Generally, any rodent of the family Sciuridae; the name is sometimes restricted to the familiar, bushy-tailed, arboreal species. Squirrels are found almost worldwide, in a variety of habitats, including forests, deserts, plains, and tundra. There are about 50 genera and 260 species, among them the ground squirrels (see photograph), marmots, and chipmunks. Many squirrels are arboreal. Some (as marmots) are terrestrial; others (flying squirrels) glide from tree to tree by means of furry flaps of skin connecting their forelegs and hind legs. All squirrels have strong hindlegs and well-developed, hairy tails. They differ widely in colour and markings and vary in form from slender (flying squirrels) to stout (marmots). Total length ranges from about 10 centimetres (4 inches) in the African pygmy squirrel (Myosciurus pumilio) to about 90 cm in the giant squirrels (Ratufa) of Asia. All squirrels except the gliders are diurnal. Tree dwellers are agile, live in tree hollows or nests built of leaves and twigs, and are usually active throughout the year. Ground dwellers live in burrows, and many become dormant in winter (hibernate) or summer (estivate). Squirrels are primarily vegetarian and are noted for their fondness for seeds and nuts. Some species eat insects or supplement their diets with animal protein. Females bear one or more litters a year, of 1 to 15 young; gestation is 22 to 45 days.
Species found in Arizona include the abert, pine, and grey squirrels.