There is a wonderful word for this sensory bubble—Umwelt. It was defined and popularized by the Baltic-German zoologist Jakob von Uexkull in 1909. Umwelt comes from the German word for “environment,” but Uexkull 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. Like the occupants of our imaginary room, a multitude of creatures could be standing in the same physical space and have completely different Umwelten. A tick, questing for mammalian blood, cares about body heat, the touch of hair, and the odor of butyric acid that emanates from skin. These three things constitute its Umwelt. Trees of green, red roses too, skies of blue, and clouds of white—these are not part of its wonderful world. The tick doesn't willfully ignore them. It simply cannot sense them and doesn't know they exist.
Uexkull compared an animal's body to a house. “Each house has a number of windows,” he wrote, “which open onto a garden: a light window, a sound window, an olfactory window, a taste window, and a great number of tactile windows. Depending on the manner in which these windows are built, the garden changes as it is seen from the house. By no means does it appear as a section of a larger world. Rather, it is the only world that belongs to the house—its [Umwelt]. The garden that appears to our eye is fundamentally different from that which presents itself to the inhabitants of the house.”
This was a radical notion at the time—and in some circles, it might still be. Unlike many of his contemporaries, Uexkull saw animals not as mere machines but as sentient entities, whose inner worlds not only existed but were worth contemplating. Uexkull didn't exalt the inner worlds of humans over those of other species. Rather, he treated the Umwelt concept as a unifying and leveling force. The human's house might be bigger than the tick's, with more windows overlooking a wider garden, but we are still stuck inside one, looking out. 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.
We cannot sense the faint electric fields that sharks and platypuses can. We are not privy to the magnetic fields that robins and sea turtles detect. We can't trace the invisible trail of a swimming fish the way a seal can. We can't feel the air currents created by a buzzing fly the way a wandering spider does. Our ears cannot hear the ultrasonic calls of rodents and hummingbirds or the infrasonic calls of elephants and whales. Our eyes cannot see the infrared radiation that rattlesnakes detect or the ultraviolet light that the birds and the bees can sense.
Even when animals share the same senses with us, their Umwelten can be very different. There are animals that can hear sounds in what seems to us like perfect silence, see colors in what looks to us like total darkness, and sense vibrations in what feels to us like complete stillness. 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. We are no sensory slouches, either. Our hearing is decent, and certainly better than that of the millions of insects that have no ears at all. Our eyes are unusually sharp, and can discern patterns on animal bodies that the animals themselves cannot see. 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.
This is also a book about animals as animals. Some scientists study the senses of other animals to better understand ourselves, using exceptional creatures like electric fish, bats, and owls as “model organisms” for exploring how our own sensory systems work. Others reverse-engineer animal senses to create new technologies: Lobster eyes have inspired space telescopes, the ears of a parasitic fly have influenced hearing aids, and military sonar has been honed by work on dolphin sonar. These are both reasonable motivations. I'm not interested in either. Animals are not just stand-ins for humans or fodder for brainstorming sessions. They have worth in themselves. We'll explore their senses to better understand their lives. “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.”
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A few terms will act as guideposts on our journey. 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 eyes, noses, and ears. And sense organs, together with the neurons that transmit their signals and the parts of the brain that process those signals, are collectively called sensory systems. The visual system, for example, includes the eyes, the photoreceptors inside them, the optic nerve, and the visual cortex of the brain. Together, these structures give most of us the sense of sight.