Squirrel

• 1. Introduction

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  Squirrels belong to a family called Sciuridae, a name that traces its roots back to Ancient Greek: skia, meaning shade, and oura, meaning tail. The long bushy tail that gives squirrels their name is also one of their most versatile tools. It keeps off rain and wind, cools the animal by pumping blood through it in hot weather, acts as a counterbalance when leaping between branches, and even serves as a parachute during falls. A creature shaped by millions of years of evolution carries its entire story on its back.

  The family spans an astonishing range. Fossil records push squirrel history back to the Eocene epoch, with the oldest known squirrel fossil, called Hesperopetes, dating to somewhere between 40 and 35 million years ago. Today the living members number around 285 species, spread across the Americas, Eurasia, and Africa, with humans having introduced them to Australia as well. From the tiny African pygmy squirrel to the Bhutan giant flying squirrel stretching up to 1.27 meters in length, this is not one animal but an entire world of animals wearing a family resemblance.

  What holds that world together? What does it take to be a squirrel, across habitats ranging from tropical rainforests to semiarid deserts? And why, after tens of millions of years, have these rodents ended up so deeply woven into human life?

• 2. A Name With a Shadow

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  The English word squirrel entered the written record in 1327. It arrived via Anglo-Norman as esquirel, which itself came from the Old French escureil, a descendant of the Latin sciurus. Behind that Latin word stood the Ancient Greek skiouros, built from the words for shade and tail, a reference to the animals' long bushy tails arching up over their backs. When a squirrel sits upright, that tail folded against its spine may disguise the silhouette of a small mammal, helping to conceal it from predators approaching from behind.

  Before esquirel arrived, Old English speakers used the word ácweorna, a term of Common Germanic origin. Versions of that older word survive in languages still spoken today: the German Eichhörnchen, the Norwegian ikorn or ekorn, the Dutch eekhoorn, the Swedish ekorre, and the Danish egern. English chose the French path; its Germanic neighbors kept the older thread.

  When a group of squirrels gathers, English offers two collective nouns: a dray or a scurry. The scurry, in particular, captures something true about how these animals move through the world, always seeming to be in motion, always somewhere between one place and the next.

• 3. The Architecture of a Squirrel

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  Tree squirrels can do something almost no other mammal can manage: descend a tree headfirst. They achieve this by rotating their ankles 180 degrees, which allows the hind feet to point backward and grip the bark from the opposite direction. That single anatomical adaptation opens up a whole vertical world.

  Their teeth follow the standard rodent blueprint: large incisors at the front that never stop growing, used for gnawing, and cheek teeth further back, set behind a wide gap called a diastema, used for grinding. Their fur is soft and silky in most species, though considerably thicker in others, and the coat color varies widely not just between species but sometimes within a single species.

  The hind limbs are longer than the forelimbs in most squirrel species, and all four feet carry either four or five toes, each tipped with sturdy claws suited for gripping. The feet have soft pads on the undersides. There is also usually a thumb, though it tends to be poorly developed. Vibrissae, the sensory whiskers, appear not just on the head but also on the limbs, giving squirrels a detailed tactile picture of whatever surface they are moving across.

• 4. How Squirrels Eat and Store

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  Squirrels cannot digest cellulose, which rules out the bulk of most plants as food. What they can process are foods dense in protein, carbohydrates, and fats: nuts, seeds, conifer cones, fruits, fungi, green vegetation, and, when pressed, insects and small vertebrates. The thirteen-lined ground squirrel, in particular, has been documented in predatory behavior on multiple occasions. Bernard Bailey, a scientist working in the 1920s, observed one preying on a young chicken. Another researcher named Wistrand later saw the same species eating a freshly killed snake.

  In 2005, a pack of black squirrels in Lazo, Russia, killed and ate a large stray dog, an event striking enough to be recorded and noted by researchers studying squirrel behavior. Whitaker examined the stomachs of 139 thirteen-lined ground squirrels and found bird flesh in four of them, plus the remains of a short-tailed shrew in one. Bradley, looking at 609 white-tailed antelope squirrel stomachs, found that at least 10% of those specimens contained some type of vertebrate, mostly lizards and rodents.

  Early spring is the hardest season for squirrels in temperate regions. The nuts they buried during autumn are beginning to sprout, making them unavailable to eat, while new food sources have not yet appeared. During this gap, squirrels rely heavily on tree buds. In subtropical and humid zones, the usual habit of burying nuts creates its own problem: the food molds, decomposes, or sprouts too soon. Some squirrels in these zones hang nuts and mushrooms on tree branches instead, a strategy thought to reduce fungal infection and food loss. The particolored flying squirrel and the Hainan flying squirrel go further still, carving grooves into nuts to wedge them tightly between intersecting twigs, a technique that echoes the mortise-tenon joint used in carpentry.

• 5. Living With People

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  When squirrels were nearly wiped out in New York during the Industrial Revolution, they were deliberately brought back, introduced to, as the record puts it, entertain and remind people of nature. That unusual episode sits at the center of what researchers call synurbanization: the process by which squirrels gradually lose their fear of humans and come to thrive in urban environments. Squirrels, alongside pigeons and feral cats, are classified as synanthropes, organisms that benefit from proximity to human settlements.

  Urban squirrel populations depend heavily on food discarded or provided by people. During harsh winters, when that supply drops, researchers have observed that competition among squirrels intensifies and aggressive resource guarding increases. The high behavioral flexibility of squirrels is what makes this rapid adaptation possible.

  Squirrel attacks on humans are recorded as exceedingly rare. The relationship runs largely in the opposite direction: squirrels benefit from human environments in ways that continue to shape their behavior across generations.

• 6. Generations and Habitats

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  Squirrels occupy nearly every terrestrial habitat, with two exceptions: the high polar regions and the driest deserts. Gestation takes anywhere from three to six weeks depending on species, and the newborns arrive naked, blind, and toothless, entirely dependent on the mother. In most squirrel species, the female alone raises the young. The offspring are weaned at six to ten weeks and reach sexual maturity by the end of their first year.

  Many juvenile squirrels die within their first year of life. Adults in the wild can expect to live five to ten years; some individuals kept in captivity have survived for ten to twenty years. Premature death can come from a nest falling out of a tree, an event that may lead the mother to abandon the young if their body temperature has dropped too far. Wildlife rehabilitators have stepped in to raise such animals, though the constant care squirrel infants require means many facilities turn them away, and the animals are routinely euthanized.

  Ground-dwelling squirrel species tend to be social, living in organized colonies, while tree-dwelling species are more solitary. Flying squirrels are mostly nocturnal, a reversal of the diurnal or crepuscular patterns seen in ground and tree squirrels, with one notable exception: lactating flying squirrels and their young shift to daytime activity during the summer months.

• 7. Origins in Deep Time

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  The squirrel fossil record points to North America as the likely origin point for the entire family. Hesperopetes, the oldest named fossil squirrel, dates to the Chadronian stage of the late Eocene, roughly 40-35 million years ago, and resembles modern flying squirrels in some respects. A range of other fossil squirrels from the late Eocene through the Miocene cannot yet be confidently assigned to any living lineage; some were probably early variants of what researchers loosely call protosquirrels.

  The three most ancient living lineages offer additional support for a North American origin. One of them, the Ratufinae or Oriental giant squirrels, contains only a handful of living species in tropical Asia. Another, the Sciurillinae, contains a single living species: the neotropical pygmy squirrel of tropical South America. The third and largest lineage has spread across nearly the whole world. If squirrels had originated in Eurasia, the reasoning goes, one would expect to find very ancient squirrel lineages in Africa. Instead, African squirrel groups appear to be of more recent origin.

  Today the living squirrels are organized into five subfamilies, with around 58 genera. The Callosciurinae count about 60 species in South East Asia. The Sciurinae hold the flying squirrels and tree squirrels, some 83 species distributed worldwide. The Xerinae, the mostly terrestrial branch, include marmots, chipmunks, and prairie dogs, grouped together in about 90 species under the tribe Marmotini. Within the debris of forest floors, across Asian canopies, and deep in burrow systems on every inhabited continent, the family Sciuridae has found a way to persist, and to diversify, across tens of millions of years.