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The western gray squirrel (Sciurus griseus) is an arboreal species in the rodent family that occurs in the far western parts of the North America. The taxon was first described by George Ord in the year 1818, based upon accounts conveyed by the Lewis and Clark expedition from observations near the mouth of the Columbia River. Although S.griseus is preyed upon by a number of apex and near-apex mammals and raptors, the chief killer of this rodent is Homo sapiens, whose overpopulation of the western USA has led to significant habitat destruction and vehicular killing.
Adult body mass ranges from approximately 450 to 950 grams, with a nose-to-tail length of 44 to 70 centimeters. This taxon manifests a coloration with dorsal fur of a gunmetal silver gray and a pure white underside, a pelage usually considered counter-shaded. All feet are pentadactyl clawed, although footprints usually display as four narrow front toes and five toes per rear foot.
The western gray squirrel is most often found in mixed oak woodland, mixed oak-conifer woodland, mixed conifer forest, walnut mixed forest and in cottonwood or sycamore dominant woodlands. Example nut foods sought are black oak and coast live oak of the Coast Ranges; interior live oak and blue oak of the hotter interior ranges; and valley oak and black walnut of the California Central Valley. The species is chiefly arboreal, but also engages in extensive terrestrial locomotion especially where canopies are less dense. Terrestrial behaviors are also associated with ground foraging for acorns, conifer seeds and to a lesser extent, berries; ground foraging is most common in mornings and late afternoon, with subsequent soil caching of nuts and seeds; however, this taxon does not possess a cheek pouch for carrying food. S.griseus is typically restricted to elevations less than 2000 meters, although some sources report occurrences at higher elevations in a portion of the northern Sierra Nevada Range and San Bernardino Mountains.
Although S.griseus is not classified as an endangered or vulnerable species, its population range has been considerably diminished over the last century. Chief threats to the species are habitat loss, overgrazing, kill by surface transportation vehicles, and introduction of alien species Release of alien species of eastern Fox squirrels in the Los Angeles Basin beginning in the 1970s has led to aggressive competition from the alien Fox squirrel, which has had the outcome of most of the local S.griseus populations to refugium mountainous areas.