Excerpted from An Immense World by Ed Yong

Introduction

The Only True Voyage

IMAGINE AN ELEPHANT IN A room. This elephant is not the proverbial weighty issue but an actual weighty mammal. Imagine the room is spacious enough to accommodate it; make it a school gym. Now imagine a mouse has scurried in, too. A robin hops alongside it. An owl perches on an overhead beam. A bat hangs upside down from the ceiling. A rattlesnake slithers along the floor. A spider has spun a web in a corner. A mosquito buzzes through the air. A bumblebee sits upon a potted sunflower. Finally, in the midst of this increasingly crowded hypothetical space, add a human. Let’s call her Rebecca. She’s sighted, curious, and (thankfully) fond of animals. Don’t worry about how she got herself into this mess. Never mind what all these animals are doing in a gym. Consider, instead, how Rebecca and the rest of this imaginary menagerie might perceive one another.

The elephant raises its trunk like a periscope, the rattlesnake flicks out its tongue, and the mosquito cuts through the air with its antennae. All three are smelling the space around them, taking in the floating scents. The elephant sniffs nothing of note. The rattlesnake detects the trail of the mouse, and coils its body in ambush. The mosquito smells the alluring carbon dioxide on Rebecca’s breath and the aroma of her skin. It lands on her arm, ready for a meal, but before it can bite, she swats it away – and her slap disturbs the mouse. It squeaks in alarm, at a pitch that is audible to the bat but too high for the elephant to hear. The elephant, meanwhile, unleashes a deep, thunderous rumble too low-pitched for the mouse’s ears or the bat’s but felt by the vibration-sensitive belly of the rattlesnake. Rebecca, who is oblivious to both the ultrasonic mouse squeaks and the infrasonic elephant rumbles, listens instead to the robin, which is singing at frequencies better suited to her ears. But her hearing is too slow to pick out all the complexities that the bird encodes within its tune.

The robin’s chest looks red to Rebecca but not to the elephant, whose eyes are limited to shades of blue and yellow. The bumblebee can’t see red, either, but it is sensitive to the ultraviolet hues that lie beyond the opposite end of the rainbow. The sunflower it sits upon has at its center an ultraviolet bullseye, which grabs the attention of both the bird and the bee. The bullseye is invisible to Rebecca, who thinks the flower is only yellow. Her eyes are the sharpest in the room; unlike the elephant or the bee, she can spot the small spider sitting upon its web. But she stops seeing much of anything when the lights in the room go out.

Plunged into darkness, Rebecca walks slowly forward, arms outstretched, hoping to feel obstacles in her way. The mouse does the same but with the whiskers on its face, which it sweeps back and forth several times a second to map its surroundings. As it skitters between Rebecca’s feet, its footsteps are too faint for her to hear, but they are easily audible to the owl perched overhead. The disc of stiff feathers on the owl’s face funnels sounds toward its sensitive ears, one of which is slightly higher than the other. Thanks to this asymmetry, the owl can pinpoint the source of the mouse’s skittering in both the vertical and horizontal planes. It swoops in, just as the mouse blunders within range of the waiting rattlesnake. Using two pits on its snout, the snake can sense the infrared radiation that emanates from warm objects. It effectively sees in heat, and the mouse’s body blazes like a beacon. The snake strikes…and collides with the swooping owl.All of this commotion goes unnoticed by the spider, which barely hears or sees the participants. Its world is almost entirely defined by the vibrations coursing through its web—a self-made trap that acts as an extension of its senses. When the mosquito strays into the silken strands, the spider detects the telltale vibrations of struggling prey and moves in for the kill. But as it attacks, it is unaware of the high-frequency sound waves that are hitting its body and bouncing back to the creature that sent them -the bat. The bat’s sonar is so acute that it not only finds the spider in the dark but pinpoints it precisely enough to pluck it from its web.

As the bat feeds, the robin feels a familiar attraction that most of the other animals cannot sense. The days are getting colder, and it is time to migrate to warmer southern climes. Even within the enclosed gym, the robin can feel Earth’s magnetic field, and, guided by its internal compass, it points due south and escapes through a window. It leaves behind one elephant, one bat, one bumblebee, one rattlesnake, one slightly ruffled owl, one extremely fortunate mouse, and one Rebecca. These seven creatures share the same physical space but experience it in wildly and wondrously different ways. The same is true for the billions of other animal species on the planet and the countless individuals within those species. I*1| Earth teems with sights and textures, sounds and vibrations, smells and tastes, electric and magnetic fields. But every animal can only tap into a small fraction of reality’s fullness. Each is enclosed within its own unique sensory bubble, perceiving but a tiny sliver of an immense world.

THERE IS A wonderful word for this sensory bubble — Umwelt. It was defined and popularized by the Baltic-German zoologist Jakob von Uexküll in 1909. Umwelt comes from the German word for “environment,” but Uexküll didn’t use it simply to refer to an animal’s surroundings. Instead, an Umwelt is specifically the part of those surroundings that an animal can sense and experience-its perceptual world. Like the occupants of our imaginary room, a multitude of creatures could be standing in the same physical space and have completely different Umwelten. A tick, questing for mammalian blood, cares about body heat, the touch of hair, and the odor of butyric acid that emanates from skin. These three things constitute its Umwelt. Trees of green, red roses too, skies of blue, and clouds of white -these are not part of its wonderful world. The tick doesn’t willfully ignore them. It simply cannot sense them and doesn’t know they exist.

Uexküll compared an animal’s body to a house. “Each house has a number of windows,” he wrote, “which open onto a garden: a light window, a sound window, an olfactory window, a taste window, and a great number of tactile windows. Depending on the manner in which these windows are built, the garden changes as it is seen from the house. By no means does it appear as a section of a larger world. Rather, it is the only world that belongs to the house —its [Umwelt]. The garden that appears to our eye is fundamentally different from that which presents itself to the inhabitants of the house.”

This was a radical notion at the time-and in some circles, it might still be. Unlike many of his contemporaries, Uexküll saw animals not as mere machines but as sentient entities, whose inner worlds not only existed but were worth contemplating. Uexküll didn’t exalt the inner worlds of humans over those of other species. Rather, he treated the Umwelt concept as a unifying and leveling force. The human’s house might be bigger than the tick’s, with more windows overlooking a wider garden, but we are still stuck inside one, looking out. Our Umwelt is still limited; it just doesn’t feel that way. To us, it feels all-encompassing. It is all that we know, and so we easily mistake it for all there is to know. This is an illusion, and one that every animal shares.We cannot sense the faint electric fields that sharks and platypuses can. We are not privy to the magnetic fields that robins and sea turtles detect. We can’t trace the invisible trail of a swimming fish the way a seal can. We can’t feel the air currents created by a buzzing fly the way a wandering spider does. Our ears cannot hear the ultrasonic calls of rodents and hummingbirds or the infrasonic calls of elephants and whales. Our eyes cannot see the infrared radiation that rattlesnakes detect or the ultraviolet light that the birds and the bees can sense.

Even when animals share the same senses with us, their Umwelten can be very different. There are animals that can hear sounds in what seems to us like perfect silence, see colors in what looks to us like total darkness, and sense vibrations in what feels to us like complete stillness. There are animals with eyes on their genitals, ears on their knees, noses on their limbs, and tongues all over their skin. Starfish see with the tips of their arms, and sea urchins with their entire bodies. The star-nosed mole feels around with its nose, while the manatee uses its lips. We are no sensory slouches, either. Our hearing is decent, and certainly better than that of the millions of insects that have no ears at all. Our eyes are unusually sharp, and can discern patterns on animal bodies that the animals themselves cannot see. Each species is constrained in some ways and liberated in others. For that reason, this is not a book of lists, in which we childishly rank animals according to the sharpness of their senses and value them only when their abilities surpass our own. This is a book not about superiority but about diversity.

This is also a book about animals as animals. Some scientists study the senses of other animals to better understand ourselves, using exceptional creatures like electric fish, bats, and owls as “model organisms” for exploring how our own sensory systems work. Others reverse-engineer animal senses to create new technologies: Lobster eyes have inspired space telescopes, the ears of a parasitic fly have influenced hearing aids, and military sonar has been honed by work on dolphin sonar. These are both reasonable motivations. I’m not interested in either. Animals are not just stand-ins for humans or fodder for brainstorming sessions. They have worth in themselves. We’ll explore their senses to better understand their lives. “They move finished and complete, gifted with extensions of the senses we have lost or never attained, living by voices we shall never hear,” wrote the American naturalist Henry Beston. “They are not brethren, they are not underlings; they are other nations, caught with ourselves in the net of life and time, fellow prisoners of the splendour and travail of the earth.”

A FEW TERMS will act as guideposts on our journey. To sense the world, animals detect stimuli-quantities like light, sound, or chemicals-and convert them into electrical signals, which travel along neurons toward the brain. The cells that are responsible for detecting stimuli are called receptors: Photoreceptors detect light, chemoreceptors detect molecules, and mechanoreceptors detect pressure or movement. These receptor cells are often concentrated in sense organs, like eyes, noses, and ears. And sense organs, together with the neurons that transmit their signals and the parts of the brain that process those signals, are collectively called sensory systems. The visual system, for example, includes the eyes, the photoreceptors inside them, the optic nerve, and the visual cortex of the brain. Together, these structures give most of us the sense of sight.

The preceding paragraph could have been pulled from a high school textbook. But take a moment to consider the miracle of what it describes. Light is just electromagnetic radiation. Sound is just waves of pressure. Smells are just small molecules. It’s not obvious that we should be able to detect any of those things, let alone convert them into electrical signals or derive from those signals the spectacle of a sunrise, or the sound of a voice, or the scent of baking bread. The senses transform the coursing chaos of the world into perceptions and experiences-things we can react to and act upon. They allow biology to tame physics. They turn stimuli into information. They pull relevance from randomness, and weave meaning from miscellany. They connect animals to their surroundings. And they connect animals to each other via expressions, displays, gestures, calls, and currents.

The senses constrain an animal’s life, restricting what it can detect and do. But they also define a species’ future, and the evolutionary possibilities ahead of it. For example, around 400 million years ago, some fish began leaving the water and adapting to life on land. In open air, these pioneers—our ancestors-could see over much longer distances than they could in water. The neuroscientist Malcolm MacIver thinks that this change spurred the evolution of advanced mental abilities, like planning and strategic thinking. Instead of simply reacting to whatever was directly in front of them, they could be proactive. By seeing farther, they could think ahead. As their Umwelten expanded, so did their minds.

An Umwelt cannot expand indefinitely, though. Senses always come at a cost. Animals have to keep the neurons of their sensory systems in a perpetual state of readiness so that they can fire when necessary. This is tiring work, like drawing a bow and holding it in place so that when the moment comes, an arrow can be shot. Even when your eyelids are closed, your visual system is a monumental drain on your reserves. For that reason, no animal can sense everything well.

Nor would any animal want to. It would be overwhelmed by the flood of stimuli, most of which would be irrelevant. Evolving according to their owner’s needs, the senses sort through an infinity of stimuli, filtering out what’s irrelevant and capturing signals for food, shelter, threats, allies, or mates. They are like discerning personal assistants who come to the brain with only the most important information.l*2] Writing about the tick, Uexküll noted that the rich world around it is “constricted and transformed into an impoverished structure” of just three stimuli. “However, the poverty of this environment is needful for the certainty of action, and certainty is more important than riches.” Nothing can sense everything, and nothing needs to. That is why Umwelten exist at all. It is also why the act of contemplating the Umwelt of another creature is so deeply human and so utterly profound. Our senses filter in what we need. We must choose to learn about the rest.

THE SENSES OF animals have fascinated people for millennia, but mysteries still abound. Many of the animals whose Umwelten are most different from ours live in habitats that are inaccessible or impenetrable —murky rivers, dark caves, open oceans, abyssal depths, and subterranean realms. Their natural behavior is hard to observe, let alone to interpret. Many scientists are limited to studying creatures that can be kept in captivity, with all the strangeness that entails. Even in labs, animals are challenging to work with. Experiments that might reveal how they use their senses are hard to design, especially when those senses are drastically different from ours.

Amazing new details-and, sometimes, entirely new senses —are being discovered regularly. Giant whales have a volleyball-sized sensor at the tip of their lower jaw, which was only discovered in 2012 and whose function is still unclear. Some of the stories in these pages are decades or centuries old; others emerged as I was writing. And there’s still so much we can’t explain. “My dad, who is an atomic physicist, once asked me a bunch of questions,” Sonke Johnsen, a sensory biologist, tells me. “After a few I don’t knows, he said: You guys really don’t know anything.” Inspired by that conversation, Johnsen published a paper in 2017 entitled “We Don’t Really Know Anything, Do We? Open Questions in Sensory Biology.”

Consider the seemingly simple question How many senses are there? Around 2,370 years ago, Aristotle wrote that there are five, in both humans and other animals-sight, hearing, smell, taste, and touch. This tally persists today. But according to the philosopher Fiona Macpherson, there are reasons to doubt it. For a start, Aristotle missed a few in humans: proprioception, the awareness of your own body, which is distinct from touch; and equilibrioception, the sense of balance, which has links to both touch and vision.

Other animals have senses that are even harder to categorize. Many vertebrates (animals with backbones) have a second sensory system for detecting odors, governed by a structure called the vomeronasal organ; is this part of their main sense of smell, or something separate? Rattlesnakes can detect the body heat of their prey, but their heat sensors are wired to their brain’s visual center; is their heat sense simply part of vision, or something distinct? The platypus’s bill is loaded with sensors that detect electric fields and sensors that are sensitive to pressure; does the platypus’s brain treat these streams of information differently, or does it wield a single sense of electrotouch?

These examples tell us that “senses cannot be clearly divided into a limited number of discrete kinds,” Macpherson wrote in The Senses. Instead of trying to shove animal senses into Aristotelian buckets, we should instead study them for what they are.I*3l Though I have organized this book into chapters that revolve around specific stimuli, like light or sound, that’s largely for convenience. Each chapter is a gateway into the varied things that animals do with each stimulus. We will not concern ourselves with counting senses, nor talk nonsensically about a “sixth sense.” We will instead ask how animals use their senses, and attempt to step inside their Umwelten.

It won’t be easy. In his classic 1974 essay, What Is It Like to Be a Bat?,” the American philosopher Thomas Nagel argued that other animals have conscious experiences that are inherently subjective and hard to describe. Bats, for example, perceive the world through sonar, and since this is a sense that the majority of humans lack, “there is no reason to suppose that it is subjectively like anything we can experience or imagine,” Nagel wrote. You could envision yourself with webbing on your arms or insects in your mouth, but you’d still be creating a mental caricature of you as a bat. “I want to know what it is like for a bat to be a bat,” Nagel wrote. “Yet if I try to imagine this, I am restricted to the resources of my own mind, and those resources are inadequate to the task.”

In thinking about other animals, we are biased by our own senses and by vision in particular. Our species and our culture are so driven by sight that even people who are blind from birth will describe the world using visual words and metaphors.*4] You agree with people if you see their point, or share their view. You are oblivious to things in your blind spots. Hopeful futures are bright and gleaming; dystopias are dark and shadowy. Even when scientists describe senses that humans lack altogether, like the ability to detect electric fields, they talk about images and shadows. Language, for us, is both blessing and curse. It gives us the tools for describing another animal’s Umwelt even as it insinuates our own sensory world into those descriptions.

Scholars of animal behavior often discuss the perils of anthropomorphism-the tendency to inappropriately attribute human emotions or mental abilities to other animals. But perhaps the most common, and least recognized, manifestation of anthropomorphism is the tendency to forget about other Umwelten —to frame animals’ lives in terms of our senses rather than theirs. This bias has consequences. We harm animals by filling the world with stimuli that overwhelm or befuddle their senses, including coastal lights that lure newly hatched turtles away from the oceans, underwater noises that drown out the calls of whales, and glass panes that seem like bodies of water to bat sonar. We misinterpret the needs of animals closest to us, stopping smell-oriented dogs from sniffing their environments and imposing the visual world of humans upon them. And we underestimate what animals are capable of to our own detriment, missing out on the chance to understand how expansive and wondrous nature truly is—the delights that, as William Blake wrote, are “clos’d by your senses five.”

Throughout this book, we’ll encounter animal abilities that others had long thought impossible or absurd. Zoologist Donald Griffin, who co-discovered the sonar of bats, once wrote that biologists have been overly swayed by what he called “simplicity filters.” That is, they seemed reluctant to even consider that the senses they were studying might be more complex and refined than whatever data they had collected could suggest. This lament contradicts Occam’s razor, the principle that states that the simplest explanation is usually the best. But this principle is only true if you have all the necessary information to hand. And Griffin’s point was that you might not. A scientist’s explanations about other animals are dictated by the data she collects, which are influenced by the questions she asks, which are steered by her imagination, which is delimited by her senses. The boundaries of the human Umwelt often make the Umwelten of others opaque to us.

Griffin’s words are not carte blanche to put forward convoluted or paranormal explanations for animal behavior. I see them, and Nagel’s essay, as a call for humility. They remind us that other animals are sophisticated, and that, for all our vaunted intelligence, it is very hard for us to understand other creatures, or to resist the tendency to view their senses through our own.

We can study the physics of an animal’s environment, look at what they respond to or ignore, and trace the web of neurons that connects their sense organs to their brains. But the ultimate feats of understanding —working out what it’s like to be a bat, or an elephant, or a spider-always require what psychologist Alexandra Horowitz calls “an informed imaginative leap.”

Many sensory biologists have backgrounds in the arts, which may enable them to see past the perceptual worlds that our brains automatically create. Sonke Johnsen, for example, studied painting, sculpture, and modern dance well before he studied animal vision. To represent the world around us, he says, artists already have to push against the limits of their Umwelt and “look under the hood.” That capacity helps him “think about animals having different perceptual worlds.” He also notes that many sensory biologists are perceptually divergent. Sarah Zylinski studies the vision of cuttlefish and other cephalopods; she has prosopagnosia and can’t recognize even familiar faces, including her mother’s. Kentaro Arikawa studies color vision in butterflies; he is red-green color-blind. Suzanne Amador Kane studies the visual and vibrational signals of peacocks; she has slight differences in her color vision in each eye, so that one gives her a slightly reddish tint. Johnsen suspects that these differences, which some might bill as “disorders,” actually predispose people to step outside their Umwelten and embrace those of other creatures. Perhaps people who experience the world in ways that are considered atypical have an intuitive feeling for the limits of typicality.

We can all do this. I began this book by asking you to conjure a room full of hypothetical animals, and I’m asking you to perform similar feats of imagination over the next 13 chapters. The task will be hard, as Nagel predicted. But there is value and glory in the striving. On this journey through nature’s Umwelten, our intuitions will be our biggest liabilities, and our imaginations will be our greatest assets.

ONE LATE MORNING in June 1998, Mike Ryan hiked into the Panamanian rainforest to search for animals with his former student Rex Cocroft. Usually, Ryan would have looked for frogs.

But Cocroft had taken a liking to sap-sucking insects called treehoppers, and he had something cool to show his friend. Heading out from their research station, the duo pulled off a road and walked along a river. Once Cocroft spotted the right kind of shrub, he turned over a few leaves and quickly found a family of tiny treehoppers of the species Calloconophora pinguis. Cocroft had found a mother surrounded by babies, their black backs capped with forward-pointing domes that looked like Elvis’s hair.

Treehoppers communicate by sending vibrations through the plants on which they stand. These vibrations are not audible but can be easily converted into sounds. Cocroft clipped a simple microphone to the plant, handed Ryan some headphones, and told him to listen. Then he flicked the leaf. Immediately the baby treehoppers ran away, while producing vibrations by contracting muscles in their abdomens. “I figured it was probably going to be some kind of scurrying noise,” Ryan recalls. “And what I heard instead was like cows mooing.” The sound was deep, resonant, and unlike anything you’d expect from an insect. As the babies settled down and returned to their mother, their cacophony of vibrational moos turned into a synchronized chorus.

Still watching them, Ryan took the headphones off. All around him, he heard birds singing, howler monkeys roaring, and insects chirping. The treehoppers were quiet. Ryan put the headphones back on, “and I was transported into a totally different world,” he tells me. Once more, the jungle noises dropped out of his Umwelt, and the mooing treehoppers returned. “It was the coolest experience,” he says. “It was sensory travel. I was in the same place, but stepping between these two really cool environments. It was such a stark demonstration of Uexküll’s idea.”

The Umwelt concept can feel constrictive because it implies that every creature is trapped within the house of its senses. But to me, the idea is wonderfully expansive. It tells us that all is not as it seems and that everything we experience is but a filtered version of everything that we could experience. It reminds us that there is light in darkness, noise in silence, richness in nothingness. It hints at flickers of the unfamiliar in the familiar, of the extraordinary in the everyday, of magnificence in mundanity. It shows us that clipping a microphone onto a plant can be an intrepid act of exploration. Stepping between Umwelten, or at least trying to, is like setting foot upon an alien planet. Uexküll even billed his work as a “travelogue.”

When we pay attention to other animals, our own world expands and deepens. Listen to treehoppers, and you realize that plants are thrumming with silent vibrational songs. Watch a dog on a walk, and you see that cities are crisscrossed with skeins of scent that carry the biographies and histories of their residents. Watch a swimming seal, and you understand that water is full of tracks and trails. “When you look at an animal’s behavior through the lens of that animal, suddenly all of this salient information becomes available that you would otherwise miss,” Colleen Reichmuth, a sensory biologist who works with seals and sea lions, tells me. “It’s like a magic magnifying glass, to have that knowledge.”

Malcolm MacIver argues that when animals moved onto land, the greater range of their vision spurred the evolution of planning and advanced cognition: Their Umwelten expanded, and so did their minds. Similarly, the act of delving into other Umwelten allows us to see further and think more deeply. I’m reminded of Hamlet’s plea to Horatio that “there are more things in heaven and Earth…than are dreamt of in your philosophy.” The quote is often taken as an appeal to embrace the supernatural. I see it rather as a call to better understand the natural. Senses that seem paranormal to us only appear this way because we are so limited and so painfully unaware of our limitations. Philosophers have long pitied the goldfish in its bowl, unaware of what lies beyond, but our senses create a bowl around us too-one that we generally fail to penetrate.

But we can try. Science-fiction authors like to conjure up parallel universes and alternate realities, where things are similar to this one but slightly different. Those exist! We will visit them one at a time, beginning with the most ancient and universal of senses — the chemical ones, like smell and taste. From there, via an unexpected route, we’ll visit the realm of vision, the sense that dominates the Umwelt of most people but that still holds surprises galore. We’ll stop to savor the delightful world of color before heading into the harsher territories of pain and heat. We’ll sail smoothly through the various mechanical senses that respond to pressure and movement-touch, vibration, hearing, and the most impressive use of hearing, echolocation. Then, as experienced sensory travelers whose imaginations have been fully primed, we’ll make our most difficult imaginative leaps yet, through the strange senses that animals use to detect the electric and magnetic fields that we cannot. Finally, at journey’s end, we’ll see how animals unify the information from their senses, how humans are polluting and distorting that information, and where our responsibilities to nature now lie.

As the writer Marcel Proust once said, “The only true voyage… would be not to visit strange lands but to possess other eyes…to see the hundred universes that each of them sees.” Let us begin.

SKIP NOTES

*1 To understand how varied senses can be in a single species, just look at humans. For some people, red and green look identical. For others, body odor smells like vanilla. For yet others, coriander (cilantro) tastes of soap.

*2 In 1987, German scientist Rüdiger Wehner described these as
“matched filters” — aspects of an animal’s sensory systems that are tuned to the sensory stimuli that it most needs to detect.

*3 If you were being maximally reductive, you could reasonably argue that there are really only two senses —chemical and mechanical.
Chemical senses include smell, taste, and vision. Mechanical senses include touch, hearing, and electrical senses. The magnetic sense might belong to either category or both. This framework will probably make absolutely no sense right now, but should become clearer as you continue in the book. I’m not especially wedded to it, but it is one possible way of thinking about the senses —and one that might appeal to the lumpers among you.

*4 Let me just say that avoiding visual metaphors when describing other senses is extremely difficult over the length of an entire book. I have tried to do so, or at least to be judicious and explicit whenever I have to resort to visual terms.