Squirrels

Squirrel Facts

Classification:

• Kingdom: Animalia
• Phylum: Chordata
• Subphylum: Vertebrata
• Class: Mammalia
• Subclass: Theria
• Infraclass: Eutheria
• Order: Rodentia
• Suborder: Sciuromorpha
• Family: Sciuridae (squirrels), with 58 genera and 285 species
• Geographical location: Worldwide, except Australia, Polynesia, and southern South America
• Habitat: All habitats except deserts
• Gestational period: Forty-four days
• Life span: Ten years in the wild, sixteen years in captivity
• Special anatomy: Large bushy tail used for balance and temperature regulation

Most people are familiar with squirrels but may not be aware that members of the squirrel family (Sciuridae) can be divided into two basic groups: tree squirrels and ground squirrels. Tree squirrels are arboreal. Not only do they nest in trees, but they also often mate and forage in trees. Ground squirrels are primarily terrestrial. Though some may climb several feet up the trunk of a tree, ground squirrels nest in burrows beneath the soil surface and forage and mate on the ground or in their burrows. Another big difference between ground-dwelling and tree-dwelling squirrels is that ground squirrels hibernate during the winter and tree squirrels do not. Typical ground squirrels include marmots, woodchucks, and chipmunks. Typical tree squirrels include fox and gray squirrels in North America and red squirrels in Europe.

Gray Squirrels

One of the most familiar tree squirrels in North America is the gray squirrel, Sciurus carolinensis. Gray squirrels live in deciduous forests and are also abundant in parks and yards in eastern North America. Their common name is somewhat misleading, in that some gray squirrels have a black or brown pelage. There are even a few populations of albino gray squirrels in North America. Nevertheless, most gray squirrels have gray backs with light-colored ventral fur and light to white fur on the backs of the ears. Like other tree squirrels, gray squirrels have large, bushy tails almost equal in length to the squirrel’s body. Though males are generally larger than females, there is no pronounced sexual dimorphism. Gray squirrels range in size from 330 to 750 grams.

Gray squirrels are not particularly social. That is, they do not form cooperative groups. Rather, gray squirrels are solitary. They do not defend territories, and home ranges of individual gray squirrels overlap widely. However, they may defend core areas in the autumn to ensure access to food.

Gray squirrels undergo one or two breeding seasons each year, depending on latitude. Squirrels in northern latitudes may only breed once a year, though squirrels in more moderate climates breed up to two times per year. Timing of the breeding season thus also varies with latitude. In the northern United States, gray squirrels begin to breed in early to mid-February. Gray squirrels are polygynous; one male may mate with several females during a single breeding season. Males compete with each other for access to females; several males may chase a female until one has an opportunity to mate with her. Some males, rather than participate in mating chases, wait behind trees and find opportunities to mate with females during times other than the mating chase. Mating takes place on the ground or in the trees and thus can be dangerous to both participants, as there is a real risk of falling during mating.

Gestation in gray squirrels lasts approximately forty-four days. Litter sizes range from one to six, with an average of two to three. Gray squirrels are born in a relatively helpless state; they are born hairless and their eyes do not open until about twenty-five days after birth. Gray squirrels nurse for eight to nine weeks, after which time they are weaned. Some litters are at this point abandoned by their mother. Young gray squirrels can fend for themselves at about eighty days of age and begin to build their own leaf nests at about eighteen weeks of age. Sexual maturity is reached at ten months of age. Gray squirrels have an average life span of only eleven to twelve months. However, many individuals live longer than this, even up to ten years in the wild (longer in captivity).

Gray Squirrel Lifestyle

Gray squirrels might best be considered opportunistic omnivores. Commonly known to consume nuts and seeds as well as buds and fruits from hardwood trees, gray squirrels have also been known to consume baby birds, insects, and fungi. Nevertheless, during the autumn and winter, gray squirrels depend almost exclusively on the mast crop from hardwood trees as their food source. Beginning in late summer to early autumn, gray squirrels begin to scatterhoard nuts. Scatterhoarding entails burying single nuts in different places around the squirrel’s home range and differs from larderhoarding in that gray squirrels do not cache large piles of nuts together in a single location. Memory and an excellent sense of smell allow the squirrels to find buried nuts later in the winter, even when buried under several inches of snow. Gray squirrels can be very selective about which foods to include in their diet. They are known to avoid nuts produced from the red subgenus of oaks, which tend to be high in tannin. When eating acorns from red oaks, squirrels generally consume those parts of the seed that are low in tannin.

Unlike ground squirrels, gray squirrels do not hibernate during the winter. Thus, even in the worst weather, they must leave the safety of their nests to obtain food. During the winter months, gray squirrels will den together in tree cavities, presumably to conserve heat. Females usually den with other females (often in mother-daughter groups) and males usually den with other males. Dens are lined with leaves as insulation. Squirrels also use a variety of anthropogenically produced materials in den construction. Foil-coated fast-food wrappers and laundry lint are not uncommon discoveries in squirrel nests. During warmer months, gray squirrels build leaf nests, called dreys, in the upper branches of hardwood trees.

Principal Terms

ground squirrels: those members of the family Sciuridae who burrow in the ground

opportunistic omnivore: an animal who includes a variety of plant and animal material in its diet, depending on the availability of different foodstuffs

polygynous: mating system in which one male mates with several females during a breeding season

Sciuridae: rodent family to which all squirrels belong

tree squirrels: those members of the family Sciuridae who nest in trees

Bibliography

Koprowski, John L. “Natal Philopatry, Communal Nesting, and Kinship in Fox Squirrels and Gray Squirrels.” Journal of Mammalogy 77 (1996): 1006-1016. Print.

Nowak, Ronald M. Walker’s Mammals of the World. Vol. 2. 6th ed. Baltimore: Johns Hopkins UP, 1999. Print.

Steele, Michael A., et al. “Tannins and Partial Consumption of Acorns: Implications for Dispersal of Oaks by Seed Predators.” The American Midland Naturalist 130 (1993): 229-38. Print.

Thorington, Richard W., Jr., et al. Squirrels of the World. Baltimore: John Hopkins UP, 2012. Print.

Wilson, Don E., and Sue Ruff, eds. The Smithsonian Book of North American Mammals. Washington, DC: Smithsonian Inst., 1999. Print.