Squirrel, (family Sciuridae), generally, any of the 50 genera and 268 species of rodents whose common name is derived from the Greek skiouros, meaning “shade tail,” which describes one of the most conspicuous and recognizable features of these small mammals. These distinctive animals occupy a range of ecological niches worldwide virtually anywhere there is vegetation. The squirrel family includes ground squirrels, chipmunks, marmots, prairie dogs, and flying squirrels, but to most people squirrel refers to the 122 species of tree squirrels, which belong to 22 genera of the subfamily Sciurinae. The North American gray squirrel (Sciurus carolinensis) has adapted to urban and suburban areas where it is regarded as aesthetic or as a minor annoyance. In northern Europe the red squirrel (S. vulgaris) is valued for its soft, thick fur. Villagers in tropical forests keep squirrels as pets. Most species are hunted for food.

General Features
Tree squirrels have slender, lanky bodies, long, muscular limbs, and furred feet. The forefeet have four long digits plus a short, stubby thumb, and the fivetoed hind feet are narrow or moderately wide. The bald soles of the feet take the form of prominent, fleshy pads. Because the ankle joints are flexible and can be rotated, squirrels can rapidly descend trees headfirst with the hind feet splayed flat against the trunk. Their large, bright eyes convey an alert demeanour, and the broad, short head tapers to a blunt muzzle adorned with long whiskers. The rounded ears, small in relation to body size, are densely covered with short, fine hairs, which form a long tuft at the tips of the ears in some species. The tail is about as long as head and body or appreciably longer. Furred from base to tip, the tail appears bushy and cylindrical when the hairs grow evenly around the tail; the tail appears flatter if the fur originates only from opposite sides. Claws are large, strong, curved, and very sharp, which enables tree squirrels to navigate vertical surfaces and slim branches.

Variation in body size is considerable. Largest are the four species of Oriental giant squirrels (genus Ratufa) native to the tropical forests of Southeast Asia. Weighing 1.5 to 3 kg (3 to almost 7 pounds), it has a body length of 25 to 46 cm (about 10 to 18 inches) and a tail about as long. Two species of pygmy squirrels are the smallest: the neotropical pygmy squirrel (Sciurillus pusillus) of the Amazon Basin weighs 33 to 45 grams (1 to 1.5 ounces), with a body 9 to 12 cm long and an equally long tail; but the African pygmy squirrel (Myosciurus pumilio) of the West African tropical forests is even smaller, at 13 to 20 grams, with a body length of 6 to 8 cm and a somewhat shorter tail.

Squirrels’ soft, dense fur is moderately long in most species but can be very long and almost shaggy in some. Colour is extraordinarily variable. Some species are plain, covered in one or two solid shades of brown or gray. A few species are striped along the sides and back; sometimes the head is also striped. Tropical species exhibit combinations of white, gray, yellow, orange, red, maroon, brown, and black, yielding a variety of complex coat patterns.

Natural History
All tree squirrels are diurnal and arboreal, but the range of vertical activity in species differs widely, especially among those living in tropical rainforests. Some, such as the Oriental giant squirrels (genus Ratufa) and the African giant squirrels (genus Protoxerus), rarely descend from the high canopy. Others, like the pygmy squirrel of Sulawesi (Prosciurillus murinus), travel and forage at intermediate levels between ground and canopy. Some large tropical squirrels, such as the Sulawesi giant squirrel (Rubrisciurus rubriventer) and the northern Amazon red squirrel (Sciurus igniventris), nest at middle levels but travel and forage low in the understory or on the ground. The African palm squirrels (genus Epixerus) are longlegged runners that forage only on the ground. Certain species, such as the redtailed squirrel (S. granatensis) of the American tropics and the African pygmy squirrel, are active from ground to canopy. In the United States, the Eastern fox squirrel (S. niger) runs along the ground from tree to tree, but others, including the Eastern gray squirrel (S. carolinensis), prefer to travel through the treetops and regularly cross rivers by swimming with the head up and tail flat on the water’s surface. Thomas’s rope squirrel (Funisciurus anerythrus) of Africa even submerges itself and swims underwater.

Most tree squirrels have strong chisellike incisors and powerful jaws, which are required for gnawing open the hard nuts that, along with fruits, are a primary component of their diet.