanks to the propolis), batumen provides an effective insulating layer designed to keep heat within the hive. The batumen seals the nest except the entrance and, in some cases, ventilation holes. The next layer within the batumen lining is a layer of storage pots for pollen and honey. This is made by a substance called cerumen, which is again made from a mixture of wax and propolis. Within this layer of storage pots is the third layer—the involucrum. This is made with thin leaves of cerumen joined to each other and to the storage pots. The involucrum encloses the fourth layer—the brood comb. The involucrum is very important for temperature regulation. Stingless bees in cooler climates build more layers of the involucrum than in warmer climates. These layers act as baffles that inhibit convection currents from forming within the nest and convecting heat away. Like the other two layers, the involucrum is also an effective insulator. Although only 1 centimeter thick, the temperature on the brood side of the involucrum can be 10°F (5°C) higher than on its outer side. The brood comb within the involucrum is the heart of the nest. This is where the queen will lay her eggs and where the larvae hatch and grow and eventually molt into adults. Each larva is provided with its own, perfectly shaped crib. Wax is used to mold a series of hexagonal cells to produce the classic honeycomb shape. Within each cell, one egg is laid so each larva will grow in its own wax pen, being fed by the industrious adults of the colony. Not all cells are reserved for larva, though. Honey bee colonies will stock some cells with honey and pollen and use them as a food store with which to feed the growing larvae.

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Even the brood combs are carefully arranged in order to help control the temperature of the nest. Closely packed, matrix arrangements of brood cells allow for heat generated by the developing larvae to dissipate easily. This is seen in stingless bee nests in very warm climates where overheating could be a problem. Nests of stingless bees in cooler climates have a looser arrangement of brood comb cells joined together in a spiral. This spiral design efficiently conserves the heat generated by the brood. Key to temperature regulation is having a suitable flow of air through the nest. The design of the comb and the size and location of the entrance hole determines how much air can flow over the brood comb. However, the cooling current of air flowing through the nest is not just caused by the nest architecture. The bees themselves can help the rate of cooling. All species of bee use their wings to fan the nest and create airflow in one direction, typically out of the entrance hole or the ventilation shafts, in order to move heat out of the nest. Some species also use a water-cooling approach to dissipate heat from the waxy structures of the nest. Individuals will carry drops of water in their mandibles and place them in empty cells in the brood comb and allow them to evaporate. Individual bees will even smooth the drop of water into a thin film over the comb using their proboscis (a tongue-like organ) in order to allow for more rapid evaporation. The careful design of a bees nest not only provides protection for the colony but has evolved to be a sophisticated structure that keeps near-constant temperature. This has proved critical to the success of these social insects who would otherwise be very much at the mercy of wildly fluctuating environmental conditions. The intricate architecture is achieved by the teamwork of individuals with very small brains, which is in itself remarkable. Blueprints for the design are hard-wired into bees themselves, although there is also further control through communication between the individuals of the colony, mostly from the queen, using pheromones and sound. Through these simple controls evolution has produced a highly advanced solution to the problem of survival. Borrowing from Nature

The basic building materials of bees nests, propolis and wax, are used often by humans. Beeswax is used as a sealant and as a polishing agent for wood furniture. Propolis can be found in many health food stores for its antibacterial properties. And of course, the honey stored in the nests are a sugary treat for humans the world over.

PAPER NESTS As with all social insects, wasps are capable of producing some very impressive structures. Whereas termites use mud to construct their mounds and bees use wax and propolis to build their nests, nest-building wasps have evolved to make their homes from paper. Although it doesn’t sound like a particularly useful

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material, wasps’ ability to make and craft paper has allowed them to flourish. It is a very versatile material and can be used to make large structures. The largest wasp nest on record measured 12 feet (3.6 m) long with a diameter of five feet and nine inches (1.7 m). As with bees, there is great diversity in the nests that different wasp species produce. Paper is produced from wellchewed wood fibers. One of the A wasp nest made from paper. [Mark Boulton / most skilled workers of the material Photo Researchers, Inc.] are the paper wasps (of the Polistinae sub-family), which are found all over the planet in both tropical and temperate climates. As with any home, wasp nests need a good foundation. Paper wasps tend to build their nests to hang from a thin stalk attached to a branch. Unfortunately, this location is easily accessed by ants, which will attack the nest to eat the eggs and young. To protect against potential ant attacks the wasps will cover the supporting stalk with a shiny, black secretion from their abdomens, which is sticky and stops potential invaders in their tracks. Thus protected, the wasps can get on with making the nest. The paper for the nest is made from fibers of dead wood, which is chewed and mixed with saliva to make a papier-mache. When wet, this papier-mache can be moulded into the classic hexagonal shapes of the cells that will house the wasps’ young. The first generation of the nest is the work of just one female. After mating she will find an appropriate site, collect the wood fibers for the paper, build the nest, and tend and collect food for the larvae that hatch from the eggs she has laid. Once these larvae develop into adults, though, they emerge as workers that look identical to the founding female but which are not capable of reproduction. The workers will then take on the tasks of collecting more wood fibers to make more paper to extend the nest. They will also forage for food and attend the developing brood of young. As with bees, wasps take careful consideration over where to build the nest. It will be ideally located to ensure the nest does not get too hot or cold. This is particularly important for the nests of the paper wasps because the cells of the brood comb are open to the elements, although some empty cells are constructed around the edge of these nests to act as a layer of insulation—this is known as the functional envelope. Other wasps, such as hornets of the Vespinae sub-family, build enclosed nests and have evolved to build various structures that help warm or cool the nest. As with bees, the size and shape of the brood comb are critical in maintaining the ideal temperature for the developing larvae inside. Wasps can also increase the nest temperature by vibrating their thorax muscles and lower it by fanning it with their wings. However, there

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are other neat adaptations that help not only to strengthen the nest but help regulate its temperature. A key ingredient of the paper is the wasp’s saliva with which it makes a papier-mache from wood fibers. The wasp saliva contains a polymer, which acts almost like a glue, that sets as a very hard material when it dries. Clearly this gives a direct strength to the paper, although it has another property that is critical for temperature regulation. The saliva polymer has a certain thermoelectric property that is able to generate and absorb heat to ensure that the brood comb maintains a certain temperature when the surrounding environment is too cool. When the surrounding temperature gets too high the saliva polymer works in reverse. It loses heat in the form of a slowly discharging electric current. This way, the nest can remain at a near-constant temperature. The larvae that grow within the hexagonal paper cells produce silk, which has a similar effect. When they pupate—that is, undergo a metamorphosis to become an adult—they will spin a silk cocoon around them. This silk has a thermoelectric property that absorbs and holds heat when temperatures are high and releases it when temperatures are too cool; thus it can help maintain a constant temperature. Critically, though, this constant temperature is higher than that maintained by the saliva polymer of the adults because the pupa requires a higher temperature (1–3°F [1–2°C] higher) to develop into an adult than is normally required for the growing larvae (around 85°F [29°C]). Intriguingly, it seems that the electrical property of the silk can also help the developing pupa communicate to the adult wasps if it is too cool. Adults are attracted to certain cells and will vibrate their muscles to warm them up. Both pheromones and electrical signals are believed to be involved in coordinating this remarkable behavior. Borrowing from Nature

As with termites, architects are exploring the paper structures of wasps to make improvements to building designs in order to allow a cool supply of air to flow through them. That is in addition to the inspiration derived from observing the paper combs of the brood chamber, which are both lightweight and strong. Building supporting structures from man-made structures in the same way similarly saves weight without any loss of strength. Such structures are also more flexible, which can be important for certain designs. The honeycomb structure can be found in materials used to make many things from squash racquets to engine parts.

CORAL REEFS The oceanic reefs produced by the organisms known as corals are the largest structures produced by any living thing. They are so enormous that islands can form on them, ships can run aground on them, and they can support entire marine ecosystems. The Great Barrier Reef off the east coast of Australia is

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composed of at least 400 species of stony and soft coral, forming a home for more than 5,000 species of mollusks and no fewer than 1,500 species of fish. It is composed of a multitude of coral colonies. The structure we see today is thought to be 6,000–8,000 years old, although the modern structure has developed on a much older reef system, thought to be 500 million years old. This enormous reef is over 1,250 miles (2000 km) long and covers more than 185,000 square miles (300,000 square kilometers), and it can be seen from space. These immense structures are the work of countless tiny individual organisms called polyps, each no more than a few millimeters in length. By extracting calcium minerals from seawater, each polyp is capable of producing its own hard skeleton, which contributes to the growing reef. Some coral polyps, called stony (or true) corals, use calcium carbonate to produce hard, external limestone shelters. Other coral polyps use the calcium mineral building blocks to An individual hard coral polyp (Tubastrea sp.). The make internal skeletons—these polyps polyp will secrete a protective calcium carbonate are called soft corals. It is the hard corals skeleton, which will help form a coral reef. [Peter that contribute most significantly to the Scoones / Photo Researchers, Inc.] growing stone structure. Over decades, centuries, and millennia, successive generations of polyps lay down layer after layer of calcium carbonate. As one polyp dies, another will grow over its predecessor’s limestone shell and will grow its own skeleton. A reef, then, is a huge structure covered with a thin living veneer on its surface. Coral polyps are small creatures, and yet the rate of growth achieved by a colony of these minute organisms is quite rapid considering they are secreting rock. Some of the branching corals can grow in height or length by as much as 4 inches (10 cm) per year (about the same rate at which human hair grows). Other corals, like the dome and plate species, are more bulky and may only grow by 0.1 to 0.8 inches (0.3–2 cm) per year. This may not seem like much, but it can be sustained for thousands of years, forming huge and complex reefs. Coral reefs are not only structural wonders of the natural world; they also offer a multitude of hiding places for a wealth of marine creatures. The coral polyps themselves feed by using their short tentacles to trap tiny particles of

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edible matter that drift on the ocean currents, and many other creatures which take up residence on the reef feed in the same way. Coral polyps also have a symbiotic relationship with a special type of algae, called zooxanthellae, that live inside their bodies. These single-celled organisms use sunlight and carbon dioxide to make energy by photosynthesis, a process that produces oxygen and other nutrients needed by the coral polyps. In return, these zooxanthellae are provided, by the polyp, with protection and carbon dioxide. Coral polyps have another impressive adaptation to help their survival on the reef. Although they are capable of sexual reproduction, for the most part new polyps are produced asexually. That is, a new individual is budded from another. Consequently, they have exactly the same genes and so would benefit from helping each other out. Despite being a simple creature, certain coral polyps from the same stock can recognize each other and transfer carbohydrates and proteins to each other when one is in some way damaged. A reef is not just a collection of individuals living in proximity; it represents a true colony with communication and interaction between each polyp. With the way that the polyps interact, a reef can almost be thought of as one huge organism in its own right. The coral polyps that can extract food from the ocean are a big draw for predatory animals such as starfish and parrotfish, whose tough lips can withstand the hard, sharp protective shell of the coral. In turn, these predators are preyed upon by larger animals and so on, until a huge variety of organisms are dependent on the reef. In short, a large coral reef represents probably the most diverse ecosystem in the ocean. They are comparable in some ways to the tropical rainforests on land. Everything about coral reefs is made possible by the tireless deposition of rock by a minute sea creature. Individual coral polyps may be small, but as a group there can be few if any organisms that can match them in terms of their building abilities. Reefs can become so enormous that they disrupt and divert the ocean currents. Impeding and diverting these currents has important implications for life in the sea and on land as these streams of water channel heat energy around the globe, and changing their course can influence weather patterns all over the world. Indeed, this modification of the environment directly benefits the reef-making polyps. The reef can dissipate the huge amounts of energy that can be carried in large waves that sweep from the middle of the ocean to the shore. This makes the waters around the reef much calmer than they otherwise might be, which means that the fragile polyp can grow without damage from the ocean, making the reef a very successful adaptation indeed. Borrowing from Nature

There is no doubting the importance of coral reefs as significant ecosystems and as important physical structures that can influence the climate of the entire planet. But can we really learn anything from tiny bricklayers? It seems that we can. In the past few years, it has been discovered that coral communities may

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contain valuable medicines that may one day lead to treatments for cancer and HIV. A recently patented and licensed chemical called eleutherobin is produced by the soft coral Eleutherobia, which has a feathery appearance. This chemical is able to bind to tiny structures called microtubules that are found inside human cells. By binding to them, eleutherobin makes the microtubules very rigid and prevents cancerous cells from dividing and multiplying.

LUMINOUS GNAT TRAPS There are many examples of highly elaborate structures built in nature. This is the culmination of many small steps in evolution resulting in a highly specialized skill that certain species possess. Although undoubtedly successful adaptations, these elaborate structures take time and energy to build and may often leave their builders vulnerable to attack by predators during their construction. In some respects, therefore, the simplest designs can be considered some of the most effective. Simple structures have the benefit that they can be built and rebuilt again and again with little effort or energy. A good example of a simple, but effective structure can be found in the pitch dark limestone caves of New Zealand. The walls and ceilings of these caves are home to the tiny larvae of the gnat, Arachnocampa luminosa. The larvae themselves are not much to look at. They are typically maggot-like, pale in color, and measure around 3 cm long. They have a hard cuticle over the head that looks like a brown helmet. Despite their appearance, these unassuming larvae have a rather dazzling method of finding food. Arachnocampa larvae feed on other insects to get the protein they need to grow. For a slow-moving larva that cannot see well in the dark, flying insects would ordinarily prove too difficult to catch in a pitch black cave. Their solution is to stay put and lure their prey to them, and they do this by light. To create the light, larvae build a simple structure from materials made in their own body. It is a simple design and works as both a lure and trap, meaning the larva needs do very little to get its food. After hatching from the egg laid by the adult on the roof of the cave, the larva first builds itself a hammock from silk produced by a silk gland located on its head. Once safely secured in its hammock it makes more silk, which hangs in threads down from the hammock. As a larva lowers the threads it will exude a small drop of mucus at regular intervals. Once complete, the thread of silk with its droplets of mucus resembles a string of pearls, which can be up to 14 inches (40 cm) in length. Excreted within the mucus are chemicals that cause the mucus to glow. Common with other instances of bioluminescence in nature, the light from the gnat’s mucus is created by the reaction of the biological molecules luciferin and luciferase. This gives the beads of mucus a faint, blue-white glow. With hundreds of these luminescent gnat larvae living together, the roof of the cave resembles a starry night sky, precisely the effect the larvae are after to catch their prey.

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Not many insects call these dark caves home, and those that do venture there do so by accident. For instance, the larvae of some flies (such as the mayflies and caddis flies) live in water. These can be swept into the caves by the streams that flow through them. When they hatch and emerge from the water they find themselves in the pitch black, not at all the environment they hoped to be in. In normal circumstances, these small flies would head into the night sky toward the stars. Of course, the stars are not visible in the caves, and they make the easy mistake of flying instead toward the pinpricks of light on the cave roof. Like a moth to a flame, these small flies soon discover that the light is not starlight at all, but the deadly, sticky mucus of the luminous gnat. As they struggle, the flies soon become inexorably entangled and their fate is sealed. As well as being very sticky, the mucus beads on the thread are mildly poisonous. Like the venom of a spider, this helps to quickly subdue the prey so it can’t wriggle itself free. The effect seems to be almost instantaneous. As soon as an insect becomes caught it ceases to struggle immediately. The brief pull on the thread from the struggling insect is enough for the luminescent gnat to identify which of its several threads has caught something. It will then move over to the appropriate thread and begin to consume it, reeling it back toward it until it has dragged its prey up to its hammock. This means that as well as getting its insect meal, the luminescent gnat can recycle the silk and mucus it produced earlier. After eating, the larva will lower another sticky, glowing thread and wait for another unwitting fly to stray into its trap. Very simple, but very effective. The larvae will eat anything that finds its way into their traps, even adult luminescent gnats. To avoid this fate, the adults tend to move away from light to prevent accidents. The gnat larvae do extremely well with their simple lure and trap, using it to exploit a habitat where hunting is all but impossible. They are an excellent example of a species being successful by making a simple but effective structure. Borrowing from Nature

As with all bioluminescent organisms, the luciferase-luciferin reaction holds a great deal of interest for producing highly efficient ‘‘cold’’ light that does not lose energy as heat. All man-made light sources lose a high proportion of the energy that powers them as heat. The reaction of luciferase and luciferin converts nearly all the energy of the reaction into light—none is lost as wasteful heat. This reaction is already used in man-made glow-sticks, but the aim is to use it in a wider range of applications.

NAKED MOLE RAT BURROWS The structures built by many animals are relatively simple in function. They tend to provide temporary shelter or a place in which to rear young.

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Other animals, though, build homes that are much more integral to their whole way of life, even to the extent of shaping the evolution of the species. One such animal is the naked mole rat, Heterocephalus glaber, which spends nearly its entire life burrowing below ground. Neither a mole nor a rat, the naked mole rat represents a unique niche among the rodents. It is a rather ugly looking creature. Its body is well adapted for burrowing and living underground and so has evolved to be hairless, short-limbed, and practically blind. Their front teeth are enlarged for scraping away at the hard soil, and their bodies are covered in sensitive whiskers and hairs to feel their surroundings. The teeth actually grow through the naked mole rat’s lips so it can keep its mouth closed when it is burrowing. They live nearly their entire lives below the baking surface of the East African deserts. Thanks to this way of life, within their tunnels they have evolved a social way of living A naked mole rat (Heterocephalus glaber) more akin to social bees and wasps than to showing the huge teeth used for burrowing. [Neil Bromhall / Photo Researchers, Inc.] rodents or other mammals. The naked mole rat burrows through the soil in search of subterranean tubers (roots packed with carbohydrate) on which it feeds. Individuals increase their chances of finding its favorite food by searching as a group. Food is scarce, and a mole rat on its own would struggle to survive. By searching as a group, when one tuber is found it can be shared amongst the colony. In fact, the colony will go one step better than this and will actively farm tubers. Once one is found and has been partially eaten, the tunnel leading to it will be blocked to allow the tuber to grow. As a group, the colony can continually harvest tubers without relying solely on finding more. The collaborative nature of naked mole rats is rare in mammals. Naked mole rats are one of only two mammals (the other being another species of mole rat) that have evolved what is called eusocial living, which is considered to be the highest form of communal living. Eusocial organisms are characterized by having only one or a few individuals responsible for reproduction. The other classes of individual (castes), such as workers, are sterile. They also have several generations living together and cooperate over the care of the young. Most eusocial organisms are insects—bees, wasps, ants, and termites. These insects have evolved this way of life thanks to some unusual genetics, called

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haplodiploidy, which mean that sisters are more closely related to each other than to their own offspring so that it makes more sense, evolutionarily, for an individual to look after a sister than to reproduce herself. (In such species, females are produced as the result of a sperm fusing with an egg, whereas if an egg is left unfertilized a male is produced. One remarkable result of the resulting genetic makeup is that a female will share half her genes with her daughters, as with any sexually reproducing organism, but three-quarters of her genes with her full sisters.) Mole rats do not have this odd genetic makeup, yet they have evolved eusocial living because of the high levels of inbreeding within the family group, which tends to remain within its own network of tunnels. As brothers and sisters are so closely related, more so than the 50 percent relatedness we see in most other sexual organisms, it makes evolutionary sense to defer breeding and help the family group. The tunnel-based home built by the naked mole rat has directly led to how the species has evolved. Within any one colony of about 80 individuals (although numbers can be as high as 300), there tends to be one female and one to three males who breed. Offspring will typically grow to become workers, digging new tunnels and maintaining old ones. Workers will divide up the work, some working to dig the tunnel and others to carry the loose soil away. Some of these workers continue to grow to become soldiers who protect the colony’s network of tunnels, mostly from other colonies of naked mole rat who may try to take over the burrow. If a breeder dies, a soldier will become sexually active to replace them. While she is still alive, though, the breeding female keeps others sterile by brute force—by butting those that show signs of becoming sexually active. It is thought that the stress from bullying can suppress sex hormones, although hormones excreted in the breeding female’s urine may also play a role. The top female uses these strong-arm tactics to run the colony, often directly pushing workers to tunnels, which have collapsed, to coerce them into working. There is even a disperser caste. These individuals have been described by their human observers as fat and unwilling to engage in work. It is thought that these lazy individuals are storing energy until they leave the colony. When they do, they will burrow to the surface (the only time a naked mole rat will see the sun) and will search for a new colony to join. Above ground, they cannot find food, so their reserves of fat are all they have to keep going. Both males and females can be dispersers, and they are capable of traveling at least 1.2 miles (2 km) above ground in their quest to find another colony or establish a new one with other dispersers from other colonies. It is thought that this behavior has evolved because of the benefits of occasional outbreeding. The frequency of disperser castes is the result of the benefits of outbreeding balanced with the risks of dispersal, which can result in death in the baking sun of the desert. The network of tunnels in which naked mole rats live are well designed and can have specialized functions. Most of the tunnels are built to search for food. Naked mole rats are particularly good at this and can dig a mile-long tunnel in

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less than three months—not bad for an animal three inches long trying to get through rock hard soil. Major highway tunnels are built in long straight lines and are big enough for two individuals to pass each other. Branch tunnels from the main highway to food supplies are much smaller and can only accommodate one individual at a time. There are also chambers for sleeping, food storage, rearing pups, and even a latrine. The latrine is an important room. It helps keep the regular tunnels and chambers clear of excrement, but also allows all individuals to cover themselves with the particular odor of the group. Being nearly blind, odor is an important way of knowing whether an individual belongs to the colony or not. Living in such an enclosed space, naked mole rats have evolved other key adaptations to survive. With no access to fresh air from outside, the tunnels can get extremely stuffy with very low levels of oxygen and potentially dangerously high levels of carbon dioxide. To cope, they have evolved a specialized type of hemoglobin that is highly efficient at binding oxygen in the naked mole rat’s red blood cells. They also have very low metabolic rates, less than half that of other rodents, which helps them conserve oxygen. In extreme conditions, these adaptations allow naked mole rats to survive, for a few hours at least, in conditions of just 3 percent oxygen. Humans, by comparison, require air with 21 percent oxygen to survive. The naked mole rat is a unique and unusual animal. It has evolved many adaptations rarely seen in animals thanks to its subterranean way of life and its home carved out of the desert soil. Again, these remarkable adaptations have enabled an organism to live in a niche not exploited by others, allowing it to flourish. Borrowing from Nature

The naked mole rat’s ability to withstand the harsh, low-oxygen environment of its tunnels has become of great interest to scientists interested in studying pain. The high levels of carbon-dioxide gives rise to a very acidic environment, which the mole rats seem oblivious to. It seems that the naked mole rat lacks a gene that produces the neurotransmitter that is responsible for transferring signals along nerves that would normally detect pain. Scientists are exploring whether these neurotransmitters could be disabled in humans who suffer from chronic pain, delivering much needed relief. There is also hope for exploiting the mole rat’s highly efficient, oxygen-grabbing hemoglobin for treating diseases caused by hypoxia (lack of oxygen), such as heart attacks, kidney disease, strokes, cancer, and diabetes.

DIATOMS In most bodies of water, in freshwater, sea water, and even in soil water, there can be found humble-looking algae called diatoms. Ecologically, diatoms are very important organisms. Although they are very tiny, being made up of

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just one cell, there are a great deal of them on this planet. There are over 70,000 species and, all told, diatoms represent a significant proportion of the biomass in our oceans. They are therefore an important source of food for many organisms. Diatoms and other microorganisms that float through the seas make up what we call plankton, which many sea creatures, even whales, feed on. Like all algae, diatoms make energy by photosynthesis. Thanks to their sheer number, this means they produce a significant amount of the oxygen we breathe. They also absorb and ‘‘lock A scanning electron micrograph of a diatom. up’’ a lot of carbon dioxide, effectively Diatoms are single-celled algae that secrete taking it out of the environment. For all an intricate glass cell wall (known as a frustule) these reasons, diatoms are known as the for protection. [Eye of Science / Photo trees of the sea—they really are as Researchers, Inc.] ecologically important as the plants on dry land. There is no doubting the importance of diatoms to our planet’s ecosystem. Recently, though, something else has been discovered about them that makes them even more remarkable. Since they were first discovered and studied under microscopes, scientists have appreciated the sheer beauty of diatoms. These single-celled organisms are encased by a highly ornate, symmetrical, silicon shell, called a frustule. Each species produces it own unique shape—circular, oval, stick-shaped, star-shaped . . . pretty much any shape imaginable. Whatever the shape, every species produces a shell that comes in two halves that fit together much like a petri dish or Camembert cheese box. In fact, the name ‘‘diatom’’ means ‘‘cut in half.’’ The larger upper half of the shell is called the epitheca, which overlaps the smaller lower half called the hypotheca. The shells themselves are intricately created from silica, making diatoms the smallest glass-workers on the planet. The shells are often punctured by many tiny pores and slits, which suggests a function beyond simple protection, although it is not clear what this function might be. Scientists are only beginning to unlock the construction of these tiny shells. As more is discovered, there is greater realization that diatoms are truly great master craftsmen. The main scaffold of the shell is silica, which is produced by the diatom within its single cell in what is called the ‘‘silica deposition vesicle.’’ A vesicle is a simple, fluid-filled sac that floats around inside a cell. This hardly sounds like a sophisticated organ to be involved in producing an important biomaterial, but it certainly manages to do the job. The silica deposition vesicle is thought to act as a mold in which the frustule will grow. Once silica has been

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produced inside the cell, the diatom must secrete it and build its shell. It is not clear exactly how silica is secreted, although it is assumed there must be some form of silica transporter molecule involved. Key building blocks for the construction of the silica shell are called silaffins (meaning a molecule with an AFFINity to SILica). Silaffins were first discovered in the diatom Cylindrotheca fusiformis. They are arranged around the diatom’s cell wall and act as a scaffold for the silica to be laid down. Silaffins precipitate liquid silica that is transported from within the cell into solid silica, which forms the frustule. It seems that the structure of silaffins allows them to bind together to form a rigid structure around which silica can be deposited. Silaffins are peptides, short sections of protein made up of fewer than 30 amino acids, the building blocks of proteins. From these short sections of protein hang molecular ‘‘side-chains’’ that are made from bio-molecules called polyamines. These side chains are of different lengths, and shapes are critical in how silaffins link together to create the scaffold to produce the unique glass wall of the species in question. The great number of possible combinations of these side chains explains why we see such huge diversity in diatom shell shapes. Although much is known about how diatoms produce such intricate and ornate shells on such a tiny scale, we have only scratched the surface. Other organic molecules as well as silaffins are likely to be involved in creating the frustule, but it is not yet known which ones or how they work. This is a fascinating field of work that will soon reveal how a simple, single-celled alga manages to be nature’s finest builder of minute structures. What is not known is the evolutionary significance of this adaptation. It almost certainly is important in protection, which may explain why there are so many diatoms in our oceans. However, there are tantalizing hints at a greater evolutionary purpose. Borrowing from Nature

There is a great deal of interest in understanding and harnessing the abilities of diatoms to produce such tiny, precise structures. In the future, diatoms could be engineered to exact specifications to create microscopic sieves; to act as gears in microscopic robots; to deliver drugs to specific parts of the body through the bloodstream; or to build tiny diffraction gratings that could be used to produce advanced holograms. Unlocking the secrets of how a diatom builds its shell could also be scaled up to help designers build strong, but lightweight, structures in aerospace and vehicle manufacture. Probably the most pursued field in which diatoms are being studied is in the computer industry. Currently, nanotechnology techniques that involve building up three-dimensional objects take a painstakingly long time. If diatoms could be engineered to produce silica structures to a design, tiny microchips could be produced without having to lift a finger. The first step toward harnessing the nano-construction abilities of diatoms has been taken. In 2004, the first diatom genome was sequenced for the species, Thalassiosira pseudonana.

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The trick is now to understand what each of the genes do and to work out how they can be manipulated. No easy task.

WEBS Of all the remarkable structures we see in nature, few can be as familiar as the spider’s web. There are 40,000 species of spider currently known to exist, and every species uses silk in some way or another, although not necessarily to make a web. Webs vary greatly in size and appearance, but all of them are elegant traps for catching prey—normally insects. Some of The different silks and glues used by spiders to prothe simplest webs are nothing duce webs. [Michael Abbey / Photo Researchers, Inc.] more than a few strands of silk radiating from a silken tube concealed in a crevice, whereas others are complex arrangements of strands running in all directions. Visually, the most impressive webs are those commonly known as orb webs, the archetypal spider’s web. The orb web varies from species to species, but essentially it is made up of three elements, all of which require slightly different types of silk. The core is made from the radial threads that run from the center of the web like the spokes of a wheel. Next are the frame threads that serve as points of attachment for the radial threads and are anchored to wherever the spider chooses to construct its trap. Lastly is the catching spiral. It is only this last element that is sticky and which directly snares the prey. Not only is the catching spiral sticky, but it also very elastic. In contrast, both the radial threads and the frame threads are non-sticky and relatively inelastic. It is not known exactly how these various parts are linked, but it seems that the orb web spiders produce a type of glue to fix the web together at the 1,000–1,500 connection points. Spiders are able to weave their silk in all sorts of ways, making all kinds of structures possible. Some species weave their silk into sheets enabling them to make tubes for hiding in and catching prey. The purse web spider (Atypus affinis) uses its silk to produce a small sock-like structure that extends from the spider’s burrow. When an unsuspecting victim strays over the purse web the female will strike from beneath and stab its prey through the web with huge fangs. Once the prey is dead, the female purse web spider will slit its web and drag her meal into her lair to eat it. After eating, the remains are discarded and the slit repaired. The trapdoor spiders use their silk to line a vertical burrow and a small camouflaged lid that fits the burrow entrance perfectly. Radiating from the burrow

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are a number of silken threads that alert the spider to the presence of prey. When it feels an insect moving near its burrow the spider flips the lid of its burrow and grabs the unfortunate victim in a lightning-fast lunge. In a similar approach, the so-called money spiders build what at first sight looks like a tangled mass of threads in low vegetation that are often revealed in their thousands by morning dew or frost. Above the domed sheet web there are numerous vertical threads, all of which serve to entangle passing insects that eventually find themselves on the domed sheet and within reach of the web’s owner. Some species of spider even team up to produce huge, three-dimensional communal webs, which can support up to 1,500 spiders living together. Spiders of the Uloboridae family will live together on a single web and will work together to wrap up prey that falls on to it. Collectively they will pour their digestive juices over the body to liquefy their food, but this is where the cooperation of these spiders ends and they fight over the prey once it has been subdued. Lynx-spiders, on the other hand, will cooperate in building the web and in feeding, the adults feeding first and the juveniles afterward. Taking things further, there are several Stegodyphus species that have a highly evolved social system. They will build a communal web, but this is populated by only a few members of the group. Most individuals will remain hidden in lairs. When prey falls into the web, the lookouts recruit help by tugging on the silken strands of the web. The rest of the group will then emerge from their lairs and come to assist, attacking the prey with bites and injecting digestive juices. Again, the group will feed together, sharing their prize. In one species, though, those that fail to help in making the kill are chased away and are not allowed to feed! All these fascinating and adaptive social behaviors stem directly from many individuals collaborating to produce a single communal web in which to live. Borrowing from Nature

Spider silk itself is the subject of a great deal of research thanks to its remarkably versatile nature, being both strong and supple. However, the ways in which spiders spin their webs is being explored as well. In recent years there have been great leaps taken in the field of nanotechnology thanks to the way spiders spin their silk. The problem faced is that once a new nano-material is produced, it is hard to replicate on an industrial scale—that is until engineers came up with electro-spinning that draws its inspiration from spiders. A tiny nozzle is used to eject the material in question, and electrically charged plates are used to direct the flow—not exactly how spiders do it, but a good way of mimicking what these creatures can do thanks to millions of years of evolution.

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SENSING THE ENVIRONMENT—HUMAN INVENTION Humans have a reasonably well-rounded set of senses. Our eyesight is the dominant sense, but hearing, touch, taste, and smell are all pretty good too—they certainly serve their purpose. None of our human senses, though, ranks highly in nature as being extraordinarily sensitive or acute. Many organisms exceed our ability to detect our own surroundings. The inquisitiveness of humans, however, pushes us to be able to detect and understand more about our environment—our home planet and even the far reaches of the universe. To do this, a whole gamut of devices have been invented over the years to improve our own senses. With sight being our dominant sense, there has been a great deal of attention paid to being able to see objects that are too small or too distant for our eyes to detect. The smallest object that the human eye can detect is about the thickness of a human hair, some 100 micrometers (0.0001 meters). Any smaller than this and we see nothing. Since the sixteenth century humans have been able to explore the world of the very tiny with the invention of the microscope. The first simple microscope made use of the glass lens that had been invented earlier in the fourteenth century to improve eyesight. The initial design placed one or two of these lenses together in a tube. Thanks to this simple design, early scientists discovered much about the cells that make up our bodies and the intricate structures of everyday objects like cork. Simple microscopes, though, were unable to magnify images indefinitely. As the lenses become more curved to increase magnification, the image they produced became blurred. Various innovations in the nineteenth century corrected these problems. For example, placing several lenses together in a microscope, each

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with only a weak magnifying power, gives an overall high degree of magnification with no blurring. These innovations allowed scientists to see even smaller objects. The next step from these microscopes, which required an illuminating light source to shine either on top of the object being viewed or behind it, was to use the electron as an alternative light source. Electrons are extremely tiny and can be fired at miniscule objects that cannot be seen by a light microscope. The way the electrons bounce off the object can be detected by an electronsensitive plate to produce a picture. Since the 1930s, electron microscopes have been used to look at viruses, the tiny organelles within cells, and even atoms. Electron microscopes are able to magnify objects by up to 1 million times their actual size. The microscope, in its various forms, allowed humans to explore the microcosms of the planet and understand in much greater detail the life that lives there. However, humans have been equally curious about exploring objects very far away from us—objects in the far reaches of the universe. The history of the telescope is much like that of the microscope. A simple arrangement of a convex lens at one end of a tube in association with a concave lens at the eyepiece allowed light from distant objects to be magnified. This initial design, built in the sixteenth century, could magnify objects by only three or four times their actual size. But in the early seventeenth century this basic design was improved by Gallileo to achieve magnification of 20 times. It was with this telescope that Gallileo mapped the moons and planets of the solar system. Newton made further improvements to the light telescope in the late seventeenth century by making use of parabolic mirrors to focus light onto the lens, but it wasn’t until the twentieth century that a significant leap forward was made. Humans can see only a narrow band of the electromagnetic spectrum, what we call the visible light or the visible spectrum. Yet objects reflect and radiate other wavelengths of light as well, including ultraviolet or infrared light. Radio telescopes that could focus and detect radio-waves reflected by distant objects allowed astronomers to explore and describe objects from across the Milky Way. Radio-waves, though, have longer wavelengths and lower energies than visible-spectrum light, so radio telescope antennae need to be very large to capture the radio-waves emitted by objects in space and give a clear resolution. This is why the radio telescope dishes seen in the middle of the desert are so huge. The aptly named Very Large Array telescope in New Mexico is really 27 large antennae linked electronically to give the effect of a single antenna 22 miles across. Using similar telescopes, infrared, ultraviolet, X-ray, and gamma radiation telescopes are used now to explore the huge expanse of space. As yet, there is not one device that can ‘‘see’’ and magnify each of these wavelengths of light—for now we must make do with specialist telescopes and build up a composite picture of the universe from the collective data received.

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With vision being the dominant sense in humans, much of our inventions for describing the environment are based on light. There are, though, uses for artificial sensors of sound, smells (chemical odors), and touch. Microphones are very simple but effective devices that convert sound into electrical energy. This electrical signal can then be converted back to sound via a speaker. Microphones work thanks to a thin membrane-like diaphragm within the device that vibrates according to the frequency of sound waves passing over it. How the diaphragm converts sound to an electric current depends on what material it is attached to. All materials used have certain electrical properties that change when they are moved by the vibrating diaphragm, allowing an electrical current to be generated. In some microphones a plastic or metal diaphragm is connected to a layer of carbon dust. Vibrations in the diaphragm compress the carbon dust, and the degree of compression varies according to the frequency at which the diaphragm vibrates. The degree of compression affects the electrical resistance of the carbon, which can be exploited to send an electrical signal to the speaker. Other microphones make use of the electricity-generating properties of an electromagnet. A magnet moving within a coil of wire will generate an electric current in the wire, so the diaphragm can be attached to either the magnet or the coil so that a current is generated when it moves that corresponds to the vibrational frequency of the diaphragm. Touch sensors have only recently been developed for their use in robotics. There are several types: touch sensors, tactile sensors, and slip sensors. In simple devices, touch sensors can detect whether two surfaces are touching or not. In more complex designs, they can sense the pressure with which the surfaces are touching. For the simple design, the sensor can work just like a mechanical switch. When two surfaces are pressed together the switch is triggered and an electrical signal is sent to a receiver. Pressure sensors rely on one surface being covered in a compressible conductive foam whose electrical properties change when compressed. This allows a variable electrical signal to be sent to the receiver depending on the pressure exerted. Typically, the material used for this is a carbon-infused rubber. Tactile sensors are used to determine variations in the structure of the object that the sensor is touching. The analogy is that our human skin can differentiate between the feel of texture of different objects like wood, fur, water, metal, and so on. Currently, tactile sensors are nowhere near as sensitive to texture as human skin. They work by stringing a series of touch sensors together and building up a collective picture of the different pressures exerted by the object at different points. The data collected is generally poor and relies heavily on the computing power of the receiver to interpret. Slip sensors, as their name suggests, detect whether two surfaces are no longer aligned. Again, this can be achieved by a series of touch sensors that detect that an object is moving across them. There are a variety of methods used to achieve the same thing in touch sensors. Currently, though, the aim is to develop a

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multi-sensor that can detect not only touch but variations in temperature and so on. Here the inspiration is once again found in nature—human skin, which is capable of detecting a range of stimuli. Finally, although there are a great number of applications for a chemical ‘‘smell’’ detector, there are very few that have been invented. Indeed, many of the detectors in development today draw their inspiration entirely from the chemical detectors that have evolved in nature, and these detectors are covered later in this chapter. Perhaps the most well-known detector used by humans was the canary in a cage, which miners took deep underground to check for odorless gas. Gas pockets could asphyxiate humans, so some form of warning was essential. Unfortunately for the canary, the birds could not smell the gas either, so the warning came from the bird being overcome by gas and crashing to the bottom of the cage. Clearly, there is still a long way to go, and evolution has a lot to teach us.

VERTEBRATE EYES The animal kingdom is dominated by visual organisms. The picture that animals, humans included, build up of the world is produced by light falling on sensitive cells in the eye, which is interpreted by the brain to create an image we see. The brains of animals are excellent at converting the information passed to them by the nerves carrying information from the eye, so the quality of the picture tends to be determined by A scanning electron micrograph of the rod and cone how well the eye detects light. photoreceptor cells found on the retina of the verteUnlike the compound eyes of brate eye. [Omikron / Photo Researchers, Inc.] insects, vertebrates have just two eyes that are well adapted to absorbing a lot of information that the brain converts to a detailed picture of the surrounding environments. There is a great deal of variation in how well vertebrate eyes do their job, yet the basic model of a vertebrate eye is the same for all vertebrates. Vertebrate eyes are typically round, with a cornea and lens at the front that lets light through into the main part of the organ. The lens is controlled by muscles that help focus the image falling on the light-sensitive part, the retina, which is located at the back of the eye. The retina itself is a complicated layer of nerves on top and light-sensitive cells (rods and cones) beneath. Light passes through the layer of nerves and falls on the rods (which are more sensitive to light) and cones (which are

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less sensitive). Humans have a mixture of rods and cones, which allows us to see in a range of light intensities. Curiously, the arrangement of the retina in vertebrates is far from perfect. The nerves of the retina all feed into the optic nerve, which transfers the information from the rods and cones to the brain. However, as the nerves lie on top of the light-sensitive cells, the optic nerve has to pass through them, which is why animals such as humans have a blind spot. This makes little difference, as we hardly notice it, but it is far from optimal. Ocean-dwelling animals such as the giant squid have a retina that is arranged with the nerves behind the light-sensitive cells, so there is no need for an optic nerve to puncture the retina. Consequently, there is no blind spot for a squid. Diurnal animals, those that are active during the day, tend to have many more cone cells than rod cells in the retina. What’s more, they will tend to have particularly densely packed clusters of cones in special, small depressions in the retina called fovea. The shape of these fovea allow light to fall on more cones than normal, which gives better visual acuity than if the retina was smooth all over. They are positioned to give better visual acuity at the center of the field of vision, and they are most notable in birds such as hawks, which can see excellently from great distances. In humans there are some 10,000 cone cells per mm 2 across most of the retina; in the densely packed fovea there are 200,000. Birds like hawks have up to 120,000 cones per mm2 across most of the eye and up to 1,000,000 in the fovea! Such a density of light-sensitive cells allows hawks to produce a clear picture of the environment from a much greater distance than humans. There are other adaptations that give birds such very good eyesight. Their eyes are typically much larger, in proportion, than mammals, which means that there is a greater area over which the light-sensitive cells are spread. Furthermore, the lens focusses light in such a way that an image is focussed equally well across the whole retina. This is not the case in humans. The periphery of our vision is quite blurred, and we need to move our eyes directly toward what we want to see to get a clear picture of it. This brings the light from the image directly on the specialized fovea in the center of our vision. This is not the case with birds. They can see perfectly out the corner of their eye. Even the way that the lens is manipulated by muscles to focus an image is an improvement on the eyes of mammals. Muscles attached to the cornea allow it to bulge outward, something which mammals cannot do, giving a much greater focal range. Birds are not the only vertebrates with specialized eyes. Some animals have what is known as a ramp retina. The eyes of these creatures are more oval than round, which causes the retina to curve in an uneven arc rather than in a regular semicircle. As a result, one-half of the retina is nearer to the lens than the other half, which is a significant adaptation because it means that the eye can focus on both nearby objects and distant objects at the same time. Horses have a ramp retina. Below the eye-line, nearby objects are in focus; above the

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eye-line distant objects are in focus. So when they dip their head to graze they can look at what they are eating and what is on the horizon, allowing them to watch out for predators and eat at the same time. Borrowing from Nature

Humans have always had a fascination with designing robots that mimic human behaviors in order to assist genuine humans in day-to-day tasks. A challenge in designing these robots is to design a visual sense that allows the robot to detect and recognize certain objects. A robot capable of this behavior could be used to help physically handicapped people. The latest wave of robots designed at MIT have eyes that can track objects in the same way as human eyes and, thanks to the robot’s computer brain, recognize faces and objects. In addition to designing eyes for robots, some early steps are being taken to design artificial eyes that enable visually impaired people to see. Simple camera-like sensors are built into ordinary-looking sunglasses. Tiny computers in the glasses interpret the visual data collected, like our own eyes. These are then sent in electric pulses directly into the brain, exactly like the optical nerve that runs from the retina. Rather than the brain creating a visual image, the data from the glasses is converted to sound, which relates to the shape of an image. This is an amazing advance, but clearly there is still a long way to go to mimic eyes that have evolved over millions of years.

INSECT EYES

A scanning electron micrograph of an insect eye showing hundreds of individual, rounded ommatidia. [Thomas Deerinck, NCMIR / Photo Researchers, Inc.]

Flying insects are amazing creatures. Unlike humans who are restricted to moving about on the ground, insects are capable of moving in three dimensions and at incredible speeds. Like the top fighter pilots who are in control of some of the most speedy and agile machines ever built by man, insects also need to absorb and process a great deal of information to avoid crashing. Given the relatively vast brain of a human pilot, not to mention the various computers, gizmos, and gadgets that are there in the cockpit to assist, it is remarkable that a tiny insect with a frankly miniscule brain can perform swoops, dives, and maneuvers with such consummate ease. To avoid crashing, the insect needs to be aware of what is going on around it by

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building up a mental picture of the world. It is little surprise, then, that insect eyes are huge organs and that 80 percent of its brain capacity is taken up with processing the information that they take in. Insects have two sorts of eyes—simple eyes, called ocelli, and a pair of compound eyes. Both are very important to the insect as it goes about living its life. The simple eyes are just that. They can detect subtle changes in light levels, which help the insect keep track of time and to detect when it is flying under cover, out in the open, or even if a potential predator has loomed over it, casting its shadow. If it is the latter, the insect can take the appropriate evasive maneuvers. Compound eyes are the actual organs of vision. These eyes are very different A higher magnification scanning electron from our own. Human eyes have a single micrograph of an insect eye that has been cut lens that projects a tiny image on the through to show the long photosensitive cell light-sensitive retina at the back of of the ommatidium, capped with a lens. [Susthe eye. The optic nerve running from umu Nishinaga / Photo Researchers, Inc.] the retina then relays the data received by the eye to the brain, where an image is built up. Insect eyes, on the other hand, are made up of thousands of tiny hexagonal lenses called ommatidia. Each ommatidium is a long lens that directs light onto a light-sensitive nerve ending at the end. Like our eyes, this nerve sends an image to the insect’s brain, albeit at a much lower resolution than we see. It is by building up each of these thousands of images that the insect can build up an image of what it is looking at. If an insect were to simply stare at an object it would not be able to see it at the same level of detail that humans can. So what benefit derives from having compound eyes if they do not produce a clear picture? To answer that, we must understand how compound eyes work in seeing moving rather than stationary objects. Insect eyes come into their own when they are moving about, which is what insects do a lot of the time. Instead of being sensitive to the detail of an image, an insect’s eye is very sensitive to movement. A moving object triggers an image in one ommatidium, then a split-second later triggers an image in an adjacent ommatidium and so on. The compound eye can therefore build up an idea of a moving object by the wave of images that it creates from one simple lens to the next. The same principle applies when an insect is flying around its environment.

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As objects pass across an insect’s field of vision, the insect can determine how close they are thanks to the speed at which they pass them. Because of their proximity to the flying insect, the image of nearby objects will pass through the insect’s field of vision more quickly than those farther away. Think of how quickly the roadside fence seems to pass by so much quicker than the trees in the distance when you are in the car. When you are in the car, though, the nearby objects pass by in a blur and those in the distance are easier to focus on. For insects it is the other way around. The faster an object moves past its field of vision, the more clearly it is registered by the insect’s brain. As nearby objects pass its eyes more quickly, an insect can build up an excellent picture of its immediate environment as it flies past. This is critically important for the flying insect, as it needs to be acutely aware of possible hazards when flying about at such high speeds. It is this excellent vision of movement, combined with lightening reflexes, that explains why house flies are so hard to swat. With all eyes, images are built up from a series of short snapshots of the surrounding environment. This is how a flick book works. Draw a series of pictures on the corner of a notebook and riffle through them and it will look like a continuous moving picture. Human eyes fuse flickering images into a continuous one at 15 frames per second. Fast-flying insects such as house flies see individual images at 100 frames per second. As a result, flies can negotiate complex environments at high speed that no fighter pilot would be able to cope with. The compound eyes of insects are an excellent example of perfect adaptation to a particular lifestyle. Insects either need to move quickly themselves or need to spot fast-moving creatures so that they can move about, hunt, and avoid predators. Human eyes just wouldn’t be up to the job, and the only image we would be able to see would be a blur of movement. For insects, their life is based on movement, so they have evolved the right equipment to survive and flourish. Borrowing from Nature

The compound eyes of insects are greatly interesting to air and naval warfare researchers. Unmanned vehicles have the benefit of carrying out surveillance without risking human life. By mimicking insect eyes, these vehicles can be piloted much more easily through tricky terrain and can also be used to build up images of very tiny objects that are not visible with conventional cameras.

ECHOLOCATION There is no doubt that vision is an important sense for most animals, but there are limitations to this sense. Eyes only work if there is some light for them to detect, which means it can be difficult for an animal to find its way in the dark. Certainly, there are some nocturnal animals that have evolved

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highly sensitive eyes that can see well in the dark, although these animals still rely on there being some light, even if it is only the weak light reflected by the moon. Some animals live in places entirely bereft of light, and they have had to evolve different senses to find their way around. Slower-moving animals might rely on smell, touch, and vibrations, but this would be no good The Californian leaf-nosed bat (Macrotus for a faster-moving animal—they would califonicus) showing large ears used for echobump into things far too frequently. To location. [Merlin D. Tuttle / Bat Conservaget around this problem some of these tion International / Photo Researchers, Inc.] species have evolved to make use of sound to navigate. Probably the best users of sound in this way are the bats. Bats are a diverse group of mammals ranging from the bumblebee bat, Craseonycteris thonglongyai (weighing between 0.003–0.004 pounds (1.5–2 g) to the giant golden crowned fruit bat, Acerodon jubatus (weighing 2.6 pounds [1.2 kg] with a wingspan of 5 feet [1.5 m]). Bats are the only mammal to have evolved true flight and are divided into two distinct sub-orders. The large megabats (Megachiroptera) feed on fruits, and the smaller microbats (Microchiroptera) feed on small insects. Megabats have very good eyesight, which they rely on to find their way around. It is the microbats that have evolved the use of echolocation to find food and navigate their environment, although they can see a little in dim light. Echolocation in the microbats is so well honed that they are able to locate and catch tiny insects on the wing, a remarkable blend of the senses and aerial ability. The elements of echolocation are simple: a sound is emitted and the echos are detected as they rebound from objects in front of the bat. To finely tune this ability, though, bats have evolved a number of highly specialized adaptations. Some bats emit sounds that are audible to humans—you might have heard these ‘‘ticking’’ sounds when out walking at dusk. These sounds are used by bats to avoid bumping into large objects, but they cannot help the bat detect much smaller objects like flying insects because these audible sounds have too low a frequency. Low-frequency sounds will simply pass through small objects, and they won’t bounce back as an echo. Therefore, to locate small objects like insects, most microbats emit very high frequency sounds to create an echo. These high-pitched sounds are produced in the larynx (as with all mammals) and are emitted through the mouth and nostrils. Many bats have even evolved modified noses that are ornate flaps of skin and cartilage. These so-called noseleaves act as an auditory lens that focuses the bat’s calls into a narrow beam. Bats are able to emit and detect such high-pitched sounds that they can navigate around wires only 0.2 mm thick.

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Once a bat has emitted a high-pitched call, it must listen for the responding echos. To achieve this, bats have excellent hearing. They have very large ears that funnel sound into their auditory canal. Their inner ear, too, is well adapted to picking up quiet sounds, having a large, spiraling cochlea that is sensitive to a range of frequencies. Unfortunately, a bat’s ear is so sensitive that it represents a major problem for its owner. Because high-pitched sounds quickly lose volume as they travel through the air, bats need to call loudly to maximize the distance over which they echolocate. Some bats are able to produce sounds that are up to 12,000 decibels— about 100 times as loud as a rock concert! This would be loud enough to deafen anything, not to mention a bat with sensitive hearing, which is why bats have evolved a way of protecting their sensitive ears. Bats can disconnect their own hearing when they give their ear-splitting calls. There are two tiny muscles in a bat’s ear that are attached to the bones, which connect the ear drum to the inner ear. (Humans have the same bones.) When a bat calls, the muscle disconnects one of these bones (the stapes) from the ear drum, breaking the connection from the outer ear to the inner ear. This means no sound travels to the very sensitive inner ear, avoiding debilitating deafness. Immediately after the call is made, the muscle relaxes and full hearing is restored, allowing the bat to detect the echo. A further benefit of using high-pitched calls is that very few animals can hear it. (It is certainly above the threshold of human hearing.) This means that potential prey cannot hear the approaching bat. This would mean that bats could hunt with stealth and more or less pick off prey at will, but it seems that things are not as simple as that. Some nocturnal moths and some species of lacewing have evolved hearing organs that detect the high-pitched frequencies used by bats. The ability to hear their predators as they hunt allows these prey species to take evasive maneuvers and escape from harm. The story doesn’t end there, though. It appears that bats have begun to evolve to use different frequencies of sounds to hunt, which the moths cannot hear in order to remain hidden from their prey. Bats and insects are locked in an evolutionary arms race—a constant struggle to find the upper hand over their nemesis. Whether or not it has been detected by its prey, once a bat has located a likely meal on the wing it will move in for the kill. This is not as easy as it might sound. Flying insects move quite erratically to avoid being plucked from the air by a predator, so locking on to these weaving insects is tricky. To assist them, bats will modify their calls during the hunt. When searching, they may emit lower-frequency, intermittent calls. Once they have detected something, they will switch to higher-frequency calls to pinpoint the prey and will begin to call more often. Once ‘‘locked on’’ to their prey, the bat will move in. Rather than heading straight for its quarry, though, a bat will perform a behavior called ‘‘parallel navigation.’’ It will keep its head pointed at the insect, emitting calls to detect its location, but it will fly toward it at a slight angle. If the insect is located

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northeast of the bat, the bat will move so as to keep the insect northeast of it, while gradually closing the distance. This helps the bat lock on to its prey’s position and predict its trajectory. It will then move on an intercept course to make the kill. This is exactly how engineers solved the problem of programming early guided missiles in the 1940s. Once locked on to its prey, the bat will zero in and intercept it in less than a second. Borrowing from Nature

Bat echolocation is being investigated as a means to help visually impaired people build up a picture of what is around them. Devices are being built that emit ultrasonic sounds like bats and detect the echos as they bounce back. These echos are translated into audible sounds, which are played to the user through a headset. Sound is played through the left and right earphones depending on where the object is relative to the user, allowing them to build up a picture of their local environment.

ELECTROSENSE Thanks to a nervous system based on sending electrical signals through the body, all animals generate a faint electric field around their body. In water, this field travels further than in air. It is perhaps not surprising, then, that many aquatic animals have evolved the ability to detect these electric signals, not to mention the behaviors to exploit it. Electric eels can generate and discharge The duck-billed platypus, Ornithorhynchus lethal blasts of electricity, but that is not anatinus. [Tom McHugh / Photo the only use of electricity in nature. The Researchers, Inc.] ability to detect and respond to electric signals, especially in water, allows for some very elegant and ingenious behaviors to have evolved. There are many species of electric fish that have evolved the ability not only to detect electric signals but to produce them as well. Unlike the electric eel, the vast majority of fish are incapable of producing a sufficiently large voltage to use electricity as a weapon. However, even weak electric signals can be used as a form of communication with a wide and varied vocabulary. One such species uses its electrical signaling ability to attract mates. Found in the Amazon basin, male Brachyhypopomus pinnicaudatus fish can be found at night making a very odd buzzing noise. This sound is reminiscent of the hum of an electricity pylon, and for good reason—the buzz is a complicated electrical signal that is the electrical equivalent of a songbird’s warble.

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It does seem to serve exactly the same purpose as a bird’s song. It is designed to attract females and to maintain a territory clear of other males. In birds, songs are often judged by females according to their complexity, and the electric song of Brachyhypopomus is no different. Female Brachyhypopomus are able to detect the electric songs produced by the males thanks to electrosensors on their body. Armed with the information, they are able to judge the songs and the males who have produced them. Complex songs are difficult to produce, and so only the best, strongest males are capable of producing them. This is a suitable signal to woo females as well as to ward off males who might compete for mates. Depending on the complexity of the song, these Amazonian fish can invest between 11 and 22 percent of their body’s energy in one night’s singing. Those able to put more energy into their song are advertising a strong body and tend to be the most successful males. Many other fish communicate in a similar way, using electric signals in social interactions and even to warn of predators. As with any signal, though, it can be hard to keep the messages quiet. Predators have evolved to pick up on the electric signals produced by other animals. By eavesdropping on electrical conversations, predators that can detect electric fields can quickly home in on a ready meal. Sharks have an especially acute electrosense. Not only can they pick up on the electric signals produced by electric fish for communication, but they can even sense the faint electric signals emitted by fish that do not communicate electrically. This allows sharks to hunt even when visibility is poor or their prey is hidden. Flat fish such as flounders that bury themselves in the sand to escape can be found with remarkable accuracy thanks to a shark’s electrosense. At the front of a shark’s head there are a number of visible pores that look like black spots. These are called ampullae of Lorenzi and they are the organs with which sharks detect electric signals. They consist of a jelly-filled cavity, which is open to the water through a pore in the skin. The cavity is filled with hairs that have an electrical charge. The hairs are sensitive enough to detect even the tiniest difference in electrical charge between the pore and the bottom of the cavity. The difference in charge causes the nerves attached to the hairs to trigger a signal to the brain. Ampullae of Lorenzi are so sensitive that they can detect 5/1,000,000,000ths of a volt in one 0.4 inch (1 cm) long ampulla. As the weak electric charges given off by other fish is very weak, sharks can only detect them over quite short distances. Nonetheless, this electrosense is a deadly addition to the shark’s already impressive arsenal of adaptations for hunting. Perhaps the strangest adaptation for detecting electrical signals, though, is found in the duck-billed platypus (Ornithorhynchus anatinus). The duckbilled platypus is a very peculiar creature indeed. It’s bill gives it the look of a bird. It lays eggs like a reptile. Yet it shows many characteristics of a mammal. It is covered in fur, it suckles its young, and it can regulate its own

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body temperature. It is such an odd collection of body parts that the first British scientists to see a specimen brought over from Australia thought it was a hoax, cunningly constructed by a taxidermist. In fact, the duck-billed platypus is a monotreme mammal, a mammal that lays eggs, and is related to spiny anteaters and, more distantly, marsupials. It is the unusual bill of the platypus that is capable of detecting electrical signals. The bill itself is very different from a duck’s beak. It is, in fact, a fleshy protuberance that sits above the mouth. As with sharks, the bill is covered with pores that open out into the water. These pores are surrounded, within the bill, by a ring of electrosensory cells, each attached to a bundle of nerves. Over the bill there are some 40,000 of these electrosensors. The pores have evolved from mucus-producing cells, the membrane of which is sensitive to changes in electrical current. Nerve endings are attached to this membrane and carry signals to the brain. Each pore is less sensitive than a shark’s ampulla, but there are many more of them, allowing the platypus to build up a complete picture of the electrical environment. The electrosensors on the platypus’s bill work in partnership with a similar number of mechano-sensors that are sensitive to touch. By waving its head about as it swims, the platypus is able to build up a three-dimensional picture of where its prey is hiding. It can therefore home in with pinpoint accuracy on its chosen meal, using its flat bill to dig out animals that are hiding within the rocks. Its hunting organ may look odd, but it is certainly effective, allowing it to fulfill a unique niche in the murky waters in which it lives. Borrowing from Nature

The electrosensory abilities of sharks can often be thrown off by the presence of metallic, man-made objects in the water, which produce an electric field. This is perhaps why sharks have been known to attack boats. This happens not out of malice, but through confusion over an object which, evolutionarily speaking, has arrived in the shark’s world only very recently. This, however, has been a source of inspiration to engineers who are using sharks as a model for robots who can detect, locate, and swim toward electrical signals given off by metallic objects. They are still a way off in replicating the threedimensional electrical awareness of sharks, but the aim is to produce aquatic robots that could be used in wreck retrieval or for defense.

FIRE AND SMOKE DETECTORS Throughout nature there have evolved a number of fascinating creatures that have adopted seemingly counterintuitive ways of life. It is hard to believe that any animal would actively seek out the highly dangerous environment of a burning forest, and yet this is exactly what the aptly named fire beetle does. When everything else is fleeing the inferno, these beetles make a beeline

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straight for it, traveling from miles around. (The black fire beetle also goes under the name ‘‘jewel beetle’’ thanks to its shiny, iridescent body. They are also known as buprestid beetles.) So why this curious behavior? The answer lies in the fire beetle’s parental duties. Its offspring, like many other insects, eat wood. To get ahead of the game adult fire beetles choose recently burned and cooled wood that will have been cleared of any other insect offspring that may otherwise compete with the fire A scanning electron micrograph of the beetle larvae for food or attack them bubble-shaped fire-detector cells of the directly. It has also been suggested that Buprestid fire beetle. [Volker Steger / Photo any chemical defenses that trees produce Researchers, Inc.] to prevent attack from insects will have been neutralized by the fire. What’s more, when the adults arrive they find an empty, predator-free environment in which to attract a partner, mate, and lay eggs—activities that are potentially very dangerous with predators nearby. Fire is quite rare in nature, and so when it happens the fire beetles need to react quickly. To do so, they are equipped with two extremely sensitive fire and smoke detectors. The first detector is located on the thorax (chest). Under the pair of middle legs on the fire beetle lie a pair of organs that are very sensitive to infrared, the radiation given out by the extreme heat of fire. When flying, the beetles hold their middle legs high in the air to expose these infrared sensors. With these sensors, the beetles can detect fire from great distances, even up to 30 miles (50 km) away, so far away that we wouldn’t be able to detect any discernible increase in temperature. The infrared sensors are backed up by a second set of finely tuned organs. Insect antennae are known to be very sensitive to touch and taste, and the fire beetle’s are no different. Sensors on the beetles’ antennae are able to detect smoke thanks to very highly sensitive chemoreceptors tuned to sense the chemicals given off by smoking wood. Not only can these antennae detect smoke per se, but they can even detect the smoke of their favorite pine trees. Research over the last five years has revealed that the fire sensors use a completely different method for detecting infrared than had been used by human engineers. Insects, like all animals, are covered with tiny receptors that trigger nerves when they move. This is essentially how we feel a breeze blowing over our skin or the touch of an object. Within the fire beetle’s infrared sensor there are 60 to 70 modified receptors that are each encased by a bubble of the insect’s cuticle, the ‘‘skin’’ of the insect. This bubble absorbs infrared radiation at exactly the right wavelength to coincide with the radiation emitted by a forest fire. So when there is a fire, the bubble absorbs the radiation, warms up,

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expands, and stimulates the receptor inside. By having a sensor on either side of the body, helped by its two antennae, the fire beetle can locate the fire by determining which side of its body is sensing more infrared radiation and home in on the source. The infrared sensor in fire beetles is unique in insects. In fact, the sense is only found in two other groups of animal—the boid snakes and the pit viper snakes. Again, it is a one-off adaptation that has evolved and which enables an organism to exploit a unique niche in nature. The value of being able to home in on a newly created resource gives fire beetles a great competitive advantage over similar insects. Borrowing from Nature

Scientists researching the infrared sensors of fire beetles have developed a low-cost prototype sensor that could be used to help detect forest fires. The sensor is highly sensitive and can automatically monitor large areas of forest and give early fire warnings. Outside of fire-fighting circles, the U.S. Department of Defense is keeping a close eye on this developing research. There is huge military potential for a new generation of supersensitive, miniature, robust, infrared detectors for missiles inspired by the small, heat-seeking beetle. Current infrared sensors need to be cooled to freezing temperatures before they work, which is expensive. The fire-beetle research is leading toward a solution that involves no cooling at all.

INFRARED VISION Most animals are highly dependent on their eyes for sensing the world around them. Many animals see the world as we do. That is, their eyes are attuned to the same frequencies of light as humans. Some animals can see a broader frequency of light, though, and many insects can detect ultraviolet (UV) radiation, which is invisible to humans. This ability is shown most strikingly when bland- The head of the western rattlesnake, looking flowers are lit with UV light and Crotalus viridis, showing the infrared senspicked up with special cameras. What ing pit below the eye. [Larry Miller / Photo Researchers, Inc.] may have looked like a plain white flower before now comes alive with patterns that glow in UV, often directing insects that can see these markings to the nectar within the flower. At the other end of the visible spectrum lies infrared radiation. When any object is warmed (either from internal or external sources) it will give off thermal radiation. This emitted radiation is made up of a range of

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frequencies from the electromagnetic spectrum, but infrared radiation makes up a significant part of it. Perhaps surprisingly, very few animals can detect infrared radiation. All animals give off heat (typically warmblooded animals, but cold-blooded animals too) and to be able to detect that heat signature would give a predator a huge advantage. No amount of clever camouflage would hide you from a hunter with that ability. One group of insects, the buprestids, or fire beetles, can detect infrared radiation, but not to any great resolution. They can detect where there is a fire, but they cannot see discrete shapes in infrared. In fact, there are only two groups of animals that can see infrared with any degree of clarity, and they are both snakes. They are the pit vipers, which include rattlesnakes, and the boids, which include boa constrictors. These snakes have the unique ability to see infrared thanks to an organ not found on any other animal—the pit organ. In a pit viper like the rattlesnake the pit organs are found beneath the eyes and just above the mouth. They are simply two small holes in the snake’s head and can easily be mistaken for nostrils. The pit organ is a very sensitive piece of equipment. It is very similar in structure to vertebrate eyes, as it is made up of layers of modified skin cells stretched out as a membrane above layers of sensitive nerve endings. There are two important layers in this membrane. The lower layer holds 2,000 or so receptors that are sensitive to infrared radiation. These are attached to nerves that send the image falling on the membrane to the brain. The top layer of the membrane helps protect the sensitive cells, in effect acting like an infrared pair of sunglasses. Without it, the sensitive layer would be dazzled by too much infrared radiation falling on it. Instead, this protective layer blocks certain frequencies of radiation and only lets other frequencies pass through. The frequencies that pass through correspond exactly to the frequencies of radiation given off by warm bodies, allowing pit vipers and boids to see the heat given off by its prey. Like the lens of an eye, the pit itself helps focus infrared light on the sensitive membrane within it. To form a clear image on the membrane, however, the pits would have to be very small. This would not let enough infrared radiation through, so the pit viper or boid has to put up with a blurry image falling on the sensitive membrane. Rather than seeing the world of infrared out of focus, though, pit vipers and boids have rather a neat trick to correct the problem. The brain of the snake can correct the image and bring it sharply into focus. Each of the 2,000 receptors will send, via nerves, a ‘‘message’’ to the brain when it detects infrared. Each of these receptors interact, however, depending on the image the snake is looking at. By interpreting these interactions, the snake’s brain can make sense of the blurry image it is looking at and turn it into a clear, crisp image. The ability to see in sharp focus is clearly very important. If a pit viper is hunting in the dark and needs to strike a tiny mouse located more than 3 feet (1 m) away, then there is no margin for error and it needs a precise picture of where its prey is if it is to eat.

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This unique adaptation gives pit vipers and boids tremendous advantages. Thanks to their pit organs they can effectively detect and target prey even at night and in pitch darkness, such as in animal burrows. They may also be used to avoid predators and to locate suitable basking spots to more effectively thermoregulate. Borrowing from Nature

There is great interest in military circles in replicating this infrared detection in snakes. The ability of pit vipers and boids to detect heat with such clarity is a much more elegant and effective solution than current heatdetecting technologies that must be super-cooled for them to work well. Attention is being paid to identifying the molecules that make up the sensitive membrane within the pit organ itself.

JACOBSON’S ORGAN OF SMELL The sense of taste and smell is the ability to detect chemicals in solids, liquids, or gases. Mostly, the organs of taste and smell are the nose and tongue. Each of these organs are covered in nerve cells that are sensitive to chemicals and which send messages to the brain when they are triggered. There is a great deal of variation between organisms in their ability to taste or smell, and this is partly due to the number of chemical receptors in the nose and tongue. A dog, for example, has 220 million odor receptors in its nose, compared with a human’s 5 million. But the story doesn’t end there. Vertebrates have another organ for sensing chemicals that is more sensitive and is linked with detecting certain volatile chemicals such as pheromones given off by other animals. Detecting these odors is important for finding a mate, avoiding predators, and finding prey. This sensitive olfactory organ is called the vomeronasal organ, or Jacobson’s organ. The Jacobson’s organ is found in all vertebrates. In many species, like

The temple viper, Tropidolaemus wagleri, showing the forked tongue used in collaboration with its Jacobson’s organ of smell (vomeronasal organ) to detect prey. [Gregory G. Dimijian / Photo Researchers, Inc.]

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humans, the organ is not well developed and is not particularly sensitive to pheromones and other volatile odors. It has often been referred to as a ‘‘sixth sense’’ organ that humans do not possess, although that is not strictly correct as humans do have some ability to detect pheromones. In all vertebrates, the organ forms when an organism is a fetus, although in many species it will not develop as the fetus grows, and even degrades in the adult. There are some species, though, that do have fully developed Jacobson’s organs. It is particularly well developed in lions. Males will yawn deeply to taste the air to detect sex hormones of receptive females. It is snakes and lizards, though, that have the most fully developed Jacobson’s organ and are very successful because of it. Snakes are remarkable animals with highly sensitive organs for detecting light, heat, and odors, making them formidable hunters. Their ability to smell is an important part of how a snake builds up a picture of its environment. A snake’s tongue is a key tool in its sense of taste and smell. It constantly flicks out of its mouth and waves from side to side, picking up subtle chemical cues in the air. As with humans, however, there are relatively few taste receptors on the snake’s tongue. Experiments have shown that it is not the tongue itself that detects odor; the real work is done by the Jacobson’s organ. As the tongue flicks out, any airborne chemicals are absorbed onto its sticky surface. When it retracts back into the mouth the tongue passes over the Jacobson’s organ and the chemicals are transferred onto it. The Jacobson’s organ is covered in chemoreceptors, specialist nerve cells that can detect volatile chemicals. These nerves pass the information to the brain, which forms a picture of the environment based on the information received. Snakes can build up such a detailed picture of their environment using the Jacobson’s organ that they can hunt by smell alone. Tiger snakes on Canac Island off the coast of Western Australia hunt the eggs and chicks of the island’s resident gulls. To get to them, the snakes have to get past the adults who defend their nests ferociously. The adult gulls will attack the tiger snake, clawing and pecking at its head and eyes, often causing blindness. A blind tiger snake can hunt just as well, though, thanks to its highly attuned sense of smell. But how does it build up such a detailed picture of its surroundings by scent alone? The first clue is with the snake’s forked tongue. The Jacobson’s organ in snakes has two open pits in the roof of the snake’s mouth. When the tongue is withdrawn it must pass over the two pits. As the snake’s tongue is forked, each fork passes over one of the pits. This is hugely significant for the snake. Because it is divided in this way, the Jacobson’s organ can analyze the chemicals landing on each pit independently. This in turn means that it analyses the chemicals stuck to each fork of the tongue independently. Detecting subtle differences in the chemicals on each fork allows the snake to build up a detailed image of its surroundings. Odor from its prey will be stronger on one fork of the tongue than another depending on where it is located relative to the snake. Snakes can detect this difference and home in on their unsuspecting victim.

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This is precisely how a blinded tiger snake can, despite its lack of vision, track a gull chick. Borrowing from Nature

The chemo-receptors of the Jacobson’s organ are different from those found in the nose of mammals. The sensitive nerves lack the tiny hairs found in nasal olfactory nerves, but instead have tiny folds in their surface called microvilli. The nerve cells in the Jacobson’s organ are also made up of a different group of proteins than found in nasal nerve cells. Despite a good understanding of the structure of the Jacobson’s organ and the genes that code for the sensitive nerve cells within it, human engineers have not yet been able to make use of this knowledge to their own end. There is a clear goal, though, to use the knowledge to build robotic smelling machines. On a domestic and commercial scale they could be an important tool for detecting food that is dangerous for human consumption. Powerful tools for detecting chemicals do exist, but these are cumbersome devices restricted to the laboratories of chemists. More portable devices are being developed, but these involve a material such as quartz or silicon that are chemically designed to absorb a certain chemical and trigger a sensor. The drawback of these devices is that they are specific to one particular chemical rather than the range of chemicals that can be detected by the Jacobson’s organ. When it comes to useful and adaptable chemical detectors, human engineers have a long way to go to better those found in a small organ in the mouth of a snakes and lizards.

ODOROUS GENES—THE MAJOR HISTOCOMPATIBILITY COMPLEX Despite the stigma attached to it, body odor plays an important role in social and sexual activities in many social species. Although we often do our best to cover up our own odor, it can even influence human behavior. In particular, there is one set of genes in humans and other animals that has an intriguing role in determining an individual’s unique odor. Over time, animals like humans have evolved to be able to detect and respond to these unique odors. What this means is that humans, in effect, have the ability to smell each others’ genes—an ability that has led to the further evolution of a fascinating set of behaviors. The group of genes that is important in body odor is a group of some 150 genes called the Major Histocompatibility Complex, or MHC for short. Their primary function is to code for proteins that are necessary for a fully functioning immune system. These proteins are responsible for recognizing whether something is part of the body or whether it is a foreign pathogen that must be killed before it can cause disease. There is a second function of the MHC genes, however, and that is to produce body odor. It is unclear exactly why this is the case.

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With any gene, there are different types of that gene, called alleles, that an individual can possess. Different alleles for the genes that make up our blood cells give rise to the different blood types that we have (A, B, AB, or O). The genes of the MHC are no different, but the MHC genes have many (50 to over 100) different alleles that can be picked from to make up the 150 or so genes that make up the MHC gene complex. With so many possible combinations, this leads to a great deal of variation between individuals. There is so much variation that we can think of each human being as having a unique MHC gene complex. This, in turn, means that we each have a unique smell. Having a unique odor may not seem that significant, but the link between genes and a detectable odor is an important one. If you can recognize a particular body odor, you can detect a particular set of genes, or rather the specific alleles that make up the MHC gene complex. Humans can smell genes! This recognition through odor has led to humans evolving a number of important behaviors in response to body odor. It allows newborn babies to recognize parents and other relatives, and it is one of the ways in which we, whether we know it or not, choose sexual partners. From an evolutionary point of view, there are certain advantages to recognizing the genes that another individual possesses. One of the most important is for an individual to recognize others that are closely related. Closely related individuals are more likely to share the same combination of alleles (remember that genes come in different ‘‘flavors’’ called alleles). It follows that it makes sense from an evolutionary point of view to help out close relatives in order to help them to pass on their genes that you are likely to share. If an individual’s genes determine their smell, relatives can detect who has a similar genetic makeup by detecting who has a similar body odor. They may then have evolved to help out individuals who smell the same. In mice, the MHC body odor is a key mechanism for individuals to detect how closely related they are. This is critical for females to recognize their offspring and to nurse the right young. Given that mice can live together in large groups of several families this is an important behavior as identical-looking young could easily get mixed up. Humans show a similar recognition of kin through smell. Studies where individuals are asked to show a preference for the smells on clothing worn by related and non-related infants have shown that fathers can recognize their babies by their smell. This is an important skill to possess. Given that parenting is a huge commitment in time and energy a father needs to be sure that he is giving his care and attention to his own child. With the risk that a male’s partner could have mated with another male it makes sense, from an evolutionary point of view, to be sure that the infant he is caring for is truly his own. Of course, infidelity may not be so high in modern society and detection of relatedness by smell is far from infallible, but even today human males behave differently toward the smell of their own child to that of an unrelated one.

SENSING THE ENVIRONMENT

We might think that human behavior in response to body odor is something that we have, evolutionarily speaking, outgrown. But this is not the case. In choosing sexual partners there are some whom we are very much attracted to and those that we are not. We tend to think that this is purely physical, but beneath all that it comes again down to body odor and its link to our genes. In picking a good mate, this time it is better to pick someone who is not closely related. Inbreeding in humans can result in certain mental and physical deficiencies in the resulting offspring. From an evolutionary point of view, then, it is best to avoid relatives in choosing a mate so that your child is fit and is more likely to pass on your genes in subsequent generations. This is why female humans tend to choose males with an odor representative of a very dissimilar set of MHC genes to their own. Interestingly, though, females prefer the odor of males with very similar MHC genes to their own (i.e., more closely related) when they are taking birth control pills. Females on birth control pills show very similar physiological signs to being pregnant. Pregnancy is a potentially dangerous time for both mother and unborn child. Given that relatives are more likely to help out an individual it makes sense for a female to seek out and form a close bond with males that are related to her. At this time, a female now prefers the odor of family. These preferences have all been shown by the same set of studies whereby individuals are asked to show a preference for the smells on T-shirts that have been worn by related and non-related individuals for several days. In humans, the relationship between body odor and the MHC genes represent a fascinating array of adaptations that are about helping out closely related individuals or mating with unrelated individuals. It demonstrates that humans are still very much a product of evolution and that we still respond to the instincts that have evolved over millions of years. Borrowing from Nature

In humans and other animals there is no particular organ of smell that as evolved specifically to detect MHC-related body odor. Both the nasal and vomeronasal smell receptors are involved. The key for the behaviors discussed above is how the brain interprets the information received by the nose and determines whether it is the smell of a relative or a not. It is this brain activity that robotics experts are trying to recreate in developing ‘‘electronic noses.’’ Progress so far has been to develop sensors that are designed to detect one particular smell (for example, in designing robots used to track down the source of a particular odor). Traditionally, these electronic noses could only detect one smell, but there are new ones available today that can distinguish between several smell compounds. They have been used in the discrimination of a range of different odor sources, such as food, biological sources for medical applications, and for environmental applications.

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MAGNETIC SENSE The ability to build up a picture of the surrounding environment is an essential skill to have. Several senses have evolved, and often a number are combined by an organism to identify what is nearby. Vision, hearing, and smell are the dominant senses, but there are other senses that are employed by organisms that live more specialist lifestyles where these other senses are of less use. One important sense for organisms navigating long distances or underground is the ability to detect the earth’s magnetic field. Several species can detect magnetic fields. Migratory birds use the skill to cover the huge distances from summer breeding grounds to over-wintering sites. Naked mole rats are dependent on detecting the earth’s magnetic field to navigate below ground where there are few, if any, cues to keep a sense of direction. Even ants employ a magnetic compass to return home after foraging. Other animals recorded as having a magnetic sense include bacteria, sharks, honey bees, and homing pigeons. Each of these organisms seems to detect magnetic fields in the same way—by tiny specialized bio-particles called magnetosomes. Magnetosomes are organelles like mitochondria or chloroplasts. They are tiny, organ-like structures that live within cells. Magnetosomes contain even smaller particles of an iron-containing substance called magnetite. Magnetite is produced by the organism from iron, oxygen, and water, and it has the highest electrical conductivity of any solid biological molecule. This is crucial because the earth’s magnetic field is very weak—no other material would be affected by it. In any magnetic field, these magnetite particles line up in the direction of the field. On a very tiny scale it is like iron filings following the magnetic field of a bar magnet. It is this property of magnetite that organisms can exploit to give them a sense of direction. In some bacteria, the shifting of the tiny magnetite particles is sufficient to physically pull the single-celled organism in the direction of the earth’s magnetic field. In multicellular organisms, though, that is simply just not possible. There must be some mechanism that translates the position of the magnetite particles into useful information that is sent to the brain. Magnetosomes, therefore, must be connected to nerves. To trigger the nerve, the magnetosome acts as a switch that can send a signal along the neuron. With many hundreds of magnetosomes, organisms can use the information they provide to orient themselves to the plane of the earth’s magnetic field. There have been several suggestions as to how these magnetic switches work, and no one is quite sure which is right. It seems that different types of switches have evolved in different species. Migratory birds and homing pigeons simply detect the angle they are moving away from the plane of the earth’s magnetic field. Certain bats, on the other hand, can more directly detect the earth’s field and can detect which way is north and which way is south—something that birds are incapable of.

SENSING THE ENVIRONMENT

There is no doubt that a magnetic sense of direction is a useful tool. Marine turtles are able to use it to navigate the oceans of the world to return to the very same beach they hatched at to lay their own eggs—not bad going for an animal that has to cross thousands of miles to get there in an environment bereft of any significant directional markers. Similarly, naked mole rats are able to find their way over the long distances of their lightless underground tunnels thanks to their extraordinary ability to detect the earth’s magnetic field. The naked mole rat is the only animal known to constantly check its position in relation to the earth’s field; migratory birds only check their direction in relation to the magnetic field once a day. Despite these remarkable feats of navigation, the earth’s magnetic field is not a very reliable thing to detect. It varies in strength and even direction over time. The earth’s poles slightly shift their geographic location every year and have even reversed entirely 10 times in the last three million years! Thus, even mole rats and marine turtles have to rely on other cues to find their way around. It was thought that all animals detected magnetic fields using magnetosomes, but recent research has given evidence to suggest that some species of bird can physically see the lines of the earth’s magnetic field. The ability to see a magnetic field seems to come from a group of light-sensitive proteins called cryptochromes, which have been found in the eyes of some birds. Normally, these molecules are involved with animal and plant body clocks. In plants, they also regulate plant growth. Plants are very responsive to magnetic fields. Seeds do not germinate in space, yet when they are grown in strong magnetic fields they grow at a much faster rate than under normal conditions. This suggests the link between cryptochromes and magnetism. Cryptochromes have been detected in the retinal nerves of migratory birds. These nerve cells are active at dusk in certain