The Secret to Squirrel Intelligence That Will Surprise You

0:00 There is something about the night that makes the small things feel worth noticing. Welcome to Sleepy Science. And tonight we are going to sit together in the quiet and [music] think about squirrels. And if somewhere along the way you feel your eyes growing heavy, [music] that is perfectly fine. That is the whole idea. If you enjoy these gentle journeys, please do leave a like, subscribe if you haven't already, and drop a [music] comment below telling us what you'd like to explore next. And if you'd like to help keep these videos going, you can find [music] us on

0:46 Patreon. The link is always in the description. Now, let us begin. Squirrels have been here for a very long time, [music] longer than you might expect. The earliest squirrel-like creatures appeared on this earth roughly 36 million years ago. 36 million years. Let that number settle quietly in your mind like a leaf drifting down to still [music] water. The world was a very different place then. The continents were shaped a little differently. The seas were warmer in some places,

1:34 cooler in others. The great ice ages had not yet come and gone. And yet something very much like a squirrel was already climbing through the trees, already storing [music] things away, already watching the world with those wide and careful eyes. There are at this moment more than 280 distinct species of squirrel living on this planet. 280 [music] They live on every continent except Antarctica [music] and Australia. Though Australia has its own small squirrelike [music]

2:20 creatures that evolved separately, shaped by [music] a different kind of silence. From the tiny African pygmy squirrel, [music] which is about the size of your thumb, to the Indian giant squirrel, which can grow to be nearly 3 ft long, [music] from nose to the tip of its extraordinary tail. The squirrel family is vast and quietly diverse. The Indian giant squirrel is worth pausing on for a moment. Its fur is not [music] the gray or brown. You might imagine when you picture a squirrel. It comes in deep maroon, rich purple,

3:08 cream, and orange. It moves through the forest canopy of southern India. Like a slow living jewel, rarely coming down to the ground at all. Most of its life is lived up in the high branches where it leaps distances of up to 20 ft between trees. 20 ft. And it [music] does this quietly without drama as though the [music] air between the trees is simply another kind of ground. [music] The word squirrel itself comes from the ancient Greek words for shadow and tail.

3:55 Skia [music] meaning shadow and ura meaning tail. Shadow tail. It is a beautiful name when you [music] think about it because the tail of a squirrel is not merely decorative. It serves as a blanket in the cold, a rudder in the air, a sun shade in the heat, and a signal to other squirrels communicating everything from alarm to contentment. The tail is in a way a second language. Squirrels communicate in many ways. >> [music]

4:40 >> They use their tails, as we've just said, but they also use a surprisingly rich vocabulary of sound. Different calls mean different things. There is a high, sharp call used when a hawk is spotted overhead. [music] A threat coming from the sky. There is a lower, slower [music] call used when a threat is on the ground. A fox, [music] a cat, a dog, and other squirrels within earshot respond differently to each call, taking [music] cover in different ways depending on which direction the danger

5:25 is coming from. This is not random noise. It is something like language. [music] The gray squirrel, the one most of us know, the one that moves through parks and gardens and the edges of quiet neighborhoods, is originally from North America. It was introduced to Britain in the 1880s and from there it spread quietly across much of Europe. It is a remarkable traveler, not by intention, but by adaptability. It [music] thrives in cities, in suburbs, in parks, in forests, [music]

6:14 adjusting its behavior to wherever it finds itself. Learning the patterns of the place, learning which humans bring [music] food and which ones are best avoided. Squirrels have a remarkably good memory, though perhaps not in the way you'd expect. They are famous for burying food, particularly nuts and seeds, to come back to later in the winter. This behavior is called scatter hoarding. [music] A single gray squirrel may bury several thousand individual food caches in a single autumn. [music] Several thousand.

7:00 And it finds a surprising number of them again. Months later, [music] under snow, under fallen leaves, in frozen ground. >> [music] >> It uses a combination of spatial memory and smell to relocate what it has hidden. But not every cache is recovered. Some are simply forgotten or left behind [music] when the squirrel does not survive the winter. And those forgotten seeds sometimes grow. Squirrels [music] by accident have planted countless trees. [music] Think about that for a moment.

7:47 The forests you walk through, the oak trees whose shade you have sat beneath. Some of them are here because a squirrel buried [music] an acorn and forgot about it a very long time ago. There is something quietly moving about that. The squirrel was not trying to plant a forest. [music] It was just trying to survive the winter. And in doing so, it [music] changed the land. Squirrels are not always working alone. Red squirrels, which are the native squirrels of Europe and parts of Asia,

8:36 have been observed sharing [music] winter cashaches with relatives. They are more social than they are often given credit for. And [music] in the coldest months, red squirrels have been seen huddling together in their dre. Those round, tightly woven nests of leaves and twigs high in the branches sharing warmth in the dark. The dre itself is a remarkable construction. From the outside, it looks rough and tangled, just a ball of debris [music] caught in the fork of a branch. But from the inside,

9:22 it is a different world entirely. [music] The squirrel weaves the outer shell from twigs and leaves and then lines the interior with moss, grass, feathers, [music] and fur. It is warm and dry even in [music] rain. It is waterproof because the squirrel knows [music] in some deep instinctive way which materials shed water and which ones hold it. A squirrel may build several dre in its home territory moving between them as the season or the weather

10:08 demands. Squirrels are dal creatures, which means they are most active [music] during the day. They wake not long after sunrise, [music] and they rest as the light fades. In the middle of the day, especially in the heat of summer, [music] they often retreat to their dre, quiet [music] rest. There is a certain wisdom in that. Work in the cool of the morning, rest in [music] the warmth of midday, return to the evening tasks [music] as the light begins to soften.

10:54 They do not hibernate. This is one of the things people most often get wrong about squirrels. They slow down in winter, spending more time in their dre, more time still and quiet, but they do not enter the deep monthsl long unconsciousness of a [music] bear or a hedgehog. On warmer winter days, they venture [music] out, digging through the snow for buried cashaches, moving carefully through a world that has gone gray and still. [music] There is something almost meditative about imagining that

11:39 a small [music] creature moving through a silent snow-covered forest [music] looking for the thing it put away back when the world was warm. Ground squirrels, however, are different. Many species of ground squirrel do hibernate [music] and they do so with a depth and commitment that is [music] quietly extraordinary. The Arctic ground squirrel, which lives in the tundra of Alaska and Canada, enters a hibernation so profound [music] that its body temperature can drop to just below freezing,

12:26 negative 3° C, below freezing. And yet, the squirrel [music] does not die. It warms itself periodically through short, intense [music] bursts of shivering that last only a few hours before it settles [music] back into its frozen stillness. Scientists have studied the Arctic ground squirrel [music] for what it might teach us about brain health. During its [music] deepest torper, the connections between neurons in its brain [music] actually shrink and retract. The brain becomes simpler,

13:11 quieter, smaller in a sense. And then as it warms in spring, those connections rebuild themselves. The brain rewires. It recovers completely without lasting damage. Researchers believe this process might one day offer clues about how to protect the human [music] brain from conditions like Alzheimer's. A sleeping squirrel in the frozen tundra might be holding without knowing it a quiet key to something vast.

13:57 Flying [music] squirrels do not actually fly. They glide. But the gliding is done [music] with such grace and such apparent ease [music] that the word flying feels almost earned. They have a [music] membrane called a patasium that stretches from their wrists to their ankles. a thin silky [music] sheet of skin that unfurs when they leap. They can glide distances of [music] up to 150 ft in a single leap. 150 ft. They steer [music] using their limbs and their tail, adjusting angle and

14:44 direction midair with [music] tiny precise movements. And they land with extraordinary accuracy, rotating their body at the last moment to slow [music] themselves and touch down gently on the bark of a tree. Flying squirrels are nocturnal, unlike most of their relatives. They move through the dark forest in near silence. Those wide black eyes gathering every scrap of available light. Their eyes are among the largest relative to body size

15:30 of any squirrel species. [music] And there is something else about flying squirrels that is easy to overlook but worth pausing on. Under ultraviolet light, their fur glows a vivid, unexpected pink, not [music] a faint tinge, a bright, warm, fluorescent [music] pink. Biologists discovered this only recently, and they are still not entirely sure why. It may help the squirrels see each other in the ultraviolet light that filters through the night forest.

16:15 Or it may serve some other purpose that we have not yet understood. The world keeps its small [music] mysteries carefully. Squirrels have front teeth called incizers that never stop growing. They grow throughout the squirrel's entire life, continuously pushing forward at a steady, quiet pace. This sounds like it might become a problem, and it would if squirrels did not gnaw. The [music] constant gnoring on nuts, on bark, on [music] twigs, on pine cones

17:01 wears the teeth down at almost exactly the same rate [music] that they grow. It is a kind of ongoing balance, a natural equilibrium, a slow negotiation between [music] growth and wear that lasts the entire life of the animal. When a squirrel gnors into a nut, [music] it is not doing so randomly. It uses a precise learned technique that is specific to the type of nut. Hazelnuts [music] walnuts acorns each has [music] a slightly different shell, a different structure, a different

17:47 [music] point of weakness. And squirrels learn through trial and repetition exactly where to apply [music] pressure and at what angle. Young squirrels are not born knowing this. They figure it out slowly, awkwardly at first, working at a nut for a long time before they crack it open. >> [music] >> And with practice they become fast and efficient. Another quiet skill earned through patience and repetition. Squirrels [music] have four toes on their front feet and five on their back.

18:36 The asymmetry helps them [music] grip. Their hind feet are capable of rotating almost entirely backwards, which allows them to descend a tree trunk head first, something most climbing animals cannot do. When you see a squirrel coming down a tree head first, those back feet have rotated around so that the claws still face the bark, still grip, still hold. It is an elegant solution to the problem of getting down safely [music] and quickly. The grip strength of a squirrel is quietly remarkable.

19:22 They can hang from a branch by a singlehinded [music] foot while using both front paws to manipulate a nut. They [music] can cling to smooth surfaces that would defeat most climbers. and they [music] can fall. A squirrel can fall from a height of up to 100 ft and land unharmed. This is because of their size and weight relative [music] to their surface area and because of the way they spread their body during a fall, slowing themselves like a tiny parachute. They reach a terminal velocity [music] that their small bodies can absorb on landing.

20:08 The physics of being small is [music] in this case a gift. Squirrels have a heart rate that hums along at around 270 [music] beats per minute when they are active. 270. Compared to the human heart, which [music] beats 60 to 100 times per minute at rest, the squirrel's heart is almost frantic. [music] And yet, the squirrel does not seem frantic. It moves through the world with a [music] kind of decisive calm, pausing often, watching, listening before acting with sudden and precise

20:55 [music] speed. Its inner world is ticking fast while its outward manner remains. Still, squirrels sleep between 12 and 14 hours a day. 12 to 14 hours. They rest in their dre or burrows, curled into themselves, warm, quiet, tucked away from the world. And when they sleep, they sleep deeply. There is no small comfort in knowing that even the most energetic, most [music] watchful of creatures

21:41 needs a great deal of rest. Rest is not [music] weakness. It is the foundation on which everything else is built. A squirrel's [music] lifespan in the wild is typically around 2 to 5 years. In captivity or in wellprotected suburban environments, some individuals have been known to live as long as 12 years. But the wild is demanding. Predators weather disease [music] competition, all take their toll. The first year of life is [music] the most dangerous.

22:27 Many young squirrels do not make it past their first winter. And yet the species endures, has endured [music] quietly for 36 million years. [music] Baby squirrels are called kittens or sometimes pups. They are born hairless and blind, entirely helpless, weighing only about as much as a coin. They are born into the dre, into that warm mosslined [music] chamber, high in the branches, and for the first weeks of their lives, they know nothing but warmth and milk

23:13 and the sound of their mother moving around them. Their eyes open at around 4 weeks. [music] And at around 8 to 10 weeks, they begin to venture out of the dre for the first time, blinking at the light, uncertain, curious. The world must seem very large. Then mother squirrels are fiercely protective. If a Dre is disturbed, a mother will carry each of her kittens [clears throat] one by one to a new location. She picks them up by the scruff of

23:59 [music] the neck and they curl themselves instinctively into a tight, compact position so they don't drag on the branches. She may move several kittens across significant [music] distances back and forth until all of them are [music] safe. It is unhurried and determined [music] and very quiet. There [music] is a squirrel species in Japan called the Japanese dwarf flying squirrel termis mamonga and it is widely [music] considered to be one of the most charmingl looking animals on earth. It has enormous eyes, a round soft face

24:48 and gray fur so smooth it looks almost unreal. It weighs between 100 and 200 [music] gram. When it glides, it tucks its legs close to its body and the potium fans out in a perfect round silhouette against the [music] sky. It lives in the mountain forests of Honshu and Kioshu and comes out only at night, moving between the conifers in near silence. Squirrels are important to the ecosystems in which [music] they live.

25:34 We have already spoken of the trees they plant by accident. The forgotten caches that grow into oaks and beaches and chestnuts. But their role goes further than that. They spread fungal spores through the forest. When they [music] dig in the earth or move through leaf litter, they disturb [music] and distribute the spores of microisal fungi. the underground networks [music] that connect the roots of trees and allow them to share [music] nutrients. A squirrel going about its [music] quiet business is tending the invisible web beneath the forest floor without the slightest

26:22 awareness of what it is doing. Squirrels also serve as prey. They are eaten by hawks, owls, foxes, weasels martins bobcats snakes and many other animals. They are a link in a long chain. Without them, the predators that depend on them, would struggle, [music] and the plants they carry and scatter would be less widely distributed. Every living thing is woven into something larger than [music] itself. The red squirrel of Europe has a tuft of

27:11 fur on each ear that grows longer in winter. These tufts are one of the things that make the red squirrel so distinctive, so immediately recognizable. They give it a slightly otherworldly look, a small fantastical quality as though it belongs in a fairy story. And perhaps it does. The red squirrel has lived in European forests long enough to have become woven into the folklore of many cultures. In Norse mythology, a squirrel named Ratatoscera runs up and down the great world tree

27:58 Idrasil, carrying messages between the eagle at the top [music] and the serpent at the roots. A messenger between worlds, running [music] always running, between the high and the low, between above and below. In many indigenous traditions of North America, the squirrel [music] represents preparation, prudence, and the wisdom of planning ahead. The squirrel that gathers in summer what it will need in winter [music] is seen as a teacher, a reminder that the good times will not

28:44 last forever [music] and that the quiet work of preparation is not a small thing. [music] It is in fact perhaps the most important thing. [music] Squirrel's eyes are placed on the sides of their heads, not the front like yours. This gives them an enormous field of view, nearly 360°. [music] They can see almost everything around them without [music] turning their heads. But this placement means their depth perception directly ahead is not as [music] strong as a forward-f facing predators.

29:30 They compensate for this by moving their heads in small bobbing motions, gaining parallax information that helps them judge distance. >> [music] >> When a squirrel tilts its head and looks at you, it is not just curious. It is computing, measuring, deciding. [music] Squirrels can detect the smell of food buried under a foot of snow. A foot of snow and still the scent reaches them. [music] The olfactory system of a squirrel is finely tuned to

30:16 the particular chemical signatures of the nuts and seeds it has cached, but it [music] is not perfectly tuned. Squirrels will sometimes dig up and steal the caches of other squirrels. And some squirrels have been observed, making [music] fake cashing motions when they suspect they are being watched, [music] pretending to bury something while actually hiding it elsewhere. This is called deceptive caching and it implies something quietly complex about the squirrel's awareness

31:02 of other minds. To perform deceptive caching, a squirrel must recognize that another squirrel is watching. [music] It must understand in some form that the watching squirrel has information that could be used against it and it must [music] take action to interfere with that information. This is a level of social cognition that we do not always associate [music] with small, seemingly simple animals. But the squirrel is not simple. It is quietly sophisticated

31:49 in the ways that matter most [music] for its survival. The tail of a squirrel [music] is important for temperature regulation as well as communication. In hot weather, [music] blood is directed to the tail where it can lose heat to the air and return to the body cooler. In cold weather, [music] the squirrel wraps its tail around itself like a [music] scarf, conserving warmth, making itself smaller against the chill. The tail is in this way a living thermostat. There are squirrels that live entirely

32:35 underground. Ground [music] squirrels, prairie dogs, which are technically a type of squirrel, and marmmets all belong to the squirrel family and all spend much of their lives beneath the surface of the earth. Prairie dog towns, [music] as the colonies are called, can be enormous. Some historical accounts describe [music] prairie dog towns stretching across hundreds of miles of grassland [music] housing hundreds of millions of individuals. Hundreds of millions. Cities beneath the plains [music]

33:21 humming with their own quiet life. Vast and ordered and largely invisible [music] to the world above, prairie dogs have one of the most complex communication systems of any animal studied. [music] Their alarm calls do not merely signal danger. They describe [music] the danger in remarkable detail. Different calls encode information about the size, shape, color, and speed of an approaching predator. A call that effectively says, "Large, brown [music]

34:06 moving fast, coming from the north." The linguist Conabodic, who [music] studied prairie dog communication for decades, has argued that their calls constitute something very close to a primitive language, not metaphorically, structurally. The marmmet, a large ground squirrel found in the mountains of Europe, Asia, and North America, lives in close family groups in burrows dug deep into alpine meadows. In winter, the entire family hibernates together,

34:53 adults and young alike, curled around each other in the dark and warmth of the burrow. Their heartbeats slow to just three or four beats per minute. Three or four. Their body temperature drops to just above the temperature of the surrounding soil. [music] They breathe only a few times per minute. For 7 months, the family lies still together in the earth, barely alive, waiting for spring. [music] There is a squirrel that is functionally colorblind or at least

35:39 close [music] to it. Most squirrels have two types of color detecting cells in their eyes called cones compared to the three types that allow humans to see the full color spectrum. Squirrels [music] can distinguish blues and yellows reasonably well, but red [music] and green appear to them as shades of yellow or gray. This does not seem to disadvantage them much. Their world is built on movement, shape, brightness, and smell more than on color. They navigate their world

36:25 differently from us, but no less effectively. Squirrels have been to space, not literally. [music] But the squirrel cage, the name given to a type of rotating exercise [music] wheel, has been used in many contexts, including in research into balance and vestibular function [music] that contributed to understanding how living things respond to microgravity. and squirrels themselves have contributed to research in ways both direct and indirect. [music] The Arctic ground squirrel's ability

37:13 to survive extreme cold and [music] recover completely has made it a subject of significant scientific interest in [music] the fields of neurology and medicine. The Kybab squirrel lives only [music] on the Kaibab Plateau on the north rim of the Grand Canyon. It is a subspecies of the Abbert squirrel, separated from its relatives by the vast chasm of the canyon itself. [music] For thousands of years, the canyon has been an uncrossable barrier. And so the squirrels on either side have evolved separately.

38:00 The kaab squirrel has developed a [music] distinctly white tail while the aad squirrel on the south rim has a gray tail. They are almost identical in every other way. These two populations separated by a mile of air and a river far below. The Grand Canyon, one of the largest things on Earth, has quietly and invisibly written its mark into the fur of a small animal. The fox squirrel is the largest tree squirrel in North America. [music] It can weigh up to about 1/2 kg.

38:47 It is, by squirrel standards, quite large. It comes in several color variations, gray, orange, black, or some combination of all three. In some parts of its range, entirely black fox squirrels live alongside entirely gray ones. as though the same species has decided to be two [music] different things at once. They coexist without apparent conflict. The black ones and the gray ones, digging in the same ground, eating from

39:32 the same trees, living the [music] same quiet lives in different shades. Squirrels can run at speeds of up to 20 mph. 20 mph in a small body over branches and rocks and roots, pivoting and accelerating with sudden [music] fluid precision. And they can stop just as fast as they start. Watch a squirrel [music] being chased and you will see it change direction in a way that seems almost instantaneous as if the change in direction costs it nothing, as if momentum

40:19 is simply not a concern. This is one reason they are so difficult for predators to catch. Not just speed, [music] but the total unpredictability of what they will do next. Squirrels engage in a behavior called food washing. Before burying a nut, some squirrels will roll it between their paws, rub their face and chin against it, and sometimes lick it. This appears to mark the food. with the squirrel's own scent, making it easier to relocate later. It also incidentally

41:05 deposits the scent glands secretions onto the nut, which may make it smell more familiar and therefore more retrievable. There is a kind of tenderness in the image. A small animal carefully tending to a single nut, holding it in its front paws [music] pressing its face against it before placing it gently in the earth. Squirrels are susceptible to several [music] diseases, including squirrel pox, a virus that is fatal to red squirrels, but which gray squirrels can carry without becoming ill themselves.

41:52 This [music] is one of the reasons the gray squirrel has displaced the red squirrel across much of Britain. [clears throat] Not through aggression, not through competition for food alone, but through the invisible transmission of a virus. The red squirrel retreats quietly, slowly to the northern forests of Scotland and a few protected [music] islands where the gray has not yet arrived. Conservation efforts for the red squirrel [music] involve everything from vaccination programs to the careful management

42:39 [music] of gray squirrel populations in protected areas. People build squirrel bridges, rope [music] bridges strung between trees, across busy roads so that squirrels can cross without danger. And cameras at these bridges [music] have captured squirrels using them regularly, trottting across with a confidence that suggests they have simply decided that the bridge belongs to them, which in a sense it does. [music] There are albino squirrels,

43:26 entirely white squirrels [music] with pale pink eyes and the faintest blush of translucence [music] in their thin skin. They are rare and because they are rare, they have attracted legend and story [music] in many of the towns where they appear. Alni, Illinois in the United States is known as the white squirrel capital of the world and has a small population of albino squirrels that are protected by local ordinance. Drivers must yield to them. Cats found [music] chasing them can be impounded.

44:12 A town organized itself around the protection of a pale [music] unusual animal. There is something quite tender about that. Black squirrels, melanistic [music] squirrels, are more common than albinos, but still noticeable when they appear in parts of Washington DC. >> [music] >> Black squirrels are so common, they have become part of the city's personality. [music] They were originally a gift from Canada in the early 1900s. A small group of black squirrels, released in the National Zoo, who eventually escaped and made the city

45:00 their home. They thrive on the heat absorbing benefits of dark fur in cold winters. An advantage that may explain why melanism appears more often in squirrel populations at higher latitudes. [music] A squirrel's front paws are almost handlike in their dexterity. Watch a squirrel eat and you will see it hold its food. with a precision and delicacy [music] that looks remarkably familiar. It rotates the nut, inspects [music] it, nor carefully at a specific point,

45:46 pauses, nor again. There is a [music] purposefulness to it that is easy to recognize, something that looks very much like paying attention. Squirrels [music] are prey animals and prey animals are almost always alert. [music] Alert in a way that we as creatures near the top of our food chain can hardly imagine. Every rustle in the bushes [music] might be significant. Every shadow passing overhead requires a moment of assessment. And yet the squirrel does not appear

46:32 anxious. It sits [music] and eats and grooms itself with apparent contentment, alert and alive and present, not troubled by the alertness itself. [music] There is something to learn there. Perhaps the grooming behavior of squirrels is extensive and careful. They groom themselves much as a cat does using their paws to wash their face, working through their fur, to remove parasites and debris, licking and combing and smoothing. >> [music]

47:18 >> They also groom each other, particularly mothers and their young, and sometimes paired squirrels that share a dre in winter. Social grooming is a bonding behavior. It says, "You are known to me. You are [music] safe. We are here together." The tufted ears of the red squirrel [music] grow longer through autumn and reach their maximum length by mid- winter, then gradually shrink back again in spring. [music] No one is entirely certain why. It may help with camouflage in the

48:03 winter forest. Or it may serve as a visual signal to other squirrels, a seasonal marker, a badge of winter. Or it may simply be a relic of something older, a feature that once served a purpose and has stayed on long after the purpose is gone. Evolution keeps things long past their apparent usefulness. The body is conservative. [music] It holds on. Squirrels are found at remarkably high altitudes. Some species of marmmet live comfortably

48:52 at altitudes above 4,000 m [music] well above the tree line in the cold thin air of alpine meadows. At these heights, the growing season is short and the winters are [music] brutal. The marmmets pack as much eating as possible [music] into the brief warm months, [clears throat] building up reserves of fat that will carry them through 7 months of sleep. They emerge in spring leaner than they entered, blinking [music] into the thin mountain light, the snow still deep around them, beginning again. Squirrels

49:39 have been observed using tools, not extensively and not in the complex ways that primates do, but there are recorded instances of squirrels [music] using small objects. A piece of bark, a leaf, a fragment of wood to help them extract or conceal food. This places them within a relatively small group of animals that can improvise solutions using [music] objects from their environment. It is a humble entry on the [music] list, but it is on the list. There is a [music] species called the

50:24 post squirrel found in Southeast Asia whose coloring is so bold it seems almost designed. [music] Three distinct bands of color. Deep black on the top, white on the sides, and a rich, warm chestnut [music] on the belly. Clear, crisp, [music] deliberate. It moves through the tropical forest like a brush stroke. Its purpose in nature is camouflage. The different patches breaking [music] up its outline in the dappled forest light. But from the outside, it looks like art.

51:13 The African pygmy squirrel, the smallest squirrel on Earth, lives in the rainforests of central and west Africa. It is about 7 to 10 cm long, not counting its tail. It weighs around 15 g. 15 g, the weight of a few sheets of paper. And yet [music] it navigates the enormous complexity of the rainforest with complete confidence. Running along branches hanging upside down, feeding on bark, lychen, and insects [music]

51:58 with the ease of something that belongs entirely to [music] where it is. Squirrels do not have a gallbladder. The gallbladder is an organ [music] that stores bile, a digestive fluid in many animals. Squirrels lack it entirely. They produce bile [music] in the liver and it flows directly to the small intestine as needed. This is not unusual among small mammals, but it is a good reminder [music] that the interior of every animal is differently arranged from what we might assume

52:44 shaped [music] by millions of years of particular pressures and particular solutions. Squirrels have a specific relationship with oak trees that has shaped entire forest landscapes. Oaks produce acorns which are a primary food source for gray and red squirrels alike. [music] But not all acorns are equal. Oak trees produce [music] two types. Acorns from white oaks, which germinate quickly, [music] and so must be eaten promptly, and acorns from red oaks, which have more tannin and germinate

53:32 [music] more slowly, making them better candidates for long-term storage. Gray squirrels have been observed eating white oak acorns [music] immediately and storing red oak acorns for winter. They are, it seems, making a kind of decision about which [music] food to keep and which to consume. Now, even more remarkably, squirrels have [music] been observed biting the tips off white oak acorns before burying them, which destroys the embryo and prevents the acorn from germinating,

54:18 keeping it [music] fresh longer underground. They are in effect [music] treating the white oak acorn, the one that would otherwise become food for a tree rather than food for a squirrel, [music] so that it will remain edible through the winter. [music] They do not do this to red oak acorns. The distinction [music] is deliberate. The choice is real. Squirrels are remarkably adaptable to human environments. In cities, [music] they have learned to use traffic lights as crosswalks, waiting at the edge of the road, and

55:04 crossing when the cars stop. This has been documented. They have learned to open latches, to work puzzle feeders that would challenge some primates, to navigate obstacle courses in exchange [music] for food. The YouTube channel Mark Robber, a former NASA engineer, built increasingly elaborate obstacle [music] courses to try to prevent squirrels from reaching bird feeders. And the squirrels solved every course, every single [music] one, with patience, with persistence, and with what looked very much like

55:51 enjoyment. [music] Squirrel's brains grow larger in autumn. The hippocampus, the part of the brain most involved in spatial memory, measurably expands in preparation for the caching season. [music] It is the brain literally growing to meet the cognitive demands of the season ahead. [music] And then in spring when the caching is done and the cash is mostly recovered, the hippocampus shrinks back again. A brain that seasonally rewires itself for the work at hand.

56:38 Human brains change over time as well, shaped by what we practice and what we need. But squirrels do it in a matter of weeks. [music] The squirrel's nose is always moving. Watch one closely and you will see the nostrils flickering [music] in rapid constant motion, sampling the air, reading information [music] that we cannot access. The smell of a predator. The smell of a buried nut. [music] The smell of another squirrel.

57:24 The smell of rain coming. The world for a squirrel is partly a world of scent that we can only imagine. Squirrels can be trained. Not [music] easily and not in the way that dogs can be trained because squirrels are not domesticated animals and they have their own strong [music] preferences, but they are responsive to positive association. [music] A squirrel that is regularly fed by a particular person will often learn to recognize that person [music] approaching them with confidence and even occasionally

58:10 taking [music] food directly from their hand. There is trust involved in this. A prey animal built for vigilance, making the decision to come [clears throat] close [music] to a being many times its size. That [music] decision is not made lightly. In Japanese culture, squirrels appear in woodblock prints and decorative art going back centuries. They are often depicted [music] climbing grape vines or sitting among autumn leaves, their tails curved over their backs.

58:57 They represent the harmony of small creatures within the larger natural world. In many of these images, the squirrel is not the subject of dramatic action. It is simply there. [music] Present belonging. Art finding beauty in the merely being of a small animal in its place. There is a disease called leptospirrosis [music] that squirrels can carry without becoming ill. They are what scientists call a reservoir host. An animal that maintains a pathogen

59:44 within a population [music] without itself suffering significantly. This is common among small mammals. It does not make them dangerous animals. It simply makes them part of a complex web of biology and disease [music] that connects all living things in ways both visible and invisible. The sound a squirrel makes is not always what people expect. Beyond the chattering alarm calls, squirrels [music] also produce softer sounds. Low murmuring chirps [music] used between a mother and her young.

1:00:32 Quiet clicking sounds used during grooming and the [music] soft, almost imperceptible rustling sounds of movement through leaves. In the company of a squirrel at close range, you might hear sounds that most people never notice [music] because they are listening for something louder. Squirrels malt twice a year. They shed their old fur and grow new coats [music] once in spring and once in autumn. >> [music] >> The winter coat is thicker, softer, more insulating.

1:01:19 The summer coat is lighter, and more sparse. [music] For a few weeks during each mol, squirrels can look a little rough, patchy, and uneven, fur coming in different lengths across their [music] bodies. They seem unbothered by this. They go about their lives in an in between coat with the same [music] quiet confidence. As always, squirrels have a fixed home range, an area they consider their territory and move through regularly. [music] For a gray squirrel,

1:02:06 this might be anywhere from [music] 2 to 7 hectares. They know this territory intimately. Every branch, every path through the undergrowth, every reliable food source, every place to hide, and every place to watch from. They carry [music] a detailed map of their world in their memory and move [music] through it with the ease of long familiarity. Squirrels mark the boundaries of their territories with scent glands located near their mouths [music] on their feet, and near their tails.

1:02:51 As they move through their range, they leave traces of themselves on branches, on tree trunks, on the ground. A scent map, invisible to us, but perfectly legible. to any squirrel that passes through. The territory of squirrel A [music] overlaps sometimes with that of squirrel B. And this overlap [music] is negotiated constantly in the small language of scent and movement and occasional direct confrontation. [music]

1:03:37 Direct confrontations between squirrels are usually brief and noisy [music] rather than physically dangerous. Two squirrels will chase each other through the branches, chattering and flicking their tails and then just as [music] quickly the pursuit ends and both animals go back to what they were doing. >> [music] >> There is a kind of efficient resolution to it. No prolonged grudge, no lasting [music] hostility, just a brief definitive argument, and then [music] the return to peace.

1:04:22 There is a species called the Bhutan giant flying squirrel. First formally described by scientists [music] only in 2021. 2021 less than 5 years ago. A squirrel with a wingspan, if you can call it that, of nearly 4 feet gliding through the mountain forests of [music] Bhutan, unrecognized by science until very recently. There are things in this world still waiting to be found, still living quietly in the high

1:05:08 forests, unnoticed. [music] Squirrels have been found at the center of several famous ecological studies. One study [music] published in the 1980s found that squirrels are capable of assessing the relative value of food sources. When given a choice between two piles of hazelnuts of different quantities, squirrels [music] consistently chose the larger pile. This sounds obvious, but demonstrating it rigorously required ruling out simpler explanations. Smell proximity familiarity

1:05:54 and the results held. Squirrels can count, or at least [music] they can distinguish between more and less. Squirrels have been observed in what researchers describe as play behavior. Young squirrels chase [music] each other, wrestle, leap at each other from branches in ways that serve no obvious survival function beyond practice [music] and apparently enjoyment. They tumble through the leaves. They [music] roll. They write themselves and do it again.

1:06:41 Play is how young animals learn the skills they will need. But it is also perhaps just what it looks like. [music] The relationship between squirrels and humans goes back further than we might think. >> [music] >> Archaeological evidence suggests that squirrels were hunted for food by early humans across much of their range. Squirrel fur was traded in medieval Europe and was [music] considered a luxury material. Ve a heraldic pattern used on coats of arms represents [music] the blue gray and white winter fur of

1:07:29 the Eurasian red squirrel which turns gray blue in winter. Cinderella's glass slipper, some historians [music] have argued, may have originally been made of squirrel fur, ve and the confusion between ve and ver, the French word for glass may be what gave us the glass slipper. Squirrels perhaps at the origin of one of the most enduring fairy tales. [music] Squirrels are sensitive to changes in the weather. Before a storm, squirrel activity often increases marketkedly as they gather and cash

1:08:16 additional food in preparation for the days of poor weather ahead. This behavior has been noted by farmers and hunters for centuries as a kind of natural barometer. A busy squirrel can mean rain on the way. They sense changes in atmospheric pressure that we cannot feel without instruments reading the air in some quiet way that science [music] is still trying to understand fully. There is something about the way squirrels move that has always attracted attention. The sudden stillness, the long pause on

1:09:04 a branch, and then the explosive movement, the leap, the sprint, [music] the disappearance into the canopy. Artists have tried to capture it for centuries. [music] Painters sculptors photographers. [music] There are squirrels on Roman mosaics. There are squirrels in medieval illuminated manuscripts. There are squirrels in Beatatri Potter and in the fairy tales of the Brothers Grim. They have always been with us. These small, [music] quick, watchful creatures

1:09:50 at the edge of our vision and [music] our imagination. The Eurasian red squirrel once lived [music] across the whole of Europe and Asia from Ireland in [music] the west to Japan in the east. Its range covered an almost unimaginable sweep of land. Mountains, plains, boreal forests, temperate woodlands. It was for most of human history one [music] of the most widely distributed mammals in the world. And now it is gone from much of that range, retreated into fragments,

1:10:36 pushed northward and upland [music] by disease, habitat loss, and the competition it could not win. The empty trees remember it in the way that empty places hold the shape of what was once there. Squirrels molt their tails more slowly than the rest [music] of their body. The tail, which is so important for balance, communication, and warmth, is replaced carefully. a few hairs at a time. A squirrel with a damaged or lost tail

1:11:21 [music] will often struggle to maintain balance on branches and may have difficulty in colder weather. The tail is not a luxury. It is a necessity. And yet squirrels with partial tails are sometimes seen adapting, adjusting, continuing. [music] Animals are remarkably good at continuing. Some squirrels sunbathe. On warm mornings, you may see a squirrel stretched out on a flat branch or a fence post, [music]

1:12:07 legs dangling, body flat against the warm surface, entirely still. It looks like a very small, very content [music] animal doing absolutely nothing, which is exactly what it is. The warmth of the sun is useful for regulating body temperature [music] and perhaps for killing skin parasites, but it also just seems to feel good. The squirrel lies there in the sun, eyes [music] half closed, breathing slowly at rest.

1:12:52 Squirrels can smell water. They can locate [music] sources of fresh water, even when those sources are hidden or distant [music] guided by a scent that we can barely detect. In drought conditions, squirrels have been observed traveling far beyond their usual home range to find [music] water. They will drink from puddles, from streams, from bird baths, from the dew [music] collected on leaves. They also get moisture from the food they eat. Fresh berries, [music] green plant material, mushrooms, absorbing water

1:13:38 not just from drinking but from every meal. >> [music] >> There is a squirrel in North America called the Douglas squirrel, also known as the chory. [music] It has a loud, scolding call that carries through the old growth forests of the Pacific Northwest. John Mureer, who walked those forests 150 years ago, wrote about the chikory with great affection. He described its call as filling [music] the forest, with a kind of wildness, a vivid, insistent liveless.

1:14:25 He called it simply the squirrel [music] that can't be quiet. In a forest that could sometimes feel cathedral still, the chory [music] kept things awake. Squirrels play a role in the spread of seeds beyond just the nuts and acorns they bury. They also eat berries and pass the seeds through their digestive system, depositing them far from the parent plant in a pocket of rich fertilized earth. They carry seeds on their fur, picking them [music] up from burrs and sticky seed heads and dropping them elsewhere.

1:15:13 [music] And they occasionally steal cashed seeds from birds, particularly jays, who themselves are seed dispersers of extraordinary importance. The squirrel and the j compete and cooperate simultaneously, each taking from the other and giving back in ways neither of them intends. The pine squirrel of North America, [music] sometimes called the red squirrel, though it is a different species from the European red squirrel, stores its winter food differently [music] from most squirrels.

1:15:58 Rather than scatter hoarding thousands of individual caches, it creates a single large midden, a pile of pine cones stored in a cool, damp [music] spot, like a hollow log or a cool corner of its territory. These mittens can contain thousands of pine cones built up over several years. The cool, slightly moist conditions keep the cones from opening and releasing [music] their seeds prematurely. The squirrel is not just storing [music] food. It is in effect

1:16:46 refrigerating it. Squirrels have been observed grieving. This is a careful word to [music] use and scientists are careful in using it. But there are documented observations of mother squirrels returning repeatedly to the site where a kitten has died and of squirrels showing behaviors consistent with distress after the loss of a companion. Whether this constitutes grief in the way humans understand it is uncertain, but the behaviors are there and they suggest

1:17:33 an interior life more complex [music] than we often assume. Squirrels have been part of human food chains in many cultures for thousands of years. In parts of Appalachia in the United States [music] squirrel remains a traditional food and squirrel hunting [music] is still practiced today. In many indigenous cultures, the red squirrel was a reliable and valued source of meat, particularly in winter when other game was scarce. The bones of squirrels appear in archaeological midens

1:18:20 across the northern hemisphere. Quiet evidence of a long partnership between two [music] species sharing the same landscape. Squirrels have been kept as pets throughout history. In the 18th century, pet squirrels were fashionable among wealthy households in Britain and [music] America. They were kept on chains or in small cages and portraits of children with pet squirrels were a common artistic subject. One such portrait, Boy with a Squirrel, was painted by the American artist John Singleton [music]

1:19:07 CPPley in 1765 and is now in the Museum of Fine Arts in Boston. In it, a young boy holds [music] a chain attached to a gray squirrel sitting on a table. The squirrel is utterly calm, [music] utterly itself, sitting in a drawing room in the new world as though [music] it has always belonged there. Squirrels can detect [music] a predator by its smell alone and will change their behavior accordingly. [music] Experiments have shown that squirrels,

1:19:53 exposed only to the scent of a fox, no fox visible, no sound, no movement, will increase their vigilance, spend more time elevated, [music] and cash food at greater distances. The information arrives as a smell and the whole behavioral response reorganizes itself around that information. One scent, one quiet change in the air and a small life restructures its choices. Squirrels have excellent object permanence. Object permanence is the understanding

1:20:39 [music] that something continues to exist even when you cannot see it. Infants develop this understanding [music] slowly. Squirrels demonstrated through their caching behavior. Burying something, moving away from it, and returning to it much later is only possible if the animal understands that the buried thing still exists. The nut [music] is in the ground. The ground is there. The nut will still be there. A simple idea and a quietly [music] important one.

1:21:27 The American red squirrel is fiercely territorial, more so than most tree squirrels. A single red squirrel will defend its entire home range against all intruders, including squirrels of its own species. It patrols the borders of its territory, calling loudly from prominent perches, making its presence known. It will chase intruders vigorously [music] and persistently, sometimes for minutes at a time. And it is [music] not bluffing. The territory matters.

1:22:14 The midden is there. The miden represents survival. The ferocity is proportionate. [music] Squirrels can swim, not gracefully, not by [music] preference, but competently. They paddle with their front legs and [music] use their tail as a rudder, keeping their head above water. [music] They have been observed crossing rivers and lakes when food or territory demands it. In some instances, large populations of squirrels have been reported [music]

1:22:59 crossing rivers together during migrations driven by food shortages. A wave of gray bodies moving across the water, determined, unhurried [music] going somewhere that has more to offer. Such mass migrations of squirrels were recorded in North America in the 19th century when accounts described rivers of squirrels crossing the Ohio River in numbers. so [music] large they darkened the water. These accounts were reported in the journals and newspapers of the

1:23:45 time with a mixture [music] of wonder and alarm. Entire populations moving at once, the forests emptying in one direction and filling in another. Today, [music] such migrations are rare because the forests themselves are fragmented and the continuous landscape that made those journeys possible [music] is largely gone. The hearing of a squirrel is [music] acute and directional. They can locate sounds with great precision, turning their head to [music] pinpoint

1:24:31 the source. of a rustle or a crack. [music] And their ears, though small and rounded, can move independently, each ear [music] swiveing slightly to gather information from different angles. A squirrel listening in a quiet forest is hearing [music] things you are not. The snap of a twig 40 ft away. The rustle of wings from a hawk circling overhead. [clears throat] All of it arriving and being processed [music] at once.

1:25:17 A squirrel's lifespan is closely related to its environment. In areas with many predators, squirrels live shorter lives [music] on average. In areas where predators are scarce, they tend to live longer, but may suffer more from disease or from the effects of overcrowding. The predators [music] keep the population from growing too large. The food supply determines whether there is enough to go around. Balance is maintained not by any single force but by the interaction of many forces all pressing against each [music]

1:26:04 other. The smallest tree squirrels in North America belong to the genus Tamascurus. [music] The pine squirrels of this genus are rarely more than 30 cm long, including [music] tail, and weigh just over 100 g. Yet, they defend territories and maintain food stores [music] that sustain them through months of deep snow. In the algebra of nature, small does not mean fragile. Small means efficient. Small means requiring less.

1:26:52 Small means finding [music] a way where a larger animal could not. [music] Squirrels have a nictitating membrane, a third eyelid. [music] It moves horizontally across the eye from the inner corner outward providing an extra layer of protection and keeping the eye moist. It is semi-transparent so the squirrel can see through it albeit somewhat blurrily. It closes during high-speed movement [music] through branches, protecting the eye from twigs and [music] debris.

1:27:37 a windshield built into the eye itself. The elegance of evolution is often in these tiny unassuming details. The solution arrived at through millions of small experiments, most of which failed and one of which worked. Squirrels have a gestation period of around 38 to 46 days depending on species. For most squirrels, the young are born twice a year, once in late winter or early spring and again in midsummer. [music]

1:28:23 This means a female squirrel may raise two litters in a single year, [music] each of two to six young. The population can grow quickly in years of abundance when the oak trees produce a [music] heavy crop. When the weather is mild, when food is plentiful, and it falls back in the hard years, quietly without drama, adjusting [music] to what the world can offer. Squirrels chew on bones. [music] Not commonly and not exclusively, but they have been observed gnoring on

1:29:09 [music] shed antlers, on bones left by other animals, [music] and occasionally on the shells of birds eggs. This is thought to be a way of supplementing their diet with calcium and other minerals. The plant matter that makes up most of their diet does not always provide sufficient minerals. And so they find other sources where they can. A reminder that even a familiar behavior knowing which we associate with nuts and seeds can serve purposes more varied than we first assume.

1:29:54 [music] There are squirrels with stripes. The chipmunk, which belongs [music] to the squirrel family, carries those distinctive stripes along its back and face. >> [music] >> The stripes are thought to provide camouflage in the dappled light of the forest floor, breaking up the outline of the animal against a background of leaf litter and shadow. In the shade of the forest, a striped animal can be remarkably hard to see. What is bold in full sun becomes invisible in the right conditions. Chipmunks, like ground squirrels,

1:30:42 hibernate through the winter. But they hibernate [music] differently from bears, waking periodically to eat from caches of food stored in their burrow. They spend the summer and autumn [music] carrying food in large cheek pouches back to the burrow, filling it steadily for the months ahead. The cheek pouches can stretch to nearly three times the width of the animal's [music] head. Stuffed full of seeds, the chipmunk looks briefly improbable, round [music] cheicked and topheavy before disappearing underground to deposit its load.

1:31:28 [music] A squirrel's teeth are orange, not white like ours. The outer layer of the incizers contains iron which gives them a warm amber orange color and makes them harder than the inner layer. When the squirrel nor the softer [music] inner surface wears down faster than the ironrich outer surface, creating a selfsharpening edge that always maintains a fine point. A chisel that sharpens itself by being used. The orange color is not incidental. It is structural.

1:32:15 It is the [music] iron that makes them strong. Squirrels can shake their tails in different ways for different purposes. >> [music] >> A slow rolling wave through the tail is a communication of general alertness. [music] A quick sharp flick signals something more urgent, irritation agitation a warning to others nearby. A full fast flagging motion [music] is the strongest signal. High alarm, something immediately dangerous. Prepare to move.

1:33:02 You can read a squirrel's state by watching its tail [music] if you know what you are looking for. Squirrels and birds have a complex relationship. Squirrels rob bird nests of [music] eggs and nestlings. This makes them unpopular with many birds and many bird watchers. But they also clear and cach seeds that birds [music] depend on in winter and the alarm calls of squirrels, alert birds to the presence of predators, providing a warning system that benefits many species.

1:33:49 Small birds will often listen for squirrel alarm calls and respond to them as they would to the alarm calls of their own species. They are eavesdropping on the squirrels communication and using it for their own survival. The skin between the toes of tree squirrels is slightly webbed, increasing the surface area [music] that can grip bark. On smooth wood, this webbing allows the toes to spread and press flat [music] against the surface, creating friction through contact.

1:34:34 On rough bark, the claws dig in at multiple angles simultaneously. A squirrel moving up a tree trunk at full speed is gripping with such precision and such strength that it appears effortless. [music] But it is not effortless. It is the result of millions of years of refined design. all pointed at the question of how to climb quickly, [music] safely in all conditions. Squirrels manufacture a paste from their own saliva and certain compounds found in snake

1:35:20 skin. Researchers have observed squirrels chewing on shed snake skin and then licking their own fur, apparently applying the snakes scented saliva [music] to themselves. The hypothesis is that this masks their own scent [music] with the smell of a predator, making them less recognizable to other predators [music] that might associate the snake smell with danger. A squirrel using the chemical signature of a snake as a kind of camouflage, wearing the smell of one predator to

1:36:05 confuse another. There is a species of squirrel that lives only on a single island [music] off the coast of Java. The Mentawi squirrel lives on the Mentoai Islands [music] and exists nowhere else on Earth. Its entire world is a small patch of island forest bounded on all sides by the sea. [music] It has never crossed to the mainland. It has never needed to. Everything it requires is contained within those few square miles of trees.

1:36:51 A whole species, a whole life contained in something that small. [music] Squirrels have inspired scientists who study robotics. The way a squirrel navigates a complex three-dimensional environment, jumping gripping falling recovering in real time with minimal error, has led researchers to study squirrel locomotion in hopes of building robots that can do the same. How does the squirrel decide? in a fraction of a second. How much

1:37:38 force to apply to a jump when the branch ahead is flexible? How does it adjust midair when the landing is different [music] from what it expected? These questions, [music] it turns out, are remarkably difficult to answer. and the squirrel solves them thousands of times a day without apparent effort. Research has shown that squirrels often adjust their jumping strategy based on experience with specific branches. [music] A squirrel that has jumped from a particular bendy branch before will jump

1:38:24 with more force on [music] subsequent attempts. Having learned that the branch deflects downward before [music] it springs back, it corrects for the anticipated deflection. It is learning in real [music] time, updating its model of the world with every jump. This [music] kind of adaptive learning based on individual experience is exactly what roboticists are trying to build [music] into their machines. The color of a squirrel's fur can vary, [music] not just between species, but within them.

1:39:11 Gray squirrels in Britain, for instance, [music] range from pale silver gray to almost brown with some individuals carrying a faint reddish tinge [music] on their flanks or faces. This variation is genetic, a quiet [music] pallet of difference within what appears from a distance to be a uniform species. [music] Each squirrel on closer inspection is itself [music] distinct, individual, recognizably particular

1:39:56 if you [music] look carefully enough. There is a behavior called the squirrel twostep. When a squirrel is being approached [music] and is uncertain whether to flee, it may move in one direction, then quickly reverse, then move in another direction. [music] This unpredictable zigzag movement is a predator confusion strategy. It makes the squirrel's next move impossible to predict. It is also why squirrels sometimes run in front of cars. The car comes close. The squirrel performs its confusion

1:40:43 maneuver. But the car, unlike a fox or a hawk, cannot [music] change direction. The squirrel's strategy evolved over millions of years against living predators [music] does not work against something that has no intentions. Squirrels in urban environments [music] have been found to have larger home ranges than squirrels in natural forests. This is because the food resources in cities [music] are distributed differently patchy unpredictable [music] dependent on the habits of humans who

1:41:31 may or may not leave food out. The urban squirrel [music] must cover more ground to find what it

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