Tree squirrels are the members of the squirrel family (Sciuridae) commonly just referred to as "squirrels". They include over a hundred arboreal species native to all continents except Antarctica and Oceania. They do not form a single natural, or monophyletic group; they are related to others in the squirrel family, including ground squirrels, flying squirrels, marmots, and chipmunks. The defining characteristic used to determine which species of Sciuridae are tree squirrels is dependent on their habitat rather than their physiology. Tree squirrels live mostly among trees, as opposed to those that live in burrows in the ground or among rocks. An exception is the flying squirrel that also makes its home in trees, but has a physiological distinction separating it from its tree squirrel cousins: special flaps of skin called patagia, acting as glider wings, which allows gliding flight. The best known genus of tree squirrels is Sciurus, which includes the Eastern gray squirrel of North America (introduced to Great Britain in 1876), the red squirrel of Eurasia, and the North American fox squirrel, among many others. Many tree squirrel species have adapted to human-altered environments such as rural farms, suburban backyards and urban parks; and because they are diurnal (active during the daytime) they have become perhaps the most familiar wildlife to most humans.
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