squirrel, (family Sciuridae), generally, any of the 50 genera and 268species ofrodents whose common name is derived from theGreekskiouros, meaning “shade tail,” which describes one of the mostconspicuous and recognizable features of these smallmammals. These distinctive animals occupy a range ofecologicalniches worldwide virtually anywhere there is vegetation. The squirrel family includesground squirrels,chipmunks,marmots,prairie dogs, andflying squirrels, but to most peoplesquirrel refers to the 122 species oftree squirrels, which belong to 22 genera of the subfamily Sciurinae. TheNorth American gray squirrel (Sciurus carolinensis) has adapted to urban and suburban areas where it is regarded asaesthetic or as a minor annoyance. In northernEurope thered squirrel (S. vulgaris) is valued for its soft, thickfur. 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, stubbythumb, and the five-toed 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 rapidlydescend 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 longwhiskers. 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 ofOriental giant squirrels (genusRatufa) native to thetropical forests ofSoutheast 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: theneotropical pygmy squirrel (Sciurillus pusillus) of theAmazon 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 theAfrican 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 (genusRatufa) and theAfrican giant squirrels (genusProtoxerus), rarely descend from the high canopy. Others, like thepygmy squirrel ofSulawesi (Prosciurillus murinus), travel and forage at intermediate levels between ground and canopy. Some large tropical squirrels, such as theSulawesi giant squirrel (Rubrisciurus rubriventer) and the northernAmazon red squirrel (Sciurus igniventris), nest at middle levels but travel and forage low in the understory or on the ground. TheAfrican palm squirrels (genusEpixerus) are long-legged runners that forage only on the ground. Certain species, such as thered-tailed squirrel (S. granatensis) of the American tropics and the African pygmy squirrel, are active from ground to canopy. In theUnited States, theEastern 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.

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Most tree squirrels have strong chisel-like incisors and powerful jaws, which are required forgnawing open the hard nuts that, along with fruits, are a primary component of their diet. They also eat seeds,fungi, insects and other arthropods, thecambium layer of tree bark, nectar, leaves, buds, flowers, and sometimesbird eggs, nestlings, and carrion. Some red squirrels (genusTamiasciurus) andSciurus species of temperate climates will stalk, kill, and eat other squirrels, mice, and adult birds and rabbits for food, but such predation in tropical tree squirrels seems rare.

Nests are constructed among branches in the forest canopy or at lower levels in tree crowns, vine tangles, tree hollows, or undergrowth near the ground. Some species of tropical tree squirrels produce severallitters per year; breeding season in the Northern Hemisphere may extend from December to September and may result in one or two litters that average three to seven young, depending upon the species.

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In the New World, tree squirrels range from theboreal forests of Canada and Alaska southward through coniferous and deciduous woodlands in the United States to the tropical rainforests ofSouth America. In Africa, tree squirrels are native to rainforests and some woodland savannas. Their distribution in the remainder of the Old World extends from the northern boreal forests of Europe andAsia to the Indonesian tropical rainforests. East of the Asiancontinental margin, tree squirrels inhabit the forests of Taiwan, some islands in the Philippines, and Sulawesi, but they do not occur naturally anywhere east of those islands. Most of the species in 20 of the 22genera are found in tropical rainforests.