Eastern Gray Squirrel
Scientific name: Sciurus carolinensis
Common name: Eastern gray squirrel
The eastern gray squirrel is a very common, North American, tree dwelling rodent. It is found in a great variety of woodland, parkland and suburban and even urban habitats. The principle requirement to make a habitat suitable for a gray squirrel is a predictable abundance of food. The primary food required by the gray squirrel is "mast" (i.e. the fruits (nuts) of forest trees such as oaks, beeches, hickories etc). This is the essential calorie source needed for the seasonal buildup of over-wintering fat and also represents an ideal storage food product for the long winter season. Gray squirrels bury large quantities of mast in communal cache zones within their overlapping territories in forest ecosystems. These communal mast burial areas not only assist the gray squirrels in their over-wintering survival and also contribute to the dispersion and germination of many hardwood trees. Gray squirrels also consume many other foods through the year (including mushrooms, tree flowers and buds, caterpillars, and plant shoots). They seem to require a diverse food base in order to accumulate needed micro-nutrients for both normal growth and for reproduction. The ideal habitat for a gray squirrel, then, must have abundant mast producing trees and a diverse array of other food sources that typically come into availability at different times of their growing seasons.
Gray squirrels are really not gray in color at all but are instead a mixture of blacks, whites and browns. Most hairs on a "gray" squirrel, in fact, are banded with all three colors but are tipped with whitish ends that generate the "gray" illusion. The back hairs of the squirrel tend to be darker and its belly hairs tend to be lighter in color. Within any population of gray squirrels lighter and darker individuals can be found. "White squirrels" or "black squirrels" may be locally favored by natural
human-generated selection forces. The undisturbed North American population of gray squirrels was, according to historical records, predominately made up of "black", gray squirrels probably due to the effectiveness of the black coloration as an aid in hiding from avian predators such as hawks or owls. The black squirrel, however, was very clearly outlined against the light colored sky when humans hunted the squirrels from the forest floor. This human hunting pressure, apparently, favored the mixed, "gray" coloration that even today predominates in most North American populations.
Gray squirrels live in two types of dwelling structures: dens, which are holes typically constructed in healthy, living trees (often by the expansion of abandoned woodpecker holes) and nests, which are densely packed masses of sticks and leaves (and a great variety of any other available natural and human-made materials). Nests are usually located high up in the forked branches of large trees. Dens are preferred for over-wintering and for brood chambers, but nests are (with constant maintenance) very weather proof and sturdy habitations.
Gray squirrels do not hibernate but instead rely on their fat reserves and cached mast stores to survive the long, cold winters. Gray squirrels can be seen out in the winter months as long as the temperatures are not too cold (not below 30 degrees F) and as long as it is not raining or snowing. Food forages are calculated risks in which the use of energy (fat) reserves must yield a "profit" or the squirrel's vital fat insulation layer will be steadily lost and the individual will be unable to survive the winter.
Gray squirrels are eaten by a large number of organisms including black snakes, rattlesnakes, weasels, skunks, and red foxes. The body of a gray squirrel was once even found in the stomach of a large mouth bass! Gray squirrels' most significant predators, though, are unquestionably birds of prey, especially hawks and owls.
Gray squirrels are diurnal animals. Their eyes are adapted to high light levels and have even greater visual acuity than human eyes. The angling of the eyes so that they are pointing slightly upward is thought to be an adaptation to help them to watch constantly for avian predators. The positioning of the "blind spot" of the eye (the part of the retina in which photoreceptors are absent) into the lower visual field is also thought to give the squirrel full view of the sky and any incoming hawk or owl.
Gray squirrels can mate twice a year of food is very abundant. The first mating occurs in January and generates the "winter born" litter in March. The second mating occurs in the late spring or early summer and results in a "summer born" litter that developed quickly to be fully weaned and independent by the winter. Only 25% of gray squirrels survive their first year. Mortality remains quite high for the first two years of life after which the mature, experienced gray squirrel has a high survival rate for another four of five years. In wild populations of gray squirrels, though, there are almost no individuals older than 7 years. The impact of disease and debilitation (tooth loss, cataracts, parasites etc) slow down older individuals just enough to compromise their ability to survive.