Western Gray Squirrel, Sciurus griseus
Western Gray Squirrel, Scurius griseus. Photograph taken in the greater San Diego area, California, June 2021. Photograph courtesy of Dr. Tom Bartol, Carlsbad, California.
The Western Gray Squirrel, Scurius griseus, is a member of the Scuridae Family of squirrels, marmots, and chipmunks. The Scuridae Family consists of two hundred eighty-six members placed into fifty-eight genera. There are twenty-eight species in the Scurius genus. In Mexico, they are known as Ardilla Gris.
Western Gray Squirrels have a steel gray back with a contrasting white belly and throat, providing them the nickname “Silver Gray Squirrel” in some parts of their range. They can be easily distinguished by their long and bushy tails that are primarily gray with white-frosted outer edges. Tails are held curved upwards in an “S” shape. Their prominent ears lack tufts and can occasionally be reddish-brown on the posterior side. This small patch of brown is only visible upon close inspection. Western Gray Squirrels shed their fur once in the late spring, and again in early fall. The fur on their tail is only shed during the spring molting. They are the largest squirrel species found within their range, measuring between 25.5 cm ( 10.4 inches) and 32.3 cm (12.7 inches) in snout to vent length. Their tail is almost as long as their body and measures between 24.0 cm (9.5 inches) and 31.0 cm (12.2 inches). Adults do not display sexual dimorphism.
Western Gray Squirrels are diurnal and arboreal. They live in hollow trees or in nests they build, called dreys. These are constructed of sticks and insulated with softer materials such as moss or fresh leaves. Dreys are located approximately a third of the way up a tree, and occur in either large conifers (especially Ponderosa Pine, Pinus ponderosa, and Douglas Fir, Pseudotsuga menziesii, or oaks averaging greater than 40.6 cm (16 in) in diameter. While they do not hibernate, they do spend significantly more time in their nest during the cold winter months. Outside of their nest, they devote their time to grooming, gathering food, and resting. Grooming can last from three to fifteen minutes, with most of their time spent on the head area. Western Gray Squirrels have a home range between 0.2 hectare (0.5 acre) and 1.0 hectare (2.5 acre), and the males generally possess a larger range than the females. Home ranges of males can overlap significantly. Reproduction begins in December and extends into July, with most breeding occurring in the spring and early summer. A mating “chase” is often involved, with several males following a female as she moves about during the day. They are polygynandrous, and both males and females have multiple partners. Gestation lasts around forty-four days. Females produce between three and five young per litter. Younger females generally produce smaller litters, while older females produce larger litters. Newborns are altricial, being born without hair and with their eyes and ears closed. Compared to the rest of their body, the head and feet of young Western Gray Squirrels are rather large. After about thirty days, the young are fully furred and make short trips out of the nest. After about sixty days, they begin eating solid foods and venturing to the ground. At about three months of age, juvenile squirrels are on their own. Sometimes, they remain close to the nest until their parent’s next breeding season. Sexual maturity is reached at ten to eleven months of age, and they live between seven and eight years in the wild.
Western Grey Squirrels are opportunistic feeders and their diet varies depending upon the availability of seasonal and local foods. They consume fungi, pine nuts, acorns, fruits, nuts, forbs, and leaves. Fungi are important spring and summer foods while acorns, when available, are very important summer, fall and winter foods. Western Gray Squirrels bury nuts individually in soil at depths between 8 cm (3 inches) and 10 cm (4 inches) across their range, before digging them up to eat in the winter. This strategy is called scattered hoarding and reduces the need to guard individual caches. They locate their caches with their keen sense of smell, although deep snow can hinder this ability. Western Gray Squirrels have many natural predators including birds of prey, bobcats, coyotes, cougars, foxes, and feral cats and dogs. When threatened, they make barking sounds while flicking their tails and stamping their feet.
The Western Gray Squirrel is found throughout the Pacific region of the western United States and northern Mexico, from northern Washington state to extreme northern Baja California. They reside in woody areas with shrubs and plenty of spaces for shelter and nesting. These areas include oak-conifer, sycamore, cottonwood, and walnut mixed forests.
Western Gray Squirrels are often confused with introduced Eastern Gray Squirrels (Sciurus carolinensis). Western Gray Squirrels are larger in size and lack any brown on their body and tail. Eastern Gray Squirrels are native to eastern North America and have brown on their face and tail.
From a conservation perspective the Western Gray Squirrel is considered considered to be of Least Concern with widely distributed stable populations. Throughout their range, they compete for food with the Acorn Woodpeckers, Melanerpes formicivorus, the California Ground Squirrel, Otospermophilus beecheyi, and the introduced Fox Squirrel, Sciurus niger, and the Gray Squirrel, Sciurus carolinensis. The Western Gray Squirrel is less tolerant to urbanized areas and their habitat is threatened by logging activities. Additionally, they are threatened by predation from house cats and traffic mortalities.