Squirrels are familiar to almost everyone. More than 200 squirrel species live all over the world, with the notable exception of Australia.
Like other rodents, squirrels have four front teeth that never stop growing so they don't wear down from the constant gnawing. Tree squirrels are the types most commonly recognized, often seen gracefully scampering and leaping from branch to branch. Other species are ground squirrels that live in burrow or tunnel systems, where some hibernate during the winter season.
Ground squirrels eat nuts, leaves, roots, seeds, and other plants. They also catch and eat small animals, such as insects and caterpillars. These small mammals must always be wary of predators because they are tasty morsels with few natural defenses, save flight. Sometimes groups of ground squirrels work together to warn each other of approaching danger with a whistling call.
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Tree squirrels are commonly seen everywhere from woodlands to city parks. Though they are terrific climbers, these squirrels do come to the ground in search of fare such as nuts, acorns, berries, and flowers. They also eat bark, eggs, or baby birds. Tree sap is a delicacy to some species.
Flying squirrels are a third, adaptable type of squirrel. They live something like birds do, in nests or tree holes, and although they do not fly, they can really move across the sky. Flying squirrels glide, extending their arms and legs and coasting through the air from one tree to another. Flaps of skin connecting limbs to body provide a winglike surface. These gliding leaps can exceed 150 feet (46 meters). Flying squirrels eat nuts and fruit, but also catch insects and even baby birds.
Whether they dwell high in a tree or in an underground burrow, female squirrels typically give birth to two to eight offspring. Babies are blind and totally dependent on their mothers for two or three months. Mothers may have several litters in a year, so most squirrel populations are robust. - in: http://animals.nationalgeographic.com/animals/mammals/squirrel/