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.
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Horowitz can’t see into the bag slung over the back of my chair, but Finn can smell into it, picking up molecules drifting from the sandwich within. Smells linger in a way that light does not, revealing history.
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In some cases, humans do better: Of 15 odorants where both species have been tested, humans outperformed our canine companions on five, including beta-ionone (cedar wood) and amyl acetate (bananas). People also excel at discriminating between smells. While it’s easy to find two colors that humans can’t tell apart, it’s very hard to find indistinguishable pairs of odors. Neuroscientist John McGann has tried, and tells me, “We tried odors that mice can’t tell apart and humans were like: No, we’ve got this.”
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Leafcutter ants are so sensitive to their trail pheromone that a milligram is enough to lay a path around the planet three times over.
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Army ants are so committed to following their pheromone trails that if those paths should accidentally loop back onto themselves, hundreds of workers will walk in an endless “death spiral” until they die from exhaustion.fn16,47,48 Many ants use pheromones to discern dead individuals: When the biologist E. O. Wilson daubed oleic acid onto the bodies of living ants, their sisters treated them as corpses and carried them to the colony’s garbage piles.49 It didn’t matter that the ant was alive and visibly kicking. What mattered was that it smelled dead.
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“We live, all the time, especially in nature, in great clouds of pheromones,” E. O. Wilson once said. “They’re coming out in spumes in millionths of a gram that can travel for maybe a kilometer.” These tailored messages drive the entire animal kingdom, from the smallest of creatures to the very biggest.
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Nor is it entirely clear why most animals have another distinct chemical sense. I’m talking, of course, about taste.
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Like smell, taste—or gustation, in the fancy scientific parlance—is a means of detecting chemicals in the environment. But beyond that, the two senses are distinct. Put your nose next to vanilla oil, and you’ll inhale a pleasing odor; drop that same oil on your tongue, and you’ll likely flinch in disgust.
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But John Caprio, a physiologist who studies catfish, says the difference between smell and taste couldn’t be clearer. Taste is reflexive and innate, while smell is not.fn29 From birth, we recoil from bitter substances, and while we can learn to override those responses and appreciate beer, coffee, or dark chocolate, the fact remains that there’s something instinctive to override. Odors, by contrast, “don’t carry meaning until you associate them with experiences,” Caprio says. Human infants aren’t disgusted by the smell of sweat or poop until they get older.
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Adults vary so much in their olfactory likes and dislikes that when the U.S. Army tried to develop a stink bomb for crowd control purposes, they couldn’t find a smell that was universally disgusting to all cultures.
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Taste, then, is the simpler sense. As we’ve seen, smell covers a practically infinite selection of molecules with an indescribably vast range of characteristics, which the nervous system represents through a combinatorial code so fiendish that scientists have barely begun to crack it. Taste, by contrast, boils down to just five basic qualities in humans—salt, sweet, bitter, sour, and umami (savory)—and perhaps a few more in other animals, which are detected through a small number of receptors.
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And while smell can be put to complex uses—navigating the open oceans, finding prey, and coordinating herds or colonies—taste is almost always used to make binary decisions about food. Yes or no? Good or bad? Consume or spit?
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But smells are more than dinner bells. In the ocean, they’re also signposts. Geological features, like submerged mountains or slopes in the seafloor, affect the levels of nutrients in the water, which in turn influence concentrations of plankton, krill, and DMS. The smellscapes that seabirds track are intimately tied to actual landscapes, and so are surprisingly predictable.95 Over time, Nevitt suspects, seabirds build up a map of these features, using their noses to learn the locations of the richest feeding spots and their home nests.
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Still recuperating, Nevitt chatted with the new chief scientist—an atmospheric chemist named Tim Bates, who had come to study a gas called dimethyl sulfide, or DMS. In the oceans, plankton release DMS when they’re eaten by krill—shrimp-like animals that are, in turn, eaten by whales, fish, and seabirds. DMS doesn’t dissolve easily in water, and eventually makes its way into the air. If it rises high enough, it seeds clouds. If it enters the nose of a sailor, it evokes an odor that Nevitt describes as “a lot like oysters” or “kind of seaweed-y.” It’s the scent of the sea. In particular, DMS is the scent of bountiful seas, where huge blooms of plankton feed equally huge swarms of krill.
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This is how all animals see—using light-sensitive proteins that are actually modified chemical sensors. In a way, we see by smelling light.
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High-resolution vision is the fourth of Nilsson’s stages. When it first appeared, it would have intensified the interactions between animals. Conflicts and courtships could play out over distances longer than touch or taste would allow and at speeds too fast for smell. Predators could now spot their prey from afar, and vice versa. Chases ensued. Animals became bigger, faster, and more mobile. Defensive armor, spines, and shells evolved. The rise of high-resolution vision might explain why, around 541 million years ago, the animal kingdom dramatically diversified, giving rise to the major groups that exist today. This flurry of evolutionary innovation is called the Cambrian explosion, and stage-four eyes might have been one of the sparks that ignited it.
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Humans outshine almost every other animal at resolving detail. Our exceptionally sharp vision, Melin realized, gives us a rarefied view of a zebra’s stripes. She and Caro calculated that on a bright day, people with excellent eyesight can distinguish the black-and-white bands from 200 yards away.` Lions can only do so at 90 yards and hyenas at 50 yards. And those distances roughly halve at dawn and dusk, when these predators are more likely to hunt. Melin was right: The stripes can’t possibly act as camouflage because predators can only make them out at close range, by which point they can almost certainly hear and smell the zebra. At most distances, the stripes would just fuse together into a uniform gray. To a hunting lion, a zebra mostly looks like a donkey.
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An animal’s visual acuity is measured in cycles per degree—a concept that, by happy coincidence, you can think of in terms of zebra stripes. Stretch out your arm and give a thumbs-up. Your nail represents roughly 1 degree of visual space, out of the 360 degrees that surround you. You should be able to paint 60 to 70 pairs of thin black-and-white stripes on that nail and still be able to tell them apart. A human’s visual acuity, then, is somewhere between 60 and 70 cycles per degree, or cpd. The current record, at 138 cycles per degree, belongs to the wedge-tailed eagle of Australia. Its photoreceptors are some of the narrowest in the animal kingdom, which allows them to be densely packed within the eagle’s retinas. With these svelte cells, the eagle effectively sees the world on a screen with over twice as many pixels as ours. It can spot a rat from a mile away.
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But eagles and other birds of prey are the only animals whose vision is substantially sharper than ours. Sensory biologist Eleanor Caves has been collating visual acuity measurements for hundreds of species, almost all of which are surpassed by humans. Aside from raptors, only other primates come close to our standards. Octopuses (46 cpd), giraffes (27 cpd), horses (25 cpd), and cheetahs (23 cpd) do reasonably well. A lion’s acuity is only 13 cpd, just above the 10 cpd threshold at which humans are considered legally blind. Most animals fall below that threshold, including half of all birds (and surprising ones like hummingbirds and barn owls), most fish, and all insects. A honeybee’s acuity is just 1 cycle per degree. Your outstretched thumbnail represents roughly one pixel of a bee’s visual world, and all the detail within that nail would collapse into a uniform smudge. Around 98 percent of insects have vision that’s even coarser.
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And paradoxically, our sharp vision muddies our appreciation of other Umwelten, because “we assume that if we can see it, they can, and that if it’s eye-catching to us, it’s grabbing their attention,” says Caves. “That’s not the case.”
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The curse of low resolution is baked into the structure of a compound eye, and having started off with eyes of this kind, insects and crustaceans are now stuck. Robber flies manage 3.7 cycles per degree, but that’s about the limit. For a fly’s eye to be as sharp as a human’s, it would have to be a meter wide.
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These qualities—sensitivity and resolution—seesaw against each other. No eye can excel at both. An eagle might be able to spot a far-off rabbit in broad daylight, but its acuity plummets as the sun sets. (There are no nocturnal eagles.) Conversely, lions and hyenas might not be able to resolve a zebra’s stripes at a distance, but their vision is sensitive enough to hunt one at night. They, and many other animals, have prioritized sensitivity over acuity. As ever, eyes evolve to suit the needs of their owners. Some animals simply don’t need to see crisp images. And some animals don’t need to see images at all.
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And yet vultures, eagles, and other large raptors often fatally crash into wind turbines. In one Spanish province alone, 342 griffon vultures collided with wind turbines over a 10-year period. How could birds that fly by day and have some of the planet’s sharpest eyes fail to avoid structures so large and conspicuous? Graham Martin, who studies bird vision, answered this question by addressing another: Where exactly do vultures look?
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This is why vultures crash into wind turbines: While soaring, they aren’t looking at what is right in front of them. For most of their history, they never had to. “Vultures would never have encountered an object so high and large in their flight path,” Martin says. It might work to turn off the turbines if the birds are near, or to lure the vultures away using ground-based markers. But visual cues on the blades themselves won’t work. (In North America, bald eagles also crash into wind turbines for the same reasons.)
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The peregrine prefers to use its right eye to track prey. Such preferences are common to birds; when eyes see distinct views, those eyes can be used for distinct tasks. The left half of a chick’s brain is specialized for focused attention and categorizing objects; the bird can spot food grains among a bed of pebbles if it uses its right eye (directed by its left brain), but not its left eye. The right half of the brain deals with the unexpected; many birds use their left eyes (directed by their right brains) to scan for predators, and are quicker to detect a threat when it approaches from the left.
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Cows and other livestock also have a somnolent air because their gaze is so fixed.60 They rarely turn to look at you in the way another human (or a jumping spider) might. But they also don’t need to. Their visual fields wrap almost all the way around their heads and their acute zones are horizontal stripes, giving them a view of the entire horizon at once. The same is true for other animals that live in flat habitats, including rabbits (fields), fiddler crabs (beaches), red kangaroos (deserts), and water striders (the surface of ponds).
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It may seem strange to talk about animals seeing at different speeds, because light is the fastest thing in the universe, and vision seems instantaneous to us. But eyes don’t work at light speed. It takes time for photoreceptors to react to incoming photons, and for the electrical signals they generate to travel to the brain. In killer flies, evolution has pushed these steps to their limits. When Gonzalez-Bellido shows these insects an image, it takes just 6 to 9 milliseconds for their photoreceptors to send electrical signals, for those signals to reach their brains, and for their brains to send commands to their muscles. By contrast, it takes between 30 and 60 milliseconds for human photoreceptors to accomplish just the first of those steps. If you looked at an image at the same moment as a killer fly, the insect would be airborne well before a signal had even left your retina. “We don’t know of a faster photoreceptor than the ones from these flies,” Gonzalez-Bellido tells me.
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This is called the critical flicker-fusion frequency, or CFF. It’s a measure of how quickly a brain can process visual information. Think of it as the frame rate of the movie playing inside an animal’s head—the point at which static images blend into the illusion of continuous motion. For humans, in good light, the CFF is around 60 frames per second (or hertz, Hz). For most flies, it’s up to 350. For killer flies, it’s probably higher still. To its eyes, a human movie would look like a slideshow. The fastest of our actions would seem languid. An open palm, moving with lethal intent, would be easily dodged. Boxing would look like tai chi.
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In general, animals tend to have higher CFFs if they’re smaller and faster.75 Compared to human vision, cats are slightly slower (48 Hz) and dogs slightly faster (75 Hz). The eyes of a scallop are positively glacial (1 to 5 Hz), and those of nocturnal toads are slower still (0.25 to 0.5 Hz). Those of leatherback turtles (15 Hz) and harp seals (23 Hz) are faster but still sluggish.
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Many birds have naturally fast vision; with a maximum CFF of 146 Hz, the pied flycatcher—a small songbird—has the fastest vision of any vertebrate that’s been tested, perhaps because its survival depends on tracking and catching flying insects. And those insects have eyes that are faster still. Honeybees, dragonflies, and flies have CFFs between 200 and 350 Hz.
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It’s possible that each of these visual speeds comes with a different sense of time’s passage. Through a leatherback turtle’s eyes, the world might seem to move in time-lapse, with humans bustling about at a fly’s frenetic pace. Through a fly’s eyes, the world might seem to move in slow motion. The imperceptibly fast movements of other flies would slow to a perceptible crawl, while slow animals might not seem like they were moving at all. “Everyone asks us how we catch the killer flies,” Gonzalez-Bellido says. “You just move toward them slowly with a vial. If you’re slow enough, you’re just part of the background.”
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The trichromats are indeed better at finding brightly colored fruit, but the dichromats surpass them at finding insects disguised as leaves and sticks. Without a riot of colors to confuse or distract them, they’re better at detecting borders and shapes, and seeing through camouflage. Melin has watched them nabbing insects that she, a trichromat, didn’t even know were there. Seeing extra colors has both drawbacks and benefits. More isn’t necessarily better, which is why some females are still dichromats and all males are.
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Many birds also have UV patterns in their feathers. In 1998, two independent teams realized that much of the “blue” plumage of blue tits actually reflects a lot of UV; as one of them wrote, “Blue tits are ultraviolet tits.” To humans, these birds all look much the same. But thanks to their UV patterns, males and females look very different from each other. The same is true for more than 90 percent of songbirds whose sexes are indistinguishable to us, including barn swallows and mockingbirds.
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If our red and blue cones are stimulated together, we see purple—a color that doesn’t exist in the rainbow and that can’t be represented by a single wavelength of light. These kinds of cocktail colors are called non-spectral. Hummingbirds, with their four cones, can see a lot more of them, including UV-red, UV-green, UV-yellow (which is red + green + UV), and probably UV-purple (which is red + blue + UV). At my wife’s suggestion, and to Stoddard’s delight, I’m going to call these rurple, grurple, yurple, and ultrapurple. Stoddard found that these non-spectral colors and their various shades account for roughly a third of those found on plants and feathers. To a bird, meadows and forests pulse with grurples and yurples. To a broad-tailed hummingbird, the bright magenta feathers of the male’s bib are actually ultrapurple.
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Picture trichromatic human vision as a triangle, with the three corners representing our red, green, and blue cones. Every color we can see is a mix of those three, and can be plotted as a point within that triangular space. By comparison, a bird’s color vision is a pyramid, with four corners representing each of its four cones. Our entire color space is just one face of that pyramid, whose spacious interior represents colors inaccessible to most of us.
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When we walk along a street or stare out a vehicle window, our eyes actually fix on specific points ahead of us, rapidly flicking from one to the next. These flicks, or saccades, are some of the fastest movements we make, which is just as well, because as they’re happening, our visual system shuts down. Our brains fill the millisecond-long gaps to create a sense of continuous vision, but that’s an illusion.
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This astonishing sensitivity means that a pit viper can detect the warmth of a rodent from up to a meter away.
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Mammalian hair might have had a similar start, appearing first as touch sensors that were only later turned into insulating coats. Some hairs still retain that original tactile function. They’re called vibrissae, from the Latin word for “vibrate.” More commonly, they’re known as whiskers.
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Dehnhardt estimated that a swimming herring should leave a trail that a harbor seal could follow from up to almost 200 yards away.
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And, as Jerome Casas has shown, they can detect the infinitesimal wind created by a charging spider. The wolf spider is the cricket’s major predator and runs down its prey. On the uneven, leaf-strewn floor of a forest, it must launch its attacks while standing on the same leaf as its target. It is fast, but Casas found that the cricket’s hairs can sense it almost as soon as it starts to run.
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The filiform hairs of crickets and the trichobothria of spiders are almost inconceivably sensitive. They can be deflected by a fraction of the energy in a single photon—the smallest possible quantity of visible light. These hairs are a hundred times more sensitive than any visual receptor that exists, or could possibly exist. Indeed, the amount of energy needed to shift a cricket’s hairs is very close to thermal noise—the kinetic energy of jiggling molecules. Put another way, it would be almost impossible to make these hairs more sensitive without breaking the laws of physics.
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And everyone who can hear does so partly through bone conduction, which is why people often think they sound strange on recordings. Those recordings reproduce the airborne components of our voices, but not the vibrations traveling through our skulls.
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Luther Standing Bear, an Oglala Lakota chief and author, offered a clue. “The Lakota … loved the earth and all things of the earth, the attachment growing with age,” he wrote in 1933. “The old people came literally to love the soil, and they sat or reclined on the ground with a feeling of being close to a mothering power …. This is why the old Indian still sits upon the earth instead of propping himself up and away from its life giving forces. For him, to sit or lie upon the ground is to be able to think more deeply and to feel more keenly; he can see more clearly into the mysteries of life and come closer in kinship to other lives about him. The earth was full of sounds which the old-time Indian could hear, sometimes putting his ear to it so as to hear more clearly.”
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To humans, whose eyes can scan the entire scene and are sharp enough to just about make out the silk of the webs, the room is a labyrinth of death traps waiting to ensnare the flies. To the spiders, which have very poor eyesight, the room doesn’t really exist: There is only the web, and whatever vibrates it. To the flies, the thin webs are imperceptible until they are ensnared in one.
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The small dewdrop spider Argyrodes is a thief, stealing from larger spiders like Nephila by hacking their webs. From a nearby hiding place, it runs several lines of silk over to the hub and spokes of a Nephila web, effectively plugging its sensory system into that of the bigger spider. It can tell when Nephila has caught something and is wrapping it in silk for storage. It then runs over and eats the insect itself, often after cutting it free from the main web so that the host spider can no longer detect it.
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Zoologist Takeshi Watanabe showed that the Japanese orb-weaver Oclonoba sybotides changes the structure of its web when it is hungry. It adds spiral decorations that increase the tension along the spokes, improving the web’s ability to transmit the weaker vibrations transmitted by smaller prey. When it is famished, every morsel counts. To capture such morsels, the spider expands the range of its senses by changing the nature of its web.
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But here’s the truly important part: Watanabe found that a well-fed spider will also go after small flies if it is placed onto a tense web built by a hungry spider. The spider has effectively outsourced the decision about which prey to attack to its web. The choice depends not just on its neurons, hormones, or anything else inside its body, but also on something outside it—something it can create and adjust. Even before vibrations are detected by its lyriform organs, the web determines which vibrations will arrive at the leg. The spider will eat whatever it’s aware of, and it sets the bounds of its awareness—the extent of its Umwelt—by spinning different kinds of webs. The web, then, is not just an extension of a spider’s senses but an extension of its cognition. In a very real way, the spider thinks with its web. Tuning the silk is like tuning its own mind.
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But unlike touch, hearing can operate over long distances. Unlike vision, hearing functions in darkness and through solid, opaque barriers. Unlike the vibrational sense from the previous chapter, hearing doesn’t need a surface and can work through all-encompassing media like air or water. And unlike smell, which is limited by the slow diffusion of molecules, hearing works at the considerably faster speed of sound. Some senses have a few of these qualities, but hearing has them all, which is why some animals rely so heavily upon it. William Stebbins once encapsulated this beautifully: “Very different from other forms of stimulation, [sound] can impart information on current events at an unseen distance,” he wrote.
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Indeed, most insects seem to be deaf, and since they handily outnumber all other animal species, it follows that most animals might be deaf. This might seem odd, especially since sound seems so omnipresent to those of us who can hear. And yet millions of deaf people do just fine without it, and many animals don’t bother with it at all. If you look at our fellow mammals and other vertebrates, you might be forgiven for thinking that hearing is invaluable. If you look at insects, you realize that it is decidedly optional.
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In the 1960s, before his work on barn owls, Masakazu Konishi found direct evidence that the processing speed of bird hearing is exceptionally fast. He played strings of rapid clicks to sparrows, while recording the electrical activity of neurons in the hearing centers of their brains. The neurons fired once per click, even when the clicks were just 1.3 to 2 milliseconds apart. At such speeds—between 500 and 770 clicks per second—a cat’s auditory neurons can only keep to the same tempo around 10 percent of the time.
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And as James Simmons and Cindy Moss have shown, the bat’s nervous system is so sensitive that it can detect differences in echo delay of just one or two millionths of a second, which translates to a physical distance of less than a millimeter.24 Through sonar, it gauges the distance to a target with far more precision than any human can with our sharp eyes.
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Volume helps, too. Annemarie Surlykke showed that the sonar call of the big brown bat can leave its mouth at 138 decibels—roughly as loud as a siren or jet engine. Even the so-called whispering bats, which are meant to be quiet, will emit 110-decibel shrieks, comparable to chainsaws and leaf blowers. These are among the loudest sounds of any land animal, and it’s a huge mercy that they’re too high-pitched for us to hear.
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The third problem is one of speed. Every echo provides a snapshot. Bats fly so quickly that they must update those snapshots regularly to detect fast-approaching obstacles or fast-escaping prey. John Ratcliffe showed that they do so with vocal muscles that can contract up to 200 times a second—the fastest speeds of any mammalian muscle.
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But as Schnitzler discovered in 1967, CF bats can compensate for Doppler shifts. When closing in on a target, they produce calls that are lower than their normal resting frequency, so the upshifted echoes hit their ears at exactly the right pitch. And they do this (quite literally) on the fly, constantly tweaking their calls so that the echoes from targets ahead stay within 0.2 percent of the ideal frequency. This is a staggering feat of motor control that’s almost unmatched in the animal kingdom.
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It is no wonder that bats are so successful. They’re found on every continent except Antarctica, and they account for one in every five mammal species. There are bats that pluck insects from the air and bats that pluck fruit from trees. There are bats that catch frogs, bats that drink blood, and bats that sip nectar with tongues more than twice as long as their bodies. There are bat-eating bats. There are bats that go fishing by echolocating on ripples. There are bats that pollinate plants by echolocating on dish-shaped leaves that are adapted to reflect sonar pulses.
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As we saw in the previous chapter, more than half of moth species have ears that can hear bat sonar. Such ears offer a considerable advantage. Bats are listening for sounds that have traveled to a moth and back again, but moths only have to detect the same sounds after their initial outward journey, when they’re much stronger. So while bats can hear small moths from no more than 9 yards away, moths can hear bats from 15 to 33 yards away. Many of them exploit this lead by executing dodges, loops, and power dives whenever they hear bat voices. Others talk back.
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The U.S. Navy started training dolphins in the 1960s to rescue lost divers, find sunken equipment, and detect buried mines. In the 1970s, it invested heavily in echolocation research, not to understand how the dolphins themselves perceived the world but to improve military sonar by reverse-engineering the animals’ superior capabilities.
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Dolphins could discriminate between different objects based on shape, size, and material. They could distinguish between cylinders filled with water, alcohol, and glycerine. They could identify distant targets from the information in a single sonar pulse. They could reliably find items buried under several feet of sediment, and they could tell if those objects were made of brass or steel—feats that no technological sonar can yet match. To date, “the only sonar that the Navy has that can detect buried mines in harbors is a dolphin,” Au says.
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Sound also interacts differently with objects underwater. Generally, sound waves reflect when they encounter a change in density. In the air, they ricochet off solid surfaces. But in water, they’ll penetrate flesh (which mostly has a density similar to water’s) and bounce off internal structures like bones and air pockets. While bats can only sense the outer shapes and textures of their targets, dolphins can peer inside theirs. If a dolphin echolocates on you, it will perceive your lungs and your skeleton. It can likely sense shrapnel in war veterans and fetuses in pregnant women. It can pick out the air-filled swim bladders that allow fish, their main prey, to control their buoyancy. It can almost certainly tell different species apart based on the shape of those air bladders
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The beaked whales, for example, are odontocetes that look dolphin-esque on the outside—but on the inside, their skulls bear a strange assortment of crests, ridges, and bumps, many of which are only found in males. Pavel Gol’din has suggested that these structures might be the equivalent of deer antlers—showy ornaments that are used to attract mates. Such ornaments would normally protrude from the body in a visible and conspicuous way, but that’s unnecessary for animals that are living medical scanners. With “internal antlers,” beaked whales could conceivably advertise to mates without needing to disrupt their sleek silhouettes.
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A striking pattern emerged: On days with the most intense solar storms, gray whales were four times more likely to beach themselves. This correlation doesn’t prove that whales have a compass, but it strongly hints that they do. More than that, it speaks to the awesome nature of magnetoreception. Here is a sense in which the forces produced by a planetary layer of molten metal collide with those unleashed by a tempestuous star, together swaying the mind of a wandering animal and determining whether it finds its way successfully or loses it for good.
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At the time of writing, magnetoreception remains the only sense without a known sensor. Magnetoreceptors are “the holy grail of sensory biology,” Eric Warrant tells me. “There may even be a Nobel Prize in finding them.”
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These lines of evidence hint at a startling conclusion: Songbirds might be able to see Earth’s magnetic field, perhaps as a subtle visual cue that overlays their normal field of view. “That’s the most likely scenario, but we don’t know because we can’t ask the birds,” Mouritsen says. Perhaps a flying robin always sees a bright spot in the direction of north. Perhaps it sees a gradient of shade painted over the landscape.
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When an animal decides to move, its nervous system issues a motor command—a set of neural signals that tell its muscles what to do. But on its way to the muscles, this command is duplicated. The copy heads to the sensory systems, which use it to simulate the consequences of the intended movement. When the movement actually occurs, the senses have already predicted the self-produced signals that they are about to experience. And by comparing that prediction against reality, they can work out which signals are actually coming from the outside world and react to them appropriately.fn5 All of this happens unconsciously, and while it isn’t intuitive, it is central to our experience of the world. The information detected by the senses is always a mix of self-produced (reafference) and other-produced (exafference), and animals can tell the two apart because their nervous systems are constantly simulating the former.
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Corollary discharges explain why you can’t tickle yourself: You automatically predict the sensations that your writhing fingers would produce, which cancels out the actual sensations that you feel. They’re why your view is stable even though your eyes are constantly darting around.fn8 They’re how chirping crickets can block out the sounds of their own calls. They’re why fish can sense the flows created by other fish without being confused by their own swimming, and why earthworms can crawl ahead without reflexively recoiling.
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Over a third of humanity, and almost 80 percent of North Americans, can no longer see the Milky Way. “The thought of light traveling billions of years from distant galaxies only to be washed out in the last billionth of a second by the glow from the nearest strip mall depresses me no end,” vision scientist Sonke Johnsen once wrote.
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We now live in the Anthropocene—a geological epoch defined and dominated by the deeds of our species. We have changed the climate and acidified the oceans by releasing titanic amounts of greenhouse gases. We have shuffled wildlife across continents, replacing indigenous species with invasive ones. We have instigated what some scientists have called an era of “biological annihilation,” comparable to the five great mass extinction events of prehistory.1 And amid this already dispiriting ledger of ecological sins, there is one that should be especially easy to appreciate and yet is often ignored—sensory pollution. Instead of stepping into the Umwelten of other animals, we have forced them to live in ours by barraging them with stimuli of our own making.2 We have filled the night with light, the silence with noise, and the soil and water with unfamiliar molecules. We have distracted animals from what they actually need to sense, drowned out the cues they depend upon, and lured them, like moths to a flame, into sensory traps.
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Our eyes are among the sharpest in the animal kingdom, but their high resolution comes with the inescapable cost of low sensitivity. Unlike most other mammals, our vision fails us at night, and our culture reflects our diurnal Umwelt. Light has come to symbolize safety, progress, knowledge, hope, and good. Darkness epitomizes danger, stagnation, ignorance, despair, and evil. From campfires to computer screens, we have craved more light, not less.fn3 It is jarring for us to think of light as a pollutant, but it becomes one when it creeps into times and places where it doesn’t belong.
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Between World War II and 2008, the global shipping fleet more than tripled, and began moving 10 times more cargo at higher speeds.40 Together, they raised the levels of low-frequency noise in the oceans by 32 times—a 15-decibel increase over levels that Hildebrand suspects were already around 15 decibels louder than in primordial pre-propeller seas. Since giant whales can live for a century or more, there are likely individuals alive today who have personally witnessed this growing underwater racket and who now only hear over a tenth of their former range.
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Our influence is not inherently destructive, but it is often homogenizing. In pushing out sensitive species that cannot abide our sensory onslaughts, we leave behind smaller and less diverse communities. We flatten the undulating sensescapes that have generated the wondrous variety of animal Umwelten.
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Consider Lake Victoria in East Africa. Once, it was home to over 500 species of cichlid fish, almost all of which were found nowhere else. That extraordinary diversity arose partly because of light. In deeper parts of the lake, light tends to be yellow or orange, while blue is more plentiful in shallower waters. These differences affected the eyes of the local cichlids and, in turn, their mating choices. Evolutionary biologist Ole Seehausen found that female cichlids from deeper waters prefer redder males, while those in the shallows have their eyes set on bluer ones. These diverging penchants acted like physical barriers, splitting the cichlids into a spectrum of differently colored forms. Diversity in light led to diversity in vision, in colors, and in species. But over the last century, runoff from farms, mines, and sewage filled the lake with nutrients that spurred the growth of clouding, choking algae. The old light gradients flattened in some places, the cichlids’ colors and visual proclivities no longer mattered, and the number of species collapsed. By turning off the light in the lake, humans also switched off the sensory engine of diversity, leading to what Seehausen has called “the fastest large-scale extinction event ever observed.”
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In the woodlands of New Mexico, Clinton Francis and Catherine Ortega found that the Woodhouse’s scrub-jay would flee from the noise of compressors used in extracting natural gas. The scrub-jay spreads the seeds of the pinyon pine tree, and a single bird can bury between 3,000 and 4,000 pine seeds a year. They are so important to the forests that in quiet areas where they still thrive, pine seedlings are four times more common than in noisy areas that they have abandoned. Pinyon pines are the foundation of the ecosystem around them—a single species that provides food and shelter for hundreds of others, including Indigenous Americans. To lose three-quarters of them would be disastrous. And since they grow slowly, “noise might have hundred-plus-year consequences for the entire ecosystem,” Francis tells me.
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IN 1995, ENVIRONMENTAL HISTORIAN William Cronon wrote that “the time has come to rethink wilderness.” In a searing essay, he argued that the concept of wilderness, especially as perceived in the United States, had become unjustly synonymous with grandeur. Eighteenth-century thinkers believed that vast and magnificent landscapes reminded people of their own mortality and brought them closer to glimpsing the divine. “God was on the mountaintop, in the chasm, in the waterfall, in the thundercloud, in the rainbow, in the sunset,” Cronon wrote. “One has only to think of the sites that Americans chose for their first national parks—Yellowstone, Yosemite, Grand Canyon, Rainier, Zion—to realize that virtually all of them fit one or more of these categories. Less sublime landscapes simply did not appear worthy of such protection; not until the 1940s, for instance, would the first swamp be honored, in Everglades National Park, and to this day there is no national park in the grasslands.”
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Equating wilderness with otherworldly magnificence treats it as something remote, accessible only to those with the privilege to travel and explore. It imagines that nature is something separate from humanity rather than something we exist within. “Idealizing a distant wilderness too often means not idealizing the environment in which we actually live, the landscape that for better or worse we call home,” Cronon wrote.
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Wonders exist in a backyard garden, where bees take the measure of a flower’s electric fields, leafhoppers send vibrational melodies through the stems of plants, and birds behold the hidden palettes of rurples and grurples. In writing this book, I have found the sublime while confined to my home by a pandemic, watching tetrachromatic starlings gathering in the trees outside and playing sniffing games with my dog, Typo. Wilderness is not distant. We are continually immersed in it. It is there for us to imagine, to savor, and to protect.
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In 1934, after considering the senses of ticks, dogs, jackdaws, and wasps, Jakob von Uexküll wrote about the Umwelt of the astronomer. “Through gigantic optical aids,” he wrote, this unique creature has eyes that “are capable of penetrating outer space as far as the most distant stars. In its [Umwelt], suns and planets circle at a solemn pace.” The tools of astronomy can capture stimuli that no animal can naturally sense—X-rays, radio waves, and gravitational waves from colliding black holes. They extend the human Umwelt across the extent of the universe and back to its very beginning.
