These examples tell us that “senses cannot be clearly divided into a limited number of discrete kinds,” Macpherson wrote in The Senses. Instead of trying to shove animal senses into Aristotelian buckets, we should instead study them for what they are. Though I have organized this book into chapters that revolve around specific stimuli, like light or sound, that's largely for convenience. Each chapter is a gateway into the varied things that animals do with each stimulus. We will not concern ourselves with counting senses, nor talk nonsensically about a “sixth sense.” We will instead ask how animals use their senses, and attempt to step inside their Umwelten.
It won't be easy. In his classic 1974 essay, “What Is It Like to Be a Bat?,” the American philosopher Thomas Nagel argued that other animals have conscious experiences that are inherently subjective and hard to describe. 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.”
In thinking about other animals, we are biased by our own senses and by vision in particular. Our species and our culture are so driven by sight that even people who are blind from birth will describe the world using visual words and metaphors. You agree with people if you see their point, or share their view. You are oblivious to things in your blind spots. Hopeful futures are bright and gleaming; dystopias are dark and shadowy. Even when scientists describe senses that humans lack altogether, like the ability to detect electric fields, they talk about images and shadows. 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.
Scholars of animal behavior often discuss the perils of anthropomorphism—the tendency to inappropriately attribute human emotions or mental abilities to other animals. But perhaps the most common, and least recognized, manifestation of anthropomorphism is the tendency to forget about other Umwelten—to frame animals' lives in terms of our senses rather than theirs. This bias has consequences. We harm animals by filling the world with stimuli that overwhelm or befuddle their senses, including coastal lights that lure newly hatched turtles away from the oceans, underwater noises that drown out the calls of whales, and glass panes that seem like bodies of water to bat sonar. We misinterpret the needs of animals closest to us, stopping smell-oriented dogs from sniffing their environments and imposing the visual world of humans upon them. And we underestimate what animals are capable of to our own detriment, missing out on the chance to understand how expansive and wondrous nature truly is—the delights that, as William Blake wrote, are “clos'd by your senses five.”
Throughout this book, we'll encounter animal abilities that others had long thought impossible or absurd. 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.
Griffin's words are not carte blanche to put forward convoluted or paranormal explanations for animal behavior. I see them, and Nagel's essay, as a call for humility. They remind us that other animals are sophisticated, and that, for all our vaunted intelligence, it is very hard for us to understand other creatures, or to resist the tendency to view their senses through our own. We can study the physics of an animal's environment, look at what they respond to or ignore, and trace the web of neurons that connects their sense organs to their brains. But the ultimate feats of understanding—working out what it's like to be a bat, or an elephant, or a spider—always require what psychologist Alexandra Horowitz calls “an informed imaginative leap.”
Many sensory biologists have backgrounds in the arts, which may enable them to see past the perceptual worlds that our brains automatically create. Sonke Johnsen, for example, studied painting, sculpture, and modern dance well before he studied animal vision. To represent the world around us, he says, artists already have to push against the limits of their Umwelt and “look under the hood.” That capacity helps him “think about animals having different perceptual worlds.” He also notes that many sensory biologists are perceptually divergent. Sarah Zylinski studies the vision of cuttlefish and other cephalopods; she has prosopagnosia and can't recognize even familiar faces, including her mother's. Kentaro Arikawa studies color vision in butterflies; he is red-green color-blind. Suzanne Amador Kane studies the visual and vibrational signals of peacocks; she has slight differences in her color vision in each eye, so that one gives her a slightly reddish tint. Johnsen suspects that these differences, which some might bill as “disorders,” actually predispose people to step outside their Umwelten and embrace those of other creatures. Perhaps people who experience the world in ways that are considered atypical have an intuitive feeling for the limits of typicality.