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.
C.Michael Hogan. 2012.
© C.Michael Hogan
Supplier: C. Michael Hogan
Mammal Species of the World
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- Original description: "Ord, G., 1818. ""Sur plusieurs animaux de l?Amérique septentrionale, et entre autres sur la Rupicapra americana, l?Antilope americana, le Cervus major ou Wapiti, etc."", in Journal de physique, de chimie et d?histoire naturelle et des arts, Tome 87, p. 152."
Biogeographic Regions: nearctic (Native )
occurs (regularly, as a native taxon) in multiple nations
Regularity: Regularly occurring
Type of Residency: Year-round
Global Range: Lake Chelan and Tacoma, Washington, southward through central and western Oregon, west-central Nevada, and the coast ranges and Sierra Nevada of California to the montane areas of southern California and extreme northern Baja California. Ranges to 2590 m in the San Bernardino Mountains, California.
The western grey squirrel ranges from about 18 inches to 24 inches in total length with the body and the tail. Their weight varies from 350 to 950 grams.
Sciurus griseus has silver grey fur on its back and a white underside. It has a long bushy tail which is the same silver grey color. Their tail may also have black in it. S. griseus has large ears without tufts.
The western grey squirrel sheds its fur once in the late spring and again in early fall. The fur on the tail is only shed during the spring molting.
Sciurus griseus lives to be between 7 and 8 years old in the wild.
Range mass: 350 to 950 g.
Length: 59 cm
Weight: 964 grams
Habitat and Ecology
Western grey squirrels are found in woodlands and coniferous forests. They can be found at elevations up to 2500 meters.
Terrestrial Biomes: forest
Comments: Fairly open oak and pine-oak forests primarily in the Upper Sonoran and Transition life zones; also in riparian woods and in lowland groves of native walnuts in California. Arboreal and terrestrial. Nests made of sticks, twigs and leaves are built in tree cavities or on the limbs of trees.
Brood dens typically are in tree cavities, often in old woodpecker holes; young may be moved to stock nest after being born in a tree-hole nest.
Non-Migrant: Yes. At least some populations of this species do not make significant seasonal migrations. Juvenile dispersal is not considered a migration.
Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).
Locally Migrant: No. No populations of this species make annual migrations of over 200 km.
Home range may range from less than one hectare to several hectares; home range may shift seasonally in response to food supply (see Carraway and Verts 1994). Due to methodological limitations, some studies may have underestimated actual home range size of this squirrel (Linders et al. 2004).
Estimates of 95% minimum convex polygon home range of radio-tagged squirrels in Washington averaged 73.0 ha for males (n = 9) and 21.6 ha for females (n = 12) for year-round use (Linders et al. 2004).
Western grey squirrel's main source of food depends largely upon local habitat characteristics. Those that live in coniferous forests feed primarily on seeds of pinecones. Those that live in hardwood forests feed largely on nuts and acorns. Sciurus griseus is also known to eat berries, fungus, bark, sap, and insects. It opens hard seeds and nuts using its incisors.
Western grey squirrels will feed on the ground as well as in trees.
Comments: Diet includes seeds, nuts, fungi, green vegetation, berries and insects; acorns, nuts, seeds of conifers, and fungi are common foods. Gathers and buries acorns.
Population density may vary with food supply and occurrence of epizootics; density was about 2-4/ha in several areas in California, about double this in one area in Oregon (see Carraway and Verts 1994).
May compete with other squirrels for food and nest sites.
Bobcats, coyotes, foxes, owls, and large hawks are important predators.
Life History and Behavior
Comments: Active throughout the year. May remain in nest during severe weather. Most active in the early morning and late afternoon.
Lifespan, longevity, and ageing
Sexual maturity of Sciurus griseus is reached at 10 to 11 months. When S. griseus is approximately one year old it will begin breeding. When a female is in estrus the vulva becomes pink and enlarged. When a male is sexually active the scrotum turns black from its original pinkish gray color. Breeding takes place once a year in the late spring. There are betwee 3 and 5 young per litter. Younger females generally have smaller litters than older females. The gestation period averages 43 days. Young are born without hair and with closed eyes and ears. The head and feet of young are large compared to the rest of the body. They are weaned at approximately 10 weeks.
Average gestation period: 44 days.
Average number of offspring: 3.
Average age at sexual or reproductive maturity (female)
Sex: female: 319 days.
Copulation occurs primarily in winter and spring. Gestation lasts about 44 days. Adult females annually produce one litter of 2-5 (average 2-3) young, mainly from February to June or July in California. Young are born naked and blind. Sexually mature in 10-11 months.
Molecular Biology and Genetics
Statistics of barcoding coverage: Sciurus griseus
Public Records: 0
Specimens with Barcodes: 1
Species With Barcodes: 1
IUCN Red List Assessment
Red List Category
Red List Criteria
Western grey squirrels are a United States species of concern, but are not currently listed as threatened or endangered under the U.S. Endangered Species Act. In late 2002 the Washington subspecies, S. griseus griseus was proposed for listing as an endangered species. In Washington they are considered threatened at the state level, in Oregon they are considered a state sensitive species.
US Federal List: no special status
CITES: no special status
IUCN Red List of Threatened Species: least concern
National NatureServe Conservation Status
Rounded National Status Rank: N5 - Secure
NatureServe Conservation Status
Rounded Global Status Rank: G5 - Secure
Relevance to Humans and Ecosystems
Economic Importance for Humans: Negative
Some people consider squirrels to be a nuisance.
Economic Importance for Humans: Positive
Sciurus griseus can be hunted and used for food. Western grey squirrels also help disperse and plant trees by burying seeds in their caches which remain uncollected.
Comments: May damage nut crops (see Carraway and Verts 1994).
Sciurid mycophagy may play an important role in forest ecology (Maser and Maser 1988).
Subject to sport hunting. Annual harvest in California was about 150,000-200,000 in the 1960s.
Western gray squirrel
In some places, this species has also been known as the silver-gray squirrel, the California gray squirrel, the Oregon gray squirrel, the Columbian gray squirrel and the banner-tail. There are three geographical subspecies: Sciurus griseus griseus (central Washington to the western Sierra Nevada in central California); S. g. nigripes (from south of San Francisco Bay to San Luis Obispo County, California; and S. g. anthonyi, which ranges from San Luis Obispo to northern Baja California).
Compared with the eastern gray squirrel S. carolinensis or the eastern fox squirrel (which have been introduced into its native range), these squirrels are shy, and will generally run up a tree and give a hoarse chirping call when disturbed. Weights vary from about 0.4 to 1 kilogram (14 to 35 oz), and length (including tail) from 45 to 60 centimetres (18 to 24 in). It is the largest native tree squirrel in the western coastal United States. Western gray squirrels exhibit a form of coloration known as counter shading. The dorsal fur is a silver gunmetal gray, with pure white on the underside; there may be black flecks in the tail. Ears are large but without tufts. The ears turn reddish-brown at the back in the winter. The tail is long and typically very bushy. Tree squirrels undergo a complete head-to-tail molt in the spring and a rump-to-head molt in the fall. Tail hair is replaced only in the spring. Nesting mothers will use their tail hair to line birthing nests. Western gray squirrels eat berries, nuts, a variety of seeds, and the eggs of small birds.
Western gray squirrels mate over an extended period ranging from December through June. Young are born after approximate 44 day gestation period. Juveniles emerge from nests between March and mid-August. Litter sizes range from one to five kits which remain in the nest for a longer period than other squirrels. The kits are relatively slow in development, and will not leave the nest for six months or more, another species disadvantage when in competition with other, more-rapidly fledging squirrels. Young gray squirrels have furled tails which will not reach fullness until adulthood. This is a good indicator of age and maturity. Mother squirrels often seem to be overworked with a stressed appearance, complete with bruised and battered nipples. Mating squirrels can be very physical and will bite and injure each other. Females can be quite territorial, and will chase others away and have fairly violent altercations between themselves.
Behavior and diet
Western gray squirrels are forest dwellers, and can be found at elevations up to at 2,000 m or more. Time on the ground is spent foraging, but they prefer to travel distances from tree to tree. They are strictly diurnal, and feed mainly on seeds and nuts, particularly pine seeds and acorns, though they will also take berries, fungus and insects. Pine nuts and acorns are considered critical foods because they are very high in oil and moderately high in carbohydrates, which help increase the development of body fat. They feed mostly in trees and on the ground. They generally forage in the morning and late afternoon for acorns, pine nuts, new tree buds, and fruits. When on alert, they will spread their tails lavishly, creating an umbrella effect that shields them and possibly provides cover from overhead predators. They are scatter-hoarders making numerous caches of food when it is abundant, and thus contribute to the seed dispersion of their food trees. Although squirrels show relatively good scent relocation abilities, some food caches are never reclaimed, becoming seedlings in the spring. Though they do not hibernate, they do become less active during the winter. Like many prey animals, they depend on auditory alerts from other squirrels or birds to determine safety. Once an alarm call is transmitted, those present will join in, and the trees become a cacophony of chirping squirrels. Tree squirrels are prey for bobcats, hawks, eagles, mountain lions, coyotes, cat, and humans.
Habitat and shelter
Squirrel nests are called dreys and can be seen in trees, built from sticks and leaves wrapped with long strands of grass. There are two stick nest types made by the western gray squirrel: the first is a large, round, covered shelter nest for winter use, birthing, and rearing young. The second is more properly termed a "sleeping platform," a base for seasonal or temporary use. Both types are built with sticks and twigs and are lined with leaves, moss, lichens and shredded bark. The birthing nest may be lined with tail hair. The nest may measure 43 to 91 cm (17 to 36 in) by up to 46 cm (18 in) and is usually found in the top third of the tree. Young or traveling squirrels will also "sleep rough" when weather permits, balanced spread-eagled on a tree limb high above the forest floor. This attitude is also adopted for cooling in hot weather, a behaviour also observed in raccoons.
Habitat loss and competition
The western gray squirrel was listed as a threatened species in Washington state in 1993. Populations of the western gray squirrel have not recovered from past reductions. They are threatened with habitat loss, road-kill mortality and disease. Habitat has been lost due to urbanization, catastrophic wild fires, and areas of forest degraded by fire suppression and overgrazing, allowing the invasion of scotch broom. Notoedric mange, a disease caused by mites, becomes epidemic in western gray squirrel populations and is a major source of mortality. Other species of eastern gray squirrels, fox squirrels, California ground squirrels and Wild Turkeys are expanding and compete with the western gray.
Listed as extirpated in some California areas, the western gray squirrel in southern California is generally found only in the mountains and surrounding foothill communities. Local rehabilitation experts recount the Eastern Fox Squirrels were released in urban regions of Los Angeles throughout the 20th Century. Fox squirrels (Sciurus niger) were introduced to the Los Angeles area in about 1904. Civil War and Spanish-American War veterans residing at the Sawtelle Veteran’s Home on Sepulveda and Wilshire Boulevards brought fox squirrels as pets to this site from their homes in the areas surrounding the Mississippi Valley (possibly Tennessee). Other introductions of fox squirrels to the Los Angeles area may have taken place during more recent times but detailed records are not available. These aggressive cousins drove the more reclusive western grays back into the mountains, where competition was not so strong. This non-native species introduction appears to be the largest threat in the southern California area.
- Linzey, A. V., Timm, R., Álvarez-Castañeda, S. T., Castro-Arellano, I. & Lacher, T. (2008). Sciurus griseus. In: IUCN 2008. IUCN Red List of Threatened Species. Retrieved 6 January 2009.
- Thorington, R.W., Jr.; Hoffmann, R.S. (2005). "Family Sciuridae". In Wilson, D.E.; Reeder, D.M. Mammal Species of the World: a taxonomic and geographic reference (3rd ed.). The Johns Hopkins University Press. pp. 754–818. ISBN 0-8018-8221-4. OCLC 26158608.
- Crane, Sara (2002). "Sciurus griseus". Animal Diversity Web. Retrieved 2008-02-03.
- Hogan, C.Michael. Sciurus griseus (Western gray squirrel). GlobalTwitcher. ed. N.Stromberg
- Linders, Mary J.; Stinson, Derek W (May 2006). Washington State Recovery Plan for the Western Gray Squirrel (.PDF). Draft. Washington Department of Fish and Wildlife Program. Archived from the original on 2007-10-31. Retrieved 2008-02-03.
- "Sciurus griseus". Integrated Taxonomic Information System. Retrieved 23 March 2006.