Bats Sing, Mice Giggle E-Book

Jagmeet Kanwal

• Herausgeber: Icon Books
• Kategorie: Lebensstil
• Sprache: Englisch

Seitenzahl: 338

Veröffentlichungsjahr: 2010

Beschreibung

"Bats Sing, Mice Giggle" tracks many years of research by hundreds of scientists that reveals how wild animals, as well as pets, have inner, secret lives of which until recently - although many animal lovers will have instinctively believed it - we have had little proof. The authors show how animal 'friends' stay in touch, and how they warn and help each other in times of danger; how some animals problem-solve as or in some instances even more effectively than humans - and how they regulate, create, and entertain themselves and others. They show how animals express grief and reverence in ways we never thought possible. From the sleep patterns of some owls, birds and horses, as well as porpoises, who go to sleep in only one half of their brains at a time; to how schools of electric fish give off complex signals of one frequency to communicate with their mates and another frequency to locate their prey, and how Polar bears tune into quantum 'radio stations' to sense prey as far away as ten miles and under the snow, "Bats Sing, Mice Giggle" provides an unparalleled insight into animals' secret lives.

Leseprobe

BATS SING, MICE GIGGLE

The Surprising Science of Animal’s Inner Lives

KAREN SHANOR, Ph.D. JAGMEET KANWAL, Ph.D.

ICON BOOKS

Published in the UK in 2010 by Icon Books Ltd, Omnibus Business Centre, 39–41 North Road, London N7 9DP email: [email protected]

This electronic edition published in 2010 by Icon Books

ISBN: 978-1-84831-226-5 (ePub format)

Printed edition (ISBN: 978-1-84831-197-8)

Text copyright © 2009, 2010 Karen Shanor and Jagmeet Kanwal The authors have asserted their moral rights.

No part of this book may be reproduced in any form, or by any means, without prior permission in writing from the publisher.

Typeset by Marie Doherty

Contents

Acknowledgments

Introduction

Part I:Sensing

1 A Supercharged World

2 Good Vibrations

3 Sounds for Tracking and Talking

4 Tasting and Touching

Part II:Surviving

5 Alarming Behavior and Survival Strategies

6 From Frogcicles to Dreamstates

7 Animal Marathons by Land and Sea

Part III:Socializing

8 Wit, Wiles and Good Fun

9 Eavesdropping and Deception

10 Rhythm, Song and Dance

11 Flirting, Courting and Coupling

Epilogue: “Human Nature” Reconsidered

Notes, References and Further Reading

Karen Shanor, Ph.D. is a neuropsychologist, a former White House consultant and an advisory member of Discovery Channel Global Education. At Stanford University, Dr. Shanor researched how rats learn, and how cats dream. Her work at NASA’s Life Sciences department included animal research on memory and information theory, and she has taught with Dr. Karl Pribram at Georgetown University since 1998. As a Peace Corps science teacher in Somalia, she was a consultant for a wildlife conservatory. A frequent lecturer at the Smithsonian Institution, Dr. Shanor also hosted an NBC radio program for five years and appears frequently on Larry King Live, CBS Nightly News, Dateline, The Today Show and Oprah, and is a regular contributor to CNN.

Jagmeet Kanwal, Ph.D. is Associate Professor in the Department of Physiology and Biophysics and the Department of Psychology at Georgetown University. He is also External Professor at the Krasnow Institute for Advanced Study in Fairfax, Virginia. Dr. Kanwal is an internationally recognized neuroethologist who was the first to perform magnetic resonance imaging in awake animals. He is an expert on cortical mechanisms for the perception of complex sounds. Dr. Kanwal discovered a left-brain dominance for species-specific calls in bats. His laboratory is engaged in cracking the code for the neural representation of social calls within and between the two cerebral hemispheres and in the amygdala. Dr. Kanwal’s early contributions on the comparative organization of chemosensory systems include the discovery of taste centers in the forebrain of fish. He uses interdisciplinary approaches to understand the functional organization of the brain from the viewpoint of behavior. He is also an ardent birdwatcher and keen nature photographer.

Both authors live in the Washington, DC area.

Acknowledgments

First and foremost, we are grateful to Icon editor Simon Flynn and his extraordinary group, especially Nick Sidwell, Andrew Furlow, Duncan Heath and Najma Finlay, who labored with patience night and day across multiple time zones to produce a book with cutting-edge science. Their understanding and skill in coordinating the tasks that needed to be performed at various stages of the production were outstanding. We thank our agents, Muriel Nellis and especially Jane Roberts of the Literary and Creative Artists, Inc. for their expertise and constant support in this endeavor. Jane’s immense enthusiasm and literary wisdom kept us feeling optimistic about completing the book during difficult times. She was always there for us when we needed a quick second opinion or a reader’s perspective. On the academic side, we are deeply indebted to Karl Pribram, who at the age of 91 continues to be a great source of inspiration. His engaging lectures and stimulating discussions at Georgetown University provided an intellectual forum for many thoughtful interactions between the authors. We are also thankful to Georgetown Professor Patrick Heelan for his guidance on the quantum physics and seismological concepts alluded to in this book. John Caprio, Thomas Finger and Nobuo Suga also have been scientific mentors whose interesting research has contributed to some of the findings reported in this book.

Marine biologist Robert Woollacott of Cambridge, Massachusetts, Ken Ferebee of the US National Park Service, Stuart Brown, author of the book Play, mathematician James Shanor, theoretical physicist Sarbmeet Kanwal, geologist Gordy Shanor, and David Wood of the Sidwell Friends science program all shared their professional expertise that helped to improve earlier drafts of the manuscript. We also thank Ian Hay Falconer and Constance Culler Falconer for their literary research, and Daniel Perry and Maxinder Kanwal for their critical input to the manuscript. Walt Ellison provided his invaluable computer expertise to keep our communication lines buzzing across the two continents.

We also want to acknowledge Srimati Kamala, Nancy Bugos, Judith Millon, Goldia and John Hodgdon, John Wusteman, Muthiah Veerappan, Niranjan M. Shah, Madhav Singh Parihar, Vera Andreeva, Gregory and Laurie Wood, and Jessleen and Mini Kanwal for their interest and constant support that helped in getting this book finished.

To all of our pets and the wildlife around us that inspired this book. And to our parents who encouraged us to indulge in and pursue our interests.

Introduction

Where Are All the Dead Animals, Sri Lanka AsksWildlife officials are stunned—the worst tsunami in memory has killed around 22,000 people along the Indian Ocean island’s coast, but they can’t find any dead animals. Giant waves washed floodwaters up to 2 miles inland at Yala National Park in the ravaged southeast, Sri Lanka’s biggest wildlife reserve and home to hundreds of wild elephants and several leopards. “The strange thing is we haven’t recorded any dead animals,” H.D. Ratnayake, deputy director of the National Wildlife Department, told Reuters Wednesday. “No elephants are dead, not even a dead hare or rabbit,” he added. “I think animals can sense disaster.”

Reuters, Sri Lanka (December 29, 2004)

What did the animals know that humans didn’t? What alarms were sent out that humans didn’t “hear”? As the Industrial Revolution and the development of urban centers moved us from nature and the land to the promise of science and technology, we detached ourselves from the understanding of the animal world. Twentieth-century scientific practices placed humans on a pedestal of superiority, further separating us from our natural roots and surroundings. Now, science is beginning to take us back to nature, providing a window into the minds of other species.

This book represents the coming together of two individuals with quite different backgrounds—a neuropsychologist and a neuroethologist—but with a common interest in the wonderful and secret lives of animals. Although the essence of this book was brewing in our psyches for most of our lives, it came as a compelling and timely surprise. Here, we provide a unique perspective on how to better understand the animal world and in so doing gain a better understanding of our own world—the inner world of our minds and the outer world that we share with all other creatures as our only home.

Bats sing, mice giggle

In a kapok tree growing in the tropical heat of a forest in Peru hangs a small male bat that has tiny sacs under each of his wings. Nine females surround him, each carrying a strong smell of a secretion that exudes from the sacs. The sacwinged bat feeds on tiny insects and interrupts its solitary existence to engage in reproductive activities. Intriguingly, pups of the species were discovered recently to babble. Four- to eight-week-old bat pups make long strings of barks, chatters and screeches that represent jumbled-up adult-like calls. Scientists now know that bats, like some primates and birds, babble as babies; and the ability to babble can even be accompanied by giggling. Not only do human infants babble and giggle as they experience feelings and try out their audiovocal abilities, so do babies of other species. New and sophisticated technology is taking our understanding into the secret world of animals where we can detect first-hand bats that do indeed sing and mice that really do giggle.

This book will take readers on a remarkable journey, during which they will discover that many of the behavioral and mental traits that have been considered to be uniquely human are in fact shared with other species. We’ll show how animal “friends” keep in touch, and how they warn and help each other in times of danger. We’ll explain how some animals problem-solve, how they build and create, and how they entertain themselves and others. Some animals have a sense of humor; for example, parrots have been known to tell jokes of their own composition. We’ll show too how parents of many different species, including bats, hug and cradle their young. And we’ll also show how animals express grief and reverence in ways we never thought possible.

What did the animals know?

We set the stage in the first two chapters with cutting-edge findings that show how animal life depends on the strong electromagnetic fields circumnavigating our planet, and the weak electric fields and even weaker electrostatic radiations emanating from animals’ bodies, as well as the vibrations they produce and detect.

All animals live in a milieu of electromagnetic waves and mechanical vibrations which they use for many of their transactions. For example, schools of electric fish generate complex electric fields and have sensors that can detect tiny distortions in these fields. Electric fish can use these electro-sensors to find food, and to determine the precise location of prey or other electric fish when socializing. To solve the mysteries of how animals negotiate their surroundings and use their brains and nervous systems, scientists are delving more thoroughly into the realm of vibratory signals and even possible quantum-level occurrences that direct and surround all life.

Our sensory experiences define our lives and separate our world from that of our fellow creatures. Almost every species occupies a unique sensory niche in which it can find food and compete with others occupying the same niche. Bumblebees find flowers so efficiently because they have the fastest color vision of any animal—five times faster than that of humans. Some species, like electric fish, become specialists in trying to overcome the competition. Others, like many species of bats, use sound pulses and their echoes to find food and probe their environment, in ways similar to those adopted by much larger creatures such as dolphins and whales in the deep and dark oceans. Yet others, like catfish, may retain an ability to live in diverse habitats as one of their senses becomes exquisitely developed.

While humans may never be able to experience the sensory niche of another species; by understanding more about the lives of different animals, we’re able to learn useful tricks to aid our own survival. We can’t, for example, imagine what a walk in the woods smells and feels like to a dog, which has high olfactory acuity. However, canines have long been used by humans to find missing people or sniff out illegal substances. Recent research has even shown that dogs can detect breast or lung cancer in a person by smelling that person’s breath. Cancerous cells produce different metabolic waste products than normal cells, and dogs can smell that difference.

Exploring these sensory capacities further in Part II, we focus on the survival strategies that animals adopt when confronted with extreme environmental conditions, or with a threat from predators or their own kind. Despite their different sensory experiences, all animals, humans included, are endowed with a deep desire to survive. We will investigate alarm behaviors in a diverse range of species, from bees to cats to sharks, connecting the dots to attempt to answer the provocative question we first posed: What do animals know that humans don’t know or heed when danger strikes?

Natural disasters are among the more dramatic threats that animals have to face. Day-to-day existence brings with it its own regular set of adverse conditions, from the fluxing and waning of food supplies to the annual cycles of seasonal weather. Less immediate than an earthquake, the winter months when life is at its lowest ebb may present the sternest of challenges to even the toughest of animals.

In Chapter 6, therefore, we discover how some animals employ hibernation behaviors, and that these can be adapted to combat extremes of heat as estivation. From the brown bear that gives birth during hibernation to the Antarctic cod that lives in the freezing waters of the Southern Ocean, we will look at what happens to animals physiologically and psychologically in these conditions. Not only that, some animals go into a light hibernative state every day to conserve energy—hummingbirds, for example, may conk out for several hours. Yet despite this, hibernation is very different from sleep and we’ll study the odd way in which they may work together. By focusing on sleep processes in the animal world, we’ll come across the many different approaches to it, including animals that go to sleep with only one half of their brains at a time. And as we draw together the latest scientific research on sleep and hibernation, we will endeavor to answer the question of not only whether animals dream, but what might be the nature of their dreams.

Chapter 7 discusses the seasonal migration of animals as they move to warmer climates or fresh food sources. Some mass journeys, such as those made every year by thousands of wildebeest in the Serengeti, are breathtaking displays of beauty and power. Other trips are just as breathtaking, but for wholly different reasons. The distances and methods involved in the long flights of butterflies and birds, for instance, have stunned scientists. The tiny brain of the monarch butterfly can calculate distances and directions that would confound the most skilled airline pilot. And migrating birds in their hundreds fly non-stop for days, even weeks at a time, yet know to stop en masse if one of the flock is sick or exhausted. Such migrating and swarming behaviors in animals have become especially important research topics over the last few years, now that we have the technology and mathematical theories to help explain how animals know when and how to flock together and where to travel.

In the last part of the book, we explore communities of animals and the emotions and desires that they produce and that we all carry with us. Our connection with all other animals really drives home here. We not only inherit these basic desires, but all of our emotional expressions stem from them—to laugh, to play, to have sex, to reproduce, to deceive, even to kill: all derive from desires existing within animal minds.

New research is discovering that it’s the deep limbic parts of our brain that secretly drive all of our thinking and our so-called “rationality”—this is the center of our emotions, of our quick evaluation and reaction against danger, of our memory; it regulates our system without the need for constant conscious analysis. The limbic brain is the primitive brain; it’s present in all vertebrate species and probably has analogous regions in most invertebrate brains that we haven’t yet discovered or even started to look for.

It’s our large neocortex, though, comprising 30 percent of the human brain and responsible for high-level thinking, as well as cognition and speech, that humans have contended sets us apart from other animals. However, recent studies in the wilds of New Guinea of the long-beaked echidna, one of the oddest and most enigmatic members of the animal world, throw a startling new light on this assumption. Belonging to the monotremes—a sub-set of mammals that lay leathery, reptile-like eggs, and which also includes the duck-billed platypus—this rarely observed creature has an electroreceptive, hairless tubular beak, webbed feet, spiny skin and, in the male’s case, a bizarre four-headed penis. Most interesting though, is that the neocortex of this peculiar animal is proportionally larger than that of a human, accounting for a remarkable 50 percent of the brain. What is this spiny monotreme, the size of a terrier, doing with such a large part of its brain devoted to what is for humans the seat of analysis, language and consciousness? What can we learn from the echidna?

As we study the neurobiology, physiology, behavior and individual experiences of animals, we have to be careful not to anthropomorphize. The fact that ants have cemeteries for their dead doesn’t mean that they mourn in the same way we do. We can’t even say that they mourn at all. Yet instinctively we also dispose of our dead and it’s important to get to the truth of our behavior. More and more studies are helping us do this. For example, a recent experiment has shown that dogs do indeed have a sense of fairness. They get jealous if they feel another dog is being treated better, and will withdraw affection or stop being cooperative. Researchers have provided examples of how animals protect each other and offer consolation to those that are upset. A variety of birds and mammals show empathy and altruistic behavior. Grief-stricken apes are known to have carried their dead babies around with them for days. Elephants have been filmed caring for the sick, and slowly walking for hours and hours around the dead body of a member of the herd. There are countless stories of pets mourning the loss of a loved one. It’s only recently that our technology has advanced and our research has become comprehensive enough to confirm some of the anecdotal evidence that has been around for so long. Many of the intuitions that humans have always had about our links to other animals are turning out to be correct.

* * *

As long as each generation continues to live closely with other animals and they with us, we will retain a modicum of sanity. Our pets and the wild birds around our houses accommodate to our ways and attempt to train us to theirs from the day we are born. Such interaction and involvement not only create awareness and modify behaviors, but have been found to change genetic information hidden within the cells of us all. As we learn deeper truths about the life around us, perhaps we will also be able to better appreciate our own being and relationships, and even expand our capacity for affection, solace, hope and love.

Part ISensing

1A Supercharged World

Not only is the universe stranger than we imagine, it is stranger than we can imagine.

Sir Arthur Eddington (1882–1944)

Electric fish jam the frequencies of rapidly changing electric fields generated by their rivals. Birds “see” the magnetic lines of the earth. Under the water, on land, and in the air there are electric and magnetic fields that affect all life. While most humans have little conscious awareness of these electromagnetic influences, medical science knows better. Physicians evaluate our health by measuring our heart and brain waves, which are also electric in nature (like those of electric fish, human brain waves change more than 100 times a second, as demonstrated by Stanford University’s Karl Pribram). Modern medicine also uses the ability of our atoms to align to magnetic fields for MRI (magnetic resonance imaging) scans. This chapter explores how various types of animals sense and use electricity and magnetic fields to communicate with each other, to get around, and to protect themselves from danger.

The world that we, as humans, live in every day seems to us an immutable expression of how things are. We can draw it in through our senses, a world of reassuring “objective” facts, full of color and music, aromas, tastes and tactile surfaces. These five key senses inform our understanding of how and what the world actually is. But think a little further about the myriad of other species that inhabit the land and sea and air and occupy the same world as we do. Our assumptions of the physical space around us become difficult to translate into the experience of other creatures.

The world as we perceive and experience it depends entirely on the range of stimuli that our senses can detect and to what we care to pay attention. For example, visible light—to which the photo-pigments in our eyes are sensitive—is a very thin slice of the vast spectrum of electromagnetic radiation (from shortwaves and microwaves to X-rays and gamma rays) present in our environment.

Rather than there being one objective world that is home to all living organisms, there are multiple subjective worlds. Nature believes in pluralistic thinking in an evolutionary sense, where different species occupy their own cognitive niches, each replete with unique angles and perspectives. Every species has its own way of perceiving and interacting with its environment based on its size, its surroundings, its sensory capacities, and how it behaves and what it remembers. The underwater experience of fish or the airborne moments of those birds and bats that fly can only be imagined by those of us who live on land. A bird’s-eye view is one that we humans often envy, but we can’t begin to imagine what the world looks like to a mantis shrimp that has many complex eyes that rotate in separate directions and are sensitive to the wide spectrum of wavelengths ranging from ultraviolet to infrared.

The senses that we are familiar with as humans are just the tip of a fantastic and fascinating sensory iceberg. It’s below the surface where we enter a realm of senses more potent than those of most humans. There are electro receptive animals for whom electric fields inform everything about the world in which they live, and there are other electrogenerative species which can not only pick up on such fields but generate their own electric charges. Completely beyond what we humans may consider the world to be, these electric fields influence everything from the way animals build and navigate with their own subjective “maps,” to the way they hunt and the way they interact with one another.

Scientists readily agree that many different animals, including millions of migrating birds, use the Earth’s magnetic fields for navigation and migration. Studies of homing pigeons suggest that they do genuinely “home in” using their own bandwidth of information about spatial patterns in the Earth’s magnetic field.

However, we’re only just beginning to understand the various ways in which this may occur. While we have studied groups of “electric fish” for decades, we’re still trying to figure out exactly how this sensory system functions and how it achieves the amazing feat of extracting information from transient distortions in electrical fields. Yet the pictures of these animal worlds being fed back from the forefront of scientific inquiry are remarkable. Sharks, skates and rays receive electrical information about the position of their prey, the drift of ocean currents and their magnetic compass heading. In training experiments, stringrays show the ability to orient themselves relative to uniform electric fields similar to those produced by ocean currents. Researchers have also discovered complex patterns of electrical discharges generated by electric fish used for “flirting” with each other and making mating choices.*

Water is well suited for electrolocation in animals. While it takes a very high voltage for electricity to cross the air barrier, water, especially brackish and sea water, is highly conductive. Consequently, much electrical field research centers on animals that live in the water or spend a great deal of time in wet areas, because it’s much easier for our present research instruments to pick up their electric signals.

In a way, therefore, humans and all other living things are permanently plugged in to a vast electromagnetic, biologically viable resource. Only certain animals, however, are able to use the electromagnetic fields directly to shape and define the worlds in which they live. While scientists at several U.S. universities have taught human subjects to focus their thoughts to affect mechanical devices like computer cursors and prosthetics for paralyzed individuals and those who have lost limbs—and it has been projected that in the not-so-far-off future, humans will be able to guide

* For example, Philine Feulner at the University of Sheffield in the U.K. and her colleagues found that female electric fish of the Campylomormyrus compressirostris species choose the males of their own species over males from very closely related species on the basis of the different electric signals they send out.

the flight of spacecraft with their minds—birds already travel thousands of miles without any mechanical devices or navigational equipment, and so do many seafaring creatures. These animals use electromagnetic information and direction to help them move, communicate and navigate over very long distances. Before we move on to looking at how they carry out this remarkable feat, we will start out by meeting two of the more fascinating electroreceptive animals around.

Animals that detect electric fields

Two unusual mammals are known to detect electrical signals. One looks like a furry beaver with reptilian-like flipper feet and huge flat bill—hence the name duck-billed platypus. It usually hunts after dark in murky river waters for crayfish and other crustacean delicacies, as well as worms or insect larvae. As the platypus sweeps its head from side to side, its huge snout, loaded with more than 40,000 electrosensors, sends signals to the brain to create a map of the electrical fields of its prey. The other electroreceptive mammal—the echidna—is peculiar-looking, with club-like feet and what resemble feathery porcupine quills stylishly covering much of its body. Its pointed beak, which comes in sizes long or short, is used for tracking down earthworms or other soil- or swamp-dwelling prey. The long-beaked echidna, mentioned in the Introduction, lives in wet tropical jungles and has around 2,000 electroreceptors in its beak that pick up the electrical signals of prey. The smaller, short-beaked echidna that lives in drier regions has more like 400 receptors at the tip of its beak. It usually waits until it rains before finding its meal—electrically, of course, in what is called electrolocation.

Whether searching for a good meal or a good mate, many animals that sense electricity around them use ampullary receptors. An ampulla is a pore in the skin that leads to a canal filled with gel. For example, the dark spots on a shark’s snout are often sense organs called ampullae of Lorenzini, after the Italian zoologist Stefano Lorenzini who described them in 1678; it wasn’t until the 20th Century, however, that scientists understood their function. And a shark can sense electrical fields such as those given off by the muscle movements of another fish, because the ampullae read the difference in voltage between sensory cells at the pore and the base of the canal—the system basically functions as a voltmeter.

Fish that generate electric fields

For many of us, the only fish we associate with electricity are electric eels, and we assume that they would zap anybody who gets too close. However, electric eels are just one of many different species of fish capable of electrogenesis. The various types of these live-wire water-dwellers fall into two distinct categories—weakly electric and strongly electric fish. In both groups, electric transmission, as in visual systems, is nearly instantaneous and is little affected by “noise” in a system, but doesn’t go far and thus is effective only over short distances. Like chemical and sound communication, an electric signal also can pass around objects.

Within the two classes, some species are known as “pulse fish” because their electric organ discharges, or EODs as scientists like to call them, are sent out at a low and irregular repetition rate (at intervals several to many times the duration of a single pulse). Other fish species are “wave fish” because the inter-pulse intervals are brief and regular (equal to or little longer than discharge duration), reaching regularities higher than any other known biological rhythm. Walter Heiligenberg, working at the Scripps Institution of Oceanography, discovered that electric fish can shift the frequency of the EOD if two fish producing the same frequency approach each other. Since the purpose of this shift is to avoid jamming of the two signals and loss of information, he called it the jamming avoidance response or the JAR.

Behavioral uses of EODs and electroreception by electric fish include: disorienting and confusing potential prey and potential predators, or finding prey (even if buried under sand); determining location (electrolocation and electroorientation) by echo or by interaction with the Earth’s magnetic or electric field; social communication (including reproductive behavior); and sensing of weather conditions, time of day, earthquakes and distant lightning. And, as we have seen, even a system of avoiding being jammed by each other’s signal has evolved in several species of fish.

Strongly electric fish produce much higher voltage pulses than their weakly electric cousins. The strongly electric fish category includes not only the electric eel, but the electric catfish and the electric or torpedo ray. Fish with electrogenesis usually produce electric charges through electrocytes or electroplaques—typically flat, disc-like modified muscle or nerve cells all stacked together. In marine fish, these cells are connected like batteries in a parallel circuit, whereas in freshwater fish they are connected like batteries in series. These latter fish are capable of producing discharges of higher voltage, because fresh water doesn’t conduct electricity as well as salt water. Each cell in the battery can produce nearly 0.15 volts by pumping out positive sodium and potassium ions. The electric eel has around 5,000 or 6,000 electroplaques in its abdomen, allowing it to generate shocks as strong as 600 volts to stun its prey.* It also can lower its voltage pulses to around 10 volts to navigate and to detect prey. As a result of its electrogenerative abilities, the electric eel is found only in freshwater habitats, as salt water can have the unhelpful effect of causing the fish to naturally short-circuit. The electric ray, which has no such problem, has a pair of special kidney-shaped organs at the base of the pectoral fins that generate and store electricity and send out charges from 8 to 220 volts to electrocute prey or to stun a possible predator. Electric catfish, common in African fresh waters, generate their electrical discharges from their skin rather than from electric organs that consist of individual electroplacques.

When a foraging torpedo ray detects prey it swims forward and upward, exposing its ventral surface toward the fish while emitting low-frequency voltage pulses. The currents passing through the victim’s body excite its nerves and muscles, stunning it and immobilizing it, whereupon the torpedo descends over it and consumes it while continuing to emit pulses. Large Atlantic torpedo rays can generate enough power to produce a shock of up to 220 volts; that’s enough voltage to run a household appliance, such as a

* Technically the electric eel is not an eel at all, but a type of knifefish.

blender or a clothes dryer. Of course, animals can’t deliver such voltages in a sustained way, as the purpose is simply to stun the prey. So it’s more like carrying a stun gun inside your body. Smaller rays, like the lesser electric ray (Narcine brasiliensis) can only muster a shock of about 37 volts because their prey are smaller in size.

Weakly electric fish, like the elephantnose fish, use their electrogenerative ability either to navigate, or locate prey, or to communicate. Instead of the electroplaques as in the electric eel, they have electric organs consisting of columns of electrocytes that generate relatively feeble electric fields. This type of active electrolocation relies on the ability of the elephantnose fish and other species that use it to detect any distortions in an electric field of less than 1 volt. Although it’s not well understood how the brain extracts all of the sensory information for active electroreception, the sensors (also called electroreceptors) in the skin are sensitive to the rate of change of voltage across their cell membrane. In contrast, sharks and rays as well as most species of catfish use passive electroreception where the animal senses the weak bioelectric fields generated by other animals. Sharks are the most electrically sensitive animals known; they respond to non-alternating current (DC) fields as low as 5 nanovolts per centimeter and can use this ability to detect a small fish buried in the sand.

Sometimes elephantnose fish deliberately get into “jamming”—not the jazz type, but similar in a way to when the musical session becomes a showy game of one-upmanship. Instead of competing or having fun with music, the electric fish try to jam the electrical signals given off by another fish. Sometimes they jam to get a mate they want, sometimes to get a meal. Ghost knifefish found in the rivers and streams of Central and South America also are known to jam the frequencies of other fish. Unlike other electric fish, in which the electric organs are derived from modified muscle tissue, in adult South American knifefish, the electric organ is a modification of the axons of spinal neurons or nerve cells. Interestingly, this transformation takes place only during the adult stage; as embryos they must rely on muscle-derived electric organs, which is therefore considered the more primitive system. Among groups of knifefish, the strongest adult male lets his presence be known by sending out the highest rate of electrical discharge—900 pulses per second. The rest of the males have lower frequency rates and are careful not to interfere with the frequencies of others. However, when a rival wants to challenge the dominant male, the challenger also will generate pulses at a frequency of 900 hertz to jam the signal. In response, the dominant male might zap his rival with a higher frequency charge of up to 1,000 hertz. But if the rival male continues to challenge and jam, the two males will probably fight it out physically, sometimes ending up locked jaw-to-jaw for an entire night. So jammers beware.

Magnetic fields

While electric pulses and waves can be used as a tool for finding your way around a localized environment, for much longer journeys it’s the ability to navigate using the Earth’s magnetic field that really counts. Unlike electric fields, magnetic fields are powerful force fields that are present globally. The Earth’s magnetic field is a geomagnetic dipole field where the poles lie in the vicinity of the geographic poles. Movement of the Earth’s fluid, outer iron core generates the magnetic fields, although details of this theory remain unclear. Despite reversals in polarity after several thousand years, as well as temporal and regional variations on a smaller scale, these fields can be considered to be fairly stable.

Migrating birds use these magnetic cues to fly hundreds or even thousands of miles on a round trip every year to escape harsh conditions and find food and safety. The pink-footed goose, for example, makes its way each winter from Iceland to the more temperate climate of Britain. Even more impressively, the bar-tailed godwit undertakes an incredible annual journey all the way from Alaska to New Zealand. Covering 7,258 miles (11,680 km), it’s the longest non-stop flight of any bird. Many more bird species also make impressive migrations.*

Birds are not the only animals to navigate across huge distances in this manner. Insects like monarch butterflies, fish, turtles and mammals of the sea travel with amazing precision. Many animals are able to detect the Earth’s magnetic field and use it for navigation and other reasons—such as the mysterious reason behind why cows have been found often to stand in north–south positions or why termites align their giant mounds north to south. Several carefully constructed experiments confirm that many animal species have internal compasses. Even the wispy monarch butterfly with its tiny brain smaller than the head of a pin has its

* The sheer exhilaration and peril of vast bird migrations is captured on film in Jacques Perrin’s magnificent Winged Migration (2001).

own compass and timekeeper. In addition to the north– south compass reckoning ability, certain migratory species are thought to also use the geographic differences in the strength of the Earth’s magnetic field as navigational guides. Birds may even be able to see these fields.

As humans, we are beginning to see more with our instruments. And an international group of scientists, for example, has been working since 2003 on developing and updating a geomagnetic map of the Earth. The Earth’s magnetic field is not uniform; it has many irregularities, such as the large one in the region of the Bermuda triangle where a number of ships and aircraft have gone missing.

Bird species such as homing pigeons and a number of types of turtles have been studied extensively to determine how they detect and orient themselves to these magnetic fields. Loggerheads and green sea turtles are favored subjects. These sea turtles swim across an area of hundreds of miles and find their ways to the same nesting and feeding sites year after year. Genetic analyses have confirmed that the adults of at least some populations do indeed return to their birthplace for nesting after first migrating to distant oceanic areas. How do they do it? Ocean waves are useful guides in shallow ocean waters, especially for hatchlings that have no previous experiences. Although these cues disappear in deeper waters as the waves become refractory, the sea turtles are able to maintain their orientation. Experiments have shown that turtles can differentiate between varying magnetic field intensities found along their migratory route; such an ability is a prerequisite for using a magnetic map. And although bats fly at night, they often use the Sun (or at least its last glow) for direction. In 2010, German researchers at the Max Planck Institute in Germany confirmed that while bats use echolocation for short trips, for their longer journeys they depend on the Earth’s magnetic fields for orientation—utilizing sunlight to calibrate this compass.

Research into finding the sensory mechanisms that enable receptivity to magnetic fields in more complex organisms has been slow because of the difficulties in designing clean experiments that generate reliable data. The phenomenon of magnetoreception, however, is now well established, and this ability is known to be present in several groups of invertebrates (molluscs, annelids and arthropods) and vertebrate species, including humans. A type of marine slug called Tritonia, a species known to use magnetic fields for orientation, holds one of the proofs of complex magnetoreception. Kenneth Lohmann and his colleagues at the University of North Carolina, Chapel Hill, recorded tiny electrical impulses in a neuron known as PE5, which subsequently increased its firing rate (the frequency at which impulses are generated) every time the scientists rotated the magnetic field by 20°, 60° and 90°. This provided clear evidence that a magnetic cue was being transduced or transformed into an electrical signal inside the nervous system. This meant that the information was now available for the nervous system and could be used to guide behavior, although how exactly this is brought about and where the sensor is remain unknown.

While scientists aren’t sure how animals perceive them, they think there are at least three different ways in which animals can orient themselves to the Earth’s magnetic fields. And scientists agree that different animals may use more than one method.

As children, many of us have played with magnets and small iron filings. In fact, the way those filings always turn around toward the magnet is similar to one way scientists think some animals respond to magnetic fields. One hypothesis is that something very similar to those little iron filings is present in the bodies of animals. Crystals of magnetite, a magnetic form of iron oxide, have been found in a number of species—including near the beak of pigeons, in the head of trout, and in the abdomen of honey bees. Clusters of these ferrimagnetic minerals seem to be affected by the intensity of the Earth’s magnetic field. This internal compass in its simplest form is seen in magnetotactic bacteria that have small particles of magnetite arranged in chains. They line up according to the Earth’s magnetic field, allowing them to be propelled northward in the Northern Hemisphere. Interestingly, bacteria found in the Southern Hemisphere have their polarity reversed and move in a southerly direction.

Scientists’ search for magnetite in animals is difficult because most, if not all, animals have iron distributed throughout their bodily tissues. This is certainly true for humans. In fact, iron is among the most common metals found in our organs, and in some degenerative processes such as hemochromatosis (where iron doesn’t get broken down and absorbed into the body properly), Parkinson’s disease and blood coagulation, iron can accumulate.

The second way in which scientists think animals may sense magnetic fields is called the “radical pair model.” This model involves certain biochemical reactions, and