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Western gray squirrel Sciurus griseus (Ord 1818)
by C.Michael Hogan PhD.
October 1, 2012
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. The earliest skeletal representative of the species was recovered in the Shasta Mountains dating to the Pleistocene era. (Kellogg. 1912)
Subspecies and Distribution
The species range is from Northern Baja California to western Canada, occurring in all three western USA states of California, Oregon and Washington. There are three geographically described subspecies: Sciurus griseus griseus (ranging from the USA state of Washington to the western slopes of the Sierra Nevada Range within Central California; S. g. nigripes (ranging from just south of San Francisco Bay to San Luis Obispo County, S. g. anthonyi, which ranges from San Luis Obispo County south to the northern portion of Baja California in Mexico.
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 and clawed, although footprint patterns display only four narrow toed toes and the rear feet five; footprint detection in the winter clearly reveals this pattern.
Habitat and behavior
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, int 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 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. Ground foraging native competitors include the California ground squirrel (Spermophilus beecheyi). S.griseus is typically restricted to elevations less than 2000 meters, although some reported occurrences at higher elevations in a portion of the northern Sierra Nevada Range and in the San Bernardino Mountains.
Nests for this species are usually found in the upper one third of the vertical forest stratification. These nests or dreys are typically constructed of twigs, leaves, tail fur, moss, shredded bark, lichens and grass fragments. Some dreys are sizable and roundish and often covered; these can be considered birthing and overwintering structures. Another type is more temporary or seasonal and is mainly for nocturnal sleeping. Nest size is large and may have a characteristic diameter of 45 to 90 centimeters.
The western gray squirrel becomes less active in winter months, but does not hibernate. Predators include: mountain lion, bobcat, coyote, martens, foxes and some species of eagle and hawk.
Mating season is typically between December and early June, with later months applying to higher elevations or more northerly latitudes. Gestation endures about six and a half weeks, with production of typically one to five kits per litter. The young are very slow to develop and reside in their nests for up to six months.
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, over zealous fire suppression and introduction of alien species; all of these phenomena are driven by the fundamental source of the human population explosion of the far western USA.
Habitat loss and habitat fragmentation are caused by the proximate causes of urbanization and conversion of forest to agricultural lands. Release of alien species of eastern fox squirrels in the Los Angeles Basin beginning in the 1970s has led to aggressive competition from this alien, which has had the outcome of most of the local S.griseus populations to refugium mountainous areas. In the case of killing by motor vehicles and trains, this species has no adaptation to such high speed instruments, since its evasive random spurts have been developed to deceive predators which have highly tuned motion detection sensory apparati, whereas humans who operate electromechanical devices (e.g. cars, trains) have slower and less sophisticated reaction times and poorer eyesight. Thus humans are not fooled by the clever and quick movements of the western gray squirrel, since Homo sapiens is not even registering the squirrel's quickness and cannot adjust vehicle speed or course rapidly enough to avoid striking a squirrel.
* Leslie N.Carraway and B.J.Verts. 1994. Mammalian Species: Sciurus griseus. American Society of Mammalogists. No.474, pp. 1-7
* Lloyd Glenn Ingles. 1947. Mammals of California. Stanford University Press. books.google.com/books ISBN 080471195X
* L.Kellogg. Pleistocene rodents of California. University of California Dept. of Geology, 7:151-168
* Mary J.Linders and Derek W.Stinson. 2006. Washington State Recovery Plan for the Western Gray Squirrel. State of Washington Department of Fish and Wildlife Program
* A.V.Linzey, R.Timm, S.T. Álvarez-Castañeda, I.Castro-Arellano and T.Lacher. 2008. Sciurus griseus. IUCN Red List of Threatened Species
* G.Ord. 1818. Sur plusieurs animaux de l’Amérique septentrionale, et entre autres sur la Rupicapra americana, l’Antilope americana, le Cervus major ou Wapiti. Journal de physique, de chimie et d’histoire naturelle et des arts, Tome 87
* R.W.Thorington and R.S.Hoffmann. 2005. Family Sciuridae. In D.E.Wilson and D.M.Reeder, Mammal Species of the World: a taxonomic and geographic reference. Johns Hopkins University Press. pp. 754–818. ISBN 08018-8221-4
* Encyclopedia of Life. 2012. Sciurus griseus. Content sources: Smithsonian Institution; Regents of the University of Michigan. Curators: Michael Frankis; Andrew Edelman
* United States National Museum. 1912. U.S Bulletin. Issue 79. Page 332. U.S Government Printing Office. Washingtonno DC books.google.com/books
The foregoing is original content of C.Michael Hogan and may not be altered or re-used without express written permission of the author.