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.
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.
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.
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 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.
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.