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Squirrels are members of the family Sciuridae (), a family that includes small or medium-sized . The squirrel family includes , (including and , among others), and . Squirrels are indigenous to the Americas, Eurasia, and Africa, and were introduced by humans to Australia. The earliest known fossilized squirrels date from the Eocene epoch, and among other living rodent families, the squirrels are most closely related to the mountain beaver and dormouse.
The Old English word for the squirrel, ācweorna, survived into Middle English as aquerne before being replaced. The Old English word is of Common Germanic origin, of which are still used in other Germanic languages, including the German language Eichhörnchen (diminutive of Eichhorn, which is not as frequently used); the Norwegian ikorn/ekorn; the Dutch language eekhoorn; the Swedish language ekorre and the Danish language egern.
A group of squirrels is called a "dray" or a "scurry". Universe in Your Pocket by Joel Levy, published by Barnes & Noble, Inc.
In most squirrel species, the hind limbs are longer than the forelimbs, while all species have either four or five toes on each foot. The feet, which include an often poorly developed thumb, have soft pads on the undersidesMilton (1984) and versatile, sturdy for grasping and tree climbing. Tree squirrels, unlike most mammals, can descend a tree headfirst. They do so by rotating their ankles 180 degrees, enabling the hind feet to point backward and thus grip the tree bark from the opposite direction. (2025). 9781421404691, Johns Hopkins University Press. ISBN 9781421404691
The teeth of sciurids follow the typical rodent pattern, with large (for gnawing) that grow throughout life, and cheek teeth (for grinding) that are set back behind a wide gap, or diastema. The typical dentition for sciurids is . The Beginning of the Age of Mammals Kenneth D. Rose (2006) p. 326
The hairs from squirrel tails are prized in fly fishing when tying fishing flies. Squirrel hair is very fine, making it better for tying fishing flies.
When the squirrel sits upright, its tail folded up its back may stop predators looking from behind from seeing the characteristic shape of a small mammal.
Many juvenile squirrels die in the first year of life. Adult squirrels can have a lifespan of 5 to 10 years in the wild. Some can survive 10 to 20 years in captivity. (2025). 9781421404691, Johns Hopkins University Press. ISBN 9781421404691 Premature death may occur when a nest falls from the tree, in which case the mother may abandon her young if their body temperature is not correct. Many such baby squirrels have been rescued and fostered by a professional wildlife rehabilitator until they could be safely returned to the wild, although the density of squirrel populations in many places and the constant care required by premature squirrels means that few rehabilitators are willing to spend their time doing this and such animals are routinely euthanized instead.
Ground squirrels and tree squirrels are usually either Diurnality or crepuscular, while the flying squirrels tend to be Nocturnality—except for lactating flying squirrels and their young, which have a period of diurnality during the summer.
During hot periods, squirrels have been documented to splooting, or lay their stomachs down on cool surfaces.
Squirrels, like other rodents, employ species-specific strategies to store food, buffering against periods of scarcity. In temperate regions, squirrels commonly cache nuts beneath leaf litter, inside hollow trees, or underground. However, in subtropical and humid environments, traditional caching can lead to mold growth, decomposition, or premature germination. To counteract these challenges, some squirrels, particularly in subtropical zones, hang nuts or mushrooms on tree branches. This behavior, believed to minimize fungal infections and reduce the risk of food loss, also inadvertently aids certain trees, like Cyclobalanopsis, in expanding their range, with forgotten or dislodged nuts sprouting in new locations, influencing forest ecology. Two species of flying squirrel, the particolored flying squirrel and Hainan flying squirrel aid such cacheing by carving grooves into the nuts to fix the nuts tightly between small intersecting twigs, akin to the mortise-tenon joint in carpentry.
Squirrels, like pigeons, feral cats, and other fauna, are synanthropes, in that they benefit and thrive from their interaction in human environments. This gradual process of successful interaction is called synurbanization, wherein squirrels lose their inherent fear of humans in an urban area environment. When squirrels were almost completely eradicated during the Industrial Revolution in New York, they were later re-introduced to "entertain and remind" humans of nature. The squirrel blended into the urban environment so efficiently that when synanthropic behavior stops (e.g. people do not leave trash outside during particularly cold winters), they can become aggressive in their search for food.
Aggression and predatory behavior has been observed in various species of ground squirrels, in particular the thirteen-lined ground squirrel. For example, Bernard Bailey, a scientist in the 1920s, observed a thirteen-lined ground squirrel preying upon a young chicken. Wistrand reported seeing this same species eating a freshly killed snake. Sometimes squirrels prey on an atypical animal: a pack of black squirrels in 2005 killed and ate a large stray dog in Lazo, Russia. Squirrel attacks on humans are exceedingly rare.
Whitaker examined the stomachs of 139 thirteen-lined ground squirrels and found bird flesh in four of the specimens and the remains of a short-tailed shrew in one; Bradley, examining the stomachs of white-tailed antelope squirrels, found at least 10% of his 609 specimens' stomachs contained some type of vertebrate, mostly and rodents. Morgart observed a white-tailed antelope squirrel capturing and eating a silky pocket mouse.
A variety of fossil squirrels, from the latest Eocene to the Miocene, have not been assigned with certainty to any living lineage. At least some of these probably were variants of the oldest basal "protosquirrels" (in the sense that they lacked the full range of living squirrels' autapomorphy). The distribution and diversity of such ancient and ancestral forms suggest the squirrels as a group may have originated in North America.Steppan & Hamm (2006)
Apart from these sometimes little-known fossil forms, the phylogeny of the living squirrels is fairly straightforward. The three main lineages are the Ratufinae (Oriental giant squirrels), Sciurillinae and all other subfamilies. The Ratufinae contain a mere handful of living species in tropical Asia. The neotropical pygmy squirrel of tropical South America is the sole living member of the Sciurillinae. The third lineage, by far the largest, has a near-cosmopolitan distribution. This further supports the hypothesis that the common ancestor of all squirrels, living and fossil, lived in North America, as these three most ancient lineages seem to have radiated from there; if squirrels had originated in Eurasia, for example, one would expect quite ancient lineages in Africa, but African squirrels seem to be of more recent origin.
The main group of squirrels can be split into five subfamilies: the Callosciurinae, 60 species mostly found in South East Asia; the Ratufinae, 4 cat-sized species found in South Asia and southeast Asia; the Sciurinae, which contains the (Pteromyini) and the Sciurini, 83 species found worldwide;Steppan, S. J. B. L. Storz, and R. S. Hoffmann. 2004. Nuclear DNA phylogeny of the squirrels (Mammalia: Rodentia) and the evolution of arboreality from c-myc and RAG1. Molecular Phylogenetics and Evolution, 30:703-719. Sciurillinae, a single species; and Xerinae, which includes three tribes of mostly terrestrial squirrels, including the Marmotini (, , , and other Holarctic ground squirrels), Xerini (African and some Eurasian ground squirrels), and Protoxerini (African tree squirrels).
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