Peg Halloran - Grey Squirrels

Eastern Grey Squirrels

Distribution - soon to be replaced with a range map

Eastern gray squirrels are found throughout the eastern United States; their natural range extends from Florida, north to Canada, and west to where the deciduous forests meet the great plains grasslands. There are 5 subspecies of eastern grey squirrels, S. carolinensis carolinensis is the subspecies found in most of the south from northern Florida, to North Carolina, west to Missouri, and eastern Texas.

General Characteristics

Eastern gray squirrels (Sciurus carolinensis) are medium sized arboreal, or tree squirrels. Their back is grizzled dark to pale gray and may be tinted with reddish coloration on their hips, feet and head. The tail is white to pale gray and as is their stomach. In the north, both ears and the soles of their feet grow heavy fur during winter. There is generally no difference between males and females with respect to size or coloration. They are usually distinguishable from their close cousin, the fox squirrel (S. niger), in that fox squirrels are at least 20% larger and have guard hairs tipped with tawny, brown, or orange instead of white as in gray squirrels.

In natural conditions, gray squirrels generally live to 7 or 8 years of age, although one individual lived to 20 years of age in captivity. They tend to be larger in the northern part of their range than in the southern part of their range. Their bodies measure 380 - 525 mm total length, 150 - 250 mm tail length, 25 - 33 mm ear length, 54 - 76 mm right hind foot length, and they range in weight from 300 - 710 grams. Eastern gray squirrels have both a summer and winter coat, and therefore molt twice each year. The spring molt begins in March, whereas the autumn molt begins in September, but tail only molts once each year in July.

Eastern gray squirrels have four sets of whiskers on the head, one set 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. Eastern gray 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 in general 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. As with other rodent species, the incisors continuously grow, to compensate for the enormous amount of wear that comes from a herbivorous diet. Behind the diastema are the cheek or grinding teeth which consist of premolars and molars. Young squirrels have milk teeth which are replaced by permanent teeth when they are between six and twelve months old.

Eastern Gray squirrels are highly adapted for climbing trees and fatal falls are rare. Adaptations for arboreal existence include tough curved claws, and due to unusual flexibility in their ankle joint, their hind foot can be reversed 180 degrees from forward to permit head first descent. Gray squirrels are excellent climbers and can leap considerable distances using powerful hindlimbs. Squirrel tails are used for balance when running and leaping between trees, and held over the back of a resting animal.

Reproduction-

Male eastern gray squirrels are sexually mature at 10 - 11 months of age. Their testes descend and regress following a semiannual cycle. Functional testes descend in the scrotum from December to February and May to July, and testes may stay descended without spermatogenesis until October. Female eastern gray squirrels have been known to produce litters as young as 5 ˝ months of age in the southern part of their range, but most do not reproduce until 1.25 years of age. An enlarged pink vulva indicates estrus and is usually visible the day before the day of behavioral estrus. Both sexes remain reproductively active throughout their lives.

Eastern gray squirrels have two breeding seasons per year, and most breeding occurs in December - February and May - June of each year. Males begin to follow females up to 5 days before estrus. As many as 34 males have been documented to follow a single female, but usually the number is between 5 and 10. A dominance hierarchy is formed among the males, and the female tends to mate first with the dominant male. However, females will mate with several males throughout the day of estrus.

Females can have two litters per year, one from each breeding season. Average litter sizes range from 2 - 3 individuals; the winter litter is generally smaller than the summer litter. Litter sizes are also correlated with food supply; smaller litters occur in years with poor food production.

Juveniles

Juvenile squirrels are born without hair (except for whiskers) 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 and ears at 21 - 28 days. Lower incisors erupt at 19 - 21 days followed by 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.

Juvenile males are most likely to leave the natal area and disperse. Dispersal usually occurs during the fall when young males move between 1 and 16 kilometers away from their natal nest, and the longest recorded dispersal is 100 km. Dispersal is a high cause of mortality among males resulting in adult sex ratios of 0.85 males : 1.6 females.

Habitat and Ecology

Eastern gray squirrels are most common in mature continuous woodlands greater than 40 hectares in size, with a woody understory. Densities are highest in habitats composed of tree species that produce winter storable foods such as oak, hickory and walnut. Due to variability in seed production, a diversity of nut trees is important to support high densities.

Nests -

Eastern gray squirrels typically use 3 different types of nests: winter dreys, summer dreys, and dens. Dreys 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, and a softer inner lining consisting of moss, bark, leaves, fur, feathers, lichen or other similar material. Summer dreys are less elaborate than winter dreys and may be no more than twig and leaf saucer shaped platforms on exposed branches. Dreys are generally built in the upper 1/3 of the canopy and seldom in isolated trees, which may serve as protection of nests from predators.

Tree dens are another type of nest used by eastern gray squirrels. These are holes or cavities in the main trunks of trees. Dens are also lined with soft material. Formation of den cavities requires 8 - 30 years, and are more common in deciduous trees than coniferous trees. Squirrels often use dens in winter months and dreys in summer months.

Food habits -

Eastern gray squirrels are generalist feeders. Eighteen plant species account for 87% of their diet, but they may feed on as many as 97 plant and 14 animal items. Squirrels feed heavily on nuts, flowers, and buds of 24 oak species, and 10 species of walnut, hickory and pecan. Other food items include the fruits, seeds, buds, or flowers of maples (Acer), mulberry (Morus), hackberry (Celtis), elms (Ulmus), buckeyes and horse chestnuts (Aesculus), wild cherries (Prunus), dogwoods (Cornus), hawthorne (Crataegus), hazelnut (Corylus americana) and ginkgo (Ginkgo biloba). Pine tree seeds and pollen cones are readily eaten including cedar (Juniperus), hemlock (Tsuga), pines (Pinus), and spruce (Picea). Fungi are also consumed when readily available in summer, as are cultivated crops in winter. Animal food items include bones, bird eggs, nestlings, and frogs. Cannibalism has also been reported.

Food consumption peaks in summer or autumn and decreases in winter, and autumn rates of food consumption exceeds energetic needs by 32%. Females tend to consume more food than males. An individual’s body mass follows trends in food consumption with maximum weight gain in autumn and minimum weight gain in spring.

Eastern gray 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. Acorns with high lipid contents are a preferred food, whereas acorns high in tannins are less desirable.

Daily activity patterns -

Eastern gray 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 and peaks 2 hours after sunrise and again 2 - 5 hours before sunset.

Gray squirrels are non-territorial. Their home ranges average 5 hectares in size, but can vary from 0.5 hectares to greater than 20 hectares. Males have slightly larger home ranges than females, due to increase in their range during the breeding season. Home range sizes also increase during spring and summer for both sexes. Home range sizes for Eastern gray squirrels are negatively correlated with food supply and squirrel population density.