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The Northern flying squirrel has silky grey and cinnamon brown fur, with white tipped and grey based belly hairs. They have a furred patagium (fleshy membrane) that extends from the wrist of the foreleg to the ankles of the hind leg. The tail is furred, flattened, rounded at the end, and long (80% of the length of the head and body). They have large black eyes, used for nighttime activity. Southern flying squirrels, which appear similar to northern flying squirrels, are distinguished by their smaller size and belly hairs which are white all the way to the base of the hair.
Northern flying squirrels are clumsy on the ground, but can glide gracefully from tree to tree. They sometimes share nests and live in groups of up to 8 adults and juveniles. Individual squirrels aggregate into single-sex groups for warmth during the winter. They are strictly nocturnal. Depending on habitat, the home range of this species ranges from 2 to 77 acres. Females are territorial, males are not. The population density can be up to ten squirrels per hectare in favorable conditions. The main predators of northern flying squirrels are owls, hawks, martens, weasels, coyotes, and the domestic cat. They avoid predation by being active at night and through their vigilance and agility in trees.
This squirrel species has a diet typical of all squirrels. They feed on nuts, acorns, truffles (above-ground parts of fungi), and lichens. They also supplement their diet with fruits, buds, sap and the occasional insect and bird egg. Northern flying squirrels diverge from many squirrels in that lichens and fungi are a large portion of the diet and are not just supplements. It is thought that northern flying squirrels hoard food for the winter, though not confirmed.
Courtship begins in March and may continue until late June, depending on the severity of the winter. One litter is born per year, and the female raises the young without the help of the male. Copulation occurs in early spring and is followed by a gestation period of 37 to 42 days. Usually, two to four young are born, though litters as small as one and as large as six have been recorded. Newborns are poorly developed and weigh only five to six grams. They have closed eyes and ears, fused toes, and a cylindrical tail. Development is slow in comparison to other mammals of similar size. By the sixth day, toes separate and the eyes open after 31 days. Young leave the nest at 40 days and are totally weaned after two months, though they may remain with the mother another month. Flying squirrels are fully grown by 240 days and can breed in their first summer after birth.
Flying squirrels have a stable population in Alaska, even though they are preyed upon by many predators. They are important to forest re-growth because they eat the above-ground fruiting parts (truffles) which grow from fungi underground. These fungi are only dispersed when animals like flying squirrels eat and expel their spores in feces as they travel the forests. When flying squirrels and other animals leave spores in cut or burned forest areas, they help to recolonize plants whose roots need the underground fungi to enhance nutrient absorption. Beyond their economic value to forest health and timber growth, flying squirrels are enjoyed by wildlife watchers who admire their habits and appearance. Logging can have serious impacts on flying squirrels, especially in clear cuts where scattered tall trees are eliminated. The squirrels need tall trees scattered through an area to travel safely across open spaces. If some tall trees and snags (dead trees) with cavities are kept in logging areas, flying squirrels will be able to use the habitat.