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

by Ed Yong

Ant pheromones are another story. There are many, and ants put them to different uses depending on their properties. Lightweight chemicals that easily rise into the air are used to summon mobs of workers that can rapidly overwhelm prey, or to raise fast-spreading alarms. Crush the head of an ant, and within seconds, nearby colony-mates will sense the aerosolized pheromones and charge into battle. Medium-weight chemicals that become airborne more slowly are used to mark trails. Workers lay these down when they find food, leading other colony-mates to foraging hotspots. As more workers arrive, the trail is strengthened. As the food runs out, the trail decays. Leafcutter ants are so sensitive to their trail pheromone that a milligram is enough to lay a path around the planet three times over. Finally, the heaviest chemicals, which barely aerosolize, are found on the surface of the ants’ bodies. Known as cuticular hydrocarbons, they act as identity badges. Ants use them to discern their own species from other kinds of ants, nestmates from other colonies, and queens from workers. Queens also use these substances to stop workers from breeding or to mark unruly subjects for punishment.

As the writer Adam Nicolson described in The Seabird’s Cry, “What may be featureless to us, a waste of undifferentiated ocean, is for them rich with distinction and variety, a fissured and wrinkled landscape, dense in patches, thin in others, a rolling olfactory prairie of the desired and the desirable, mottled and unreliable, speckled with life, streaky with pleasures and dangers, marbled and flecked, its riches often hidden and always mobile, but filled with places that are pregnant with life and possibility.”

Directionality comes more easily to a paired detector, which also explains the distinctive shape of one of nature’s least likely but most effective smell organs—the forked tongue of snakes.

As a timber rattlesnake slithers over the forest floor, its tongue turns the world into both map and menu, revealing the crisscrossing tracks of scurrying rodents and discerning the scents of different species. Amid the tangled trails, it can pick out those of its favorite prey[*27] and find sites where those tracks are common and fresh. It hides nearby, coiled in ambush. When a rodent runs past, the snake explodes outward four times faster than a human can blink. It stabs the rodent with its fangs and injects venom. The toxins usually take a while to work, and since rodents have sharp teeth, the snake avoids injury by releasing its prey and letting it run off. After several minutes, it starts flicking its tongue to track down the now-dead victim. The venom helps. Aside from lethal toxins, rattlesnake venom also includes compounds called disintegrins, which aren’t toxic but react with a rodent’s tissues to release odorants. The snakes can use these aromas to distinguish envenomated rodents from healthy ones and to tell rodents envenomated by their own species from those bitten by other kinds of rattlesnakes.

But John Caprio, a physiologist who studies catfish, says the difference between smell and taste couldn’t be clearer. Taste is reflexive and innate, while smell is not.[*29] From birth, we recoil from bitter substances, and while we can learn to override those responses and appreciate beer, coffee, or dark chocolate, the fact remains that there’s something instinctive to override. Odors, by contrast, “don’t carry meaning until you associate them with experiences,” Caprio says. Human infants aren’t disgusted by the smell of sweat or poop until they get older. 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. Even animal pheromones, which are traditionally thought to trigger hardwired responses, are surprisingly flexible in their effects, which can be sculpted through experience.

At the start of this chapter, we saw that dogs and other animals detect smells using proteins called odorant receptors. These are part of a much larger group of proteins called G-protein-coupled receptors, or GPCRs. Ignore the convoluted name; it doesn’t matter. What matters is that they are chemical sensors. They sit on the surface of cells, grabbing specific molecules that float past. Through their actions, cells can detect and react to the substances around them. This process is temporary: After the GPCRs are done, they either release or destroy the molecules that they’ve grabbed. But one group of them bucks this trend: opsins. They are special because they keep hold of their target molecules, and because those molecules absorb light. This is the entire basis of vision. This is how all animals see—using light-sensitive proteins that are actually modified chemical sensors. In a way, we see by smelling light.

One possible exception is the puff adder, a venomous African snake. It sits in ambush for weeks at a time, and protects itself by visually blending into its environment. But somehow, it seems to blend in chemically, too. In 2015, Ashadee Kay Miller found that keen-nosed animals, including dogs, mongooses, and meerkats, can’t detect a puff adder, even when they walk over one. Dogs can detect the scent of shed skin, but for reasons that no one understands, the living snakes are undetectable to their noses.

Unless you actually stuck your nose over some benzaldehyde, you couldn’t guess that it smells like almonds. If you saw dimethyl sulfide drawn on a page, you couldn’t foresee that it carries the scent of the sea. Even similar molecules can produce immensely different smells. Heptanol, with a backbone of seven carbon atoms, smells green and leafy. Add another carbon atom to the chain and you get octanol, which smells more like citrus. Carvone exists in two forms that contain exactly the same atoms but are mirror images of each other: One smells of caraway seeds and the other of spearmint. Mixtures are even more confusing. When mixed, some pairs of odors still smell distinct, while others produce a third smell that’s unlike the two parents. Meanwhile, perfumes that contain hundreds of chemicals don’t smell any more complex than individual odorants, and people typically struggle to name more than three ingredients in a blend. Noam Sobel, a neurobiologist who studies olfaction, has come closer than anyone else to wrangling this complexity. While I was writing this book, he and his team developed a measure that analyzes 21 features of odorant molecules and collapses these into a single number. The closer this smell metric is for any two molecules, the more similar their odors. This isn’t quite the same as predicting scent from structure, but it’s the next best thing—predicting scent from similarity to other scents.

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.

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. “When chickens look at you, you never know what the other eye is doing,”

In 2012, evolutionary biologist Megan Porter compared almost 900 opsins from different species, and confirmed that they share a single ancestor. That original opsin arose in one of the earliest animals and was so efficient at capturing light that evolution never conjured up a better alternative. Instead, the ancestral protein diversified into a wide family tree of opsins, which now underlie all vision. Porter draws that tree as a circle, with branches radiating outward from a single point. It looks like a giant eye.

There’s always at least one person who writes in with a pompous and incorrect corrective, so let’s get this out of the way: 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.

In 1994, Nilsson and Susanne Pelger simulated the evolution of a sharp stage-four eye from a simple stage-three one. The simulation began with a small, flat patch of photoreceptors. With every generation, the patch slowly thickens and curves into a cup. It gains a crude lens, which gradually improves. Assuming pessimistically that the eye improves by just 0.005 percent every generation, and that each generation lasts for a year, it would take just 364,000 years for the blurry stage-three eye to become something like ours. As far as evolution goes, that’s a blink of an eye.

It’s not the case, either, that advanced eyes always exist in advanced creatures and simple eyes always in simple ones. There are some microbes that consist entirely of single cells and which also double as surprisingly complex eyes. 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. The warnowiids, a group of single-celled algae, also seem to be living eyes, and each cell has components that resemble a lens, an iris, a cornea, and a retina. What they see, and whether they see at all, are open questions.

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.

The photoreceptors in a killer fly’s eye fire quickly and reset quickly. Both traits demand a lot of energy. Compared to the photoreceptors of a fruit fly, those of a killer fly have three times more mitochondria—the bean-shaped batteries that supply animal cells with power.

Other predatory insects, like dragonflies and robber flies, have large, high-resolution eyes with distinctive acute zones. As they pursue their targets, they turn their heads to keep the prey within the sharpest part of their visual field. Killer flies “have to pay attention in all directions,” Gonzalez-Bellido says, so they don’t have an acute zone, and their visual resolution isn’t especially high. Despite that, they seem to have a more demanding hunting strategy. Dragonflies hunt against the sky, spotting the silhouettes of prey that fly above them. But killer flies somehow “do the impossible thing of hunting against the ground,” Gonzalez-Bellido says. They’ll pick out prey moving in front of complex backgrounds, and then chase those targets through leaves and other cluttered environments.

There are many ways to break an eye, and evolution has explored them all. Lenses have degenerated. Visual pigments have disappeared. Eyeballs have sunk beneath the skin or been covered by it. One species alone, the Mexican cavefish, has lost its eyes several times over, as different sighted populations moved from bright rivers to dark caves and independently abandoned vision. As Eric Warrant tells me, “Why Gollum in The Hobbit had extra-big eyes makes no scientific sense.”

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). At my wife’s suggestion, and to Stoddard’s delight, I’m going to call these rurple, grurple, yurple, and ultrapurple.

An orb-weaver not only builds its own vibrational landscape but also can adjust it as if tuning a musical instrument. The range of that instrument is immense. By using gas guns to fire projectiles at individual silk fibers and analyzing the threads with high-speed cameras and lasers, Mortimer concluded that some silks can transmit vibrations over a wider range of speeds than any known material. A spider can theoretically change the speed and strength of those vibrations by altering the stiffness of its silk, the tension in the strands, and the overall shape of the web. It can do this every time it builds a new web, by pulling silk out of its body at different speeds, by creating fibers of different thicknesses, or by adding tension to the new strands. It can adjust webs that have already been spun by adding, removing, or tugging on specific threads. It can rely on silk’s natural tendency to contract in humidity, and then stretch out these tightened threads to just the right degree. It’s not clear when orb-weavers might decide to do any of this, but they certainly have the option of tuning their own senses and defining their own Umwelt according to their needs. Zoologist Takeshi Watanabe showed that the Japanese orb-weaver Oclonoba sybotides changes the structure of its web when it is hungry. It adds spiral decorations that increase the tension along the spokes, improving the web’s ability to transmit the weaker vibrations transmitted by smaller prey. When it is famished, e...

When the environment fluctuates from one season to the next, the information that’s relevant also changes.[*15] For a North American bird, spring often means sex. The air fills with courtship calls that are absent in other times of year and must now be carefully judged. Fall brings openness: Bare branches make little birds more visible to predators. The ability to localize the sound of approaching danger, which is inextricably linked to fast hearing, becomes paramount. An animal’s Umwelt cannot be static, because an animal’s world isn’t static.

It was a thrilling and formative time. Camped on a beach beneath the Southern Cross, with penguins bumbling past and albatrosses wheeling overhead, Clark began listening to whales.

He has seen whales slaloming between underwater mountain ranges, zigging and zagging between landmarks hundreds of miles apart. “When you watch these animals move, it’s as if they have an acoustic map of the oceans,” he says. He also suspects that the animals can build up such maps over their long lives, accruing

The scale of a whale’s hearing is hard to grapple with. There’s the spatial vastness, of course, but also an expanse of time. Underwater, sound waves take just under a minute to cover 50 miles. If a whale hears the song of another whale from a distance of 1,500 miles, it’s really listening back in time by about half an hour, like an astronomer gazing upon the ancient light of a distant star. If a whale is trying to sense a mountain 500 miles away, it has to somehow connect its own call with an echo that arrives 10 minutes later. That might seem preposterous, but consider that a blue whale’s heart beats around 30 times a minute at the surface, and can slow to just 2 beats a minute on a dive. They surely operate on very different timescales than we do. If a zebra finch hears beauty in the milliseconds within a single note, perhaps