Overview: Enter a new dimension—the world as it is truly perceived by other animals—from the Pulitzer Prize-winning, New York Times bestselling author of I Contain Multitudes.
“A stunning achievement, steeped in science but suffused with magic.”—Siddhartha Mukherjee, author of The Gene
The Earth teems with sights and textures, sounds and vibrations, smells and tastes, electric and magnetic fields. But every kind of animal, including humans, is enclosed within its own unique sensory bubble, perceiving but a tiny sliver of our immense world.
In An Immense World, author and Pulitzer Prize–winning science journalist Ed Yong coaxes us beyond the confines of our own senses, allowing us to perceive the skeins of scent, waves of electromagnetism, and pulses of pressure that surround us. We encounter beetles that are drawn to fires, turtles that can track the Earth’s magnetic fields, fish that fill rivers with electrical messages, and even humans who wield sonar like bats. We discover that a crocodile’s scaly face is as sensitive as a lover’s fingertips, that the eyes of a giant squid evolved to see sparkling whales, that plants thrum with the inaudible songs of courting bugs, and that even simple scallops have complex vision. We learn what bees see in flowers, what songbirds hear in their tunes, and what dogs smell on the street. We listen to stories of pivotal discoveries in the field, while looking ahead at the many mysteries that remain unsolved.
Funny, rigorous, and suffused with the joy of discovery, An Immense World takes us on what Marcel Proust called “the only true voyage . . . not to visit strange lands, but to possess other eyes.”
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| An Immense World How Animal Senses Reveal the Hidden Realms Around Us by Ed Yong Book |
An Immense World How Animal Senses Reveal the Hidden Realms Around Us by Ed Yong Book Read Online Chapter One
Leaking Sacks of Chemicals
Smells and Tastes
“I don’t think he’s been in here before,” Alexandra Horowitz tells me. “So it should be very smelly.”
By “he,” she means Finnegan—her ink-black Labrador mix, who also goes by Finn. By “here,” she means the small, windowless room in New York City in which she runs psychological experiments on dogs. By “smelly,” she means that the room should be bursting with unfamiliar aromas, and thus should prove interesting to Finn’s inquisitive nose. And so it does. As I look around, Finn smells around. He explores nostrils-first, intently sniffing the foam mats on the floor, the keyboard and mouse on the desk, the curtain draped over a corner, and the space beneath my chair. Compared to humans, who can explore new scenes by subtly moving our heads and eyes, a dog’s nasal explorations are so meandering that it’s easy to see them as random and thus aimless. Horowitz thinks of them differently. Finn, she notes, is interested in objects that people have touched and interacted with. He follows trails and checks out spots where other dogs have been. He examines vents, door cracks, and other places where moving air imports new odorants—scented molecules.[*1] He sniffs different parts of the same object, and he’ll sniff them at different distances, “like he’s approaching the Van Gogh and seeing what the brushstrokes look like up close,” says Horowitz. “They’re in that state of olfactory exploration all the time.”
Horowitz is an expert on dog olfaction—their sense of smell—and I’m here to talk with her about all things sniffy and nasal. And yet, I’m so relentlessly visual that when Finn finishes nosing around and approaches me, I’m instantly drawn to his eyes, which are captivating and brown like the darkest chocolate.[*2] It takes concerted effort to refocus on what’s right in front of them—his nose, prominent and moist, with two apostrophe-shaped nostrils curving to the side. This is Finn’s main interface with the world. Here’s how it works.
Take a deep breath, both as demonstration and to gird yourself for some necessary terminology. When you inhale, you create a single airstream that allows you to both smell and breathe. But when a dog sniffs, structures within its nose split that airstream in two. Most of the air heads down into the lungs, but a smaller tributary, which is for smell and smell alone, zooms to the back of the snout. There it enters a labyrinth of thin, bony walls that are plastered with a sticky sheet called the olfactory epithelium. This is where smells are first detected. The epithelium is full of long neurons. One end of each neuron is exposed to the incoming airstream and snags passing odorants using specially shaped proteins called odorant receptors. The other end is plugged directly into a part of the brain called the olfactory bulb. When the odorant receptors successfully grab their targets, the neurons notify the brain, and the dog perceives a smell. You can breathe out now.
Humans share the same basic machinery, but dogs just have more of everything: a more extensive olfactory epithelium, dozens of times more neurons in that epithelium, almost twice as many kinds of olfactory receptors, and a relatively larger olfactory bulb.[*3] And their hardware is packed off into a separate compartment, while ours is exposed to the main flow of air through our noses. This difference is crucial. It means that whenever we exhale, we purge the odorants from our noses, causing our experience of smell to strobe and flicker. Dogs, by contrast, get a smoother experience, because odorants that enter their noses tend to stay there, and are merely replenished by every sniff.
The shape of their nostrils adds to this effect. If a dog is sniffing a patch of ground, you might imagine that every exhalation would blow odorants on the surface away from the nose. But that’s not what happens. The next time you look at a dog’s nose, notice that the front-facing holes taper off into side-facing slits. When the animal exhales while sniffing, air exits through those slits and creates rotating vortices that waft fresh odors into the nose. Even when breathing out, a dog is still sucking air in. In one experiment, an English pointer (who was curiously named Sir Satan) created an uninterrupted inward airstream for 40 seconds, despite exhaling 30 times during that period.
With such hardware, it’s no wonder that dog noses are incredibly sensitive. But how sensitive? Scientists have tried to find the thresholds at which dogs can no longer smell certain chemicals, but their answers are all over the place, varying by factors of 10,000 from one experiment to another.[*4] Rather than focusing on these dubious statistics, it’s more instructive to look at what dogs can actually do. In past experiments, they have been able to tell identical twins apart by smell. They could detect a single fingerprint that had been dabbed onto a microscope slide, then left on a rooftop and exposed to the elements for a week. They could work out which direction a person had walked in after smelling just five footsteps. They’ve been trained to detect bombs, drugs, landmines, missing people, bodies, smuggled cash, truffles, invasive weeds, agricultural diseases, low blood sugar, bedbugs, oil pipeline leaks, and tumors.
Migaloo can find buried bones at archeological sites. Pepper uncovers lingering oil pollution on beaches. Captain Ron detects turtle nests so that the eggs can be collected and protected. Bear can pinpoint hidden electronics, while Elvis specializes in pregnant polar bears. Train, who flunked out of drug detection school for being too energetic, now uses his nose to track the scat of jaguars and mountain lions. Tucker used to hang off the bow of boats and sniff for orca poop; he has since retired, and his duties now fall to Eba. If it has a scent, a dog can be trained to detect it. We redirect their Umwelten in service of our needs, to compensate for our olfactory shortcomings. These feats of detection are worth marveling at, but they are also parlor tricks. They allow us to abstractly appreciate that dogs have a great sense of smell, without truly appreciating what that means for their inner lives or how their olfactory world differs from a visual one.
Unlike light, which always moves in a straight line, smells diffuse and seep, flood and swirl. When Horowitz observes Finn sniffing a new space, she tries to ignore the clear edges that her vision affords, and instead pictures “a shimmering environment, where nothing has a hard boundary,” she says. “There are focal areas, but everything is sort of seeping together.” Smells travel through darkness, around corners, and in other conditions that vex vision. Horowitz can’t see into the bag slung over the back of my chair, but Finn can smell into it, picking up molecules drifting from the sandwich within. Smells linger in a way that light does not, revealing history.[*5] The past occupants of Horowitz’s room have left no ghostly visual traces, but their chemical imprint is there for Finn to detect. Smells can arrive before their sources, foretelling what’s to come. The scents unleashed by distant rain can clue people in to advancing storms; the odorants emitted by humans arriving home can send their dogs running to a door. These skills are sometimes billed as extrasensory, but they are simply sensory. It’s just that things often become apparent to the nose before they appear to the eyes. When Finn sniffs, he is not merely assessing the present but also reading the past and divining the future. And he is reading biographies. Animals are leaking sacks of chemicals, filling the air with great clouds of odorants.[*6] While some species deliberately send messages by releasing smells, all of us inadvertently do so, giving away our presence, position, identity, health, and recent meals to creatures with the right noses.[*7]
“I never thought much about the nose at all,” says Horowitz. “It didn’t occur to me.”[*8] When she started studying dogs, she focused on things like their attitudes to unfairness—the kind of topic that’s interesting to psychologists. But after reading Uexküll and thinking about the Umwelt concept, she shifted her attention to smell—the kind of topic that’s interesting to dogs.
She notes, for example, that many dog owners deny their animals the joys of sniffing. To a dog, a simple walk is an odyssey of olfactory exploration. But if an owner doesn’t understand that and instead sees a walk as simply a means of exercise or a route to a destination, then every sniffy act becomes an annoyance. When the dog pauses to examine some invisible trace, it must be hurried along. When the dog sniffs at poop, a carcass, or something the owner’s senses find displeasing, it must be pulled away. When the dog sticks its nose in the crotch of another dog, it’s being indecorous: Bad dog! After all, in Western cultures at least, humans don’t smell each other.[*9] “You could give someone a hug, but if you actually sniffed them, that would be very weird,” says Horowitz. “I could say that your hair smells great, but I can’t say that you smell great, unless we’re intimate.” Time and again, people impose their values—and their Umwelt—onto their dogs, forcing them to look instead of sniff, dimming their olfactory worlds and suppressing an essential part of their caninehood. That was never clearer to Horowitz than when she took Finn to a nosework class.
Oddly billed as a sport, these classes simply train dogs to find hidden scents, under increasingly difficult conditions. That should come naturally, but it didn’t to many of the animals in Finn’s class. Several seemed to lack any agency: They had to be pulled from box to box by their owners, or were completely unsure what to do. Others became agitated in the presence of other dogs and barked at them. But after a summer of sniffing, those behavioral quirks diminished. The reticent dogs regained their volition. The reactive dogs became tolerant. All seemed more easygoing. Fascinated, Horowitz and her colleague Charlotte Duranton ran their own experiment with 20 dogs. In front of each animal, Duranton placed a bowl in one of three locations: one where the bowl always contained food, a second where it was always empty, and a third where the outcome was ambiguous. The dogs quickly learned to approach the food-filled bowl and ignore the empty one. What about the ambiguous one? A dog’s willingness to approach that bowl indicates what a cognitive psychologist might call positive judgment bias and what everyone else might call optimism. Horowitz found that dogs became more optimistic after just two weeks of nosework. As their sense of smell brightened, so did their outlook. (By contrast, dogs didn’t change after two weeks of heelwork—an owner-led obedience activity that involves neither olfaction nor autonomy.)
For Horowitz, the implications are clear: Let dogs be dogs. Appreciate that their Umwelt is different, and lean into that difference. She does this by taking Finn on dedicated smell walks, when he’s allowed to sniff to his olfactory bulb’s content. If he stops, she stops. His nose sets the pace. The walks are slower, but she has no destination in mind. We go on such a walk together, heading a few blocks west of her office and into Manhattan’s Riverside Park. It’s a hot summer’s day, and the air is redolent with garbage, urine, and exhaust—and that’s only what I can smell. Finn detects more. He runs his nose along the cracks in the pavement. He investigates a traffic sign. He pauses to sniff a hydrant “because it’s been visited by all the other dogs of Columbia University,” Horowitz says. Sometimes she’ll see Finn sniff a fresh patch of urine, raise his head, look around (or smell around), and find the dog that just left it. The smell isn’t just an object unto itself but a reference point, and the walk isn’t just an intermediate state between points A and B but a tour of Manhattan’s layered, unseen stories.
Once we’re inside the park, the air fills with greenery, cut grass, mulch, and barbecues. Another dog walks past and Finn turns to breathe in an odor sample, puffing his cheeks out like a cigar smoker. Two large poodles approach, but before they can get close, their owner pulls them away and body-checks them against a fence. Horowitz looks sad. She’s happier when a female Australian shepherd arrives and circles Finn, both enthusiastically sniffing each other’s genitals, while we make small talk with the owner. We glean the other dog’s sex through pronouns; Finn worked it out through smell. We ask about her age; Finn can guess. We don’t ask about her health or readiness to mate; Finn doesn’t need to ask. “There was a time when I would try to smell what he’s smelling, but I do that less often simply because I know I’m not getting what he’s getting,” Horowitz says. But there’s room for improvement. Though the human nose lacks the anatomical complexity of a dog’s and is unhelpfully farther from the ground, it is also underused. By taking more sniffs herself, and paying closer attention to odors, Horowitz says that she has become a better smeller (and a more socially awkward one). “We have perfectly good noses. We just don’t use them as well as the dog.”
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A funny thing happens when you mention dogs to neuroscientists who study olfaction in humans, as Horowitz learned while writing her book Being a Dog. They get a little territorial, a little…well…sniffy. Some dislike that dogs get treated like special olfactory paragons when many other mammals are excellent smellers, including rats (which can also detect landmines), pigs (whose olfactory epithelium can be twice as large as a German shepherd’s), and elephants (which we’ll get to later). Others point to massive discrepancies in studies that test dogs’ ability to detect specific odors. These have variously claimed that dogs are a billion times more sensitive than humans, or a million times, or just ten thousand times. In some cases, humans do better: Of 15 odorants where both species have been tested, humans outperformed our canine companions on five, including beta-ionone (cedar wood) and amyl acetate (bananas). People also excel at discriminating between smells. While it’s easy to find two colors that humans can’t tell apart, it’s very hard to find indistinguishable pairs of odors. Neuroscientist John McGann has tried, and tells me, “We tried odors that mice can’t tell apart and humans were like: No, we’ve got this.”
Yet textbooks still claim our sense of smell is terrible. McGann traced the origin of this pernicious myth to the nineteenth century. In 1879, neuroscientist Paul Broca noted that our olfactory bulbs are relatively puny compared to those of other mammals. He reasoned that smell is a base and animalistic sense, and the loss of it was necessary for us to have higher thought and free will. He then classified us (along with other primates and whales) as non-smellers. The label stuck, even though Broca never actually measured how well animals smell, relying instead on sketchy inferences based on the dimensions of their brains. Compared to a mouse, a human has an olfactory bulb smaller relative to other parts of the brain, but also physically bigger, with roughly as many neurons. It’s not clear what any one of these metrics says about an animal’s experience of smell.[*10]
The textbook perspective is also a Western one, based on cultures where smell has long been undervalued. Plato and Aristotle argued that olfaction was too vague and ill-formed to produce anything other than emotional impressions. Darwin deemed it to be “of extremely slight service.” 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.”
The Jahai people of Malaysia would disagree, as would the Semaq Beri, the Maniq, and the many other hunter-gatherer groups who have dedicated smell vocabularies. The Jahai use a dozen words for smells and smell alone. One describes the scent in gasoline, bat droppings, and millipedes. Another is for some quality shared by shrimp paste, rubber tree sap, tigers, and rotten meat. Yet another refers to soap, the pungent durian fruit, and the popcorn-like twang of the binturong.[*11] They “have this ease of talking about smells,” says psychologist Asifa Majid, who found that the Jahai can name smells as easily as English-speakers can name colors. Just as tomatoes are red, the binturong is ltpit. Smell is also a fundamental part of their culture. Once, Majid was told off by Jahai friends for sitting too close to her research partner and allowing their smells to mingle. Another time, she tried to name the smell of a wild ginger plant; children mocked her not only for failing but also for treating the whole plant as a single object, when the stem and flowers obviously had distinct smells. The myth of poor human olfaction “might have been overridden much earlier if the humans under consideration had been Jahai instead of Brits and Americans,” Majid tells me.
Even Westerners can pull off surprising olfactory feats when given the chance. In 2006, neuroscientist Jess Porter took blindfolded students to a park in Berkeley and asked them to follow a 10-meter trail of chocolate oil that she had drizzled on the grass. The students got down on all fours, snuffled about like dogs, and looked ridiculous. But they succeeded, and got better with practice.
When I visit Alexandra Horowitz, she challenges me to the same test and lays some chocolate-scented string on the floor. Eyes closed and nostrils open, I kneel down and sniff away. I quickly pick up the smell of chocolate and follow it. When I lose the scent, I cast my head from side to side, exactly like a dog would. But there end the similarities. A dog can sniff six times a second, wafting a steady conveyor of air over its olfactory receptors. I start to hyperventilate after several consecutive sniffs, and when I pause to exhale, I lose the trail. I succeed in tracking the string, but it takes me a minute to do what Finn manages in half a second. Even if I practiced regularly, I couldn’t come close; I don’t have the hardware. And crucially, Horowitz adds after whipping away the string, a dog can still follow a trail once the odor source is gone. We both try, bending down to sniff. “I don’t smell anything left,” she says. We humans underestimate our sense of smell, but it’s also clear that we simply don’t live in the same olfactory world as a dog. And that world is so complicated that it’s a wonder we can make sense of it at all.
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Many living things can sense light. Some can respond to sound. A select few can detect electric and magnetic fields. But every thing, perhaps without exception, can detect chemicals. Even a bacterium, which consists of just one cell, can find food and avoid danger by picking up on molecular clues from the outside world. Bacteria can also release their own chemical signals to communicate with each other, launching infections and performing other coordinated actions only when their numbers are large enough. Their signals can then be detected and exploited by bacteria-killing viruses, which have a chemical sense even though they are such simple entities that scientists disagree about whether they’re even alive. Chemicals, then, are the most ancient and universal source of sensory information. They’ve been part of Umwelten for as long as Umwelten have existed. They’re also among the hardest parts of it to understand.
Scientists who work on vision and hearing have it comparatively easy. Light and sound waves can be defined by clear and measurable properties like brightness and wavelength, or loudness and frequency. Shine wavelengths of 480 nanometers into my eyes, and I’ll see blue. Sing a note with a frequency of 261 hertz (Hz), and I’ll hear middle C. Such predictability simply doesn’t exist in the realm of smells. The variation among possible odorants is so wide that it might as well be infinite. To classify them, scientists use subjective concepts like intensity and pleasantness, which can only be measured by asking people. Even worse, there are no good ways of predicting what a molecule smells like—or even if it smells at all—from its chemical structure.[*12] And yet, many animals naturally grapple with the intricacy of olfaction, without any training in chemistry or neuroscience. Their noses are kings of infinite space. How do they work?
The basics became clearer after Linda Buck and Richard Axel made a pivotal discovery in 1991. In work that would earn them a Nobel Prize, the duo identified a large group of genes that produce odorant receptors—the proteins that initially recognize smelly molecules.[*13] We encountered them earlier in this chapter while discussing dogs, but they underlie the sense of smell throughout the animal kingdom. The odorant receptors probably recognize their target molecules, like electric sockets accepting certain cables.[*14] When this happens, the neurons that harbor these receptors send signals to the smell centers of the brain, and the animal perceives a scent. But the details of this process are still murky. There aren’t enough receptors to account for the huge range of possible odorants, so the perception of scent must depend on the combination of olfactory neurons that are firing. If one group goes off, you delight at the scent of a rose. If another group activates, you wince at the whiff of vomit. Such a code must exist, but its nature is still mostly mysterious.
Odorant receptors can also vary from one individual to another in dramatic ways. For example, the OR7D4 gene creates a receptor that responds to androstenone, the chemical behind the stench of sweaty socks and body odor. To most people, it’s repulsive. But to a lucky few who inherit a slightly different version of OR7D4, androstenone smells like vanilla. That’s just one receptor out of hundreds, and all exist in varied forms, bestowing every individual with their own subtly personalized Umwelt. Everyone likely smells the world in a slightly different way. And if it’s that hard to appreciate the olfactory Umwelt of another human, imagine how hard the task becomes for another species.
We should be skeptical of any claim that pits one animal’s sense of smell against another’s. I have repeatedly read that an elephant’s sense of smell is five times more sensitive than a bloodhound’s, but that’s an utterly meaningless statement. Does that mean the elephant detects five times more chemicals? Does it sense certain chemicals at a fifth the concentration, or from five times the distance? Does it remember smells for five times as long? Such comparisons will always be flawed because smell is diverse and often unquantifiable. 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?”
Male moths, for example, are tuned to sexual chemicals released by females. They pick up these odorants from miles away using feathery antennae, and slowly flutter over to the source. Smell is so important to them that when scientists transplanted the antennae of female sphinx moths onto males, the recipients behaved like females, seeking out the scent of egg-laying sites instead of mates. Their sense of smell is clearly amazing, as evidenced by the continued existence of moths. But they only put this amazing sense toward a few specific tasks. Moths have been described as “odor-guided drones,” and that’s not an exaggeration. Many males don’t even have mouthparts when they reach adulthood. Freed from the need to feed, their short lives are devoted to flying, finding, and…mating. Their behaviors are simple enough that they can be easily diverted. By mimicking female moth odors, bolas spiders can lure male moths into fatal ambushes, while farmers can lure them into traps. Other insects, however, process smells in more sophisticated ways.
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In a lab in New York City, Leonora Olivos Cisneros pulls out a large Tupperware container and lifts the lid to reveal a writhing sea of dark-red dots. They’re ants. Specifically, they’re clonal raiders—an obscure species that’s stockier than most ants and, unusually, has neither queens nor males. Every individual is female and every one can reproduce by cloning herself. About 10,000 of them are scurrying around the container. Most have formed a makeshift nest from their own bodies and are tending to their young grubs. The rest are wandering around in search of food. Olivos Cisneros feeds them on other ants, including escamoles—the larvae of a much larger species that she brings over from Mexico.
The clonal raiders are so small that it’s hard to focus on any one of them. Under the microscope, they’re much easier to see, not just because they’ve been magnified but also because Olivos Cisneros has painted them. With practiced hands, she uses insect pins to dab splotches of yellow, orange, magenta, blue, and green onto the insects’ backs, giving each individual a unique color code that can be tracked by an automated camera system. The colors also make them easier to observe by eye. Every now and then, I notice one of them tapping at another with the tips of its clubby antennae. This action, delightfully known as antennating, is the ant equivalent of a sniff. It’s the means through which they inspect the chemicals on each other’s bodies and discern colony-mates from interlopers. These ants normally live underground and are completely blind. “There’s nothing visual going on,” Daniel Kronauer, who leads the lab, tells me. “In terms of their communication, everything is chemical.”
The chemicals they use are pheromones—an important term that is frequently misunderstood. It refers to chemical signals that carry messages between members of the same species. Bombykol, which female moths use to attract males, is a pheromone; the carbon dioxide that draws mosquitoes to my body is not. Pheromones are also standardized messages, whose use and meaning do not vary between individuals of a given species. All female silk moths use bombykol and all males are attracted to it; by contrast, the smells that distinguish one person’s scent from another’s are not pheromones. Indeed, despite the existence of pheromone parties where singletons sniff each other’s clothes, or pheromone sprays that are marketed as aphrodisiacs, it’s still unclear if human pheromones even exist. Despite decades of searching, none have been identified.[*15]
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.
Pheromones hold such sway over ants that they can force the insects to behave in bizarre and detrimental ways, in disregard of other pertinent sensory cues. Red ants will look after the caterpillars of blue butterflies, which look nothing like ant grubs but smell exactly like them. Army ants are so committed to following their pheromone trails that if those paths should accidentally loop back onto themselves, hundreds of workers will walk in an endless “death spiral” until they die from exhaustion.[*16] Many ants use pheromones to discern dead individuals: When the biologist E. O. Wilson daubed oleic acid onto the bodies of living ants, their sisters treated them as corpses and carried them to the colony’s garbage piles. It didn’t matter that the ant was alive and visibly kicking. What mattered was that it smelled dead.
“The ant world is a tumult, a noisy world of pheromones being passed back and forth,” Wilson said. “We don’t see it, of course. We don’t see anything more than these little ruddy creatures scurrying around on the ground, but there’s a huge amount of activity, of coordination and communication going on.” That’s all based on pheromones. These smelly substances allow ants to transcend the limits of individuality and act as a superorganism, producing complex and transcendent behaviors from the unknowing actions of simple individuals. They allow army ants to act as unstoppable predators, Argentine ants to create supercolonies that extend for miles, and leafcutter ants to develop their own agriculture by gardening fungi. Ant civilizations are among the most impressive on Earth, and as ant researcher Patrizia d’Ettorre once wrote, their “genius is definitely in their antennae.”
Kronauer’s research with the clonal raider ant shows how that genius might have evolved. Ants are essentially a group of highly specialized wasps that evolved between 140 and 168 million years ago and rapidly transitioned from a solitary existence to an extremely social one. Along the way, their repertoire of odorant receptor genes—the ones that allow them to sense smelly chemicals—ballooned in size. While fruit flies have 60 of these genes and honeybees have 140, most ants have between 300 and 400, and the clonal raider has a record-breaking 500.[*17] Why? Here are three clues. First, a third of the clonal raiders’ odorant receptors are only produced on the underside of their antennae—the parts that they pat each other with during antennation. Second, these receptors specifically detect the heavyweight pheromones that ants wear as identity badges. Third, these 180 or so receptors all arose from just one gene, which was repeatedly duplicated at roughly the time that ancestral ants went from living alone to living in colonies. Putting these clues together, Kronauer reasons that all that extra olfactory hardware might have helped ants to better recognize their nestmates. After all, they are not only looking for the presence or absence of one pheromone but weighing up the relative proportions of a few dozen of them. That’s a challenging computation, but one that undergirds everything else that ants do. By expanding their powers of smell, they gained the means of regulating their sophisticated societies.
It becomes especially obvious how much ants rely on smell when they are disconnected from that sense. When Kronauer deprived his clonal raiders of a gene called orco, which odorant receptors need to detect their target molecules, the mutant ants behaved in entirely un-ant-like ways. “Right from the beginning, there was something wrong with those ants,” Olivos Cisneros tells me. “It was super-easy to spot.” They wouldn’t follow pheromone trails. They ignored barriers whose intense smells would ward off normal ants, like lines drawn by Sharpies. They ignored the grubs that they’re normally duty-bound to care for. They ignored their colonies altogether, and went walkabout on their own for days at a time. If they accidentally found themselves within a colony, their presence was disruptive. Sometimes they’d release alarm pheromones without provocation, sending their nestmates into an unnecessary panic. “They can’t tell that there are other ants there,” Kronauer says. “They just can’t sense them at all.” It’s hard not to feel sorry for them. An ant without olfaction is an ant without a colony, and an ant without a colony is barely an ant at all.[*18]
Ants are perhaps the most dramatic example of the power of pheromones, but they’re hardly the only ones. Female lobsters urinate into the faces of males to tempt them with a sex pheromone. Male mice produce a pheromone in their urine that makes females especially attracted to other components in their odor; this substance is called darcin, after Pride and Prejudice’s male hero. The early spider-orchid deceives male bees into carrying its pollen by mimicking their sexual pheromones. “We live, all the time, especially in nature, in great clouds of pheromones,” E. O. Wilson once said. “They’re coming out in spumes in millionths of a gram that can travel for maybe a kilometer.” These tailored messages drive the entire animal kingdom, from the smallest of creatures to the very biggest.
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In 2005, Lucy Bates arrived in Kenya’s Amboseli National Park to study its elephants. On her first day out, her experienced field assistants told her that these animals, which had been observed by scientists since the 1970s, would almost certainly realize that a fresh face had joined the research group. Bates was skeptical. How would they know? Why would they care? But as soon as the team found one of the herds and switched off their vehicle’s engine, the elephants immediately turned toward them. “One of them came up, stuck her trunk in my window, and had a good sniff,” Bates tells me. “They knew someone new was inside.”
Over the next few years, Bates came to realize what anyone who spends time with elephants knows: Their lives are dominated by smell. You don’t need to know about an elephant’s record-breaking catalog of 2,000 olfactory receptor genes, or the size of its olfactory bulb. Just watch the trunk. No other animal has a nose so mobile and conspicuous, and so no other animal is as easy to watch in the act of smelling. Whether an elephant is walking or feeding, alarmed or relaxed, its trunk is constantly in motion, swinging, coiling, twisting, scanning, sensing. Sometimes the entire 6-foot organ periscopes dramatically to inspect an object. Sometimes its movements are subtle. “You can approach a feeding elephant who’s heard you coming, and without turning its head, it’ll flick just the tip of its trunk toward you,” says Bates.
African elephants can use their trunks to detect their favorite plants, even when obscured in lidded boxes, and even when hidden among a messy botanical buffet. They can learn unfamiliar smells: After being briefly taught to detect TNT, which is supposedly odorless to humans, three African elephants could identify the substance more skillfully than highly trained detection dogs. Two of those same elephants, Chishuru and Mussina, could sniff a human and identify the matching scent from a row of nine jars laced with the odors of different people. Asian elephants are no slouches, either. In one study, they could correctly identify which of two covered buckets contained more food through smell alone—a feat that humans can’t duplicate and that (in one of Alexandra Horowitz’s experiments) even dogs struggled with.[*19] “We could tell the difference if we looked, but if we were just smelling it, there’s no way,” says Bates. “The level of information they can get is just so far beyond what we can comprehend.”
Elephants can also smell danger. Some time after Bates arrived in Amboseli, one of her colleagues gave a ride to a couple of Maasai men in a jeep that the team had used for decades. The next day, when the team drove out, the elephants were unexpectedly cautious around the familiar vehicle. Young Maasai men will sometimes spear elephants, and Bates reasoned that the creatures were disconcerted by the lingering scents in the jeep—some combination of the cows that the Maasai raise, the dairy products they eat, and the ochre they daub on their bodies. To test this idea, she hid various bundles of clothes in elephant country. When the animals approached washed garments or those worn by the Kamba, who pose no threat to them, they were curious but unconcerned. But every time they got wind of clothes worn by the Maasai, their reactions were unmistakable. “Once the first trunk went up, the whole group ran away as fast as they could, and almost always into long grass,” Bates tells me. “It was incredibly stark—every group, every time.”
Food and foes aside, few sources of odor are as pertinent to an elephant as other elephants. They’ll regularly inspect each other with their trunks, probing away at glands, genitals, and mouths. When African elephants reunite after a prolonged separation, they go through intense greeting rituals. Human observers can see their flapping ears and hear their throaty rumbles, but for the elephants themselves, the experience must also be olfactory pandemonium. They vigorously urinate and defecate, while aromatic liquid pours forth from glands behind their eyes, filling the air around them with scents.
Few people have done more to study elephant odors than Bets Rasmussen,[*20] a biochemist who was once crowned “the queen of elephant secretions, excretions and exhalations.” If an elephant produced it, Rasmussen likely sniffed it and possibly tasted it. Those secretions, she realized, are full of pheromones, and thus full of meaning. In 1996, after 15 years of work, she isolated a chemical called Z-7-dodecen-1-yl acetate, which females release in their urine to inform bulls that they’re ready to mate. It was astonishing that just one compound could so greatly affect the sex lives of so complex an animal. It was even more astonishing that female moths attract males with the same substance. Fortunately, male moths aren’t drawn to female elephants, because the attractant is just one of several compounds on their search list. Luckier still, male elephants don’t try to mate with female moths, because the latter produce piddling amounts of the pheromone. Other elephants, however, shine like odorous beacons. Rasmussen eventually discovered that elephants can tell, through smell, when females are at different parts of their estrus cycles, or when bulls are in the hyperaggressive sexual state called musth. They can also identify individuals. As they walk the time-worn trails that connect their home ranges, they leave dung and urine behind—not waste, but personal stories to be read by the trunks of others around them.
In 2007, Lucy Bates found a clever way of testing this idea. She followed family groups of elephants and waited for one to urinate. Once the herd had left, she drove over, scooped up the urine-soaked soil with a trowel, and placed it in an ice cream tub. She then drove around the savannah until she found either the same herd of elephants or a different one. Cutting them off, she emptied the container of soil onto the path ahead of them, sped off to a distant vantage point, and waited. “It was not the most pleasant experiment,” she tells me. “Often, you’d think you know where they were going and put the sample out, and they’d change direction. That was quite soul-destroying.” When she got it right, the elephants would always inspect the urine as they approached. If it came from a different family group, they quickly ignored it. If it came from a family member who wasn’t part of the current unit, they showed more interest. But if it came from an elephant who was part of the same group and walking behind them, they were especially curious. They knew exactly who had left the urine, and since that individual couldn’t possibly have teleported ahead, they seemed confused and carefully investigated the displaced scent. Elephants move in large family groups, and it seems they know not only who’s around but where those individuals are. Scent cements that awareness. “The amount of information that they must be picking up all the time as they’re walking along, from all the different smells they’re taking in…I think it just must be overwhelming,” says Bates.
The exact nature of that information is hard to discern. Smells aren’t easily captured, so while scientists can photograph an animal’s displays and record its calls, those who care about olfaction have to do things like scoop up urine-soaked soil. Smells aren’t easily reproduced, either: You can’t play back an odor through a speaker or a screen, so researchers have to do things like drive piss-soaked soil in front of elephant herds. And that’s if they think about olfaction at all. In many cases, elephant researchers have tested the brains of these animals through experiments that are implicitly visual and involve objects like mirrors. How much have we missed about an elephant’s mind because we’ve ignored its primary senses?
When they walk their favorite routes and encounter the smelly deposits of other elephants, what are they getting besides identity? Do they know the emotional states of those previous passers-by? Can they sense stress or diagnose illnesses? What of their wider environment? Elephants that have returned to postwar Angola seem to skirt around the millions of landmines that still dot the land—unsurprising, perhaps, given how quickly they can be trained to detect TNT. They’ve been known to dig wells in times of drought, and George Wittemyer, who has also worked in Amboseli, is sure that they’re using the smell of buried water to do so. He also thinks that they can detect approaching rain from the smells it unleashes as it splashes onto faraway soils. “That smell is exhilarating,” he tells me. “It makes me feel excited and alive, and you’ll also see elephants rising up to it.”
Rasmussen once speculated that elephants might guide their long migrations using “chemical memories of landscapes, terrain, pathways, mineral and salt sources, waterholes, the scenting of rain or flooding rivers, and tree odors signifying seasons.” No one has tested these claims, but they make sense. After all, dogs, humans, and ants can all track trails of scent. Salmon can return to the very streams in which they were born by homing in on the distinctive scents of those natal waters.[*21] Whip spiders use the smell sensors on the tips of their extremely long, thread-like front legs to find their way back to their shelters amid the clutter of a rainforest. Polar bears might be able to navigate across thousands of miles of indistinct ice because glands in their paws leave scent behind with every step. These examples are so common that some scientists believe the main purpose of animal olfaction isn’t to detect chemicals but to use them in navigating through the world. With the right noses, landscapes can be mapped as odorscapes, and fragrant landmarks can show the way to food and shelter. Ironically, the best evidence for such feats comes from animals that, until recently, were thought to be unable to smell.
—
John James Audubon, the avid naturalist and artist, was best known for painting North America’s birds, and compiling those pieces into a seminal ornithological tome. But he was also responsible for seeding a centuries-long falsehood about birds through some truly abysmal experiments involving vultures.
Since Aristotle, scholars believed that vultures had a keen sense of smell. Audubon thought differently. When he left a putrefying pig carcass in the open, no vultures came to eat. By contrast, when he put out a deerskin stuffed with straw, a turkey vulture swooped in and pecked away. These birds, he claimed in 1826, find their food with sight, not smell. His supporters bolstered that claim with equally dodgy evidence. One noted that vultures would attack a painting of an eviscerated sheep, and that captive vultures refused to eat after being blinded. Another showed that a turkey—not a turkey vulture, mind you; an actual turkey—would still eat food that was tainted with sulfuric acid and potassium cyanide, a strong-smelling concoction that proved violently fatal. These bizarre studies struck a chord. Never mind that vultures prefer fresh carcasses and ignore overly stinky meat like the kind Audubon used. Forget that Audubon confused black vultures (which are less reliant on smell) with turkey vultures, or that oil paints at the time gave off certain chemicals also found in decaying flesh. Disregard the many reasons a mutilated animal might not feel very peckish. The idea that turkey vultures—and by dubious extension, all birds—can’t smell became textbook wisdom. Evidence to the contrary was ignored for decades, and the study of avian olfaction lapsed into neglect.[*22]
Betsy Bang revitalized it. An amateur ornithologist and medical illustrator, she dissected the nasal passages of bird after bird and sketched what she saw. And what she saw—large cavities filled with convoluted scrolls of thin bone, much like what lurks within a dog’s snout—convinced her that birds must be able to smell. Why else would they have all that hardware? Concerned that the textbooks were spouting misinformation, Bang spent the 1960s carefully examining the brains of more than a hundred species and measuring their olfactory bulbs. She showed that these smell centers were especially large in turkey vultures, the kiwis of New Zealand, and the tubenoses—a group of seabirds that includes albatrosses, petrels, shearwaters, and fulmars. Tubenoses are named for the obvious nostrils on their beaks, which were originally thought to be channels for expelling salt. Bang’s work suggested another purpose: The tubes draw air into the nose, allowing the birds to catch the scent of food while soaring over the ocean. For them, “olfaction is of primary importance,” Bang wrote.[*23] (“She didn’t mind taking on a fight, even if it meant taking on Audubon,” her son Axel later said.)
Elsewhere in California, Bernice Wenzel had come to the same conclusion. A physiology professor (and one of the few women in the United States to hold such a position in the 1950s), Wenzel showed that when homing pigeons catch a whiff of scented air, their hearts beat faster and the neurons in their olfactory bulbs buzz excitedly. She repeated that test with other birds—turkey vultures, quails, penguins, ravens, ducks—and all reacted similarly. She proved what Bang deduced: Birds can smell. Both Bang and Wenzel, who have since passed away, have been described as “mavericks of their generation” who pushed against incorrect dogma and allowed others to explore a sensory world that was deemed nonexistent. And because of the examples they set and the mentorship they offered, many of the scientists who followed in their footsteps were also women.
One, Gabrielle Nevitt, was in the audience when Wenzel discussed her seabird studies in one of her final pre-retirement talks. Inspired, Nevitt began a career-long quest to find out how tubenoses make use of smell. Beginning in 1991, she would get onto any Antarctic voyage that she could, while trying “to figure out how to test birds from the deck of an icebreaker without getting killed,” she tells me. She’d soak tampons in fish oils and fly them from kites. She’d release slicks of pungent oils from the sterns of ships. And every time, tubenoses arrived quickly. Nevitt suspected that the birds were drawn to a specific chemical within the pungent glop, but she didn’t know what it might be, or how the birds found it across featureless water. She only learned the answer on a later Antarctic voyage, and in unexpected circumstances.
During the trip, a fierce storm rocked Nevitt’s ship, throwing her across her room and slamming her into a tool chest. She tore her kidney and was confined to her bunk, even after her ship had docked and a fresh crew had come on board. Still recuperating, Nevitt chatted with the new chief scientist—an atmospheric chemist named Tim Bates, who had come to study a gas called dimethyl sulfide, or DMS. In the oceans, plankton release DMS when they’re eaten by krill—shrimp-like animals that are, in turn, eaten by whales, fish, and seabirds. DMS doesn’t dissolve easily in water, and eventually makes its way into the air. If it rises high enough, it seeds clouds. If it enters the nose of a sailor, it evokes an odor that Nevitt describes as “a lot like oysters” or “kind of seaweed-y.” It’s the scent of the sea.
In particular, DMS is the scent of bountiful seas, where huge blooms of plankton feed equally huge swarms of krill. As Nevitt talked to Bates, it dawned on her that DMS was exactly the chemical she had envisioned—an olfactory dinner bell that alerted seabirds when waters were teeming with prey. Bates cemented this impression by giving Nevitt a map that showed DMS levels across parts of Antarctica. In the varying levels of the chemical, Nevitt saw a seascape of odorous mountains and unscented valleys. She realized that the ocean wasn’t as featureless as she had once imagined; rather, it had a secret topography that was invisible to the eye but evident to the nose. She began to perceive the sea the way a seabird might.
Once back on her feet, Nevitt carried out a string of studies that confirmed the DMS hypothesis. She found that tubenoses will flock to slicks of the chemical. She calculated that they can detect it at the kind of low, feeble traces that might realistically drift on the wind. She showed that some tubenoses are drawn to DMS before they can even fly.[*24] Many species nest in deep burrows, and their chicks, which resemble grapefruit-sized balls of lint, hatch into a world of darkness. Their early Umwelt is bereft of light but awash in odor, wafting in from the burrow entrance or carried in on the beaks and feathers of their parents. These hatchlings have no knowledge of the ocean, but they know to head toward DMS. And even after they emerge into the light, trading their claustrophobic nurseries for the immensity of the sky, smells remain their north star. They soar for thousands of miles, searching for diffuse plumes of scent that might betray the presence of krill beneath the surface.[*25]
But smells are more than dinner bells. In the ocean, they’re also signposts. Geological features, like submerged mountains or slopes in the seafloor, affect the levels of nutrients in the water, which in turn influence concentrations of plankton, krill, and DMS. The smellscapes that seabirds track are intimately tied to actual landscapes, and so are surprisingly predictable. Over time, Nevitt suspects, seabirds build up a map of these features, using their noses to learn the locations of the richest feeding spots and their home nests.
This is a hard idea to test, but Anna Gagliardo found compelling evidence for it. She transported a few Cory’s shearwaters—a kind of tubenose—to locations 500 miles from their nesting colonies and temporarily shut down their sense of smell with a nasal wash. When released, these birds struggled to travel home, taking weeks or months to do what normal shearwaters did in mere days. Without smell, they lost their way. Without smell, the ocean was stripped of landmarks. 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.”
—
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. Even humans can do this: The string-tracking task that Alexandra Horowitz asked me to try is much harder if one nostril is blocked. 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.
Snake tongues come in shades of lipstick red, electric blue, and inky black. Outstretched and splayed, they can be longer and wider than their owners’ heads. Kurt Schwenk has been fascinated by them for decades, and he often finds that he’s alone in that. In the second year of his PhD, he told a fellow student what he was working on, eager to revel in the joys of scientific pursuits with a like-minded soul. The student (who is now a famous ecologist) burst out laughing. “That would have been enough to hurt my feelings, but this was a guy who studied the mites that hang out in the nostrils of hummingbirds,” Schwenk tells me, still slightly outraged. “Someone who studied hummingbird nostril mites thought that what I did was funny! For some reason, people find tongues funny.”
Perhaps there’s something unseemly about studying organs that are linked to carnal delights like sex and food. Perhaps it’s weird to seriously investigate things that we protrude in jest or defiance. Or perhaps it’s that the forked tongue has become a symbol of malevolence and duplicity. Whatever the case, serious scholars have put forward some very strange hypotheses for how snakes use their tongues, or for why those tongues are forked. Some have described them as venomous stingers, or fly-catching forceps, or tactile organs akin to hands, or even nostril-cleaning tools. Aristotle suggested that the fork doubled the pleasure that a snake gets from its food—but the snake’s tongue has no taste buds and conveys no sensory information on its own. Instead, as scientists finally discovered in the 1920s, it’s a chemical collector. When it darts into the world, its tips snag odor molecules that lie on the ground or drift through the air. When it retracts, saliva sweeps the chemical bounty into a pair of chambers—the vomeronasal organ—that connect to the brain’s smell centers.[*26] With the aid of its tongue, a snake smells the world. Each flick is the equivalent of a sniff. Indeed, the very first thing that a hatchling serpent does upon breaking out of its egg is to flick its tongue. “That tells you something about the primacy of the sense,” Schwenk says.
Using its tongue, a male garter snake can track a slithering female by following the trail of pheromones she leaves behind. By comparing what she deposited on different sides of objects she pushed against, he can work out her direction. Once he finds her, he can gauge her size and health, possibly with just one or two flicks. He can do this all in the dark. A male can even be fooled into vigorously mating with a paper towel that has been imbued with a female’s scent. But all of these feats could be just as easily accomplished with a paddle-shaped, human-esque tongue. So why do snakes have forked ones? Schwenk reasoned that the fork allows snakes to smell in stereo, by comparing chemical traces at two points in space. If both tips detect trail pheromones, the snake stays on course. If the right tip gets a hit but the left one doesn’t, the snake veers right. If both come up empty, it swings its head from side to side until it regains the trail. The fork allows the snake to precisely define the edges of the path.
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. They can even track the specific individual that they attacked because they instantly learn the victim’s scent at the moment of a bite. “There are presumably odors of multiple mice around, but they know which trail to follow,” Schwenk says.
Snakes can also catch trails of scent on the breeze. Chuck Smith, one of Schwenk’s former students, demonstrated this by implanting copperheads with radio transmitters and tracking their movements. Twice, he released a female snake into the wild and watched as she stayed in exactly the same place. She couldn’t have left a scent trail, but she still managed to attract males who were randomly wandering hundreds of yards away, then suddenly crawled directly to her in a straight line.
Schwenk guessed that their secret lies in the way they flick. Lizards, the group from which snakes evolved, also smell with their tongues, which are also sometimes forked. But when lizards stick their tongues out, they usually flick once. The tips extend, scrape the ground, and retract. Snakes, without exception, flick repeatedly and rapidly, often never touching the ground. The tongues bend in the middle as if moving on a hinge, and the tips carve out a wide circular arc, 10 to 20 times a second. Bill Ryerson, another of Schwenk’s students, analyzed those movements by getting snakes to tongue-flick into clouds of cornstarch. He illuminated the clouds with laser light, and filmed the swirling particles with high-speed cameras. When Schwenk saw the footage, “my brain nearly exploded,” he says.
It turns out that the tongue’s tips splay out at the ends of each flick and get closer at the midpoint. This motion creates two donut-shaped rings of continuously moving air that draw in odorants from the left and right sides of the snake. It’s as if the snake temporarily conjures up two large fans that suck in odors from either side, concentrating diffuse odor molecules onto the tips of its tongue. And since the odors come in from left and right, the fork can still provide a sense of direction, even when flicking in air.
This style of smelling is unusual in two ways. First, it involves a tongue, which is traditionally an organ of taste—a sense that snakes barely use, for reasons I’ll get to. Second, it involves an organ that, in most other animals, is either nonexistent or of secondary importance. Many backboned animals have two distinct systems for detecting odors. The main one includes all the structures, receptors, and neurons that I described in the head of a dog at the start of this chapter. The vomeronasal organ is its sidekick; it has its own kinds of odor-sensing cells, its own sensory neurons, and its own connections to the brain. It’s usually found inside the nasal cavity, just above the roof of the mouth. Don’t bother trying to feel around for yours, though. For some reason, humans lost our vomeronasal organ during our evolution, as did other apes, along with whales, birds, crocodiles, and some bats.
Most other mammals, reptiles, and amphibians have kept theirs. When one elephant touches another with its trunk and brings the pheromone-coated tip into its mouth, those molecules head to the vomeronasal. When horses or cats curl back their upper lip to expose their teeth, they’re cutting off their nostrils and sending inhaled odorants to the vomeronasal. And when a snake retracts its tongue and squeezes the tips between the floor and roof of its mouth, the collected molecules are squirted to the vomeronasal. In snakes, this sidekick is the star. Without it, garter snakes stop following trails and stop eating, while rattlesnakes botch half their strikes and fail to capture what they hit. These snakes can still inhale odorants through their nostrils, but their “main” olfactory system can’t seem to do much with that information. It has been relegated to a passive role, informing the brain if there’s something interesting around to tongue-flick at.
Snakes are unusual not just because their vomeronasal organ is so important but also because we actually understand what it does. In other animals, the organ is a mystery, albeit one that seems to attract confident claims.[*28] For the moment, no one really knows why some species have two separate systems for smelling. Nor is it entirely clear why most animals have another distinct chemical sense. I’m talking, of course, about taste.
—
Every April, the Association for Chemoreception Sciences holds its annual meeting in Florida, and, per tradition, scientists who study smell square off against those who study taste in a heated softball game. “Smell usually wins,” smell scientist Leslie Vosshall tells me, “because the field is vastly larger. It’s like four or five to one.” Like smell, taste—or gustation, in the fancy scientific parlance—is a means of detecting chemicals in the environment. But beyond that, the two senses are distinct. Put your nose next to vanilla oil, and you’ll inhale a pleasing odor; drop that same oil on your tongue, and you’ll likely flinch in disgust.
The difference between smell and taste is surprisingly complicated. You might reasonably say that animals smell with noses and taste with tongues, but snakes use their tongues to collect odors, and other animals (which we’ll meet shortly) taste with unusual body parts. You could also argue (and many scientists do) that we smell molecules that drift through the air, but taste those that stay in liquid or solid form. Smell works at a distance; taste works through contact. That’s a better distinction, but it has several problems. First, the receptors that are responsible for recognizing smells are always covered in a thin layer of liquid, so odorant molecules must first dissolve to be detected. So smell—like taste—always involves a liquid step and always involves close contact even if those smells have traveled from afar. Second, as we’ve seen, ants and other insects can smell by contact, using their antennae to pick up pheromones that are too heavy to go airborne. Third, fish can smell even though everything they’re smelling is dissolved in water. For creatures like these that are constantly immersed in liquid, the distinction between taste and smell can be so confusing that one neuroscientist just told me, “I avoid thinking about it.”
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.
Taste, then, is the simpler sense. As we’ve seen, smell covers a practically infinite selection of molecules with an indescribably vast range of characteristics, which the nervous system represents through a combinatorial code so fiendish that scientists have barely begun to crack it. Taste, by contrast, boils down to just five basic qualities in humans—salt, sweet, bitter, sour, and umami (savory)—and perhaps a few more in other animals, which are detected through a small number of receptors. And while smell can be put to complex uses—navigating the open oceans, finding prey, and coordinating herds or colonies—taste is almost always used to make binary decisions about food. Yes or no? Good or bad? Consume or spit?
It’s ironic that we associate taste with connoisseurship, subtlety, and fine discrimination when it is among the coarsest of senses. Even our ability to taste bitter, which warns us of hundreds of potentially toxic compounds, isn’t built to distinguish between them. There’s only one sensation of bitter because you don’t need to know which bitter thing you’re tasting—you just need to know to stop tasting it. Taste is mostly a final check before consumption: Should I eat this? That’s why snakes barely bother with taste. With their flickering tongues, they can make decisions about whether something is worth eating through smell well before their mouths make contact.[*30] It’s almost unheard of for a snake to strike a prey animal and then spit it out. (We tend to wrongly equate taste with flavor, when the latter is more dominated by smell. That’s why food seems bland when you have a cold: Its taste is the same, but the flavor dims because you can’t smell it.)
Reptiles, birds, and mammals taste with their tongues. Other animals aren’t so restricted. If you’re very small, food isn’t just something you put in your mouth, but something you can walk upon. As such, most insects can taste with their feet and legs. Bees can detect the sweetness of nectar just by standing on a flower. Flies can taste the apple you’re about to eat by landing on it. Parasitic wasps can use taste sensors on the tips of their stings to carefully implant their eggs in the bodies of other insects. One species can even taste the difference between hosts that have already been parasitized by other wasps and those that are currently vacant.[*31]
If a mosquito lands on a human arm, “it’s a delight of the senses,” says Leslie Vosshall. “Human skin has a taste to it, which gives them more confirmation that they made it to the right place.” But if that arm is covered with bitter-tasting DEET, the receptors on their feet force them to take off before they get a chance to bite. Vosshall has videos in which a mosquito lands on a gloved hand and walks over to a small patch of exposed but DEET-covered skin. Its leg touches the skin, and immediately withdraws. It circles, tries again, and retreats again. “It’s poignant,” she tells me, in a strange display of sympathy for a mosquito. “It’s also really psychedelic. We have no idea what it’d be like to taste with our fingers.” Insects can taste with other body parts, too, which expands the uses to which they can put this typically limited sense. Some can find good sites for laying their eggs using taste receptors on their egg-laying tubes. Some have taste receptors on their wings, which might alert them to traces of food as they fly. Flies will start grooming themselves if they taste the presence of bacteria on their wings. Even decapitated flies will do this.
The most extensive sense of taste in nature surely belongs to catfish. These fish are swimming tongues. They have taste buds spread all over their scale-free bodies, from the tips of their whisker-like barbels to their tails. There’s hardly a place you can touch a catfish without brushing thousands of taste buds. If you lick one of them, you’ll both simultaneously taste each other.[*32] “If I were a catfish, I’d love to jump into a vat of chocolate,” John Caprio tells me. “You could taste it with your butt.” With their body-wide buds, catfish have turned taste into an omnidirectional sense—albeit one that’s still devoted to evaluating food. They eat meat, and if you put a piece anywhere on their skin (or add meat juices to the water around them), they’ll turn and snap at the right place. They’re exquisitely sensitive to amino acids—the building blocks of proteins and flesh.[*33] They aren’t great at detecting sugars, though: Unfortunately for Caprio, his chocolate fantasy would be underwhelming.
This inability to sense sugar and other classic tastes is surprisingly common, and varies according to an animal’s diet. Cats, spotted hyenas, and many other mammals that eat meat and nothing else similarly lack a sweet tooth. Vampire bats, which drink only blood, have also lost their taste for sweetness, and for umami. Pandas have no need to sense umami either, since they only eat bamboo, but they gained an expanded set of bitter-sensing genes to warn them of the myriad possible toxins in their mouthfuls.[*34] Other leaf-eating specialists, like koalas, have also gained more bitter detectors, while mammals that swallow their prey whole, including sea lions and dolphins, have lost most of theirs. Repeatedly and predictably, the gustatory Umwelten of animals have expanded and contracted to make sense of the foods they most often encounter. And sometimes those changes altered their destinies.
Like cats and other modern carnivores, small predatory dinosaurs probably lost the ability to taste sugar. They passed their restricted palate on to their descendants, the birds, many of which still have no sense for sweetness. Songbirds—the vocal and hugely successful group that includes robins, jays, cardinals, tits, sparrows, finches, and starlings—are an exception. In 2014, evolutionary biologist Maude Baldwin showed that some of the earliest songbirds regained their sweet tooth by tweaking a taste receptor that normally senses umami into one that also senses sugar. This change occurred in Australia, a land whose plants produce so much sugar that its flowers overflow with nectar and its eucalyptus trees exude a syrupy substance from their bark. Perhaps these abundant sources of energy allowed the newly sweet-toothed songbirds to thrive in Australia, to endure marathon migrations to other continents, to find nectar-rich flowers wherever they arrived, and to diversify into a massive dynasty that now includes half the world’s bird species. This story is unproven but nonetheless beguiling. It’s possible that if a random Australian bird hadn’t expanded its Umwelt tens of millions of years ago, none of us would be waking up to the melodic sounds of birdsong today.[*35]
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You can split the senses into different groups depending on the stimuli that they detect. Smell, its vomeronasal variant, and taste are chemical senses, which detect the presence of molecules. They are ancient, universal, and seem to sit apart from the others, which is partly why I chose them as the first stop on our journey. But they aren’t entirely distinct. On closer inspection, they share common ground with at least one other sense, in an unexpected way.
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.
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