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Overview

Brief Summary

Species Abstract

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.

  • A.V.Linzey, R.Timm, S.T. Álvarez-Castañeda, I.Castro-Arellano and T.Lacher. 2008. Sciurus griseus. IUCN Red List of Threatened Species
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Description

Although Western Gray Squirrels are diurnal, they are secretive by nature and stay away from humans as much as possible. However, they are comparatively frequently hit by automobiles—perhaps because they lack experience of human behavior. The Western Gray Squirrel has a silvery-gray back, a pure-white belly, and a beautiful long, bushy, blackish-gray tail edged with white. These squirrels prefer oak and conifer forests, traveling the arboreal pathways made by interconnecting tree canopies and feeding on pine cones. They also forage on the ground, eating fungi, pine nuts, acorns, bark, and some vegetation. Like Eastern Gray Squirrels, they bury acorns in the fall, presumably finding them later by scent. This behavior is called scatter-hoarding. Western Gray Squirrels usually produce only one litter per year. The female builds a nest of sticks for raising her young, and the squirrels often sleep in nests in tree cavities. They are not territorial about their nests. Only one squirrel occupies a nest at a time, except for a mother with her young, but several squirrels may nap in the same cavity in the course of a day.

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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."
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Distribution

Sciurus griseus lives on the west coast of the United States. S. griseus can be found in Washington, Oregon, California, and in a very small part of Nevada.

Biogeographic Regions: nearctic (Native )

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Range Description

This species is found throughout Pacific region of the western United States and northern Mexico, from northern Washington state to extreme northern Baja California.
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occurs (regularly, as a native taxon) in multiple nations

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National Distribution

United States

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

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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.

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Physical Description

Morphology

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.

Other Physical Features: endothermic ; bilateral symmetry

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Size

Length: 59 cm

Weight: 964 grams

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Size in North America

Sexual Dimorphism: None

Length:
Average: 623 mm
Range: 510-770 mm

Weight:
Average: 793 g
Range: 500-950 g
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Ecology

Habitat

Western grey squirrels are found in woodlands and coniferous forests. They can be found at elevations up to 2500 meters.

Terrestrial Biomes: forest

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Habitat and Ecology

Habitat and Ecology
This species occurs in oak-conifer woodlands and in mixed conifer forests. Also found in sycamore, cottonwood and walnut mixed forest.

Systems
  • Terrestrial
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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.

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Migration

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).

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Trophic Strategy

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.

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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.

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General Ecology

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.

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Life History and Behavior

Behavior

Perception Channels: tactile ; chemical

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Cyclicity

Comments: Active throughout the year. May remain in nest during severe weather. Most active in the early morning and late afternoon.

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Life Expectancy

Lifespan, longevity, and ageing

Observations: Little is known about the longevity of these animals. There are anecdotal reports of animals living up to 11 years, which is plausible. Record longevity for an individual in captivity, though, is 6.3 years (Richard Weigl 2005).
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Reproduction

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.

Key Reproductive Features: gonochoric/gonochoristic/dioecious (sexes separate); sexual

Average gestation period: 44 days.

Average number of offspring: 3.

Average age at sexual or reproductive maturity (female)

Sex: female:
319 days.

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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.

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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage: Sciurus griseus

Barcode of Life Data Systems (BOLDS) Stats
Public Records: 0
Specimens with Barcodes: 1
Species With Barcodes: 1
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Conservation

Conservation Status

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

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IUCN Red List Assessment


Red List Category
LC
Least Concern

Red List Criteria

Version
3.1

Year Assessed
2008

Assessor/s
Linzey, A.V., Timm, R., Álvarez-Castañeda, S.T., Castro-Arellano, I. & Lacher, T.

Reviewer/s
McKnight, M. (Global Mammal Assessment Team), Amori, G., Koprowski, J. & Roth, L. (Small Nonvolant Mammal Red List Authority)

Contributor/s

Justification
This species is listed as Least Concern in view of its wide distribution, presumed large population, and because it is unlikely to be declining at nearly the rate required to qualify for listing in a threatened category.
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National NatureServe Conservation Status

United States

Rounded National Status Rank: N5 - Secure

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NatureServe Conservation Status

Rounded Global Status Rank: G5 - Secure

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Population

Population
This species is considered uncommon. Density estimates vary from 0.25 individuals/ha in the foothills of the Sierra Nevada range to 2.8 individuals/ha in mixed coniferous forest.

Population Trend
Unknown
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Threats

Major Threats
This species competes for food with acorn woodpeckers, California ground squirrels and introduced tree squirrels (S. niger and S. carolinensis) Its habitat is also threatened by logging activities. This species is less tolerant to urbanized areas compared to eastern squirrels. This species is also threatened by predation from house cats and is one of the more common squirrel species in traffic mortalities.
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Management

Conservation Actions

Conservation Actions
There are no known conservation measures specific to this species. However, there are several protected areas within its range.
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Relevance to Humans and Ecosystems

Benefits

Some people consider squirrels to be a nuisance.

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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.

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Economic Uses

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.

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