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Tree squirrel

Tree squirrels include over a hundred species that are found on all continents except Antarctica, and are the members of the squirrelfamily (Sciuridae) most commonly referred to as "squirrels".^[1] They do not form a single natural, or monophyletic, group, but instead are related to the various other animals in the squirrel family, including ground squirrels, flying squirrels, marmots, and chipmunks. The defining characteristic that is used to determine which of the various species of Sciuridae are tree squirrels is therefore not so dependent on their physiology, but their habitat. Tree squirrels live mostly among trees, as opposed to other squirrels that live in burrows in the ground or among rocks. However, there is one exception to this rule, as physiological distinction does make a difference in regard to flying squirrels, who also make their home in trees, but have unique physical characteristics that separate them from their tree squirrel cousins (specifically, special flaps of skin that act as glider wings, allowing them to "fly").

The most well known genus of tree squirrels is Sciurus, which includes the Eastern gray squirrel of North America (and which wasintroduced^[when?] to Great Britain^{[citation needed]}), the red squirrel of Eurasia, and the North American fox squirrel, among many others. Since many tree squirrel species have readily adapted to human-altered environments (including intensely cultivated farms and urban cities), and because they are mostly diurnal (active during the daytime), when most people are outdoors to see them, they are perhaps the most familiar members of the rodent family to most humans. Indeed, in some larger cities, they are often the only wild animals (not counting birds) that most people ever see. It is no surprise, then, that tree squirrels and humans have had a very complex and long-lasting relationship.