A rodent in the family Sciuridae. This family includes all tree squirrels and flying squirrels, as well as squirrels that live on the ground such as the woodchuck, marmot, chipmunk, and prairie dog. There are 51 living genera and 272 species living in a wide variety of habitats ranging through tropical rainforests, tundra, alpine meadows, and semiarid deserts in Europe, Asia, Africa, North America, and most of South Africa. They are absent from Australia, Madagascar, Polynesia, and the Sahara Desert. Locomotion is plantigrade (walking with the whole sole of the foot touching the ground). Sensitive vibrissae are present on the head, feet, and outsides of the legs. The dental formula is I 1/1, C 0/0, Pm 1-2/1, M 3/3 for a total of 20 or 22 teeth. The incisors grow continually throughout the animal's life.