Squirrels are members of the family Sciuridae, a family that includes small or medium-size rodents. The squirrel family includes tree squirrels, ground squirrels, chipmunks, marmots, flying squirrels, and prairie dogs, amongst other rodents. Squirrels are indigenous to the Americas, Eurasia, and Africa, and were introduced by humans to Australia.The earliest known squirrels date from the Eocene period and are most closely related to the mountain beaver and to the dormouse, among other living rodent families.
Squirrels are generally small animals, ranging in size from the pygmy squirrel,at 7–10 cm in length and just 10 g in weight, to the giant flying squirrel, at 1.08 m in length and the marmot, which weighs from 5 to 8 kg. Squirrels typically have slender bodies with bushy tails and large eyes. In general, theirfur is soft and silky, although much thicker in some species than others. The colour of squirrels is highly variable.
In general, the hind limbs are longer than the fore limbs, and they have four or five toes on each paw. Their paws include an often poorly developed thumb and have soft pads on the undersides. Unlike most mammals, tree squirrels can descend a tree head-first. They do so by rotating their ankles 180 degrees so the hind paws are backward-pointing and can grip the tree bark. Squirrels live in almost every habitat from tropical rain forest to semiarid desert, avoiding only the high polar regions and the driest of deserts. They are predominantly herbivorous, subsisting on seeds and nuts, but many will eat insects and even small vertebrates.
As their large eyes indicate, in general squirrels have an excellent sense of vision, which is especially important for tree-dwelling species. They also have very versatile and sturdy claws for grasping and climbing.Many also have a good sense of touch, with hairs on their heads and limbs. The teeth of sciurids follow the typical rodent pattern, with large gnawing incisors that grow throughout life, and grinding cheek teeth set back behind a wide gap.
Many juvenile squirrels die in the first year of life. Adult squirrels can have a lifespan of 5 to 10 years in the wild. Some can survive 10 to 20 years in captivity. Some causes of premature death include nests which have fallen out of trees, of which the mother will leave the young if the body temperature is not correct. Many squirrels have been rescued by people prompting fostering of baby squirrels or wildlife rehabilitators before turning them back into the wild.
Squirrels breed once or twice a year and give birth to a varying number of young after three to six weeks, depending on species. The young are born naked, toothless, and blind. In most species of squirrel, only the female looks after the young, which are weanedat around six to ten weeks of age and become sexually mature at the end of their first year. In general, ground-dwelling species are social animals, often living in well-developed colonies, but the tree-dwelling species are more solitary.
Ground and tree squirrels are typically diurnal, while flying squirrels tend to be nocturnal—except for lactating flying squirrels and their offspring, which have a period of diurnally during the summer.
Squirrels cannot digest cellulose, so they must rely on foods rich in protein, carbohydrates and fats. In temperate regions, early spring is the hardest time of year for squirrels, because buried nuts begin to sprout and are no longer available for the squirrel to eat, and new food sources have not become available yet. During these times, squirrels rely heavily on the buds of trees. Squirrels’ diets consist primarily of a wide variety of plants, including nuts, seeds, conifer, cones, fruits, fungi and vegetations. However, some squirrels also consume meat, especially when faced with hunger. Squirrels have been known to eat insects, eggs, small birds, young snakes, and smaller rodents. Indeed, some tropical species have shifted almost entirely to a diet of insects
Predatory behaviour has been noted by various species of ground squirrels, For example, Bailey, a scientist in the 1920s, observed a thirteen-lined ground squirrel preying upon a young chicken. Wistrand reported seeing this same species eating a freshly killed snake.Whitaker examined the stomachs of 139 thirteen-lined ground squirrels and found bird flesh in four of the specimens.