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The senses transform the coursing chaos of the world into perceptions and experiences—things we can react to and act upon. They allow biology to tame physics. They turn stimuli into information. They pull relevance from randomness, and weave meaning from miscellany. They connect animals to their surroundings. And they connect animals to each other via expressions, displays, gestures, calls, and currents.

The senses constrain an animal’s life, restricting what it can detect and do. But they also define a species’ future, and the evolutionary possibilities ahead of it. For example, around 400 million years ago, some fish began leaving the water and adapting to life on land. In open air, these pioneers—our ancestors—could see over much longer distances than they could in water. The neuroscientist Malcolm MacIver thinks that this change spurred the evolution of advanced mental abilities, like planning and strategic thinking. Instead of simply reacting to whatever was directly in front of them, they could be proactive. By seeing farther, they could think ahead. As their Umwelten expanded, so did their minds. An Umwelt cannot expand indefinitely, though. Senses always come at a cost. Animals have to keep the neurons of their sensory systems in a perpetual state of readiness so that they can fire when necessary. This is tiring work, like drawing a bow and holding it in place so that when the moment comes, an arrow can be shot.

Writing about the tick, Uexküll noted that the rich world around it is “constricted and transformed into an impoverished structure” of just three stimuli. “However, the poverty of this environment is needful for the certainty of action, and certainty is more important than riches.” Nothing can sense everything, and nothing needs to. That is why Umwelten exist at all. It is also why the act of contemplating the Umwelt of another creature is so deeply human and so utterly profound. Our senses filter in what we need. We must choose to learn about the rest.

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

Bats, for example, perceive the world through sonar, and since this is a sense that the majority of humans lack, “there is no reason to suppose that it is subjectively like anything we can experience or imagine,” Nagel wrote. You could envision yourself with webbing on your arms or insects in your mouth, but you’d still be creating a mental caricature of you as a bat. “I want to know what it is like for a bat to be a bat,” Nagel wrote. “Yet if I try to imagine this, I am restricted to the resources of my own mind, and those resources are inadequate to the task.”

Language, for us, is both blessing and curse. It gives us the tools for describing another animal’s Umwelt even as it insinuates our own sensory world into those descriptions.

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

“When you look at an animal’s behavior through the lens of that animal, suddenly all of this salient information becomes available that you would otherwise miss,” Colleen Reichmuth, a sensory biologist who works with seals and sea lions, tells me. “It’s like a magic magnifying glass, to have that knowledge.”

Senses that seem paranormal to us only appear this way because we are so limited and so painfully unaware of our limitations.

When you inhale, you create a single airstream that allows you to both smell and breathe. But when a dog sniffs, structures within its nose split that airstream in two. Most of the air heads down into the lungs, but a smaller tributary, which is for smell and smell alone, zooms to the back of the snout. There it enters a labyrinth of thin, bony walls that are plastered with a sticky sheet called the olfactory epithelium. This is where smells are first detected. The epithelium is full of long neurons. One end of each neuron is exposed to the incoming airstream and snags passing odorants using specially shaped proteins called odorant receptors. The other end is plugged directly into a part of the brain called the olfactory bulb. When the odorant receptors successfully grab their targets, the neurons notify the brain, and the dog perceives a smell. You can breathe out now.

If it has a scent, a dog can be trained to detect it. We redirect their Umwelten in service of our needs, to compensate for our olfactory shortcomings.

These skills are sometimes billed as extrasensory, but they are simply sensory. It’s just that things often become apparent to the nose before they appear to the eyes. When Finn sniffs, he is not merely assessing the present but also reading the past and divining the future. And he is reading biographies. Animals are leaking sacks of chemicals, filling the air with great clouds of odorants.[* 6] While some species deliberately send messages by releasing smells, all of us inadvertently do so, giving away our presence, position, identity, health, and recent meals to creatures with the right noses.[* 7]

Time and again, people impose their values—and their Umwelt—onto their dogs, forcing them to look instead of sniff, dimming their olfactory worlds and suppressing an essential part of their caninehood.

Of the five Aristotelian senses, four have vast and specific lexicons. Smell, as Diane Ackerman wrote, “is the one without words.”

Many living things can sense light. Some can respond to sound. A select few can detect electric and magnetic fields. But every thing, perhaps without exception, can detect chemicals.

Even a bacterium, which consists of just one cell, can find food and avoid danger by picking up on molecular clues from the outside world. Bacteria can also release their own chemical signals to communicate with each other, launching infections and performing other coordinated actions only when their numbers are large enough. Their signals can then be detected and exploited by bacteria-killing viruses, which have a chemical sense even though they are such simple entities that scientists disagree about whether they’re even alive.

Chemicals, then, are the most ancient and universal source of sensory information. They’ve been part of Umwelten for as long as Umwelten have existed.

That’s just one receptor out of hundreds, and all exist in varied forms, bestowing every individual with their own subtly personalized Umwelt. Everyone likely smells the world in a slightly different way.

pheromones—an important term that is frequently misunderstood. It refers to chemical signals that carry messages between members of the same species.

Pheromones are also standardized messages, whose use and meaning do not vary between individuals of a given species.

it’s still unclear if human pheromones even exist. Despite decades of searching, none have been identified.[*

the heaviest chemicals, which barely aerosolize, are found on the surface of the ants’ bodies. Known as cuticular hydrocarbons, they act as identity badges. Ants use them to discern their own species from other kinds of ants, nestmates from other colonies, and queens from workers. Queens also use these substances to stop workers from breeding or to mark unruly subjects for punishment.

pheromones. These smelly substances allow ants to transcend the limits of individuality and act as a superorganism, producing complex and transcendent behaviors from the unknowing actions of simple individuals.

“We live, all the time, especially in nature, in great clouds of pheromones,” E. O. Wilson once said. “They’re coming out in spumes in millionths of a gram that can travel for maybe a kilometer.” These tailored messages drive the entire animal kingdom, from the smallest of creatures to the very biggest.

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 right noses, landscapes can be mapped as odorscapes, and fragrant landmarks can show the way to food and shelter.

As a timber rattlesnake slithers over the forest floor, its tongue turns the world into both map and menu, revealing the crisscrossing tracks of scurrying rodents and discerning the scents of different species. Amid the tangled trails, it can pick out those of its favorite prey[* 27] and find sites where those tracks are common and fresh. It hides nearby, coiled in ambush. When a rodent runs past, the snake explodes outward four times faster than a human can blink. It stabs the rodent with its fangs and injects venom. The toxins usually take a while to work, and since rodents have sharp teeth, the snake avoids injury by releasing its prey and letting it run off. After several minutes, it starts flicking its tongue to track down the now-dead victim. The venom helps. Aside from lethal toxins, rattlesnake venom also includes compounds called disintegrins, which aren’t toxic but react with a rodent’s tissues to release odorants. The snakes can use these aromas to distinguish envenomated rodents from healthy ones and to tell rodents envenomated by their own species from those bitten by other kinds of rattlesnakes. They can even track the specific individual that they attacked because they instantly learn the victim’s scent at the moment of a bite. “There are presumably odors of multiple mice around, but they know which trail to follow,” Schwenk says.

Repeatedly and predictably, the gustatory Umwelten of animals have expanded and contracted to make sense of the foods they most often encounter.

our sharp vision muddies our appreciation of other Umwelten, because “we assume that if we can see it, they can, and that if it’s eye-catching to us, it’s grabbing their attention,” says Caves. “That’s not the case.”

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.[* 18] (In North America, bald eagles also crash into wind turbines for the same reasons.)

“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.”[* 19]

Color-blindness shouldn’t be a disability, but it can be because humans have built cultures that are predicated on trichromacy.

The first primates were almost certainly dichromats. They had two cones, short and long. They saw in blues and yellows, like dogs. But sometime between 29 and 43 million years ago, an accident occurred that permanently changed the Umwelt of one specific lineage of primates: They gained an extra copy of the gene that builds their long opsin. Such duplications often happen when cells divide and DNA is copied. They’re mistakes, but fortuitous ones, for they provide a redundant copy of a gene that evolution can tinker with without disrupting the work of the original. That’s exactly what happened with the long-opsin gene. One of the two copies stayed roughly the same, absorbing light at 560 nanometers. The other gradually shifted to a shorter wavelength of 530 nanometers, becoming what we now call the medium (green) opsin. These two genes are 98 percent identical, but the 2 percent gulf between them is also the difference between seeing only in blues and yellows and adding reds and greens to the mix.[* 3] With the new medium opsins joining the earlier long and short ones, these primates had evolved trichromacy. And they passed their expanded vision to their descendants—the monkeys and apes of Africa, Asia, and Europe, a group that includes us.

Colors are not inherently magical. They become magical when and if animals derive meaning from them.

people who have lost their lenses to surgeries or accidents can perceive UV as whitish blue. This happened to the painter Claude Monet, who lost his left lens at the age of 82. He began seeing the UV light that reflects off water lilies, and started painting them as whitish blue instead of white.

Ultraviolet vision is so ubiquitous that much of nature must look different to most other animals.[* 10] Water scatters UV light, creating an ambient ultraviolet fog, against which fish can more easily see tiny UV-absorbing plankton. Rodents can easily see the dark silhouettes of birds against the UV-rich sky. Reindeer can quickly make out mosses and lichens, which reflect little UV, on a hillside blanketed by UV-reflective snow. I could go on. I’m going to go on. Flowers use dramatic UV patterns to advertise their wares to pollinators. Sunflowers, marigolds, and black-eyed Susans all look uniformly colored to human eyes, but bees can see the UV patches at the bases of their petals, which form vivid bullseyes. Usually, these shapes are guides that indicate the position of nectar. Occasionally, they are traps. Crab spiders lurk on flowers to ambush pollinators.

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.

Frustrating though it might be, most of us simply cannot imagine what many animals actually look like to each other, or how varied their sense of color can be.

all of us—monochromat, dichromat, trichromat, or tetrachromat—take the colors that we see for granted. Each of us is stuck in our own Umwelt.

Imagine that you’re a mantis shrimp. It is a truth universally acknowledged that you are in want of something to punch.

Since eyes define nature’s palette, an animal’s palette tells you whose eyes it is trying to catch.

Nociception is an ancient sense. It is so widespread and consistent across the animal kingdom that the same chemicals, opioids, can quell the nociceptors of humans, chickens, trout, sea slugs, and fruit flies—creatures separated by around 800 million years of evolution.

Pain (or nociception, if you prefer) is the unwanted sense. It is the only one whose absence (in naked mole-rats or grasshopper mice) feels like a superpower. It is the only one that we try to avoid, that we dull with medication, and that we try to avoid inflicting upon others.

Our brains are topped by a thick mushroom cap of neural tissue called the neocortex. It’s organized like an orchestra, with many specialized sections that act together to produce the music of consciousness and the lament of pain. But fish brains lack a neocortex, much less a highly organized one.

Consciousness isn’t an inherent property of all life. It arises from nervous systems, and while those systems might not need a neocortex, they do need enough processing power.

What matters is not just the total tally of neurons but the connections between them. In human brains, hundreds of thousands of neurons connect the different sections of our cortical orchestra. These links allow us to play the full symphony of a painful experience, melding sensory cues with negative emotions, bad memories, and more.

Evolution has pushed the nervous systems of insects toward minimalism and efficiency, cramming as much processing power as possible into small heads and bodies. Any extra mental ability—say, consciousness—requires more neurons, which would sap their already tight energy budget.

insect nervous systems have evolved to pull off complex behaviors in the simplest possible ways, and robots show us how simple it is possible to be.

The fire-chasing Melanophila beetles, however, are drawn to heat. These black, half-inch-long insects have been found in what entomologist Earle Gorton Linsley described as “unbelievable numbers” in smelting plants, the kilns of cement factories, and the vats of hot syrup in sugar refineries. One summer, Linsley saw them swarming an outdoor barbecue where “large quantities of deer meat were being prepared.” In the 1940s, the insects would regularly bother football fans in Berkeley’s California Memorial Stadium “by alighting on the clothing or even biting the neck or hands,” Linsley wrote. It’s possible that “the beetles are attracted by the smoke from some twenty thousand (more or less) cigarettes which on still days sometimes hangs like a haze over the stadium.” These incidents are unfortunate for both species, because industrial plants, barbecues, and football stadiums are unhelpful distractions that waylay the beetles from their true targets: forest fires. Arriving at a fire, the beetles have perhaps the most dramatic sex in the animal kingdom, mating as a forest burns around them. Later, the females lay their eggs on charred, cooled bark. When the wood-eating grubs hatch, they find an Eden. The trees they devour are too injured to defend against insect larvae feeding within them. The predators that might eat them are put off by the smoke and heat emitted from the embers and ashes. In peace, they thrive, mature, and eventually fly off in search of their own blazes. But forest fires are rare and unpredictable, and the beetles must have some means of detecting them from afar.

The atoms and molecules in all objects are constantly jiggling about, and this motion produces electromagnetic radiation. As an object gets hotter, its molecules move faster, and it emits more radiation at higher frequencies. That radiation includes some visible light—think about the glow of heated metal—but most of it lies in the infrared spectrum.[* 7]

To detect the infrared light from a distant source, either the source must be extremely intense (like the sun) or you need specialized equipment. Melanophila beetles have the latter. Below their wings and just behind their middle legs, these insects have a pair of pits. Each one contains a cluster of around 70 spheres that together look like a malformed raspberry. When zoologist Helmut Schmitz examined these spheres under a microscope, he saw that each is filled with fluid and encloses the tip of a pressure-sensitive neuron. When infrared radiation hits the spheres, the fluid inside them heats up and expands. It can’t bulge outward because the spheres have hard exteriors, so instead it squeezes the nerves, causing them to fire.

During flight, their beating wings create vibrations that travel into the nearby pits, shake the spherical sensors, and push the sensory neurons within to the edge of firing. It now takes much less infrared radiation to fully push them over that edge.

a fire-chaser beetle’s beating wings prime its heat sensors in a similar way, setting them up to detect sources of infrared that would normally be too weak. A beetle that’s sitting on a tree would be relatively insensitive. But as soon as it takes off in search of fires, its body automatically widens its search area and transforms even faint traces of distant heat into blazing beacons.[* 8]

To understand another animal’s Umwelt, you have to watch its behavior. But a pit viper’s behavior mostly consists of waiting. Since they don’t generate their own body heat, they can go without eating for months and can sit in ambush until exactly the right moment. The few researchers brave enough to study them end up with animals that mostly sit around doing nothing, which makes them very hard to train—or comprehend. After all, even animals that we already understand and that we know how to train can sense heat in ways that are hard to explain.

Imagine that, right now, a sea otter is about to search for food. Floating on its back on the surface of the sea, it rolls and dives. It will only stay submerged for a minute—roughly the time it will take you to read this paragraph. The descent eats up many of the precious seconds, so once the otter reaches the right depth, it has no time for indecisiveness. In a few frantic moments, it presses its knobby mittens over the seafloor, inspecting whatever it can find. The water is dark, but darkness doesn’t matter. To some of the most sensitive paws in the world, the ocean is bright with shapes and textures to be felt, grasped, pressed, prodded, squeezed, stroked, and manhandled—or perhaps otterhandled. Hard-shelled prey nestle among the similar hard rocks, but in a split second, the otter feels the difference between the two, and pulls the former from the latter. With its sense of touch, its dexterous paws, and its overabundant mustelid confidence, it snatches that clam, yanks that abalone, grabs that sea urchin, and finally ascends to eat its catches, breaking the water at the end of this sentence.

At a broad level, we understand how these mechanoreceptors work. Despite their variety, they all consist of a nerve ending enclosed in some kind of touch-sensitive capsule. A tactile stimulus bends or deforms the capsule, causing the nerve inside to fire.

Light may be the fastest thing in the universe, but light sensors have their limits, and the star-nosed mole’s sense of touch blows past them all.

The filiform hairs of crickets and the trichobothria of spiders are almost inconceivably sensitive. They can be deflected by a fraction of the energy in a single photon—the smallest possible quantity of visible light. These hairs are a hundred times more sensitive than any visual receptor that exists, or could possibly exist.

To the spiders, which have very poor eyesight, the room doesn’t really exist: There is only the web, and whatever vibrates it.

The web, then, is not just an extension of a spider’s senses but an extension of its cognition. In a very real way, the spider thinks with its web. Tuning the silk is like tuning its own mind.

both hearing and touch are mechanical senses, which detect movements in the outside world using receptors that send electrical signals when they’re bent, pressed, or deflected.

Echolocation differs from the senses we have met so far, because it involves putting energy into the environment. Eyes scan, noses sniff, whiskers whisk, and fingers press, but these sense organs are always picking up stimuli that already exist in the wider world. By contrast, an echolocating bat creates the stimulus that it later detects. Without the call, there is no echo. As bat researcher James Simmons explained to me, echolocation is a way of tricking your surroundings into revealing themselves. A bat says, “Marco,” and its surroundings can’t help but say, “Polo.” The bat speaks, and a silent world shouts back.

Most bats echolocate in a way broadly similar to that of the archetypal big brown. They send out short sonar pulses that last between 1 and 20 milliseconds and are separated by relatively longer silences. Those pulses also sweep down across a broad band of frequencies, which is why these bats are known as FM, or frequency-modulated, bats. But around 160 species—the horseshoes, hipposiderids, and Parnell’s mustached bat—do something very different. Their calls are much longer, lasting for many tens of milliseconds in some species, and separated by much shorter gaps. And instead of covering a range of frequencies, these species hold one particular note. For that reason, they are called CF, or constant-frequency, bats. And they are listening out for a very specific kind of echo. When a sonar pulse hits an insect’s flapping wing, the echo strength varies as the wing moves up and down. But at one particular moment, when the wing is exactly perpendicular to the incoming sound, an especially loud and sharp echo bounces straight back at the bat. This is called an acoustic glint. It’s a dead giveaway that an insect is flying nearby. FM bats can theoretically detect these glints, but they’re unlikely to. Their brief sonar pulses are separated by long gaps, so an FM bat has to get very lucky to hit an insect’s wing at exactly the right moment to return a glint. By contrast, the pulses of CF bats are long enough to cover an entire wingbeat. They catch glints galore. And since leaves and other background objects don’t flap in the same rhythmic way as wings, a CF bat can use glints to distinguish fluttering insects against cluttering foliage. They must be the auditory equivalent of flashes of light.

CF bats can compensate for Doppler shifts. When closing in on a target, they produce calls that are lower than their normal resting frequency, so the upshifted echoes hit their ears at exactly the right pitch. And they do this (quite literally) on the fly, constantly tweaking their calls so that the echoes from targets ahead stay within 0.2 percent of the ideal frequency. This is a staggering feat of motor control that’s almost unmatched in the animal kingdom.

more than half of moth species have ears that can hear bat sonar.

it is clear that the knifefishes and elephantfishes use their electric fields to sense their surroundings, and even to communicate with each other. Electricity is to them what echoes are to bats, smells are to dogs, and light is to humans—the core of their Umwelt.

Life exists within that planetary electric field and is affected by it. Flowers, being full of water, are electrically grounded, and bear the same negative charge as the soil from which they sprout. Bees, meanwhile, build up positive charges as they fly, possibly because electrons are torn from their surface when they collide with dust and other small particles. When positively charged bees arrive at negatively charged flowers, sparks don’t fly, but pollen does. Attracted by their opposing charges, pollen grains will leap from a flower onto a bee, even before the insect lands.

Alongside the bright colors that we can see (and the ultraviolet ones we cannot), flowers are also surrounded by invisible electric halos. And bumblebees can sense these.

Spider silk picks up a negative charge as it leaves a spider’s body, and is repelled by the negatively charged plants on which they sit. That force, though tiny, is enough to launch the spider into the air. And since the electric fields around plants are strongest at points and edges, spiders can ensure a vigorous takeoff by ballooning from twigs and blades of grass.

Distinguishing self from other isn’t a given; it’s a difficult problem that nervous systems have to solve. “This is largely what sentience is,” neuroscientist Michael Hendricks tells me. “And perhaps it’s why sentience is: It’s the process of sorting perceptual experiences into self-generated and other-generated.”

Animals cannot make sense of what’s around them without first making sense of themselves. And this means that an animal’s Umwelt is the product not just of its sense organs but of its entire nervous system acting in concert.

Our experiences of the world feel disconnected from the very sense organs that produce them, which makes it easy to believe that they are purely mental constructs divorced from physical reality.

the Anthropocene—a geological epoch defined and dominated by the deeds of our species.

“The thought of light traveling billions of years from distant galaxies only to be washed out in the last billionth of a second by the glow from the nearest strip mall depresses me no end,” vision scientist Sonke Johnsen once wrote.

Light at night, however, is a uniquely anthropogenic force.

As those species go extinct, so too do their Umwelten. With every creature that vanishes, we lose a way of making sense of the world.