Squirrels are members of the family Sciuridae, a family that includes small or medium-size rodents. The squirrel family includes tree squirrels, ground squirrels, chipmunks, marmots, flying squirrels, Squirrels are indigenous to the Americas, Eurasia, and Africa, and were introduced by humans to Australia. The earliest known date from the Eocene period and are most closely related to the mountain beaver. That word squirrel, first attested in 1327, comes from the Anglo-Norman esquirel which is from the Old French escurel and this Latin word was borrowed from the Ancient Greek word σκίουρος, skiouros, which means shadow-tailed, referring to the bushy appendage possessed by many of its members. The native Old English word for the squirrel, ācweorna, survived only into Middle English before being replaced, 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 limbs are longer than the fore limbs. Their paws include a poorly developed thumb, and have soft pads on the undersides. Unlike most mammals, Tree squirrels can descend a tree head-first and they do so by rotating their ankles 180 degrees so the hind paws are backward-pointing and can grip the tree bark. Squirrels live in almost every habitat from tropical rainforest to desert, avoiding only the high polar regions. They are predominantly herbivorous, subsisting on seeds and nuts, but many will eat insects, as their large eyes indicate, in general squirrels have an excellent sense of vision, which is especially important for tree-dwelling species. They also have very versatile and sturdy claws for grasping and climbing, many also have a good sense of touch, with vibrissae on their heads and limbs. The teeth of sciurids follow the typical rodent pattern, with large gnawing incisors that grow throughout life, the typical dental formula for sciurids is 188.8.131.52.0.1.3. 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, Squirrels breed once or twice a year and give birth to a varying number of young after three to six weeks, depending on species. The young are naked, toothless, and blind. In most species of squirrel, only the female looks after the young, in general, ground-dwelling species are social animals, often living in well-developed colonies, but the tree-dwelling species are more solitary. Squirrels cannot digest cellulose, so they must rely on foods rich in protein, carbohydrates, during these times, squirrels rely heavily on the buds of trees
Various members of the family Sciuridae
Reaching out for food on a garden bird feeder, this squirrel can rotate its hind feet, allowing it to descend a tree head-first.