Squirrels are members of the family Sciuridae, a family that includes small or medium-size rodents. The squirrel family includes tree squirrels, ground squirrels (including chipmunks and prairie dogs, among others), and flying squirrels. Squirrels are indigenous to the Americas, Eurasia, and Africa, and were introduced by humans to Australia.[1] 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 to the dormice.[citation needed]
The word squirrel, first attested in 1327, comes from the Anglo-Norman esquirel which is from the Old French escurel, the reflex of a Latin word sciurus, which was taken from the Ancient Greek word σκίουρος (skiouros; from σκία-ουρος) 'shadow-tailed', referring to the long bushy tail which many of its members have.[2][3]
The native Old English word for the squirrel, ācweorna, survived only into Middle English (as aquerne) before being replaced.[4] The Old English word is of Common Germanic origin, cognates of which are still used in other Germanic languages, including the German Eichhörnchen (diminutive of Eichhorn, which is not as frequently used), the Norwegian ikorn/ekorn, the Dutch eekhoorn, the Swedish ekorre and the Danish egern.
Squirrels are generally small animals, ranging in size from the African pygmy squirrel and least pygmy squirrel at 10–14 cm (3.9–5.5 in) in total length and just 12–26 g (0.42–0.92 oz) in weight,[7][8] to the Bhutan giant flying squirrel at up to 1.27 m (4 ft 2 in) in total length,[9] and several marmot species, which can weigh 8 kg (18 lb) or more.[10][11] Squirrels typically have slender bodies with very long very bushy tails and large eyes. In general, their fur is soft and silky, though much thicker in some species than others. The coat color of squirrels is highly variable between—and often even within—species.[12]
In most squirrel species, the hind limbs are longer than the fore limbs, 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 undersides[13] and versatile, sturdy claws for grasping and climbing.[14] Tree squirrels, unlike most mammals, can descend a tree head-first. 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.[15]
Squirrels live in almost every habitat, from tropical rainforest to semiarid desert, avoiding only the high polar regions and the driest of deserts. They are predominantly herbivorous, subsisting on seeds and nuts, but many will eat insects and even small vertebrates.[16]