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

by Ed Yong

Color, then, is fundamentally subjective. There’s nothing inherently “green” about a blade of grass, or the 550-nanometer light that it reflects. Our photoreceptors, neurons, and brains are what turn that physical property into the sensation of green.

A small proportion of people, and entire species of animals, also see only in shades of gray, not because of brain damage but because their retinas aren’t set up for color vision. They are called monochromats. Some, like sloths and armadillos, only have rod cells, which work well in dim light but aren’t geared toward color. Others, like raccoons and sharks, only have one cone, and since color vision depends on opponency, having one cone is effectively like having none. Whales have just one cone, too: To paraphrase the vision scientist Leo Peichl, for a blue whale, the ocean is not blue.

Dogs have two cones—one with a long, yellow-green opsin and another with a short, blue-violet one. They see mostly in shades of blue, yellow, and gray.

In some countries, they might be disqualified from flying planes, joining the military, or even driving. Color-blindness shouldn’t be a disability, but it can be because humans have built cultures that are predicated on trichromacy. And what’s so special about trichromacy, other than that most people have it?

He reminds us that seeing more colors isn’t advantageous in and of itself. Colors are not inherently magical. They become magical when and if animals derive meaning from them. Some are special to us because, having inherited the ability to see them from our trichromatic ancestors, we imbued them with social significance.

Our lenses typically block out UV, but 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.

Many birds also have UV patterns in their feathers. In 1998, two independent teams realized that much of the “blue” plumage of blue tits actually reflects a lot of UV; as one of them wrote, “Blue tits are ultraviolet tits.” To humans, these birds all look much the same. But thanks to their UV patterns, males and females look very different from each other. The same is true for more than 90 percent of songbirds whose sexes are indistinguishable to us, including barn swallows and mockingbirds.

dinosaurs, which were almost certainly tetrachromats and “probably saw all kinds of cool non-spectral colors,” Stoddard says. It’s ironic that for the longest time, illustrators and filmmakers portrayed dinosaurs in dull shades of brown, gray, and green. Only recently have artists started painting these animals with bright colors, inspired by the revelation that they are the ancestors of birds.

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.

The real glory of colors isn’t that some individuals see more of them, but that there’s such a range of possible rainbows.

Primates, for example, evolved trichromacy to better spot young leaves and ripe fruits. And once they added red to their Umwelt, they began evolving patches of bare skin that could convey messages by flushing with blood. The red faces of rhesus macaques, the red rumps of mandrills, and the comically red and bald heads of uakaris are all sexual signals made possible by trichromatic vision.

In 1992, Lars Chittka and Randolf Menzel analyzed 180 flowers and worked out what kind of eye would be best at discriminating their colors. The answer—an eye with green, blue, and UV trichromacy—is exactly what bees and many other insects have. You might think that these pollinators evolved eyes that see flowers well, but that’s not what happened. Their style of trichromacy evolved hundreds of millions of years before the first flowers appeared, so the latter must have evolved to suit the former. Flowers evolved colors that ideally tickle insect eyes.

Guided by evolution, eyes are living paintbrushes. Flowers, frogs, fish, feathers, and fruit all show that sight affects what is seen, and that much of what we find beautiful in nature has been shaped by the vision of our fellow animals. Beauty is not only in the eye of the beholder. It arises because of that eye.

There’s only a narrow Goldilocks zone of wavelengths that are useful for vision, and they range from 300 to 750 nanometers. Our eyes, which work from 400 to 700 nanometers, already cover much of that available visual space. But in the margins, a lot can happen.

Why don’t most humans see UV? It might be the cost of having sharp eyesight. When light passes through our lenses, shorter wavelengths are bent at sharper angles. Even if the lens admitted UV, it would focus these wavelengths at a point well in front of the others, blurring the image on the retina. This is called chromatic aberration. It’s less of an issue for small eyes, or for those that don’t need to be very acute. But for big-eyed animals with sharp vision, it’s a problem.

Naked mole-rats are exceptionally long-lived for rodents, with life spans of up to 33 years.

They’ve also been forced to tolerate carbon dioxide, which builds up in the nesting chambers with every exhalation. Carbon dioxide normally makes up 0.03 percent of the air in an average room. If levels shot up to 3 percent, you’d hyperventilate and panic. Meanwhile, the gas would dissolve in the wet surfaces of your mucous membranes, acidifying them.

Our experience of pain depends on a class of neurons called nociceptors. (The word is pronounced with a soft c, and comes from the Latin word nocere, meaning “to harm.”)

People often assume that pain feels the same across the entire animal kingdom, but that is not true. Much like color, it is inherently subjective and surprisingly variable. Just as wavelengths of light aren’t universally red or blue, and odors aren’t universally fragrant or pungent, nothing is universally painful, not even chemicals in scorpion venom that specifically evolved to inflict pain.

Sherrington wanted a separate term to describe the act of sensing harmful stimuli as distinct from the painful feelings they produce—a term that would have “the advantage of greater objectivity.” He came up with nociception. Over a century later, scientists and philosophers still make the distinction between nociception and pain. Nociception is the sensory process by which we detect damage. Pain is the suffering that ensues.

thanks to the unfortunate persistence of dualism—the outdated belief that the mind and body are separate—people often equate subjective with woolly, and psychological with imagined. This is harmfully wrong. It’s not the case that nociception is a physical process of the body, while pain is a psychological process of the mind. Both arise from the firing of neurons. It’s just that in humans, nociception can be confined to the peripheral nervous system, while with pain, the brain is always involved. Pain requires some degree of conscious awareness. Nociception can exist without it.

Scientists who study human pain still largely rely on people’s own accounts, and animals obviously can’t talk about their feelings.[*5] Our only recourse is to read the tea leaves of their behavior.

the philosopher and priest Nicolas Malebranche wrote that “animals eat without pleasure, cry without pain, grow without knowing it: they desire nothing, fear nothing, know nothing.” Such views have changed in recent decades, and most scientists would now agree that mammals can feel pain.

The controversies about animal pain often assume that they either feel exactly what we feel or nothing at all, as if they’re either little people or sophisticated robots. This dichotomy is false, but it persists because it’s difficult to imagine an intermediate state.

when Crook damaged one of the squid’s fins, the nociceptors on the opposite fin were just as excitable as those on the wounded side. Imagine if your entire body became delicate to the touch whenever you stubbed your toe: That’s a squid’s reality. “When they’re injured, their whole body becomes hypersensitive,” Crook tells me. “They go from being normal to this potential world of pain.” This might explain why they don’t groom their wounds. They can sense that they’ve been hurt, but they might not be able to tell where.

Thinking through the ethics of animal research, especially when that research is about pain, is not easy, “but I think it should be hard,” she says. “You should be distressed by what you’re doing to an animal in an experiment, even if it’s not painful. Animals don’t sign up for this. Even if my broader goal is to alleviate animal suffering, the animal sitting in the tank doesn’t know that.”

Rather than asking if cephalopods experience pain, we might ask which ones experience it, and how. The same goes for the 34,000 known species of fish, the 67,000 known species of crustaceans, and the who-knows-how-many-million species of insects. It’s ridiculous to treat these groups as monolithic when we know, from other senses like vision and smell, that even closely related animals differ in how they perceive the world.

Instead of focusing on whether pain even exists, we might ask, as physiologist Catherine Williams told me, “In which conditions and from which stimuli is it an advantage to have it, experience it, and display it?” And we would find that pain manifests differently in a burrowing mole-rat than in a scorpion-hunting mouse, or in a long-armed octopus than in a short-armed squid.

Naked mole-rats are so weird that their bizarre traits have often been mythologized, and many of the claims that surround them are untrue. I highly recommend the paper “Surprisingly Long Survival of Premature Conclusions About Naked Mole-Rat Biology,” which is an important corrective to some of these myths.

subjective pain is just one thing to consider when thinking about animal welfare, and may not even be the most important. “We could simply accept that nociception itself is more than enough to affect an animal’s welfare, and thus may require treatment,” the veterinarian Frederic Chatigny wrote. “Pain, while defined by consciousness, is not necessary for an animal’s wellbeing to be negatively affected.”

In the 1790s, the Italian priest and biologist Lazzaro Spallanzani realized that bats could still navigate in spaces too dark for a captive owl. In a series of cruel experiments, he showed that bats could orient when blinded, but would blunder into objects when deafened or gagged. He never fully grasped the meaning of these curious findings and could only write that “the ear of the bat serves more efficiently for seeing, or at least for measuring distances, than do its eyes.”

Annemarie Surlykke showed that the sonar call of the big brown bat can leave its mouth at 138 decibels—roughly as loud as a siren or jet engine. Even the so-called whispering bats, which are meant to be quiet, will emit 110-decibel shrieks, comparable to chainsaws and leaf blowers. These are among the loudest sounds of any land animal, and it’s a huge mercy that they’re too high-pitched for us to hear.

But bats can hear their own calls, which creates an obvious second challenge: They must avoid deafening themselves with every scream. They do so by contracting the muscles of their middle ears in time with their calls. This desensitizes their hearing while they shout and restores it in time for the echo.

They’re relying on memory and instinct. Humans behave in the same way: Most car accidents occur close to home, in part because drivers are less watchful when going down familiar routes.

It is no wonder that bats are so successful. They’re found on every continent except Antarctica, and they account for one in every five mammal species. There are bats that pluck insects from the air and bats that pluck fruit from trees. There are bats that catch frogs, bats that drink blood, and bats that sip nectar with tongues more than twice as long as their bodies. There are bat-eating bats. There are bats that go fishing by echolocating on ripples. There are bats that pollinate plants by echolocating on dish-shaped leaves that are adapted to reflect sonar pulses.

Earth’s core is a solid iron sphere surrounded by molten iron and nickel. The churning movements of that liquid metal turn the entire planet into a giant bar magnet. Its magnetic field can be depicted in the style of a school textbook: Lines emerge near the south pole, curve around the globe, and reenter near the north pole. This geomagnetic field is always present. It doesn’t change across the day or through the seasons. It’s not affected by weather or obstacles. Consequently, it is a boon for travelers, who can always use it to establish their bearings. Humans have done so for more than a thousand years, using compasses. Other animals—sea turtles, spiny lobsters, songbirds, and many others—have done so for millions of years, without help.

When the time comes for birds to migrate, they become visibly restless. Even in captivity, they’ll hop, flit, and flutter. These frantic movements are known as Zugunruhe—a German word that means “migration anxiety.” The birds know it’s time.

Earth’s magnetic field is extremely weak. It is so faint that the random jiggling movements of an animal’s molecules can carry 200 billion times more energy. No creature should be able to sense such an absurdly weak stimulus. And yet the robins clearly could.

mosquitoes are drawn to the carbon dioxide in our breath and the odors emanating from our skin. They can smell us.

When animals move, their sense organs provide two kinds of information. There’s exafference, signals produced by stuff happening in the world. There’s also reafference, signals produced by an animal’s own actions.

Controlling a human body is relatively simple for a human brain because our bones and joints constrain our movements. There are only so many ways in which, for example, you can pick up a mug. But as philosopher Peter Godfrey-Smith wrote in Other Minds, an octopus has “a body of pure possibility.”

The octopus, then, arguably has two distinct Umwelten. The arms live in a world of taste and touch. The head is dominated by vision. There’s undoubtedly some cross-talk between these sides, but Grasso suspects that the information exchanged between the head and the arms is simplified.

When I touched their tanks, I didn’t feel the mechanoreceptors in my fingers reacting to pressure. I simply felt. 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. That’s why our stories and myths are so full of characters who can transfer their consciousness into the bodies of animals—the Norse god Odin, for example, or Bran from the once-popular series Game of Thrones. Such feats, in which humans literally step into the sensory worlds of other animals, feel like the ultimate form of Umwelt-appreciation. But they also fundamentally misunderstand the concept. An animal’s sensory world is the result of solid tissues that detect real stimuli and produce cascades of electrical signals. It is not separate from the body, but of it. You can’t simply imagine how a human mind would work in a bat’s body or an octopus’s, because it wouldn’t work.

More than 83 percent of the continental United States lies within a kilometer of a road.

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.

A bogong moth will never know what a zebra finch hears in its song, a zebra finch will never feel the electric buzz of a black ghost knifefish, a knifefish will never see through the eyes of a mantis shrimp, a mantis shrimp will never smell the way a dog can, and a dog will never understand what it is like to be a bat. We will never fully do any of these things either, but we are the only animal that can even come close. We may not ever know what it is to be an octopus, but at least we know that octopuses exist, and that their experiences differ from ours. Through patient observation, through the technologies at our disposal, through the scientific method, and, above all else, through our curiosity and imagination, we can try to step into their worlds. We must choose to do so, and to have that choice is a gift. It is not a blessing we have earned, but it is one we must cherish.