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

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.

“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.”

To sense the world, animals detect stimuli—quantities like light, sound, or chemicals—and convert them into electrical signals, which travel along neurons toward the brain.

The cells that are responsible for detecting stimuli are called receptors: Photoreceptors detect light, chemoreceptors detect molecules, and mechanoreceptors detect pressure or movement.

These receptor cells are often concentrated in sense organs, like e...

And sense organs, together with the neurons that transmit their signals and the parts of the brain that process those signals, are...

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.

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.

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

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. 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.”

Why should nociception suck? Some scientists suggest that unpleasant emotions might have intensified and calcified the effect of nociceptive sensations, so that animals not only avoid what is currently hurting them but also learn to avoid it in the future. Nociception says, “Get away.” Pain says, “…and don’t go back.”

Magnetic fields are also deeply counterintuitive. As the Insane Clown Posse famously noted, “Fuckin’ magnets, how do they work?” Or as Warrant said to me, “I have enough trouble even understanding the stimulus, never mind trying to understand what an animal might perceive from it.” Other unusual senses like echolocation and electroreception can at least be compared to more familiar ones like hearing or touch. But I have no idea how to begin thinking about the Umwelt of a loggerhead turtle.

In 1995, environmental historian William Cronon wrote that “the time has come to rethink wilderness.” In a searing essay, he argued that the concept of wilderness, especially as perceived in the United States, had become unjustly synonymous with grandeur. Eighteenth-century thinkers believed that vast and magnificent landscapes reminded people of their own mortality and brought them closer to glimpsing the divine. “God was on the mountaintop, in the chasm, in the waterfall, in the thundercloud, in the rainbow, in the sunset,” Cronon wrote. “One has only to think of the sites that Americans chose for their first national parks—Yellowstone, Yosemite, Grand Canyon, Rainier, Zion—to realize that virtually all of them fit one or more of these categories. Less sublime landscapes simply did not appear worthy of such protection; not until the 1940s, for instance, would the first swamp be honored, in Everglades National Park, and to this day there is no national park in the grasslands.” Equating wilderness with otherworldly magnificence treats it as something remote, accessible only to those with the privilege to travel and explore. It imagines that nature is something separate from humanity rather than something we exist within. “Idealizing a distant wilderness too often means not idealizing the environment in which we actually live, the landscape that for better or worse we call home,” Cronon wrote.

The majesty of nature is not restricted to canyons and mountains. It can be found in the wilds of perception—the sensory spaces that lie outside our Umwelt and within those of other animals. To perceive the world through other senses is to find splendor in familiarity, and the sacred in the mundane. Wonders exist in a backyard garden, where bees take the measure of a flower’s electric fields, leafhoppers send vibrational melodies through the stems of plants, and birds behold the hidden palettes of rurples and grurples. In writing this book, I have found the sublime while confined to my home by a pandemic, watching tetrachromatic starlings gathering in the trees outside and playing sniffing games with my dog, Typo. Wilderness is not distant. We are continually immersed in it. It is there for us to imagine, to savor, and to protect.