Top Quotes: “An Immense World: How Animal Senses Reveal the Hidden Realms Around Us” — Ed Yong

Introduction

“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.”

“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.”

“Aristotle missed a few senses 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?”

Smell

“The scents unleashed by distant rain can clue people in to advancing storms; the odorants emitted by humans arriving home can send their dogs running to a door.”

“The English language confirms his view with just three dedicated smell words: stinky, fragrant, and musty. Everything else is a synonym (aromatic, foul), a very loose metaphor (decadent, unctuous), a loan from another sense (sweet, spicy), or the name of a source (rose, lemon). Of the five Aristotelian senses, four have vast and specific lexicons. Smell, as Diane Ackerman wrote, “is the one without words.”

The Jahai people of Malaysia would disagree, as would the Semaq Beri, the Maniq, and the many other hunter-gatherer groups who have dedicated smell vocabularies. The Jahai use a dozen words for smells and smell alone. One describes the scent in gasoline, bat droppings, and millipedes. Another is for some quality shared by shrimp paste, rubber tree sap, tigers, and rotten meat. Yet another refers to soap, the pungent durian fruit, and the popcorn-like twang of the binturong. They “have this ease of talking about smells,” says psychologist Asifa Majid, who found that the Jahai can name smells as easily as English-speakers can name colors.

Just as tomatoes are red, the binturong is Itpit. Smell is also a fundamental part of their culture. Once, Majid was told off by Jahai friends for sitting too close to her research partner and allowing their smells to mingle. Another time, she tried to name the smell of a wild ginger plant; children mocked her not only for failing but also for treating the whole plant as a single object, when the stem and flowers obviously had distinct smells. The myth of poor human olfaction “might have been overridden much earlier if the humans under consideration had been Jahai instead of Brits and Americans,” Majid tells me.

Even Westerners can pull off surprising olfactory feats when given the chance. In 2006, neuroscientist Jess Porter took blindfolded students to a park in Berkeley and asked them to follow a 10-meter trail of chocolate oil that she had drizzled on the grass. The students got down on all fours, snuffled about like dogs, and looked ridiculous. But they succeeded, and got better with practice.”

“Light and sound waves can be defined by clear and measurable properties like brightness and wavelength, or loudness and frequency. Shine wavelengths of 480 nanometers into my eyes, and I’ll see blue. Sing a note with a frequency of 261 hertz (Hz), and I’ll hear middle C. Such predictability simply doesn’t exist in the realm of smells. The variation among possible odorants is so wide that it might as well be infinite. To classify them, scientists use subjective concepts like intensity and pleasantness, which can only be measured by asking people. Even worse, there are no good ways of predicting what a molecule smells like — or even if it smells at all — from its chemical structure.”

“Male moths, for example, are tuned to sexual chemicals released by females. They pick up these odorants from miles away using feathery antennae, and slowly flutter over to the source. Smell is so important to them that when scientists transplanted the antennae of female sphinx moths onto males, the recipients behaved like females, seeking out the scent of egg-laying sites instead of mates. Their sense of smell is clearly amazing.”

“Lightweight chemicals that easily rise into the air are used to summon mobs of workers that can rapidly overwhelm prey, or to raise fast-spreading alarms. Crush the head of an ant, and within seconds, nearby colony-mates will sense the aerosolized pheromones and charge into battle.”

“Pheromones hold such sway over ants that they can force the insects to behave in bizarre and detrimental ways, in disregard of other pertinent sensory cues. Red ants will look after the caterpillars of blue butterflies, which look nothing like ant grubs but smell exactly like them. 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. 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. It didn’t matter that the ant was alive and visibly kicking. What mattered was that it smelled dead.”

“Female lobsters urinate into the faces of males to tempt them with a sex pheromone. Male mice produce a pheromone in their urine that makes females especially attracted to other components in their odor.”

“Young Maasai men will sometimes spear elephants, and Bates reasoned that the creatures were disconcerted by the lingering scents in the jeep — some combination of the cows that the Maasai raise, the dairy products they eat, and the ochre they daub on their bodies. To test this idea, she hid various bundles of clothes in elephant country. When the animals approached washed garments or those worn by the Kamba, who pose no threat to them, they were curious but unconcerned. But every time they got wind of clothes worn by the Maasai, their reactions were unmistakable. “Once the first trunk went up, the whole group ran away as fast as they could, and almost always into long grass,” Bates tells me. “It was incredibly stark — every group, every time.””

“In 2007, Lucy Bates found a clever way of testing this idea. She followed family groups of elephants and waited for one to urinate. Once the herd had left, she drove over, scooped up the urine-soaked soil with a trowel, and placed it in an ice cream tub. She then drove around the savannah until she found either the same herd of elephants or a different one. Cutting them off, she emptied the container of soil onto the path ahead of them, sped off to a distant vantage point, and waited. “It was not the most pleasant experiment,” she tells me. “Often, you’d think you know where they were going and put the sample out, and they’d change direction. That was quite soul-destroying.” When she got it right, the elephants would always inspect the urine as they approached. If it came from a different family group, they quickly ignored it. If it came from a family member who wasn’t part of the current unit, they showed more interest. But if it came from an elephant who was part of the same group and walking behind them, they were especially curious. They knew exactly who had left the urine, and since that individual couldn’t possibly have teleported ahead, they seemed confused and carefully investigated the displaced scent.”

“Salmon can return to the very streams in which they were born by homing in on the distinctive scents of those natal waters.”

“Polar bears might be able to navigate across thousands of miles of indistinct ice because glands in their paws leave scent behind with every step. These examples are so common that some scientists believe the main purpose of animal olfaction isn’t to detect chemicals but to use them in navigating through the world.”

“With the aid of its tongue, a snake smells the world. Each flick is the equivalent of a sniff. Indeed, the very first thing that a hatchling serpent does upon breaking out of its egg is to flick its tongue. “That tells you something about the primacy of the sense,” Schwenk says.

Using its tongue, a male garter snake can track a slithering female by following the trail of pheromones she leaves behind. By comparing what she deposited on different sides of objects she pushed against, he can work out her direction. Once he finds her, he can gauge her size and health, possibly with just one or two flicks. He can do this all in the dark. A male can even be fooled into vigorously mating with a paper towel that has been imbued with a female’s scent. But all of these feats could be just as easily accomplished with a paddle-shaped, human-esque tongue. So why do snakes have forked ones? Schwenk reasoned that the fork allows snakes to smell in stereo, by comparing chemical traces at two points in space. If both tips detect trail pheromones, the snake stays on course. If the right tip gets a hit but the left one doesn’t, the snake veers right. If both come up empty, it swings its head from side to side until it regains the trail. The fork allows the snake to precisely define the edges of the path.”

Taste

“Taste is reflexive and innate, while smell is not. 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. 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. Even animal pheromones, which are traditionally thought to trigger hardwired responses, are surprisingly flexible in their effects, which can be sculpted through experience.

Taste, then, is the simpler sense.”

“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?”

“There’s only one sensation of bitter because you don’t need to know which bitter thing you’re tasting — you just need to know to stop tasting it. Taste is mostly a final check before consumption: Should I eat this? That’s why snakes barely bother with taste. With their flickering tongues, they can make decisions about whether something is worth eating through smell well before their mouths make contact. It’s almost unheard of for a snake to strike a prey animal and then spit it out. (We tend to wrongly equate taste with flavor, when the latter is more dominated by smell. That’s why food seems bland when you have a cold: Its taste is the same, but the flavor dims because you can’t smell it.) Reptiles, birds, and mammals taste with their tongues.

Other animals aren’t so restricted. If you’re very small, food isn’t just something you put in your mouth, but something you can walk upon. As such, most insects can taste with their feet and legs. Bees can detect the sweetness of nectar just by standing on a flower. Flies can taste the apple you’re about to eat by landing on it.”

“If a mosquito lands on a human arm, “it’s a delight of the senses,” says Leslie Vosshall. “Human skin has a taste to it, which gives them more confirmation that they made it to the right place.” But if that arm is covered with bitter-tasting DEET, the receptors on their feet force them to take off before they get a chance to bite. Vosshall has videos in which a mosquito lands on a gloved hand and walks over to a small patch of exposed but DEET-covered skin. Its leg touches the skin, and immediately withdraws. It circles, tries again, and retreats again. “It’s poignant,” she tells me, in a strange display of sympathy for a mosquito. “It’s also really psychedelic. We have no idea what it’d be like to taste with our fingers.””

“The most extensive sense of taste in nature surely belongs to catfish. These fish are swimming tongues. They have taste buds spread all over their scale-free bodies, from the tips of their whisker-like barbels to their tails. There’s hardly a place you can touch a catfish without brushing thousands of taste buds. If you lick one of them, you’ll both simultaneously taste each other. If I were a catfish, I’d love to jump into a vat of chocolate,” John Caprio tells me. “You could taste it with your butt.””

“Like cats and other modern carnivores, small predatory dinosaurs probably lost the ability to taste sugar. They passed their restricted palate on to their descendants, the birds, many of which still have no sense for sweetness. Songbirds – the vocal and hugely successful group that includes robins, jays, cardinals, tits, sparrows, finches, and starlings – are an exception. In 2014, evolutionary biologist Maude Baldwin showed that some of the earliest songbirds regained their sweet tooth by tweaking a taste receptor that normally senses umami into one that also senses sugar. This change occurred in Australia, a land whose plants produce so much sugar that its flowers overflow with nectar and its eucalyptus trees exude a syrupy substance from their bark. Perhaps these abundant sources of energy allowed the newly sweet-toothed songbirds to thrive in Australia, to endure marathon migrations to other continents, to find nectar-rich flowers wherever they arrived, and to diversify into a massive dynasty that now includes half the world’s bird species. This story is unproven but nonetheless beguiling. It’s possible that if a random Australian bird hadn’t expanded its Umwelt tens of millions of years ago, none of us would be waking up to the melodic sounds of birdsong today.”

Vision

“The perimeter revealed that a vulture’s visual field covers the space on either side of its head but has large blind spots above and below. When it flies, it tilts its head downward, so its blind spot is now directly ahead of it. 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.)

When I think about Martin’s study, I’m suddenly and acutely aware of the large space behind my head that I cannot see and that I seldom think about. Humans and other primates are rather odd in having two eyes that point straight ahead. The left eye gets a very similar view to the right, and their visual fields overlap a lot. This arrangement gives us excellent depth perception. It also means we can barely see things to our sides, and we can’t see what’s behind us without turning our heads. For us, seeing is synonymous with facing, and exploration is achieved through gazing and turning. But most birds (except for owls) tend to have side-facing eyes and don’t need to point their heads at something to look at it.

A soaring vulture that’s scanning the ground can also see other vultures flying next to it, without having to turn. A heron’s visual field covers 180 degrees in the vertical; even when standing upright with its beak pointing straight ahead, it can see fish swimming near its feet. A mallard duck’s visual field is completely panoramic, with no blind spot either above or behind it. When sitting on the surface of a lake, a mallard can see the entire sky without moving. When flying, it sees the world simultaneously moving toward it and away from it. We use the phrase “bird’s-eye view” to mean any vista seen from on high. But a bird’s view is not just an elevated version of a human one. “The human visual world is in front and humans move into it,” Martin once wrote. But “the avian world is around and birds move through it.””

“Most birds also have circular acute zones, but theirs point outward, not forward. If they want to examine objects in detail, they have to look sideways, with just one eye at a time. When a chicken investigates something new, it will swing its head from side to side to look upon it with the acute zone of each eye in turn. “When chickens look at you, you never know what the other eye is doing,” says Almut Kelber, a zoologist who studies bird vision. “They must have at least two centers of attention, which is very hard to imagine.”

Many birds of prey, like eagles, falcons, and vultures, actually have two acute zones in each eye — one that looks forward, and another that looks out at a 45-degree angle. The side-facing one is sharper, and it’s the one that many raptors use when hunting.”

“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.”

“Cows and other livestock also have a somnolent air because their gaze is so fixed. 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). Except for the occasional aerial predators, up and down are largely immaterial to them. There is only across, in every possible direction. A cow can simultaneously see a farmer approaching it from the front, a collie walking up from behind, and the herdmates at its side. Looking around, which is inextricable from our experience of vision, is actually an unusual activity, which animals do only when they have restricted visual fields and narrow acute zones.”

“The fish Anableps anableps, which lives at the surface of South American rivers, also partitions its eyes. The top half sticks out of the water and is adapted for air vision, and the bottom half stays below the surface and is adapted for aquatic vision. It’s also known as the four-eyed fish.”

“The deep-sea crustacean Streetsia challengeri has fused its eyes into a single horizontal cylinder, which looks like a corn dog. It can see in almost every direction circumferentially — above, below, and to the sides — but not ahead or behind.”

“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.””

“Others unsubscribe from vision entirely. In underground realms, in caves, and in other dark corners of Earth where vision cannot earn its worth, eyes are often lost.”

“The crew estimated that it was a 10-foot-long juvenile, which was nowhere close to the species’ maximum size of 43 feet. Still, it was a giant squid — an almost mythic animal, and one with the largest and most sensitive eyes on the planet.

The eyes of a giant squid (and the equally long but much heavier colossal squid) can grow as big as soccer balls, with diameters up to 10.6 inches. These proportions are perplexing. Yes, bigger eyes are more sensitive, and it makes sense for an animal in the dark ocean to have them. But no other creature, including those that live in the deep sea, has eyes that are even in the same ballpark as a giant or colossal squid’s. The next-largest eyes, which belong to the blue whale, are less than half the size. A swordfish’s eye, which is the largest of any fish at 3.5 inches, could fit inside a giant squid’s pupil. The squid’s eyes are not just big; they are absurdly and excessively bigger than those of any other animal. What does it need to see that it can’t see with a swordfish-sized eye?

Sonke Johnsen, Eric Warrant, and Dan-Eric Nilsson think they know the answer. They calculated that in the deep ocean, eyes suffer from diminishing returns. As they get bigger, they cost more energy to run but offer little extra visual power. Once they get past 3.5 inches — that is, swordfish-sized — there’s little point in enlarging them further. But the team found that extra-large eyes are better at one task, and one task alone: spotting large, glowing objects in water deeper than 500 meters. There’s an animal that fits those criteria, and it is one that giant squid really need to see: the sperm whale.

The largest toothed predators in the world, sperm whales are the giant squid’s main nemeses.”

“The cephalopods — octopuses, squid, and cuttlefish — have just one class of these, which means they are also monochromats. They can rapidly change the colors of their skin yet are unable to see their own shifting hues.”

“Even with experiments like this, it is easy to underestimate what other birds can see. They don’t just have human vision plus ultraviolet, or bee vision plus red. Tetrachromacy doesn’t just widen the visible spectrum at its margins. It unlocks an entirely new dimension of colors. Remember that dichromats can make out roughly 1 percent of the colors that trichromats see — tens of thousands, compared to millions. If the same gulf exists between trichromats and tetrachromats, then we might be able to see just 1 percent of the hundreds of millions of colors that a bird can discriminate. 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.”

“Birds aren’t the only tetrachromats. Reptiles, insects, and freshwater fish, including the humble goldfish, have four cones as well. By looking at tetrachromats among modern animals and working backward, scientists can deduce that the first vertebrates were likely tetrachromats, too. Mammals, probably because they were all initially nocturnal, lost two of their ancestral cones and became dichromats. But they scurried beneath the feet of dinosaurs, which were almost certainly tetrachromats and “probably saw all kinds of cool non-spectral colors.””

“Most of the fish in coral reefs are also trichromats. But since red light is strongly absorbed by water, their sensitivities are shifted toward the blue end of the spectrum. This explains why so many reef fish, like the blue tang that stars in Pixar’s Finding Dory, are blue and yellow. To their version of trichromacy, yellow disappears against corals, and blue blends in with the water. Their colors look incredibly conspicuous to snorkeling humans, because our particular trio of cones excels at discriminating blues and yellows. But the fish themselves are beautifully camouflaged to each other, and to their predators.

The color vision of predators diversified the patterns of Central America’s strawberry poison frog — a single species that comes in 15 incredibly different forms. One is lime green with cyan stockings. Another is orange with black spots. These colors are so varied as to seem almost random, but there’s method to the visual madness.

These frogs are poisonous, and the most toxic ones are also the most conspicuous. But as Molly Cummings and Martine Maan discovered, they are conspicuous only to birds and not to other predators like snakes. It is likely that tetrachromatic avian eyes drove the evolution of the outlandish amphibian skins. This makes sense: The colors are intended as warnings, and across the generations, frogs whose hues were best suited to the vision of their predators were more likely to go unattacked. And Cummings and Maan showed that you can work out who those predators are — in this case, birds — by studying the colors of their prey. Since eyes define nature’s palette, an animal’s palette tells you whose eyes it is trying to catch.

You can apply the same logic to flowers. In 1992, Lars Chittka and Randolf Menzel analyzed 180 flowers and worked out what kind of eye would be best at discriminating their colors. The answer — an eye with green, blue, and UV trichromacy — is exactly what bees and many other insects have. You might think that these pollinators evolved eyes that see flowers well, but that’s not what happened. Their style of trichromacy evolved hundreds of millions of years before the first flowers appeared, so the latter must have evolved to suit the former. Flowers evolved colors that ideally tickle insect eyes.”

Touch

“Naked mole-rats are exceptionally long-lived for rodents, with life spans of up to 33 years. Their lower incisors can splay apart and come together to grasp objects. Their sperm are misshapen and sluggish. They can survive for up to 18 minutes without oxygen, a hardship that no mouse can endure for more than a minute. They live in cooperative colonies like those of ants and termites, with one or more breeding queens and dozens of sterile workers. A single naked mole-rat, like the one I am holding, is an unusual sight. So is a naked mole-rat in the open. They normally live within labyrinthine underground tunnels, which they constantly expand, remodel, and patrol in their quest for nutritious tubers.”

“When fish nociceptors fire, the signals travel to parts of the brain that deal with learning and other behaviors more complex than simple reflexes. Sure enough, when the animals are pinched, shocked, or injected with toxins, they’ll behave differently for hours or days — or until they get painkillers. They’ll make sacrifices to get those drugs, or to avoid further discomfort. In one experiment, Sneddon showed that zebrafish prefer to swim in an aquarium full of plants and gravel than in one that’s empty. But if she injected the fish with acetic acid and dissolved a painkiller in the water of the barren aquarium, they abandoned their normal preferences and chose the boring but soothing environment instead. In another study, Sarah Millsopp and Peter Laming trained goldfish to feed in a specific part of an aquarium, and then gave them an electric shock. The fish fled and stayed away for days, forgoing food in the process. They eventually returned, but did so more quickly if they were hungry or if the shock had been mild. Their initial escape might have been reflexive, but they then weighed up the pros and cons of avoiding further harm. As Braithwaite wrote in her book, Do Fish Feel Pain?, “There is as much evidence that fish feel pain and suffer as there is for birds and mammals.””

“Even more surprisingly, Crook found that injured squid behave as if their entire bodies were sore. When humans and other mammals get cut or bruised, the damaged area is painful but the rest of the body isn’t. If I singe my hand, it hurts when I prod the burn but not when I poke my foot. But when Crook damaged one of the squid’s fins, the nociceptors on the opposite fin were just as excitable as those on the wounded side. Imagine if your entire body became delicate to the touch whenever you stubbed your toe: That’s a squid’s reality. “When they’re injured, their whole body becomes hypersensitive,” Crook tells me. “They go from being normal to this potential world of pain.” This might explain why they don’t groom their wounds. They can sense that they’ve been hurt, but they might not be able to tell where.

For mammals, the localized nature of pain allows us to protect and clean vulnerable body parts, while getting on with the rest of our lives. Why should squid lack such a useful source of information? One possibility, Crook says, “is that everything in the ocean will eat a squid.” Injured squid are especially attractive to predatory fish, either because they are more conspicuous or because they look (or smell) like easier prey. By setting their entire bodies on high alert, they might be better at evading attacks that could come from any direction. Body-wide sensitivity also makes sense for animals that cannot physically reach most of their bodies. What good would it do them to know that a fin has been injured when they cant do anything about it?

Octopuses are different. Unlike squid, they can touch every part of their bodies. They can even reach inside themselves to groom their own gills — the equivalent of a human putting a hand down their throat to scratch their lungs. And unlike squid, which are stuck in open-water groups and can’t take a day off, octopuses can hole up in solitary dens until they feel better. Since they have the time and dexterity to tend to their injuries, it would make sense for them to know where their wounds are. And as Crook showed, they do. Octopuses will sometimes break off an arm if its tip is injured. When that happens, the stump will be more sensitive than the arms around it, and octopuses will cradle that stump in their beaks. In her latest study, published in 2021, Crook found that octopuses will avoid places where they’ve been injected with acetic acid, but gravitate to places where they receive painkillers. And once they’re injected with local anesthetic, they stop grooming their injured arms. In her latest paper, Crook is unambiguous: “Octopuses are capable of experiencing pain.””

“Chili peppers burn because the capsaicin within them triggers TRPV1 — a TRP channel that detects painfully high temperatures. Mint cools because it contains menthol, which activates the cold sensor called TRPM8.”

“This concept is intuitive, and yet when we watch extremophiles, from emperor penguins braving the Antarctic chill to camels trekking over scorching sands, it’s easy to think that they are suffering throughout their lives. We admire them not just for their physiological resilience but also for their psychological fortitude. We project our senses onto theirs and assume that they’d be in discomfort because we’d be in discomfort. But their senses are tuned to the temperatures in which they live. A camel likely isn’t distressed by the baking sun, and penguins probably don’t mind huddling through an Antarctic storm. Let the storm rage on. The cold doesn’t bother them, anyway.”

“Gallio showed that flies could easily stay with spaces that are kept to 25°C (77°F), which they love, while avoiding neighboring zones of 30°C (86°F), which they dislike, or 40°C (104°F), which kills them. They could also make these decisions at incredible speed. Whenever they’d hit the edge of a hot zone, they’d immediately execute a sharp midair U-turn, as if they’d run into an invisible wall.

Such maneuvers are possible because the chitin that makes up a fly’s antennae is very good at conducting heat and because the antennae themselves are tiny. They can so quickly equilibrate with their surroundings that a fly can instantly tell if it has blundered into air that’s too hot or cold. Gallio found that it can even use its antennae as stereo thermometers to track gradients of heat, much as a dog uses its paired nostrils for odors. The fly can tell if one antenna is just 0.1°C hotter than the other, and uses those comparisons to steer toward the more comfortable temperature. When Gallio tells me about these results, I suddenly reconsider the movements of every fly I’ve ever seen. Their paths, which always seemed so random and chaotic, now take on an air of purpose, as if the insect is threading its way through an obstacle course of hot and cold that I can’t perceive, don’t care about, and oafishly wade through.”

“The beetles spherical sensors must be extraordinarily sensitive, since the insects frequently travel to burning forests and other hot places from dozens of miles away. The Coalinga oil depot that was struck by lightning in 1925 lies in the middle of an arid, treeless region, and most of the beetles that arrived there likely came from forests that lay 80 miles to the east.”

“The ancestors of birds and mammals independently evolved the ability to produce and control their own body heat, divorcing their temperatures from the temperatures of their surroundings. This ability, known technically as endothermy and colloquially as warm-bloodedness, endowed birds and mammals with speed and stamina, durability and possibility. It allowed them to survive in extreme environments and stay active over long durations and distances. It also made them very easy to track. Their unwavering body heat made them perpetually blaring beacons, which parasites could use to find hosts, and especially blood vessels. Blood, after all, is a superb source of food — rich in nutrients, well balanced, and usually sterile. It’s no surprise that at least 14,000 animal species have evolved to feed on it, or that many of these — bedbugs, mosquitoes, tsetse flies, and assassin bugs — are attuned to heat.

Among mammals, only three species of vampire bats feed exclusively on blood. Two mostly drink from birds, but the common vampire targets mammals, especially large ones like cows or pigs. Its a small animal that measures 3 inches from nose to tail and has a flattened, pug-like face. On the ground, its wings fold back, and it adopts a sprawling, four-legged stance. It approaches targets like this, either landing directly on their backs or alighting nearby and crawling over in a most un-bat-like way. Once near, it painlessly inflicts a small cut with its blade-like incisors and laps up the blood that flows out. A compound in its saliva, aptly known as draculin, stops the blood from clotting, allowing the bat to feed for up to an hour. It can drink its own body weight in blood and must do so once a night to survive. Other senses help it to track a target from afar, but once it gets at least 6 inches away, it uses a thermal sense to pick a good bite site.”

“Rattlesnakes will strike at warm objects, preferring freshly killed mice over long-deceased ones, and they’ll hit their targets in complete darkness. Even a congenitally blind rattlesnake that was born without eyes could kill mice as effectively as a sighted individual. Thanks to its pits, its aim was good enough not only to hit the rodents but to specifically strike them in the head.”

“Crocodiles can detect the gentlest ripples at the water’s surface, crickets can sense the faint breeze produced by a charging spider, and seals can track fish by the invisible currents that they leave as they swim. But most such signals are undetectable to us: I can feel the strong air currents created by my ceiling fan, but little else. For humans (and sea otters), touch is primarily a sense of direct contact.”

“LIKE THE STAR-NOSED mole, many animals that specialize in touch work in conditions where vision is limited. They’re often searching for things that are hidden or hard to find, which forces them to root around with body parts that can probe, press, and explore. Whether we’re talking about a sea otter’s paw or a human’s finger, an elephant’s trunk or an octopus’s arm, animals discover the world by deliberately moving tactile organs over it. And as the mole shows, those organs don’t have to be hands.

The beaks of birds are made of bone and sheathed in the same hard keratin that constitutes your fingernails. They seem inanimate and insensitive — hard, face-mounted tools for grabbing and pecking. But in many species, the tip of the bill contains a smattering of mechanoreceptors, sensitive to vibrations and movements.”

“This simple experiment revealed that the knots could still detect clams that were buried beyond the reach of their bills. They could even sense stones, so they clearly weren’t relying on smells, sounds, tastes, vibrations, heat, or electric fields. Instead, Piersma thinks that they use a special form of touch that works at a distance.

As a knot’s bill descends into the sand, it pushes on the thin rivulets of water between the grains, creating a pressure wave that radiates outward. If there’s a hard object in the way — say, a clam or a rock — the water must flow around it, which distorts the pattern of pressure. The pits on the knot’s bill tip can sense those distortions, detecting surrounding objects without having to make contact with them. This ability, which Piersma calls “remote touch,” is impressive enough, but the knot improves it even further by probing the same areas repeatedly, stabbing its beak up and down several times a second. This stirs up the sand grains, which settle into a denser configuration, heightening the buildup of pressure from the beak and making the distortions more obvious. Every time the knot lowers its head, the food around it becomes more obvious, as if it were using a kind of sonar based on touch instead of hearing.

The emerald jewel wasp also has a long, probing organ with a touch-sensitive tip, but its goals and methods are far grislier than a red knot’s. The wasp — a beautiful inch-long creature with a metallic green body and orange thighs — is a parasite that raises its young on cockroaches. When a female finds a roach, she stings it twice — once in its midsection to temporarily paralyze its legs, and a second time in its brain. The second sting targets two specific clusters of neurons and delivers venom that nullifies the roach’s desire to move, turning it into a submissive zombie. In this state, the wasp can lead the roach to her lair by its antennae, like a human walking a dog. Once there, she lays an egg on it, providing her future larva with a docile source of fresh meat. This act of mind control depends on that second sting, which the wasp must deliver to exactly the right location. Just as a red knot has to find a clam hidden somewhere in the sand, an emerald jewel wasp has to find the roach’s brain hidden somewhere within a tangle of muscles and internal organs.

Fortunately for the wasp, her stinger is not only a drill, a venom injector, and an egg-laying tube but also a sense organ. Ram Gal and Frederic Libersat showed that its tip is covered in small bumps and pits that are sensitive to both smell and touch. With them, she can detect the distinctive feel of a roach’s brain.”

“Many other birds have stiff bristles on their heads and faces. These are often wrongly billed as nets that help birds to snag flying insects. It’s more likely that they’re touch sensors, which the birds use when handling prey, feeding chicks, or maneuvering around dark nests. Such uses might explain why birds have feathers at all. It’s clear that birds evolved from dinosaurs, and that many dinosaurs were covered in bristly proto-feathers or dino-fuzz. These structures were too simple for flight, so they must have evolved for some other reason. The most common explanation is that they provided insulation, but that would only be true if they suddenly appeared in large numbers. Alternatively, and perhaps more plausibly, they could have initially evolved to provide tactile information. As the whiskered auklet shows, an animal only needs a few bristles to extend its sense of touch in useful ways. Perhaps feathers first appeared as small clumps on the heads or arms of dinosaurs, helping them first to feel and only later to fly.

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 vibrisse, from the Latin word for “vibrate.” More commonly, they’re known as whiskers.”

“In 2012, Bauer tested Hugh and Buffett to see if they could distinguish between plastic boards with differently spaced ridges, much as Sarah Strobel later did with Selka the sea otter and various human volunteers. The two manatees performed just as well as the other species. Their faces were the equals of human fingertips.”

“As a fish swims, it leaves behind a hydrodynamic wake — a trail of swirling water that continues to whirl long after the animal has passed. Seals, with their sensitive whiskers, can detect and interpret these trails.”

“To us, touch is rooted in the present, in the instants when a sensor makes contact with a surface. But to Sprouts, touch extends into the recent past, just as smell does to Finn. His whiskers can feel what was, rather than simply what is.”

“The bumps, she discovered, are pressure receptors that can detect vibrations at the water’s surface. They might work like little buttons, akin to the Eimer’s organs on moles. They’re so sensitive that if Soares let a single drop of water fall into an (unsedated) alligator’s tank, the animal would turn and lunge toward the disturbance, even when its eyes and ears were covered. But if Soares covered its snout in a plastic sheet, the drops went unnoticed. The animals use the bumps to scan the thin horizontal layer where air and water meet. They sit in ambush in that layer, waiting for something to land in the water or to arrive at its edge for a drink.”

“These results suggest that a peahen that stands in front of a courting male might be able to detect the air disturbances produced by his tail. As well as seeing his efforts, she might feel them. (This also works in reverse, since females will sometimes display back to males.)”

“Embryonic tadpoles can hatch early when attacked. Warkentin even saw them bursting out of eggs that were held in a snake’s mouth.”

“Light can pass through the translucent eggs, and chemicals can diffuse into them. But vibrations are what really matter. They pass into the eggs and into the embryos, which can distinguish between bad vibes and benign ones without any previous experience of either. A bite from a snake will trigger hatching. Rain, wind, and footsteps will not.”

“Once the tadpoles have transformed into frogs and are ready to make tadpoles of their own, males compete for access to mates. By watching them with infrared cameras, Warkentin and their colleague Michael Caldwell saw that males would square off along a branch, raise their bodies, and vigorously shake their backsides. These displays are meant to be visually captivating, but males will also perform when their lines of sight are obscured. They might not be able to see each other, but they can still feel the vibrations created by their rival’s quivering bum and use those vibrations to assess size and motivation. In these contests, the victors are usually those that shake for more time and create longer-lasting vibrations.

Many other animals probably communicate in this way. Male fiddler crabs attract mates by thumping their gigantic claws on the sand. Termite soldiers drum their heads against the walls of their mounds to create vibrational alarms that attract more soldiers. Water striders — insects that skate along the surface of ponds and lakes — can coerce partners into sex by making ripples that summon vibration-sensitive predators. All of these creatures create and respond to vibrations that travel along the surfaces around them, whether branch or beach.”

“Sand scorpions are some of the Mojave’s most common residents and will eat anything they can successfully grab and sting, including other sand scorpions. In the 1970s, Philip Brownell and Roger Farley realized that the scorpions would readily attack anything that walks or lands within 20 inches of them. “Gentle disturbances of the sand with a twig also triggered a vigorous attack,” Brownell later wrote in Scientific American, “but a moth held squirming in the air a few centimeters from the scorpion did not attract its attention.” It seemed to track its prey using surface waves.

Brownell and Farley tested this idea by placing scorpions in a cunningly designed arena. It looked smooth and continuous on the surface, but a buried air gap blocked vibrations from traveling between the two halves. If a scorpion stood on one half, it was completely oblivious when the researchers prodded the other half with a stick, even at a point just an inch away. But if even one of the scorpion’s legs straddled the gap, it became aware of the entire arena and would turn to face any disturbance.

Its sensors lie in its feet. On the joint that could be loosely described as an “ankle,” there’s a cluster of eight slits, as if the exoskeleton had been scored by a sharp knife. These are the slit sensilla — vibration-detecting organs common to all arachnids. Each slit is spanned by a membrane and connected to a nerve cell. When a surface wave reaches the scorpion, the rising sand pushes against its feet. This compresses the slits by an infinitesimal amount, but enough to squeeze the membrane and cause the nerves to fire. By sensing the tiniest changes in its own exoskeleton, the scorpion can feel the steps of passing prey. The first time this happens, it shifts into its hunting stance. It raises its body, opens its pincers, and arranges its eight feet into a near-perfect circle. In this position, it can work out where surfaces waves are coming from by noting when those waves hit each of its feet. It turns and runs before pausing and waiting for another wave. When one arrives, it turns and runs again, getting closer to its target with each successive tremor. If its pincers collide with something, the scorpion seizes and stings. If it arrives at the source of the waves and can’t find anything, it knows that its prey is underground, and digs it out.”

“The orb web isn’t just another substrate, like soil, sand, or plant stems. It is built by the spider and it is part of the spider. It is as much a part of the creature’s sensory system as the slits on its body.

Like the Nephila in Mortimer’s arachnarium, most orb-weavers sit in the middle of their webs and rest their legs on the radial spokes that funnel vibrations toward them. From this position, they can distinguish the vibrations generated by rustling wind or falling leaves from those created by struggling prey. They can probably work out where those struggles are coming from by comparing the strength of the vibrations hitting each of their legs. They can assess the size of their prisoners, and will approach the larger ones more carefully or not at all. If the prey stops moving, they can find it by deliberately plucking the silk and “listening” to the returning vibrational echoes. When it comes to capturing prey, vibrations supersede other stimuli. If a tasty fly buzzes above an orb-weaver, the spider will simply wave it away with its legs. The fly only becomes recognizable as food if it shakes the web.

This dependency on vibrations is so absolute that many animals can exploit orb-weavers by camouflaging their footsteps.”

“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.”

“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.”

“Since getting a puppy, I’ve been spending a lot more time on the floor than I used to. From that position, I can feel surface vibrations that I hadn’t ever noticed before. I can feel the footsteps of my neighbors as they come in and out. I can feel the rumbles of garbage trucks as they drive past outside.”

Hearing

“The barn owl’s ear shares the same basic structure: The outer ear collects, the middle ear amplifies and transmits, and the inner ear detects. But while your outer ears are a pair of fleshy flaps, the owl’s are effectively its entire face. The feathers of the conspicuous facial disc that makes owls look owlish are thick, stiff, and densely packed. They act like a radar dish that collects incoming sound waves and funnels them toward the ear holes.”

“Over their 480-million-year history, they did so on at least 19 independent occasions, and on almost every imaginable body part. Ears exist on the knees of crickets and katydids, the abdomens of locusts and cicadas, and the mouths of hawkmoths. Mosquitoes hear with their antennae. Monarch caterpillars hear with a pair of hairs on their midsection. The bladder grasshopper has six pairs of ears running down its abdomen, while mantises have a single cyclopean ear in the middle of their chests.”

“When the environment fluctuates from one season to the next, the information that’s relevant also changes. For a North American bird, spring often means sex. The air fills with courtship calls that are absent in other times of year and must now be carefully judged. Fall brings openness: Bare branches make little birds more visible to predators. The ability to localize the sound of approaching danger, which is inextricably linked to fast hearing, becomes paramount. An animal’s Umwelt cannot be static, because an animal’s world isn’t static.

Bird songs don’t lie beyond the reach of human senses, like the circularly polarized patterns of mantis shrimps or the vibrational songs of treehoppers. We can very much hear them. The fee-bee-fee-bay of chickadees and the wha-wha-wha of nuthatches are obvious enough that we can transcribe them. And yet, we still don’t appreciate these signals in the same way as their intended audiences can. To us, a chickadee song sounds the same whether we listen to it in October or March. To a chickadee, it does not.”

“He has seen whales saloming between underwater mountain ranges, zigging and zagging between landmarks hundreds of miles apart. “When you watch these animals move, it’s as if they have an acoustic map of the oceans,” he says. He also suspects that the animals can build up such maps over their long lives, accruing sound-based memories that lurk in their mind’s ear.”

“The scale of a whale’s hearing is hard to grapple with. There’s the spatial vastness, of course, but also an expanse of time. Underwater, sound waves take just under a minute to cover 50 miles. If a whale hears the song of another whale from a distance of 1,500 miles, it’s really listening back in time by about half an hour, like an astronomer gazing upon the ancient light of a distant star. If a whale is trying to sense a mountain 500 miles away, it has to somehow connect its own call with an echo that arrives 10 minutes later. That might seem preposterous, but consider that a blue whale’s heart beats around 30 times a minute at the surface, and can slow to just 2 beats a minute on a dive. They surely operate on very different timescales than we do.”

“Whales weren’t always big. They evolved from small, hoofed, deer-like animals that took to the water around 50 million years ago. Those ancestral creatures probably had vanilla mammalian hearing. But as they adapted for an aquatic life, one group of them — the filter-feeding mysticetes, which include blues, fins, and humpbacks — shifted their hearing to low infrasonic frequencies. At the same time, their bodies ballooned into some of the largest Earth has ever seen. These changes are probably connected. The mysticetes achieved their huge size by evolving a unique style of feeding, which allows them to subsist upon tiny crustaceans called krill. Accelerating into a krill swarm, a blue whale expands its mouth to engulf a volume of water as large as its own body, swallowing half a million calories in one gulp. But this strategy comes at a cost. Krill aren’t evenly distributed across the oceans, so to sustain their large bodies, blue whales must migrate over long distances. The same giant proportions that force them to undergo these long journeys also equip them with the means to do so — the ability to make and hear sounds that are lower, louder, and more far-reaching than those of other animals.

Back in 1971, Roger Payne speculated that foraging whales could use these sounds to stay in touch over long distances. If they simply called when fed and stayed silent when hungry, they could collectively comb an ocean basin for food and home in on bountiful areas that lucky individuals have found. A whale pod, Payne suggested, might be a massively dispersed network of acoustically connected individuals, which seem to be swimming alone but are actually together.”

“They’d noticed that elephant families would often move in the same directions for weeks at a time, even though they were separated by several miles. In the early evenings, different groups would also converge on the same waterholes at the same time, but from different directions. Infrasound carries over long distances, even in air, and if elephants use it to communicate, that would explain how they can synchronize their movements across a savannah. Poole and Moss invited Payne to join them. She accepted, and in 1986, the team showed that African elephants use infrasound just like their Asian counterparts — and in every conceivable context. There are contact rumbles that help individuals find each other.

There are greeting rumbles that they make when reuniting after a separation. Males make rumbles when in heat, and females make rumbles in response to them. There’s a “let’s go” rumble, and an “I just had sex” rumble. At close range, most of these rumbles contain frequencies audible to human ears, but some became apparent only when the team sped up their recordings, or visualized them.

These infrasonic rumbles are airborne sounds, so they’re partly distinct from the surface-borne signals that Caitlin O’Connell more recently identified. Both are mostly imperceptible to us, and both can be detected by other elephants over long ranges. The low-frequency parts of the rumbles range between 14 and 35 Hz — about the same as a large whales. Those calls don’t carry as far in the air as underwater, and atmospheric conditions dictate how far they can travel: The colder, clearer, and calmer the air, the greater the range. In the heat of midday, an elephant’s auditory world shrinks. A few hours after sunset, it expands tenfold, theoretically allowing elephants to hear each other over several miles.”

“In the winter of 1877, Joseph Sidebotham was staying in a hotel at Menton, France, when he heard what sounded like a canary singing on his balcony. He soon discovered that the singer was actually a mouse. He fed it with biscuits, and it reciprocated by singing for hours by the fireplace, cranking out a tune as beautiful as that of any bird. His son suggested that all mice might sing similar melodies at pitches too high for humans to hear. Sidebotham disagreed. “I am inclined to think the gift of singing in mice is but of very rare occurrence,” he wrote to the journal Nature.

He was wrong. Roughly a century later, scientists realized that mice, rats, and many other rodents do indeed make a wide repertoire of “ultrasonic” calls, with frequencies too high to be audible to humans. They make these sounds when playing or mating, when stressed or cold, when aggressive or submissive. Pups that are separated from their nests make ultrasonic “isolation calls” that summon their mothers. Rats that are tickled by humans make ultrasonic chirps that have been compared to laughter. Richardson’s ground squirrels produce ultrasonic alarm calls when they detect a predator (or a tan fedora repeatedly thrown by a scientist to mimic a predator). Male mice that sniff female hormones produce ultrasonic songs that are remarkably similar to those of birds, complete with distinctive syllables and phrases. Females attracted to these serenades join their chosen partners in an ultrasonic duet.”

“Calls demand listeners. If the hummingbirds’ tunes lie beyond their own Umwelten, who’s the audience?

Maybe it’s insects? Even though most insects can’t hear at all, many of those with ears can hear ultrasonic frequencies. More than half of the 160,000 species of moths and butterflies are so equipped. The greater wax moth can even hear frequencies near 300 kHz — the highest limit of any animal by some margin. Hummingbirds eat insects as well as nectar, so perhaps they produce ultrasonic calls that they can’t hear to flush out the insects that can.”

“Echolocation is mentally demanding, especially since bats do everything they do at speed. Often they simply don’t have the time to use their sonar to its fullest capacity, which is why they often make ridiculous mistakes that seem beneath them. They can distinguish two grades of sandpaper whose grains differ by half a millimeter, but will also plow headlong into a newly installed cave door. They can discern flying insects by shape, but will go after a pebble launched into the air. Bats are fully capable of avoiding such errors. They’re just not paying attention. They’re relying on memory and instinct.”

“In their adult form, they have no mouths and little time. In a week, they’ll be dead. Until then, “all they do is mate and evade bats,” Barber says. They have no noxious chemicals. They can’t make jamming clicks. They can’t even hear bats coming because they have no ears. But those long tails that grow from their hindwings flap and spin behind them as they fly, producing echoes that distract echolocating bats into attacking an inessential body part. On average, a luna moth without tails is nine times more likely to be eaten than one whose tails are intact. “When I discovered that, I thought: This can’t be real,” Barber says. “Echolocation is such a remarkable sense. How can a spinning piece of membrane fool the bats? But we see it, and consistently.””

“1987, Nachtigall’s team started working with a false killer whale — an 18-foot-long, black-skinned dolphin species known for being smart and sociable. The animal, Kina, could use her sonar to tell the difference between hollow metal cylinders that looked identical to the human eye and that differed in thickness by the width of a hair. On one memorable occasion, the team tested Kina using two cylinders that had been manufactured to the same specifications. To everyone’s confusion, Kina repeatedly indicated that the objects were different. When the team had the cylinders remeasured, they realized that one had a minuscule taper and was 0.6 millimeters wider at one end than the other. “It was incredible,” Nachtigall recalls. “We ordered them to be the same, the machinists said they were the same, and the animal said, No, they’re different. And she was right.”

Dolphins can also echolocate on a concealed object and then recognize the same object visually — even on a television screen. This might seem like an obvious feat, but stop to consider what it involves. The animal isn’t just working out the object’s position but constructing a mental representation of that object, which can be translated to its other senses. And it’s doing that with sound – a stimulus that doesn’t naturally carry rich, three-dimensional information. If you heard a saxophone, you might recognize the instrument and work out where its music is coming from, but good luck predicting its shape from sound alone. You could, however, touch a saxophone and get a solid impression of what it should look like. So it is with echolocation. This sense is often described as “seeing with sound,” but you could just as easily think of it as “touching with sound.” It’s as if a dolphin is reaching out and squeezing its surroundings with phantasmal hands.”

“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 waters) 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. And it can tell if a fish has something weird inside it, like a metal hook. In Hawaii, false killer whales often pluck tuna off fishing lines, and “they’ll know where the hook is inside that fish,” Aude Pacini, who studies these animals, tells me. “They can ‘see’ things that you and I would never consider unless we had an X-ray machine or an MRI scanner.””

“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 Goldin 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.”

“At night, spinner dolphins — a small and especially acrobatic species — capture prey by working together in teams of up to 28 individuals. Kelly Benoit-Bird and Whitlow Au showed that these hunts go through several distinct phases. First, the spinners patrol in a widely spaced line. Then, once they’ve found a group of fish or squid, they cluster together into a tight row and bulldoze their prey. The victims pile on top of each other, and the spinners encircle them to cut off any escapees. Pairs of dolphins then take turns darting into the circle from opposite ends, picking off the trapped animals. Throughout this sequence, the spinners switch formations seamlessly and simultaneously, and at those transition points they’re especially likely to click. Are they shouting commands at each other? Are they echolocating on their teammates to track their positions? Could they be using each other’s echoes to extend their own perceptions? Whatever the case, their coordinated, intelligent behavior is made possible by sonar — a sense that works over distances longer than a single dolphin. The pod might be spaced over 40 meters of water, but they’re connected by sound and can act as one.”

“WHEN I TRY to click with my tongue, the sound has a muffled wetness to it, like a stone being thrown into a pond. When Daniel Kish clicks, the sound is sharper, crisper, and much louder. It is the sound of someone snapping their fingers, a sound that will make you snap to attention. It’s a sound that Kish has been practicing for almost all of his life.

Born in 1966 with an aggressive form of eye cancer, Kish had his right eye removed at 7 months, and his left at 13 months. Shortly after he lost his second eye, he started clicking. At the age of two, he would routinely climb out of his crib and explore his house. One night, he crawled out of his bedroom window, dropped into a flower bed, and toddled around the backyard, clicking as he went. He remembers sensing the acoustically transparent chain-link fence, and the large house on the other side. He remembers climbing the fence, and then others like it, until a neighbor finally called the police, who brought him home. It wasn’t till much later that Kish learned what echolocation was, or that he’d been doing it for about as long as he’d been walking.

Now in his 50s, Kish is still clicking and still using the rebounding echoes to perceive the world. I meet him at his house in Long Beach, California, where he lives by himself. Inside, he doesn’t need to echolocate; he knows exactly where everything is. But when we go for a walk, the clicks come into play. Kish walks briskly and confidently, using a long cane to sense obstacles at ground level and echolocation to sense everything else. As we head down a residential street, he accurately narrates everything that we pass. He can tell where each house begins and ends. He can locate porches and shrubbery. He knows where cars are parked along the road.”

“A tree, Kish tells me on our walk, sounds like a solid vertical post that is topped by a larger, softer blob. A wooden fence will sound softer than a wrought iron one, and both will sound more solid than a chain-link fence. On his street the crisp sound of the hardwood door sandwiched between the fuzzier sounds of the surrounding bushes tells him when he’s back at his house. Occasionally, unexpected combinations of texture confuse him. We pass a car that’s parked in an incompletely paved driveway, with concrete beneath its tires but turf beneath its undercarriage. Kish pauses as we pass it, and asks me if someone has parked on their lawn.

For Kish, echolocation is freedom. He walks around the city, rides his bike, and goes on solo hikes. And he’s not unusual in that: Since at least 1749, there have been anecdotes about blind people who could walk unassisted through crowded streets, or (in later centuries) cycle around obstacles and skate in busy rinks. Humans had been echolocating for hundreds of years before anyone had even defined echolocation as a concept. The ability was historically described as “facial vision” or an “obstacle sense.” As with bats, researchers believed that practitioners were sensing subtle changes in airflow over their skin. The practitioners, meanwhile, were mostly mystified about the nature of their perceptions.

Take Michael Supa. A psychology student, Supa had been blind since childhood. He would regularly detect distant obstacles in his daily life but couldn’t explain how he did it. He suspected that hearing was involved, since hed often snap his fingers or click his heels to find his way around. In the 1940s, he tested that idea. In a large hall, Supa showed that he and other students — one also blind, and two sighted but blindfolded — could use their hearing to detect a large Masonite screen. This worked best if they wore shoes on a hardwood floor, less well if they wore socks on carpet, and not at all if their ears were plugged.”

“When Griffin coined the term echolocation in 1944, he was describing the skills of both bats and blind people, citing Supa. But while bat sonar became a common part of popular knowledge, human echolocation did not. To this day, Kish will meet echolocation researchers “who have no idea that humans can echolocate,” he says. “Human biosonar has been dismissed as too crude to be worthy of study.” I suspect that’s because blindness still carries so much stigma. To be blind to something is to be oblivious to it. To have a blind spot is to have a zone of ignorance. To lack vision is to lack creativity. These ableist phrases equate lack of sight with lack of awareness. And yet blind people are profoundly aware of their surroundings. With echolocation, Kish can do things that sighted people cannot, like perceive objects behind him, around corners, or through walls. But some tasks that are easy with vision are very hard through sonar. Large objects in the background will mask the echoes of smaller objects in the foreground. Just as bats struggle to detect insects on leaves, Kish and other echolocators struggle to locate objects on tabletops.”

“In 2000, Kish founded a nonprofit called World Access for the Blind to teach other blind people to echolocate. He and his fellow instructors, who are also blind, have trained thousands of students in dozens of countries. Echolocation is still a niche skill and one that’s frowned upon by some parts of the blind community for being socially inappropriate, counter to tradition, or too hard for all but a prodigious few. Kish disagrees. Echolocation could be more common if only more echolocators were allowed to teach. Kish himself was the first fully blind person in the United States to be certified as an orientation and mobility specialist — someone who helps blind people learn to get around. “There is active resistance to blind people teaching other blind people how to be blind,” he tells me. “It’s a sort of reinforced custodialism.” Kish says that many blind children will naturally try to explore through noise. If they’re not using their tongues, they might snap their fingers or stomp their feet. But parents often see these behaviors as weird or antisocial, and put a stop to them before they can bloom into a sophisticated sonar sense. Kish’s parents never did that. They allowed him to click. They bought him a bicycle.”

Electricity

“Around 350 species of fish can produce their own electricity, and humans have known about their ability since long before anyone knew what electricity was. Around 5,000 years ago, the Egyptians carved depictions of Blubby’s ancestors onto tombs. The Greeks and Romans wrote about the torpedo ray’s “benumbing” power — a strange force that could kill small fish, run up a spear into the arm of the person fishing, and treat everything from headaches to hemorrhoids. The true nature of these discharges only became clearer in the seventeenth and eighteenth centuries, when scientists defined electricity as a physical entity and realized that animals can produce it.

The study of electric fish then became entwined with the study of electricity itself. These animals inspired the design of the first synthetic battery. They fueled the discovery that muscles and nerves in all animals run on minute currents. Indeed, electric fish evolved their unique powers by modifying their own muscles or nerves into special electric organs.”

“The electric organ in the fish’s tail is like a small battery. When it switches on, it creates an electric field that envelops the animal. Current flows through the water from one end of the electric organ to the other. Nearby conductors, like animals (whose cells are essentially bags of salty liquid), increase the flow of that current. Insulators, like rocks, reduce it. These changes affect the voltage on different parts of the fish’s skin. The fish can detect these differences using sensory cells called electroreceptors. The black ghost knifefish has 14,000 of these scattered over its body, and it uses them to work out the position, size, shape, and distance of the objects around it. Just as sighted people create images of the world from patterns of light shining onto their retinas, an electric fish creates electric images of its surroundings from patterns of voltage dancing across its skin. Conductors shine brightly upon it. Insulators cast electric shadows.”

“It is also omnidirectional. Since an electric fish’s field extends in every direction, so does its awareness. That’s why the black ghost knifefish that I saw, and the African knifefish that entranced Hans Lissmann, could avoid obstacles behind them. These fish have been filmed swimming backward for meters at a time. “Imagine walking backward for five meters- you just wouldn’t,” Fortune tells me. “Electric fish can.””

“if the lateral line already existed, why evolve electrolocation on top of it? It might be that electric fields are more reliable than almost any other stimulus. They aren’t distorted by turbulence, so electric fish can thrive in fast-flowing rivers, where torrents and eddies befuddle the lateral line. Electric fields aren’t obscured by darkness or murkiness, so electric fish can stay active in turbid waters and nighttime hours. Electric fields aren’t blocked by barriers as light and smells are, so electric fish can sense through solid objects to detect hidden treasures. Indeed, it’s very hard to hide from these animals. They are sensitive not only to conductance, which is an object’s ability to carry a current, but also to capacitance, which is its ability to store a charge. And in natural environments, “capacitance is a mark of the living,” Maciver says. Prey animals can freeze, hide, and hush to fool predators that rely on vision and hearing. But stillness, concealment, and silence don’t work against electrolocation. To an electric fish, all that’s alive stands out against all that isn’t.”

“They use electric fields not just to sense their environment but also to communicate. They court mates, claim territory, and settle fights with electric signals in the same way other animals might use colors or songs.”

“Some species of electric fish turn their fields on and off to produce strong staccato pulses, like drumbeats. The shape of these pulses — their duration and how their voltage changes over time — contains information about the animal’s species, sex, status, and sometimes identity. Over short timescales, every individual produces the same pulses again and again: “I like to think of it as the sound of your voice,” says Bruce Carlson. The timing of the pulses, however, can vary considerably. If the shape of the pulse conveys identity, the timing of the pulses conveys meaning. One rhythm might be as attractive as birdsong; another could be as threatening as a snarl.”

“It can detect an electric field of just one nanovolt — a billionth of a volt — across a centimeter of water. A shark’s electric sense only works at short range, however. It can’t sense a buried fish (or electrode) from across an ocean, or even from across a pool. It has to be within an arm’s length of its target. Over mile distances, a shark sniffs out its food. As it draws near, vision takes over. Nearer still, the lateral line chips in. Its electric sense only enters the fray at the close of the hunt, to pinpoint the exact position of its prey and guide its strike. That’s why the ampullae of Lorenzini are usually concentrated around the mouth.

Passive electroreception is especially useful for finding hidden prey. Animals, after all, can’t turn off their natural electric fields. But if a shark can’t rely on other senses — say, when its prey are buried, as in Kalmijn’s experiment — it has to swim around until its ampullae of Lorenzini are close enough to a target. Some species have expedited that search by enlarging their heads. Instead of conical snouts, hammerhead sharks have broad, flattened heads that look like car spoilers. The undersides of their “hammers” are loaded with ampullae, and the sharks use these as one might use metal detectors, sweeping them over the seafloor in search of buried (edible) riches. They’re not more electrically sensitive than other sharks, but their heads allow them to scan a wider area in a given time.”

Magnetism

“Every spring, billions of bogongs emerge from their pupal stage in the dry plains of southeastern Australia. Anticipating the arrival of the baking summer, they flee toward cooler climes. And somehow, despite never having flown before, let alone migrated, they know which way to go. They fly over 600 miles and arrive at a few select alpine caves. Within these caves, every square meter of wall might be tiled by 17,000 bogongs, their wings overlapping like the scales of a fish. Safe and cool, they ride out the summer in a state of dormancy before making the return trip in autumn. On some nights when Warrant goes out to collect them with the Eye of Sauron he is “literally inundated by thousands of them,” he says.

The only other insect known to make such long migrations to such specific destinations is the monarch butterfly of North America. But while monarchs navigate during the day by using the sun as a compass, the bogongs only fly at night. How do they know the right direction? Warrant, who grew up among the Snowy Mountains and has loved the local insects since he was a child, has always wanted to find out. At first, he thought they might be using their sensitive eyes to observe the stars. And while he was right about that, on his first night of observing captive bogongs he noticed that they could still fly in the right direction without being able to see the sky. Warrant realized that they must be able to sense Earth’s magnetic field.

Earth’s core is a solid iron sphere surrounded by molten iron and nickel. The churning movements of that liquid metal turn the entire planet into a giant bar magnet. Its magnetic field can be depicted in the style of a school textbook: Lines emerge near the south pole, curve around the globe, and reenter near the north pole. This geomagnetic field is always present. It doesn’t change across the day or through the seasons. It’s not affected by weather or obstacles. Consequently, it is a boon for travelers, who can always use it to establish their bearings. Humans have done so for more than a thousand years, using compasses. Other animals — sea turtles, spiny lobsters, songbirds, and many others — have done so for millions of years, without help.”

“To test this idea, Granger collated 33 years’ worth of records of healthy, uninjured gray whales inexplicably stranding themselves. She compared the timing of these incidents to data on solar activity, wrangled by her astronomer colleague Lucianne Walkowicz. 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.”

“Hatching from an egg that was buried in a sandy beach, a baby turtle must run a gauntlet of crab claws and bird beaks on its ungainly crawl toward the ocean. Once in the water, it must flee from the coastal shallows, where it can be easily grabbed from above by seabirds and from below by predatory fish. To find some semblance of safety, it must reach the open ocean as quickly as possible. For a turtle that hatches in Florida, that means swimming due east until it reaches the North Atlantic gyre — a clockwise current that spans the ocean between North America and Europe. The hatchling somehow stays within this loop for 5 to 10 years, hiding out among clumps of floating seaweed and slowly gaining in size. By the time it completes its full (and very slow) lap of the Atlantic and returns to North American waters, it is invulnerable to all but the largest sharks. By the 1990s, no one had worked out how inexperienced turtles could pull off such grand migrations — a state of ignorance that the late Archie Carr lamented as “an insult to science.” At first, Ken Lohmann couldn’t understand the fuss. Armed with a newly acquired PhD and the hubris of youth, he thought the answer was obvious: The turtles must use a magnetic compass.”

“Many animals, including salmon, turtles, and Manx shearwaters (a kind of seabird), can also imprint on the magnetic signature of their birthplaces, etching it deep within their memory so they can find the same sites as adults. Turtles use these imprints to lay eggs on the same beaches from which they hatched. Their accuracy is uncanny. Green turtles that nest on Ascension Island can find that same tiny nub of land in the middle of the Atlantic after a 1,200-mile journey to and from Brazil.

This “natal homing” instinct is so strong that turtles will sometimes swim for hundreds of miles to their beach of birth, even though there’s a perfectly good alternative right next to them. Perhaps that’s because good nest sites are hard to find. They must be accessible from the water. The sand grains must be large enough to let oxygen through. The temperature must be exactly right, since turtles develop as males or females depending on how hot or cold their eggs are. “A turtle might say: Well, the one place in the world I know works is the beach where I developed myself,” Lohmann says. And its magnetic map allows it to relocate that sure-bet nursery after years away at sea.”

Multi-Senses

“The noisy and erratic nature of magnetoreception might also explain why no animal relies on it alone. Instead, they seem to use it as a backup sense in case more reliable ones like vision fail. “If you’re a migrating animal, magnetoreception is probably the least important sense, unless you’re completely lost,” Keays says. In the absence of magnetic cues, bogong moths can still navigate by looking at the pattern of stars in the night sky. Turtle hatchlings ignore magnetic fields when they first enter the water and use the direction of the waves to guide them out to sea.

Animals never use a single sense exclusively. “They use every damn piece of information they can get their hands on,” Warrant tells me. “They are multisensory in every possible way.””

“Venkataraman tells me that the mosquitoes are drawn to the carbon dioxide in our breath and the odors emanating from our skin. They can smell us. To demonstrate this, she picks up a different cage, and I exhale along one side of it. Within minutes, almost all the mosquitoes have swarmed onto that side and are probing away.

Leslie Vosshall, who runs the lab where Venkataraman works, spent years trying to protect people from Aedes aegypti by befuddling its olfactory abilities. First, she tried to disable a gene called orco, which seems to underlie a mosquito’s entire sense of smell. This approach worked when Daniel Kronauer, who works down the hall from Vosshall, tried it in clonal raider ants, as we saw earlier. But it failed when Vosshall tried it on mosquitoes: Without orco, they ignored human body odor but they were still drawn to carbon dioxide. Switching tactics, Vosshall’s team tried to create mutant mosquitoes that, could no longer smell carbon dioxide. That didn’t work either: The insects could still easily home in on humans. “The results kinda sucked,” Vosshall tells me.

Mosquitoes can’t be thrown off with any one strategy because they aren’t beholden to any one sense. Instead, they use a multitude of cues that interact in complicated ways. They’re attracted to the heat of warm-blooded hosts, but only if they first smell carbon dioxide. When Vosshall’s student Molly Liu placed the insects in a chamber and slowly heated one of the walls, most of them had buzzed off by the time the surface hit human body temperature. But if Liu sprayed a puff of carbon dioxide into the chamber, the mosquitoes swarmed the hot wall and stayed there. In carbon dioxide’s absence, heat is repulsive and a sign of danger. In its presence, heat is attractive and a sign of a meal. Vosshall still believes she can find a way of cloaking humans from mosquitoes, but she’ll need to consider many senses at once — smell, vision, heat, taste, and more. Aedes aegypti has “a plan B at every point,” she tells me.”

“This mosquito is now among the planet’s most effective hunters of humans, and it is extremely picky about anything else. That’s why, to feed captive mosquitoes, scientists like Venkataraman often just stick their arms inside their insect cages. “It takes about 10 minutes,” she says. “I don’t do it regularly, so I still react to the bites, but if you don’t scratch, it’s fine.” It’s hard to imagine not scratching.”

“The octopus, then, arguably has two distinct Umwelten. The arms live in a world of taste and touch. The head is dominated by vision. There’s undoubtedly some crosstalk between these sides, but Grasso suspects that the information exchanged between the head and the arms is simplified. To extend Uexküll’s metaphor of animal bodies as houses with sensory windows, the octopus’s body consists of two semidetached houses with utterly different architectural styles and a small connecting door between them.”

Threats

“In 2001, when astronomer Pierantonio Cinzano and his colleagues created the first global atlas of light pollution, they calculated that two-thirds of the world’s population lived in light-polluted areas, where the nights were at least 10 percent brighter than natural darkness. Around 40 percent of humankind is permanently bathed in the equivalent of perpetual moonlight, and around 25% percent constantly experiences an artificial twilight that exceeds the full moon. “‘Night’ never really comes for them,” the researchers wrote. In 2016, when the team updated their atlas, they found that the problem was even worse. By then, around 83 percent of people — and more than 99 percent of Americans and Europeans — were living under light-polluted skies. Every year, the proportion. of the planet covered by artificial light gets 2 percent bigger and 2 percent brighter. A luminous fog now smothers a quarter of Earth’s surface and is thick enough in many places to blot out the stars. Over a third of humanity, and almost 80 percent of North Americans, can no longer see the Milky Way.”

“Almost 7 million birds a year die in the United States and Canada after flying into communication towers. The red lights of those towers are meant to warn aircraft pilots, but they also disrupt the orientation of nocturnal avian fliers, which then veer into wires or each other. Many of these deaths could be avoided simply by replacing steady lights with blinking ones.”

“Colors matter, too. Red can disrupt migrating birds but is better for bats and insects. Yellow doesn’t bother insects and turtles but can disrupt salamanders. No wavelength is perfect, Longcore says, but blue and white are worst of all. Blue light disrupts body clocks and strongly attracts insects. It is also easily scattered, increasing the spread of light pollution. It is, however, cheap and efficient to produce. The new generation of energy-efficient white LEDs contain a lot of blue light, and, if the world switches to them from traditional yellow-orange sodium lights, the amount of global light pollution would increase by two or three times. “We can make better choices by tuning lights with intention,” Longcore says. “And we shouldn’t use full-spectrum at night. We shouldn’t want to give everything the signal, that it’s constantly daytime.””

“They found that human activity has doubled the background noise levels in 63 percent of protected spaces, and increased them tenfold in 21 percent. In the latter places, “if you could have heard something 100 feet away, now you can only hear it 10 feet away,” Rachel Buxton of the NPS tells me. Aircraft and roads are the main culprits, but so are industries like oil and gas extraction, mining, and forestry.”

“In 2012, Jesse Barber, Heidi Ware, and Christopher McClure built a phantom road. On a ridge in Idaho that acts as a stopover for migrating birds, the team set up a half-mile corridor of speakers and played looped recordings of passing cars. At the sound of these disembodied noises, a third of the usual birds stayed away. Many of those that stayed paid a price for persisting. With tires and horns drowning out the sounds of predators, the birds spent more time looking for danger and less time looking for food. They put on less weight, and were weaker as they continued their arduous migrations. The phantom road experiment was pivotal in showing that wildlife could be deterred by noise and noise alone, detached from the sight of vehicles or the stench of exhaust. Hundreds of studies have come to similar conclusions.”

“Between World War II and 2008, the global shipping fleet more than tripled, and began moving 10 times more cargo at higher speeds. 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. As ships pass in the night, humpback whales stop singing, orcas stop foraging, and right whales, become stressed. Crabs stop feeding, cuttlefish change colors, damselfish are more easily caught. “If I said that I’m going to increase the noise level in your office by 30 decibels, OSHA would come in and say you’d need to wear earplugs,” Hildebrand tells me.”

“Smooth vertical surfaces, which don’t exist in nature, return echoes that sound like open air; perhaps that’s why bats so often crash into windows. DMS, the seaweed-y chemical that once reliably guided seabirds to food, now also guides them to the millions of tons of plastic waste that humans have dumped into the oceans; perhaps that’s why an estimated 90 percent of seabirds eventually swallow plastic. The currents produced by objects moving in the water can be detected by the body-wide hairs of manatees, but not with enough notice to avoid a fast-moving speedboat; boat collisions are responsible for at least a quarter of deaths among Florida’s manatees. Odorants in river water can guide salmon back to their streams of birth, but not if pesticides in that same water weaken their sense of smell. Weak electric fields at the bottom of the sea can guide sharks to buried prey, but also to high-voltage cables.

Some animals have come to tolerate the sights and sounds of modernity. Others even flourish among them. Some urban moths have evolved to become less attracted to light. Some urban spiders have gone in the opposite direction, spinning webs beneath streetlights to feast on the attracted insects. In the towns of Panama, nighttime lights drive frog-eating bats away, allowing male túngara frogs to add more sexy chucks to their songs without the risk of attracting predators. Animals can adapt, either by changing their behavior over an individual lifetime or by evolving new behaviors over many generations.

But adaptation is not always possible. Species with slow lives and long generations can’t evolve quickly enough to keep pace with levels of light and noise. pollution that double every few decades.”

“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.””

“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.”

“A heat wave had forced the corals to expel the symbiotic algae that give them nutrients and colors. Without these partners, the corals starved and whitened in the worst bleaching event on record, and the first of several to come. Snorkeling through the rubble, Gordon found that the reefs had been not only bleached but also silenced. Snapping shrimps no longer snapped. Parrotfish no longer crunched. Those sounds normally help to guide baby fish back to the reef after their first vulnerable months out at sea. Soundless reefs were much less attractive. Gordon feared that if fish avoided the degraded reefs, the seaweed they normally eat would run amok, overgrowing the bleached corals and preventing them from rebounding. But in 2017, “we went back and thought: Can we flip that on its head?” he says.

He and his colleagues set up loudspeakers that continuously played recordings of healthy reefs over patches of coral rubble. The team would dive every few days to survey the local animals. “And on day 30,” Gordon says, “I remember moseying around with my dive buddies and saying, ‘There’s a big pattern here, isn’t there?’” After 40 days, he ran the numbers and saw that the acoustically enriched reefs had twice as many young fish as silent ones and 50 percent more species. They had not only been attracted by the sounds but stayed and formed a community. “It was a lovely experiment to do,” Gordon says. It showed what conservationists can accomplish by “seeing the world through the perceptions of the animals you’re trying to protect.””

“The pesticide DDT can thread their way through the bodies of animals long after they are banned. Plastics will continue to despoil the oceans for centuries even if all plastic production halts tomorrow. But light pollution ceases as soon as lights are turned off. Noise pollution abates once engines and propellers wind down. Sensory pollution is an ecological gimme — a rare example of a planetary problem that can be immediately and effectively addressed.”

“In the summer of 2007, Kurt Fristrup and his colleagues did a simple experiment at Muir Woods National Monument in California. On a random schedule, they stuck up signs that declared one of the most popular parts of the park a quiet zone and encouraged visitors to silence their phones and lower their voices. These simple steps, with no accompanying enforcement, reduced the noise levels in the park by 3 decibels, equivalent to 1,200 fewer visitors.

But personal responsibility cannot compensate for societal irresponsibility. To truly make a dent in sensory pollution, bigger steps are needed. Lights can be dimmed or switched off when buildings and streets are not in use. They can be shielded so that they stop shining above the horizon. LEDs can be changed from blue or white to red. Quiet pavements with porous surfaces can absorb the noise from passing vehicles. Sound-absorbing barriers, including berms on land and bubble nets in the water, can soften the din of traffic and industry. Vehicles can be diverted from important areas of wilderness, or they can be forced to slow down: In 2007, when commercial ships in the Mediterranean began slowing down by just 12 percent, they produced half as much noise. Such vessels can also be fitted with quieter hulls and propellers, which are already used to muffle military ships (and would make commercial ones more fuel-efficient). Many helpful technologies already exist, but the economic incentives to make them cheaper or to deploy them en masse are lacking. We could regulate industries causing sensory pollution, but there’s not enough societal will.”