Squirrels are members of the family Sciuridae, consisting of small or medium-size rodents. The family includes tree squirrels, ground squirrels, chipmunks, marmots (including woodchucks), flying squirrels, and prairie dogs. Squirrels are indigenous to the Americas, Eurasia, and Africa, and have been introduced to Australia. The earliest known squirrels date from the Eocene and are most closely related to the mountain beaver and to the dormouse among living rodent families.
Squirrels are generally small animals, ranging in size from the African pygmy squirrel at 7–10 cm (2.8–3.9 in) in length and just 10 g (0.35 oz) in weight, to the Alpine marmot, which is 53–73 cm (21–29 in) long and weighs from 5 to 8 kg (11 to 18 lb). Squirrels typically have slender bodies with bushy tails and large eyes. In general, their fur is soft and silky, although much thicker in some species than others. The color of squirrels is highly variable between—and often even within—species.
In general, the hind limbs are longer than the fore limbs, and they have four or five toes on each paw. Their paws include an often poorly developed thumb, and have soft pads on the undersides. Unlike most mammals, Tree squirrels can descend a tree head-first. They do so by rotating their ankles 180 degrees so the hind paws are backward-pointing and can grip the tree bark.