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

by Ed Yong

Our eyes are unusually sharp, and can discern patterns on animal bodies that the animals themselves cannot see.

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.

proprioception, the awareness of your own body, which is distinct from touch; and equilibrioception, the sense of balance, which has links to both touch and vision.

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.

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.

Kant said that “smell does not allow itself to be described, but only compared through similarity with another sense.” The English language confirms his view with just three dedicated smell words: stinky, fragrant, and musty. Everything else is a synonym (aromatic, foul), a very loose metaphor (decadent, unctuous), a loan from another sense (sweet, spicy), or the name of a source (rose, lemon). Of the five Aristotelian senses, four have vast and specific lexicons. Smell, as Diane Ackerman wrote, “is the one without words.”

We need to stop asking “How good is an animal’s sense of smell?” Better questions would be “How important is smell to that animal?” and “What does it use its sense of smell for?”

Leafcutter ants are so sensitive to their trail pheromone that a milligram is enough to lay a path around the planet three times over.

Shearwaters, dogs, elephants, and ants all smell with different organs, but they all smell in stereo, using a pair of nostrils or antennae. By comparing the odorants that land on each side, they can track the source of a scent.

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.

dogs and other animals detect smells using proteins called odorant receptors.

Dogs have a facial muscle that can raise their inner eyebrows, giving them a soulful, plaintive expression. This muscle doesn’t exist in wolves. It’s the result of centuries of domestication, in which dog faces were inadvertently reshaped to look a bit more like ours. Those faces are now easier to read, and better at triggering a nurturing response.

It is dangerous to assess an animal’s sensory abilities by counting its genes. Dogs have twice the number of working odorant receptor genes as humans, but that doesn’t mean that their sense of smell is twice as good.

Humans are such a visual species that those of us with sight instinctively equate active eyes with an active intellect.

The eyes of the giant squid are as big as soccer balls; those of fairy wasps are the size of an amoeba’s nucleus.

Squid, jumping spiders, and humans have all independently evolved camera-like eyes, in which a single lens focuses light onto a single retina.

The first step to understanding another animal’s Umwelt is to understand what it uses its senses for.

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

For a fly’s eye to be as sharp as a human’s, it would have to be a meter wide.

scallops have eyes when most other bivalves like mussels and oysters do not.

We don’t create a tactile scene of the world, even though we can feel with every part of our skin. Indeed, we largely ignore those sensations until something pokes us (or vice versa). And when we feel something unexpected, our most common reaction is to turn and look at it.

Humans and other primates are rather odd in having two eyes that point straight ahead. The left eye gets a very similar view to the right, and their visual fields overlap a lot. This arrangement gives us excellent depth perception. It also means we can barely see things to our sides, and we can’t see what’s behind us without turning our heads. For us, seeing is synonymous with facing, and exploration is achieved through gazing and turning.

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.

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. When a chicken investigates something new, it will swing its head from side to side to look upon it with the acute zone of each eye in turn.

An animal’s visual field determines where it can see. Its acute zones determine where it sees well. Without considering both traits, we can seriously misinterpret an animal’s actions.

Cows and other livestock also have a somnolent air because their gaze is so fixed. They rarely turn to look at you in the way another human (or a jumping spider) might. But they also don’t need to. Their visual fields wrap almost all the way around their heads and their acute zones are horizontal stripes, giving them a view of the entire horizon at once.

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.

If you looked at an image at the same moment as a killer fly, the insect would be airborne well before a signal had even left your retina.

The fly’s vision also updates more quickly. 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. Boxing would look like tai chi.

To dive into the ocean is to enter the largest habitat on the planet—a realm with over 160 times more living space than all the ecosystems on the surface combined. Most of that space is dark.

Turn off the lights, and our world becomes monochromatic. This shift occurs because our eyes contain two types of photoreceptors—cones and rods. The cones allow us to see colors, but they only work in bright light. In the dark, the more sensitive rods take over, and a kaleidoscope of daytime hues is replaced by the blacks and grays of the night.

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.

Consider the freshwater bacterium Synechocystis. Light that hits one side of its spherical cell becomes focused on the opposite side. The bacterium can sense where that light is coming from, and move in that direction. It is effectively a living lens, and its entire boundary is a retina.

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.

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.

Reflections from the tapetum are responsible for the eyeshine of dogs, cats, deer, and other animals illuminated by car headlights or camera flashes. The structure of a reindeer’s tapetum changes in the dark winter to reflect even more light. Coincidentally, this also changes the tapetum’s color, and thus the color of reindeer eyes, from golden yellow in the summer to a rich blue in the winter.

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.

Color-blindness shouldn’t be a disability, but it can be because humans have built cultures that are predicated on trichromacy.

Most animals that can see color can see UV. It’s the norm, and we are the weirdos.[*9]

If our red and blue cones are stimulated together, we see purple—a color that doesn’t exist in the rainbow and that can’t be represented by a single wavelength of light. These kinds of cocktail colors are called non-spectral. Hummingbirds, with their four cones, can see a lot more of them, including UV-red, UV-green, UV-yellow (which is red + green + UV), and probably UV-purple (which is red + blue + UV).

Tetrachromats also have a different concept of white. White is what we perceive when all our cones are equally stimulated. But you’d need a different blend of wavelengths to excite a bird’s quartet of cones than you would a human’s trio. Paper is treated with dyes that happen to absorb UV, so it wouldn’t look white to a bird. Many supposedly “white” bird feathers reflect UV and wouldn’t necessarily look white to birds, either.

Human tetrachromats are usually women, because the genes for the long and medium opsins both sit on the X chromosome. Since most women have two X chromosomes, they can inherit two slightly different versions of either gene. They would then end up with four different kinds of opsins that are tuned to different wavelengths—short,

Both medium and long genes lie on the X chromosome. If someone with two X chromosomes inherits a faulty copy of either gene, they usually have a working backup. But if someone with an X and a Y chromosome inherits a faulty copy, they’re stuck with it. This is why red-green color-blindness, which is typically caused by the loss of either the M or L cones, is much more common in men than in women.

human color vision is far more varied than what she and others have seen in chimps, baboons, and other primates. It’s unclear why, but it might be that our survival is now less closely tied to the colors we see, allowing for variants that might once have been detrimental to remain.