An Immense World: How Animal Senses Reveal the Hidden Realms Around Us

by Ed Yong

How do you know but ev’ry Bird that cuts the airy way, Is an immense world of delight, clos’d by your senses five? —William Blake

Rebecca, who is oblivious to both the ultrasonic mouse squeaks and the infrasonic elephant rumbles, listens instead to the robin, which is singing at frequencies better suited to her ears. But her hearing is too slow to pick out all the complexities that the bird encodes within its tune.

Earth teems with sights and textures, sounds and vibrations, smells and tastes, electric and magnetic fields. But every animal can only tap into a small fraction of reality’s fullness. Each is enclosed within its own unique sensory bubble, perceiving but a tiny sliver of an immense world.

There is a wonderful word for this sensory bubble—Umwelt. It was defined and popularized by the Baltic-German zoologist Jakob von Uexküll in 1909. Umwelt comes from the German word for “environment,” but Uexküll didn’t use it simply to refer to an animal’s surroundings. Instead, an Umwelt is specifically the part of those surroundings that an animal can sense and experience—its perceptual world.

Our Umwelt is still limited; it just doesn’t feel that way. To us, it feels all-encompassing. It is all that we know, and so we easily mistake it for all there is to know. This is an illusion, and one that every animal shares.

Each species is constrained in some ways and liberated in others. For that reason, this is not a book of lists, in which we childishly rank animals according to the sharpness of their senses and value them only when their abilities surpass our own. This is a book not about superiority but about diversity.

Animals are not just stand-ins for humans or fodder for brainstorming sessions. They have worth in themselves.

“They move finished and complete, gifted with extensions of the senses we have lost or never attained, living by voices we shall never hear,” wrote the American naturalist Henry Beston. “They are not brethren, they are not underlings; they are other nations, caught with ourselves in the net of life and time, fellow prisoners of the splendour and travail of the earth.”

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.

Senses always come at a cost. Animals have to keep the neurons of their sensory systems in a perpetual state of readiness so that they can fire when necessary. This is tiring work, like drawing a bow and holding it in place so that when the moment comes, an arrow can be shot. Even when your eyelids are closed, your visual system is a monumental drain on your reserves. For that reason, no animal can sense everything well.

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.

Occam’s razor, the principle that states that the simplest explanation is usually the best. But this principle is only true if you have all the necessary information to hand.

A scientist’s explanations about other animals are dictated by the data she collects, which are influenced by the questions she asks, which are steered by her imagination, which is delimited by her senses. The boundaries of the human Umwelt often make the Umwelten of others opaque to us.

Perhaps people who experience the world in ways that are considered atypical have an intuitive feeling for the limits of typicality.

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.

the act of delving into other Umwelten allows us to see further and think more deeply. I’m reminded of Hamlet’s plea to Horatio that “there are more things in heaven and Earth…than are dreamt of in your philosophy.” The quote is often taken as an appeal to embrace the supernatural. I see it rather as a call to better understand the natural. Senses that seem paranormal to us only appear this way because we are so limited and so painfully unaware of our limitations. Philosophers have long pitied the goldfish in its bowl, unaware of what lies beyond, but our senses create a bowl around us too—one that we generally fail to penetrate.

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

Female lobsters urinate into the faces of males to tempt them with a sex pheromone.

After being briefly taught to detect TNT, which is supposedly odorless to humans, three African elephants could identify the substance more skillfully than highly trained detection dogs.

Birds can smell. Both Bang and Wenzel, who have since passed away, have been described as “mavericks of their generation” who pushed against incorrect dogma and allowed others to explore a sensory world that was deemed nonexistent.

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.

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.

It’s ironic that we associate taste with connoisseurship, subtlety, and fine discrimination when it is among the coarsest of senses. Even our ability to taste bitter, which warns us of hundreds of potentially toxic compounds, isn’t built to distinguish between them. 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.

(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.)

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.

Pandas have no need to sense umami either, since they only eat bamboo, but they gained an expanded set of bitter-sensing genes to warn them of the myriad possible toxins in their mouthfuls.

Eyes can come in eights or hundreds. The eyes of the giant squid are as big as soccer balls; those of fairy wasps are the size of an amoeba’s nucleus.

Sonke Johnsen opens his book The Optics of Life by noting that vision “is about light, so perhaps we should start with what light is.” And then, with admirable candor: “I have no idea.” Though it surrounds us almost constantly, light’s true nature is not intuitive. Physicists contend that it exists both as an electromagnetic wave and as particles of energy known as photons. The specifics of this dual nature needn’t concern us. What matters is that neither guise is something living things should obviously be able to detect. From a biological perspective, perhaps the most wondrous thing about light is that we can sense it at all.

Primates, for example, probably evolved big, sharp eyes to capture tree-dwelling insects sitting on branches. We humans have inherited that acute vision, which sighted people now use to guide their dexterous fingers, to read symbols that they imbue with meaning, and to assess the cues hidden in subtle facial expressions. Our eyes suit our needs. They also give us a singular Umwelt that most other animals do not share.

eagles and other birds of prey are the only animals whose vision is substantially sharper than ours.

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.

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.”

Many animals have an area in their retinas where their photoreceptors (and the attendant neurons) are densely packed, increasing the resolution of their vision. This region goes by many names. In invertebrates, it’s called an acute zone. In vertebrates, it’s an area centralis. If that area is also inwardly dimpled, as it is in our eyes, it’s a fovea. For all our sakes (except the vision scientists, to whom I apologize), I’m just going to stick with acute zone. In humans, it’s a bullseye—a round spot in the center of our visual field. It’s what you are training upon these letters as you read them. 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.

An animal’s visual field determines where it can see. Its acute zones determine where it sees well.

A seal’s visual field is more similar to ours but with excellent coverage above its head and poor coverage below, presumably to spot fish silhouetted against the sky. A seal that swims upside down might look relaxed to a human observer, but is actually scanning the seafloor for food.

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.

It may seem strange to talk about animals seeing at different speeds, because light is the fastest thing in the universe, and vision seems instantaneous to us. But eyes don’t work at light speed. It takes time for photoreceptors to react to incoming photons, and for the electrical signals they generate to travel to the brain.

Imagine looking at a light that flickers on and off. As the flickering gets faster, there will come a point when the flashes merge into a steady glow. This is called the critical flicker-fusion frequency, or CFF. It’s a measure of how quickly a brain can process visual information. Think of it as the frame rate of the movie playing inside an animal’s head—the point at which static images blend into the illusion of continuous motion. For humans, in good light, the CFF is around 60 frames per second (or hertz, Hz). For most flies, it’s up to 350. For killer flies, it’s probably higher still. To its eyes, a human movie would look like a slideshow. The fastest of our actions would seem languid. An open palm, moving with lethal intent, would be easily dodged.

It’s possible that each of these visual speeds comes with a different sense of time’s passage. Through a leatherback turtle’s eyes, the world might seem to move in time-lapse, with humans bustling about at a fly’s frenetic pace.

The tarsiers—small primates from Southeast Asia that look like gremlins—have eyes that are each larger than their brains.

The deep ocean’s consummate darkness creates a problem for the scientists who want to study its denizens. Researchers can’t see what’s around them unless they turn on their submersible’s lights, but doing so is devastating for creatures that have adapted to a lightless life. Even moonlight can blind a deep-sea shrimp in a few seconds. A submersible’s headlights will do much worse. Some deep-sea animals end up doing kamikaze runs at subs. Startled swordfish ram them with their swords. Other creatures freeze or flee. “The way to think about ocean exploration is that we probably create a sphere a hundred yards wide that keeps away anything that can get away,” says Sonke Johnsen. “Most of the time, we’re seeing terror and blindness. We see how animals behave when they think they’re being killed by some glowing god.”

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.

The largest toothed predators in the world, sperm whales are the giant squid’s main nemeses. Their stomachs have been found full of the squid’s parrot-like beaks, and their heads often bear circular scars inflicted by the serrated rims of the squid’s suckers. They do not produce their own light, but just like a descending submersible, they trigger flashes of bioluminescence when they bump against small jellyfish, crustaceans, and other plankton.

There’s always at least one person who writes in with a pompous and incorrect corrective, so let’s get this out of the way: The word octopus is derived from Greek and not Latin, so the correct plural is not octopi. Technically, the formal plural would be octopodes (pronounced ock-toe-poe-dees) but octopuses will do.

So why are zebras striped? Caro has a definitive answer: to ward off bloodsucking flies. African horseflies and tsetse flies carry a number of diseases that are fatal to horses, and zebras are especially vulnerable because their coats are short. But stripes, for some reason, confuse the biting pests. By filming actual zebras, as well as normal horses dressed in zebra-striped coats, Caro showed that flies would approach the animals and then fumble their landings. It’s not yet clear why this happens.

Why don’t vultures just have wider visual fields that allow them to look ahead while flying? Martin thinks it’s because their large, sharp eyes are vulnerable to dazzling glare from the sun. In general, he says, birds with large eyes tend to have larger blind spots. Birds with panoramic vision, like ducks, tend to have smaller and less acute eyes that can better tolerate the presence of the sun.

Turning the eyes is out of the question because birds of prey can barely move their eyes without turning their heads. Indeed, their eyes are so big that they almost touch each other inside the skull.

Traditional fluorescent lights flicker at 100 Hz—that is, 100 times a second. That’s too fast for humans to see, but not for many birds like starlings, for whom the lights must be stressful and irritating.

There are many ways to break an eye, and evolution has explored them all. Lenses have degenerated. Visual pigments have disappeared. Eyeballs have sunk beneath the skin or been covered by it. One species alone, the Mexican cavefish, has lost its eyes several times over, as different sighted populations moved from bright rivers to dark caves and independently abandoned vision. As Eric Warrant tells me, “Why Gollum in The Hobbit had extra-big eyes makes no scientific sense.”

We need to spend some time in the weeds to appreciate the flowers.