The family Sciuridae contains two subfamilies, the sciurinae (the tree and ground squirrels), with about 225 species, and the Petauristinae (the Flying squirrels), with about 35 species. Squirrels are distributed throughout the world except for the Australian region, Madagascar, southern. South America, and certain desert regions.
The American eastern gray squirrel, Sciurus Carolinensis, typically of the tree squirrels, is native to the forests of the eastern half of the United States. Introduced into the United Kingdom. It has displaced the smaller native red squirrel, S. vulgris, in many regions. Grey squirrel normally nest in tree hollows, they may also construct leaf nets, which are used mainly as feeding shelters. They may breed in midwinter and again in late spring. Gestation is about 44 days, with usually two to four young per litter.
Ground squirrels are similar in appearance to tree squirrels. They typically inhabit open or brushy regions, with some species occurring in tundra or desert. Most are burrows, and those living in colder climates hibernate, whereas others, in hot, dry areas, may estivate.