Western Grey Squirrels

Photo courtesy of The Squirrel Cam



Western Grey Squirrels (Sciurus griseus) are also known as California Grey Squirrels, and are the only large grey tree squirrel in their range on the Western Coast of the United States. Western Grey Squirrels are found in the Pacific States from Washington to California.

General Characteristics

Western grey squirrels are medium sized tree or arboreal squirrels. They have grey backs with numerous white tipped hairs and a white belly. The backs of their ears are reddish-brown and they have long bushy tails with bands of grey white and black, especially below.

The life span of Western grey squirrels is equivalent to that of Eastern grey squirrels; 7 - 8 years in the wild. Their bodies measure 450 - 600 mm total length, 240 - 310 mm tail length, 75 - 100 mm right hind foot, and they range in weight from 350 - 950 grams. Western grey squirrels have both a summer and winter coat, and therefore molt twice each year. The spring molt begins in May or June and the autumn molt begins in September, but the tail only molts once each year in the summer.

Western grey squirrels have four sets of whiskers located above and below the eyes, on the underside of the head in front of the throat, and on the nose. Whiskers, also known as vibrissae are touch receptors that provide the animal with information about its immediate surroundings. Western grey squirrels have very good eyesight even in dim light, and a wide field of vision. They also have a well developed sense of smell and hearing.

Squirrels have upper and lower incisor teeth followed by a gap called a diastema. The diastema is where the canine teeth would normally be found in carnivorous animals such as cats or dogs, or omnivorous animals such as monkeys. Behind the diastema are the cheek or grinding teeth which consist of premolars and molars. As with other rodent species, the incisors continuously grow to compensate for the enormous amount of wear that comes from a herbivorous diet. Young squirrels have milk teeth which are replaced by permanent teeth when they are between six and twelve months old.

Western grey squirrels are highly adapted for climbing trees and fatal falls are rare. Adaptations for climbing trees include tough curved claws, and the ability to leap considerable distances using powerful hindlimbs. Tails are used for balance when running and leaping between trees, and held over the back of a resting animal.



Female Western grey squirrels have one litter per year, and most breeding occurs between March and June of each year. Average litter sizes range from 3 - 5 individuals. Both sexes remain reproductively active throughout their lives. Female Western Grey squirrels usually reproduce as yearlings. An enlarged pink vulva is usually visible the day before the onset of estrus. Male Western grey squirrels are sexually mature as yearlings and testes descend in the scrotum from February to June.

Juvenile squirrels are born without hair and weigh between 13 and 18 grams. Hair begins to grow on the tail and dorsum by 21 days, and on their ventrum by 42 days. Eyes open at 24 - 42 days, ears at 21 - 28 days, lower incisors erupt at 19 - 21 days, upper incisors in week 4 and cheek teeth in week 6. Weaning begins at 7 weeks and is complete by 10 weeks. Adult body mass is reached after 8 - 9 months.


Habits and Ecology

Western grey squirrels are active year round during the daytime. Even during the most severe winter weather they will leave their nests for short periods of time to forage for food. Activity is bimodal from late spring to autumn with peaks 2 hours after sunrise and again 2 - 5 hours before sunset.

Western grey squirrels are non-territorial. They have small overlapping home ranges which average 3 hectares in size, but can vary from 0.5 hectares to greater than 7 hectares. There is little difference in home range size between males and females, although males having slightly larger home ranges than females. For most arboreal squirrel species, home range size is negatively correlated with food supply and squirrel population density.

Western Grey squirrels are generalist feeders. They rely on pine cones, acorns and other nuts, some fungi, berries and occaisionally insects. Food consumption peaks in summer or autumn and decreases in winter. Autumn rates of food consumption exceed energetic needs by 32% so that the animals can increase their weight before the onset of winter. Western grey squirrels are classic scatterhoarders. They carry nuts in their jaws and bury them in various locations within their home ranges. Olfaction and memory are used in locating their caches.


Nests -

During the summer Western grey squirrels typically use dreys, which are are round conspicuous twig and leaf nests built in trees between 25 and 45 cm in diameter. They are waterproof, and made of an outer layer of interwoven twigs with a softer inner lining consisting of moss, bark, leaves, fur, feathers, lichen or other similar material. Dreys are generally built in the upper 1/3 of the canopy and seldom in isolated trees, which may serve to protect nests from predators. In the winter Western grey squirrels will often use dens in hollow trees.

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This page was developed and is maintained by Peg Halloran, Ph.D.
email: halloran@colorado.edu
Copyright 1997, 1998, 1999; all rights reserved of course.
Last Update: March 16, 1999 by Peg Halloran, Ph.D.