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

Introduction: The Only True Voyage
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The senses transform the coursing chaos of the world into perceptions and experiences—things we can react to and act upon. They allow biology to tame physics. They turn stimuli into information. They pull relevance from randomness, and weave meaning from miscellany. They connect animals to their surroundings. And they connect animals to each other via expressions, displays, gestures, calls, and currents.
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The senses constrain an animal’s life, restricting what it can detect and do. But they also define a species’ future, and the evolutionary possibilities ahead of it. For example, around 400 million years ago, some fish began leaving the water and adapting to life on land. In open air, these pioneers—our ancestors—could see over much longer distances than they could in water. The neuroscientist Malcolm MacIver thinks that this change spurred the evolution of advanced mental abilities, like planning and strategic thinking. Instead of simply reacting to whatever was directly in front of them, they could be proactive. By seeing farther, they could think ahead. As their Umwelten expanded, so did their minds. An Umwelt cannot expand indefinitely, though. Senses always come at a cost. 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.
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Writing about the tick, Uexküll noted that the rich world around it is “constricted and transformed into an impoverished structure” of just three stimuli. “However, the poverty of this environment is needful for the certainty of action, and certainty is more important than riches.” Nothing can sense everything, and nothing needs to. That is why Umwelten exist at all. It is also why the act of contemplating the Umwelt of another creature is so deeply human and so utterly profound. Our senses filter in what we need. We must choose to learn about the rest.
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Consider the seemingly simple question How many senses are there? Around 2,370 years ago, Aristotle wrote that there are five, in both humans and other animals—sight, hearing, smell, taste, and touch. This tally persists today. But according to the philosopher Fiona Macpherson, there are reasons to doubt it. For a start, Aristotle missed a few in humans: 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.
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Bats, for example, perceive the world through sonar, and since this is a sense that the majority of humans lack, “there is no reason to suppose that it is subjectively like anything we can experience or imagine,” Nagel wrote. You could envision yourself with webbing on your arms or insects in your mouth, but you’d still be creating a mental caricature of you as a bat. “I want to know what it is like for a bat to be a bat,” Nagel wrote. “Yet if I try to imagine this, I am restricted to the resources of my own mind, and those resources are inadequate to the task.”
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Language, for us, is both blessing and curse. It gives us the tools for describing another animal’s Umwelt even as it insinuates our own sensory world into those descriptions.
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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. It reminds us that there is light in darkness, noise in silence, richness in nothingness. It hints at flickers of the unfamiliar in the familiar, of the extraordinary in the everyday, of magnificence in mundanity. It shows us that clipping a microphone onto a plant can be an intrepid act of exploration. Stepping between Umwelten, or at least trying to, is like setting foot upon an alien planet. Uexküll even billed his work as a “travelogue.”
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“When you look at an animal’s behavior through the lens of that animal, suddenly all of this salient information becomes available that you would otherwise miss,” Colleen Reichmuth, a sensory biologist who works with seals and sea lions, tells me. “It’s like a magic magnifying glass, to have that knowledge.”
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Senses that seem paranormal to us only appear this way because we are so limited and so painfully unaware of our limitations.
Chapter 1: Leaking Sacks of Chemicals | Smells and Tastes
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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.
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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.
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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]
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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.
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Of the five Aristotelian senses, four have vast and specific lexicons. Smell, as Diane Ackerman wrote, “is the one without words.”
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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.
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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.
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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.
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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.
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pheromones—an important term that is frequently misunderstood. It refers to chemical signals that carry messages between members of the same species.
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Pheromones are also standardized messages, whose use and meaning do not vary between individuals of a given species.
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it’s still unclear if human pheromones even exist. Despite decades of searching, none have been identified.[*
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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.
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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.
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“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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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.
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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.
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How much have we missed about an elephant’s mind because we’ve ignored its primary senses?
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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.
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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.”
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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.
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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)—
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Repeatedly and predictably, the gustatory Umwelten of animals have expanded and contracted to make sense of the foods they most often encounter.
Chapter 2: Endless Ways of Seeing | Light
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monitor, I can watch a spider paying attention and losing interest. I can observe it observing. By watching its gaze, I can get as close as possible to glimpsing its mind. And, despite many similarities, I can see just how different its vision is from mine.
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Every animal that sees does so with opsins, which work by tightly embracing a partner molecule called a chromophore, usually derived from vitamin A. The chromophore can absorb the energy from a single photon of light. When it does, it instantly snaps into a different shape, and its contortions force its opsin partner to reshape itself, too. The opsin’s transformation then sets off a chemical chain reaction that ends with an electrical signal traveling down a neuron. This is how light is sensed. Think of the chromophore as a car key and the opsin as an ignition switch. The two fit together, light turns the key, and the engine of vision whirs into life.
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Primates, for example, probably evolved big, sharp eyes to capture tree-dwelling insects sitting on branches. We humans have inherited that acute vision, which sighted people now use to guide their dexterous fingers, to read symbols that they imbue with meaning, and to assess the cues hidden in subtle facial expressions. Our eyes suit our needs. They also give us a singular Umwelt that most other animals do not share.
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But eagles and other birds of prey are the only animals whose vision is substantially sharper than ours.
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our sharp vision muddies our appreciation of other Umwelten, because “we assume that if we can see it, they can, and that if it’s eye-catching to us, it’s grabbing their attention,” says Caves. “That’s not the case.”
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animals can achieve sharper vision by having smaller and more densely packed photoreceptors. But each receptor now collects light over a smaller area and is thus less sensitive. These qualities—sensitivity and resolution—seesaw against each other. No eye can excel at both.
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humans are such visually driven creatures that trying to conceive of these completely alien systems is very hard.”
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a vulture’s visual field covers the space on either side of its head but has large blind spots above and below. When it flies, it tilts its head downward, so its blind spot is now directly ahead of it. This is why vultures crash into wind turbines: While soaring, they aren’t looking at what is right in front of them. For most of their history, they never had to. “Vultures would never have encountered an object so high and large in their flight path,” Martin says. It might work to turn off the turbines if the birds are near, or to lure the vultures away using ground-based markers. But visual cues on the blades themselves won’t work.[* 18] (In North America, bald eagles also crash into wind turbines for the same reasons.)
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“The human visual world is in front and humans move into it,” Martin once wrote. But “the avian world is around and birds move through it.”[* 19]
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The left half of a chick’s brain is specialized for focused attention and categorizing objects; the bird can spot food grains among a bed of pebbles if it uses its right eye (directed by its left brain), but not its left eye. The right half of the brain deals with the unexpected; many birds use their left eyes (directed by their right brains) to scan for predators, and are quicker to detect a threat when it approaches from the left.
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A seal’s visual field is more similar to ours but with excellent coverage above its head and poor coverage below, presumably to spot fish silhouetted against the sky. A seal that swims upside down might look relaxed to a human observer, but is actually scanning the seafloor for food.
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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.
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Vision can extend in any direction and every direction. It can envelop and surround. And it can vary in time as well as space. It can fill not just the empty voids around us but also the fleeting gaps between moments.
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eyes don’t work at light speed. It takes time for photoreceptors to react to incoming photons, and for the electrical signals they generate to travel to the brain.
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“The way to think about ocean exploration is that we probably create a sphere a hundred yards wide that keeps away anything that can get away,” says Sonke Johnsen. “Most of the time, we’re seeing terror and blindness. We see how animals behave when they think they’re being killed by some glowing god.”
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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.
Chapter 3: Rurple, Grurple, Yurple | Color
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Color, then, is fundamentally subjective. There’s nothing inherently “green” about a blade of grass, or the 550-nanometer light that it reflects. Our photoreceptors, neurons, and brains are what turn that physical property into the sensation of green. Color exists in the eye of the beholder—and also in their brain.
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Color-blindness shouldn’t be a disability, but it can be because humans have built cultures that are predicated on trichromacy.
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The first primates were almost certainly dichromats. They had two cones, short and long. They saw in blues and yellows, like dogs. But sometime between 29 and 43 million years ago, an accident occurred that permanently changed the Umwelt of one specific lineage of primates: They gained an extra copy of the gene that builds their long opsin. Such duplications often happen when cells divide and DNA is copied. They’re mistakes, but fortuitous ones, for they provide a redundant copy of a gene that evolution can tinker with without disrupting the work of the original. That’s exactly what happened with the long-opsin gene. One of the two copies stayed roughly the same, absorbing light at 560 nanometers. The other gradually shifted to a shorter wavelength of 530 nanometers, becoming what we now call the medium (green) opsin. These two genes are 98 percent identical, but the 2 percent gulf between them is also the difference between seeing only in blues and yellows and adding reds and greens to the mix.[* 3] With the new medium opsins joining the earlier long and short ones, these primates had evolved trichromacy. And they passed their expanded vision to their descendants—the monkeys and apes of Africa, Asia, and Europe, a group that includes us.
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Colors are not inherently magical. They become magical when and if animals derive meaning from them.
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people who have lost their lenses to surgeries or accidents can perceive UV as whitish blue. This happened to the painter Claude Monet, who lost his left lens at the age of 82. He began seeing the UV light that reflects off water lilies, and started painting them as whitish blue instead of white.
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Ultraviolet vision is so ubiquitous that much of nature must look different to most other animals.[* 10] Water scatters UV light, creating an ambient ultraviolet fog, against which fish can more easily see tiny UV-absorbing plankton. Rodents can easily see the dark silhouettes of birds against the UV-rich sky. Reindeer can quickly make out mosses and lichens, which reflect little UV, on a hillside blanketed by UV-reflective snow. I could go on. I’m going to go on. Flowers use dramatic UV patterns to advertise their wares to pollinators. Sunflowers, marigolds, and black-eyed Susans all look uniformly colored to human eyes, but bees can see the UV patches at the bases of their petals, which form vivid bullseyes. Usually, these shapes are guides that indicate the position of nectar. Occasionally, they are traps. Crab spiders lurk on flowers to ambush pollinators.
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To humans, these birds all look much the same. But thanks to their UV patterns, males and females look very different from each other. The same is true for more than 90 percent of songbirds whose sexes are indistinguishable to us, including barn swallows and mockingbirds.
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Picture trichromatic human vision as a triangle, with the three corners representing our red, green, and blue cones. Every color we can see is a mix of those three, and can be plotted as a point within that triangular space. By comparison, a bird’s color vision is a pyramid, with four corners representing each of its four cones. Our entire color space is just one face of that pyramid, whose spacious interior represents colors inaccessible to most of us.
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Frustrating though it might be, most of us simply cannot imagine what many animals actually look like to each other, or how varied their sense of color can be.
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all of us—monochromat, dichromat, trichromat, or tetrachromat—take the colors that we see for granted. Each of us is stuck in our own Umwelt.
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Imagine that you’re a mantis shrimp. It is a truth universally acknowledged that you are in want of something to punch.
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Since eyes define nature’s palette, an animal’s palette tells you whose eyes it is trying to catch.
Chapter 4: The Unwanted Sense | Pain
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Nociception is the sensory process by which we detect damage. Pain is the suffering that ensues.
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Nociception is an ancient sense. It is so widespread and consistent across the animal kingdom that the same chemicals, opioids, can quell the nociceptors of humans, chickens, trout, sea slugs, and fruit flies—creatures separated by around 800 million years of evolution.
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Pain (or nociception, if you prefer) is the unwanted sense. It is the only one whose absence (in naked mole-rats or grasshopper mice) feels like a superpower. It is the only one that we try to avoid, that we dull with medication, and that we try to avoid inflicting upon others.
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Our brains are topped by a thick mushroom cap of neural tissue called the neocortex. It’s organized like an orchestra, with many specialized sections that act together to produce the music of consciousness and the lament of pain. But fish brains lack a neocortex, much less a highly organized one.
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Consciousness isn’t an inherent property of all life. It arises from nervous systems, and while those systems might not need a neocortex, they do need enough processing power.
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What matters is not just the total tally of neurons but the connections between them. In human brains, hundreds of thousands of neurons connect the different sections of our cortical orchestra. These links allow us to play the full symphony of a painful experience, melding sensory cues with negative emotions, bad memories, and more.
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Evolution has pushed the nervous systems of insects toward minimalism and efficiency, cramming as much processing power as possible into small heads and bodies. Any extra mental ability—say, consciousness—requires more neurons, which would sap their already tight energy budget.
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insect nervous systems have evolved to pull off complex behaviors in the simplest possible ways, and robots show us how simple it is possible to be.
Chapter 5: So Cool | Heat
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The fire-chasing Melanophila beetles, however, are drawn to heat. These black, half-inch-long insects have been found in what entomologist Earle Gorton Linsley described as “unbelievable numbers” in smelting plants, the kilns of cement factories, and the vats of hot syrup in sugar refineries. One summer, Linsley saw them swarming an outdoor barbecue where “large quantities of deer meat were being prepared.” In the 1940s, the insects would regularly bother football fans in Berkeley’s California Memorial Stadium “by alighting on the clothing or even biting the neck or hands,” Linsley wrote. It’s possible that “the beetles are attracted by the smoke from some twenty thousand (more or less) cigarettes which on still days sometimes hangs like a haze over the stadium.” These incidents are unfortunate for both species, because industrial plants, barbecues, and football stadiums are unhelpful distractions that waylay the beetles from their true targets: forest fires. Arriving at a fire, the beetles have perhaps the most dramatic sex in the animal kingdom, mating as a forest burns around them. Later, the females lay their eggs on charred, cooled bark. When the wood-eating grubs hatch, they find an Eden. The trees they devour are too injured to defend against insect larvae feeding within them. The predators that might eat them are put off by the smoke and heat emitted from the embers and ashes. In peace, they thrive, mature, and eventually fly off in search of their own blazes. But forest fires are rare and unpredictable, and the beetles must have some means of detecting them from afar.
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The atoms and molecules in all objects are constantly jiggling about, and this motion produces electromagnetic radiation. As an object gets hotter, its molecules move faster, and it emits more radiation at higher frequencies. That radiation includes some visible light—think about the glow of heated metal—but most of it lies in the infrared spectrum.[* 7]
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To detect the infrared light from a distant source, either the source must be extremely intense (like the sun) or you need specialized equipment. Melanophila beetles have the latter. Below their wings and just behind their middle legs, these insects have a pair of pits. Each one contains a cluster of around 70 spheres that together look like a malformed raspberry. When zoologist Helmut Schmitz examined these spheres under a microscope, he saw that each is filled with fluid and encloses the tip of a pressure-sensitive neuron. When infrared radiation hits the spheres, the fluid inside them heats up and expands. It can’t bulge outward because the spheres have hard exteriors, so instead it squeezes the nerves, causing them to fire.
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During flight, their beating wings create vibrations that travel into the nearby pits, shake the spherical sensors, and push the sensory neurons within to the edge of firing. It now takes much less infrared radiation to fully push them over that edge.
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a fire-chaser beetle’s beating wings prime its heat sensors in a similar way, setting them up to detect sources of infrared that would normally be too weak. A beetle that’s sitting on a tree would be relatively insensitive. But as soon as it takes off in search of fires, its body automatically widens its search area and transforms even faint traces of distant heat into blazing beacons.[* 8]
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Jakob von Uexküll, coiner of the Umwelt concept, wrote that ticks track their hosts through scent and use temperature only to check if they’ve landed on bare skin. But this isn’t true. Ann Carr and Vincent Salgado recently found that ticks can detect body heat from up to 13 feet away. More surprisingly, the duo showed that common repellents like DEET and citronella don’t disrupt a tick’s sense of smell but do stop them from tracking heat. This discovery might lead to new ways of preventing tick bites, and it might force scientists to reevaluate a lot of previous tick studies. How many past experiments have been misinterpreted because researchers have had an inaccurate picture of a tick’s Umwelt? In hindsight, the tick’s thermal sense should have been clear. The organs at the tips of their questing legs were mostly thought to be odor detectors. But these structures also include tiny spherical pits with neurons at their bases, much like those on a vampire bat’s face. Tellingly, these pits are covered with a thin sheet that has a small hole in it. That’s a terrible design for a nose, because the sheet would block most odorants from reaching the underlying neurons. It is, however, an excellent design for an infrared sensor. Infrared radiation, emanating from the blood of a distant host, would be mostly blocked by the sheet, but some would pass through the hole to partly illuminate the pit below. By analyzing which bits were lit up, the tick could work out the direction of the radiation, and the whereabouts of its source. This idea still needs to be confirmed, but it makes sense. After all, it’s how the most sophisticated heat sensors in nature work. To find them, you need a little courage, some shin guards, and a long pole.
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A pit viper’s thermal sensitivity comes from the structure of its pits (which are similar to those on a tick’s legs). To get an idea of their shape, imagine placing a miniature trampoline on the bottom of a goldfish bowl and turning the whole thing on its side. There’s a narrow opening, leading into a wider air-filled chamber, across which a thin membrane is stretched. When infrared radiation passes through the opening, it falls upon the membrane and heats it up. This happens readily because the membrane is exposed to the elements, is suspended in midair, and is a sixth as thick as a page of this book. It is also riddled with some 7,000 nerve endings that detect the slightest rise in temperature. Those nerves, as Elena Gracheva discovered, are packed with the heat sensor TRPA1, carrying 400 times as much of it as neurons elsewhere in the snake’s body. They’ll respond if the membrane rises in temperature by as little as 0.001 ° C. This astonishing sensitivity means that a pit viper can detect the warmth of a rodent from up to a meter away. A blindfolded rattlesnake that’s sitting on your head could sense the warmth of a mouse on the tip of your outstretched finger.[* 11] The pits are structurally similar to eyes. The membrane, which detects infrared light, is like a retina. The opening, which allows that light to enter, is like a pupil. And just like a pupil, the opening is narrow, which means that some regions of the membrane are heated by incoming infrared while others lie in cool shadow. The snake can use these patterns of hot and cold to map a heat source in its vicinity just as it uses the light falling on its retina to construct an image of a scene. These similarities aren’t just metaphorical. Some scientists think that the pits really are a second pair of eyes, tuned to the infrared wavelengths of light that are invisible to the main pair. Signals from the two organs are initially processed by different parts of the brain but eventually feed into a single region called the optic tectum. There, the two streams are combined, and information inputs from the visible and infrared spectrums are seemingly fused together by neurons that respond to both. It’s possible that the snakes really are seeing infrared, treating it as just another color.
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To understand another animal’s Umwelt, you have to watch its behavior. But a pit viper’s behavior mostly consists of waiting. Since they don’t generate their own body heat, they can go without eating for months and can sit in ambush until exactly the right moment. The few researchers brave enough to study them end up with animals that mostly sit around doing nothing, which makes them very hard to train—or comprehend. After all, even animals that we already understand and that we know how to train can sense heat in ways that are hard to explain.
Chapter 6: A Rough Sense | Contact and Flow
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Sea otters have neither the large heat-retaining bodies nor the insulating blubber of seals and whales. They do have the densest fur in the animal kingdom, with more hairs per square centimeter than humans have on our heads, but even that isn’t enough to stop heat from rapidly bleeding off their bodies. To stay warm, they need to eat a quarter of their own weight every day; hence their frenetic nature. They’re always diving, day and night. Almost everything’s on the menu, and almost everything is grabbed by hand.
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a region called the somatosensory cortex
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In sea otters, the part of the somatosensory cortex that receives signals from the paws is disproportionately big compared to those of other mustelids, and even compared to those of other otters.
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Imagine that, right now, a sea otter is about to search for food. Floating on its back on the surface of the sea, it rolls and dives. It will only stay submerged for a minute—roughly the time it will take you to read this paragraph. The descent eats up many of the precious seconds, so once the otter reaches the right depth, it has no time for indecisiveness. In a few frantic moments, it presses its knobby mittens over the seafloor, inspecting whatever it can find. The water is dark, but darkness doesn’t matter. To some of the most sensitive paws in the world, the ocean is bright with shapes and textures to be felt, grasped, pressed, prodded, squeezed, stroked, and manhandled—or perhaps otterhandled. Hard-shelled prey nestle among the similar hard rocks, but in a split second, the otter feels the difference between the two, and pulls the former from the latter. With its sense of touch, its dexterous paws, and its overabundant mustelid confidence, it snatches that clam, yanks that abalone, grabs that sea urchin, and finally ascends to eat its catches, breaking the water at the end of this sentence.
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Our own fingertips are among nature’s most sensitive touch organs. They allow us to wield tools with fine precision, to read patterns of raised dots when our vision is impaired, and to control screens with taps, swipes, and touches. Their sensitivity depends on mechanoreceptors—cells that respond to light tactile stimulation. These cells come in several varieties, each of which responds to a different kind of stimulus. Merkel nerve endings respond to continuous pressure: They help you gauge the shape and material properties of this book as you squeeze its pages. Ruffini endings respond to tension and stretch in the skin: They help you adjust your grip, and recognize when objects slip from your grasp. Meissner corpuscles respond to slow vibrations: They produce the feelings of slip and flutter as your fingers move over surfaces, and they allow Braille readers to make sense of raised dots. Pacinian corpuscles respond to faster vibrations: They’re useful in assessing finer textures or in sensing objects through tools, like hairs that are gripped by tweezers or soil that crunches beneath a spade.
Page 160 · Location 3078
At a broad level, we understand how these mechanoreceptors work. Despite their variety, they all consist of a nerve ending enclosed in some kind of touch-sensitive capsule. A tactile stimulus bends or deforms the capsule, causing the nerve inside to fire.
Page 162 · Location 3115
what the star-nosed mole does with its nose. As it scurries through its dark underground world, it constantly presses its star against the walls of its tunnels, a dozen times a second. With every press, its environment comes into focus in a starburst of textures. I imagine that each one adds to a continuous model of the tunnel that builds in the mole’s mind, like a pointillist image appearing dot by dot. The mole’s somatosensory cortex—the touch center of its brain—is disproportionately devoted to the star, much as a human’s touch center is especially devoted to our hands.
Page 163 · Location 3135
Catania shows me a video that he filmed from below as a star-nosed mole investigated a glass slide containing a piece of worm. When the video is slowed by 50 times, I can see the animal dabbing its star against the glass, detecting the morsel, bringing the tactile fovea across to inspect it more thoroughly, and finally swallowing it. In real time, it’s impossible to work out what is happening. The mole simply appears, and the worm disappears. By analyzing such footage, Catania and his colleague Fiona Remple found that the mole can identify its prey, swallow it, and begin searching for the next mouthful in an average of 230 milliseconds and as little as 120 milliseconds. That’s as fast as a human blink. Imagine that your eye starts to close at the exact moment that a foraging mole first touches an insect with its star. Before your lashes cross the midline of your eye, the mole’s brain has already recognized what it has touched and sent motor commands to reposition the star. By the time your eye is fully shut, the mole has touched the insect a second time with its supersensitive 11th rays. By the time your eye is half-open, the mole has processed the information from that second touch and decided on a course of action. When your eye is fully open, the insect is gone and the mole is looking for another.
Page 164 · Location 3147
Light may be the fastest thing in the universe, but light sensors have their limits, and the star-nosed mole’s sense of touch blows past them all.
Page 165 · Location 3155
“They’re little vacuum cleaners,” says Catania. “They eat things so small that you might think: Why even bother?” They bother because they have no competition. Thanks to the star—a nose that works like a hand and scans like an eye—the underground world appears in glorious detail, and abounds with food that its competitors can’t even perceive.
Page 167 · Location 3193
The emerald jewel wasp also has a long, probing organ with a touch-sensitive tip, but its goals and methods are far grislier than a red knot’s. The wasp—a beautiful inch-long creature with a metallic green body and orange thighs—is a parasite that raises its young on cockroaches. When a female finds a roach, she stings it twice—once in its midsection to temporarily paralyze its legs, and a second time in its brain. The second sting targets two specific clusters of neurons and delivers venom that nullifies the roach’s desire to move, turning it into a submissive zombie. In this state, the wasp can lead the roach to her lair by its antennae, like a human walking a dog. Once there, she lays an egg on it, providing her future larva with a docile source of fresh meat. This act of mind control depends on that second sting, which the wasp must deliver to exactly the right location.
Page 169 · Location 3231
realize that a whisking mouse or rat uses its vibrissae in a way that’s far closer to what I do with my eyes. The rodent constantly scans and re-scans the area in front of it, building up an awareness of a scene. If it senses something with the long, mobile whiskers on its snout, it investigates further with the shorter, immobile whiskers on its chin and lips, which are more numerous and more sensitive. This behavior is similar to that of a star-nosed mole pressing its nose along a tunnel, detecting objects with its star, and finally bringing the small and most sensitive rays into play. It’s also similar to a human sweeping their eyes over a scene, detecting something in their peripheral vision, and focusing on it with their high-resolution foveae.
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Manatees are the only known mammals that only have vibrissae and no other kinds of hair.
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a system of sensors called the lateral line. The lateral line is found in all fish (and some amphibians). It usually includes a smattering of visible pores on a fish’s head and flanks, along with fluid-filled canals running just below its skin.
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When a predator lunges, the rush of incoming water triggers the lateral lines of the nearest individuals, which dart away. Their startled movements trigger the lateral lines of their neighbors, which trigger their neighbors, and so on. Waves of panic spread outward, and the school seamlessly parts around the predator. Each fish only attends to the small volume of water around it, but the sense of touch connects them all and allows them to act as a coordinated whole.
Page 180 · Location 3414
The animals use the bumps to scan the thin horizontal layer where air and water meet. They sit in ambush in that layer, waiting for something to land in the water or to arrive at its edge for a drink. This strategy demands stillness, so they can’t engage in the comparatively hectic explorations of moles, mice, or even manatees. Unmoving, they use their touch sensors to monitor everyone else’s movements.[* 21]
Page 186 · Location 3513
The filiform hairs of crickets and the trichobothria of spiders are almost inconceivably sensitive. They can be deflected by a fraction of the energy in a single photon—the smallest possible quantity of visible light. These hairs are a hundred times more sensitive than any visual receptor that exists, or could possibly exist.
Chapter 7: The Rippling Ground | Surface Vibrations
Page 205 · Location 3921
To the spiders, which have very poor eyesight, the room doesn’t really exist: There is only the web, and whatever vibrates it.
Page 205 · Location 3927
Spiders have been around for almost 400 million years, and they’ve likely been producing silk for all that time. Their silk is a marvel of engineering. Though light and elastic, it can be stronger than steel and tougher than Kevlar. Spiders use it to wrap their eggs, construct shelters, hang in the air, and soar through the skies (more on that later). Most famously, many species fashion it into a flat, circular shape—the orb web. The orb web is a trap, which intercepts and immobilizes flying insects. It’s also a surveillance system, which extends the range of the spider’s senses well beyond the reach of its body. That body is covered in thousands of slit sensilla—vibration-sensing cracks similar to those that sand scorpions use to detect the seismic activity of their prey. On spiders, these slits are also concentrated around the joints, where they’re grouped into clusters called lyriform organs. Using these exquisitely sensitive organs, all spiders can sense the vibrations coursing through whatever they’re standing upon. For the tiger wandering spider of the previous chapter, that surface is the ground. For orb-weavers like Nephila, it’s the web. These spiders construct the surfaces that they then sense vibrations through. For that reason, the orb web isn’t just another substrate, like soil, sand, or plant stems. It is built by the spider and it is part of the spider. It is as much a part of the creature’s sensory system as the slits on its body.
Page 206 · Location 3939
Like the Nephila in Mortimer’s arachnarium, most orb-weavers sit in the middle of their webs and rest their legs on the radial spokes that funnel vibrations toward them. From this position, they can distinguish the vibrations generated by rustling wind or falling leaves from those created by struggling prey. They can probably work out where those struggles are coming from by comparing the strength of the vibrations hitting each of their legs. They can assess the size of their prisoners, and will approach the larger ones more carefully or not at all. If the prey stops moving, they can find it by deliberately plucking the silk and “listening” to the returning vibrational echoes. When it comes to capturing prey, vibrations supersede other stimuli. If a tasty fly buzzes above an orb-weaver, the spider will simply wave it away with its legs. The fly only becomes recognizable as food if it shakes the web. This dependency on vibrations is so absolute that many animals can exploit orb-weavers by camouflaging their footsteps. The small dewdrop spider Argyrodes is a thief, stealing from larger spiders like Nephila by hacking their webs. From a nearby hiding place, it runs several lines of silk over to the hub and spokes of a Nephila web, effectively plugging its sensory system into that of the bigger spider. It can tell when Nephila has caught something and is wrapping it in silk for storage. It then runs over and eats the insect itself, often after cutting it free from the main web so that the host spider can no longer detect it. Argyrodes acts carefully to avoid creating its own telltale vibrations. It runs only when Nephila is moving and treads more slowly when Nephila is still. It also holds on to any strands it cuts to avoid any sudden releases in tension. Through such subterfuge, this thief is almost never caught. As many as 40 of them might be plugged into a single Nephila web. Other creatures have more lethal intentions than pillaging food. Some assassin bugs walk so stealthily that they can creep right up to a spider and kill it on its own web. Portia, a jumping spider that eats other spiders, will violently twang a web to mimic the impact of a twig and use this vibrational smoke screen to charge at its prey. Both Portia and the assassin bugs can pluck webs to mimic the vibrations of ensnared prey and lure spiders to them. These predators are all visually conspicuous, but as long as their vibrations feel like those of an insect, a twig, or a breeze, an orb-weaver can’t tell the difference. It lives in what Friedrich Barth calls “a small woven world full of vibrations.”
Page 207 · Location 3959
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, every morsel counts. To capture such morsels, the spider expands the range of its senses by changing the nature of its web. But here’s the truly important part: Watanabe found that a well-fed spider will also go after small flies if it is placed onto a tense web built by a hungry spider. The spider has effectively outsourced the decision about which prey to attack to its web. The choice depends not just on its neurons, hormones, or anything else inside its body, but also on something outside it—something it can create and adjust. Even before vibrations are detected by its lyriform organs, the web determines which vibrations will arrive at the leg. The spider will eat whatever it’s aware of, and it sets the bounds of its awareness—the extent of its Umwelt—by spinning different kinds of webs.[* 13] The web, then, is not just an extension of a spider’s senses but an extension of its cognition. In a very real way, the spider thinks with its web. Tuning the silk is like tuning its own mind. A spider can also tune its body. Biophysicist Natasha Mhatre showed that the infamous black widow can adjust the lyriform organs on its joints to different vibrational frequencies by changing its posture. The widow spins a messy horizontal web, and normally hangs upside down from it with legs outstretched. But when it’s hungry, it can also draw its legs into a “crouch”—a sensory power pose that retunes its joints to higher frequencies. Like the tense web of Watanabe’s orb-weaver, this stance might shift the spider’s Umwelt toward the movements of smaller prey. It might also help it to ignore the low frequencies of wind. It’s like a postural squint, which allows the spider to focus its attention. The analogy isn’t exact, though, since squinting helps us to focus on particular parts of space. Here, the black widow’s posture focuses on different parts of information space. It’s as if a human could emphasize the red parts of our vision by squatting, or single out high-pitched sounds by going into downward dog.
Chapter 8: All Ears | Sound
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both hearing and touch are mechanical senses, which detect movements in the outside world using receptors that send electrical signals when they’re bent, pressed, or deflected.
Chapter 9: A Silent World Shouts Back | Echoes
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Echolocation differs from the senses we have met so far, because it involves putting energy into the environment. Eyes scan, noses sniff, whiskers whisk, and fingers press, but these sense organs are always picking up stimuli that already exist in the wider world. By contrast, an echolocating bat creates the stimulus that it later detects. Without the call, there is no echo. As bat researcher James Simmons explained to me, echolocation is a way of tricking your surroundings into revealing themselves. A bat says, “Marco,” and its surroundings can’t help but say, “Polo.” The bat speaks, and a silent world shouts back.
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Most bats echolocate in a way broadly similar to that of the archetypal big brown. They send out short sonar pulses that last between 1 and 20 milliseconds and are separated by relatively longer silences. Those pulses also sweep down across a broad band of frequencies, which is why these bats are known as FM, or frequency-modulated, bats. But around 160 species—the horseshoes, hipposiderids, and Parnell’s mustached bat—do something very different. Their calls are much longer, lasting for many tens of milliseconds in some species, and separated by much shorter gaps. And instead of covering a range of frequencies, these species hold one particular note. For that reason, they are called CF, or constant-frequency, bats. And they are listening out for a very specific kind of echo. When a sonar pulse hits an insect’s flapping wing, the echo strength varies as the wing moves up and down. But at one particular moment, when the wing is exactly perpendicular to the incoming sound, an especially loud and sharp echo bounces straight back at the bat. This is called an acoustic glint. It’s a dead giveaway that an insect is flying nearby. FM bats can theoretically detect these glints, but they’re unlikely to. Their brief sonar pulses are separated by long gaps, so an FM bat has to get very lucky to hit an insect’s wing at exactly the right moment to return a glint. By contrast, the pulses of CF bats are long enough to cover an entire wingbeat. They catch glints galore. And since leaves and other background objects don’t flap in the same rhythmic way as wings, a CF bat can use glints to distinguish fluttering insects against cluttering foliage. They must be the auditory equivalent of flashes of light.
Page 257 · Location 4916
CF bats can compensate for Doppler shifts. When closing in on a target, they produce calls that are lower than their normal resting frequency, so the upshifted echoes hit their ears at exactly the right pitch. And they do this (quite literally) on the fly, constantly tweaking their calls so that the echoes from targets ahead stay within 0.2 percent of the ideal frequency. This is a staggering feat of motor control that’s almost unmatched in the animal kingdom.
Page 258 · Location 4937
more than half of moth species have ears that can hear bat sonar.
Chapter 10: Living Batteries | Electric Fields
Page 278 · Location 5368
it is clear that the knifefishes and elephantfishes use their electric fields to sense their surroundings, and even to communicate with each other. Electricity is to them what echoes are to bats, smells are to dogs, and light is to humans—the core of their Umwelt.
Page 296 · Location 5654
Life exists within that planetary electric field and is affected by it. Flowers, being full of water, are electrically grounded, and bear the same negative charge as the soil from which they sprout. Bees, meanwhile, build up positive charges as they fly, possibly because electrons are torn from their surface when they collide with dust and other small particles. When positively charged bees arrive at negatively charged flowers, sparks don’t fly, but pollen does. Attracted by their opposing charges, pollen grains will leap from a flower onto a bee, even before the insect lands.
Page 296 · Location 5670
Alongside the bright colors that we can see (and the ultraviolet ones we cannot), flowers are also surrounded by invisible electric halos. And bumblebees can sense these.
Page 298 · Location 5696
Spider silk picks up a negative charge as it leaves a spider’s body, and is repelled by the negatively charged plants on which they sit. That force, though tiny, is enough to launch the spider into the air. And since the electric fields around plants are strongest at points and edges, spiders can ensure a vigorous takeoff by ballooning from twigs and blades of grass.
Chapter 12: Every Window at Once | Uniting the Senses
Page 328 · Location 6382
Distinguishing self from other isn’t a given; it’s a difficult problem that nervous systems have to solve. “This is largely what sentience is,” neuroscientist Michael Hendricks tells me. “And perhaps it’s why sentience is: It’s the process of sorting perceptual experiences into self-generated and other-generated.”
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Animals cannot make sense of what’s around them without first making sense of themselves. And this means that an animal’s Umwelt is the product not just of its sense organs but of its entire nervous system acting in concert.
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Our experiences of the world feel disconnected from the very sense organs that produce them, which makes it easy to believe that they are purely mental constructs divorced from physical reality.
Chapter 13: Save the Quiet, Preserve the Dark | Threatened Sensescapes
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the Anthropocene—a geological epoch defined and dominated by the deeds of our species.
Page 338 · Location 6588
“The thought of light traveling billions of years from distant galaxies only to be washed out in the last billionth of a second by the glow from the nearest strip mall depresses me no end,” vision scientist Sonke Johnsen once wrote.
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Light at night, however, is a uniquely anthropogenic force.
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As those species go extinct, so too do their Umwelten. With every creature that vanishes, we lose a way of making sense of the world.