Squirrels are classified as tree, flying and ground. These types evolved during the animals' migration to Europe, Asia, Africa and South America. Tree and flying squirrels showed up in Eurasia 30 million years ago, traveling over land bridges like those where the Bering Strait is now. These land masses disappeared below sea level periodically. When land resurfaced, no trees would be present. This ultimately produced the ground squirrel. Africa collided with Eurasia about 18 million years ago. Migration and diversification of squirrels from Eurasia began shortly after this event. Squirrels entered South America less than 3 million years ago over the Isthmus of Panama. All South American squirrels, except one type, are believed to have one common ancestor from North America.
Biologists have identified about 280 species of squirrels. Their habitats range from burrows, caves, frozen tundra and treetops to city parks and home attics. Squirrels can range from the minute, tree-dwelling African pygmy squirrel, which is the size of a mouse, to a ground squirrel such as the marmot, which lives in burrows and can weigh over 10 lbs.
Squirrels will eat anything. Of course, they eat nuts, berries and insects; they also eat bird eggs, baby birds, birdseed, tree sap, human food and garbage. In North America, gray squirrels eat mostly nuts, acorns and berries. They store nuts for the wintertime. The flying squirrel, Northern and Southern, is completely omnivorous. It eats birds and even carrion. Flying squirrels also store nuts. Ground squirrels hibernate, but tree and flying squirrels only enter a state of torpor. Ground squirrels in the desert practice a type of heat hibernation called estivation. Hibernation, torpor and estivation reduce body functions to the minimum to avoid extreme cold or heat, and to conserve energy when food is scarce.
In the wild, squirrels are prey for any animal that eats meat. For squirrels in North America, this includes owls, hawks, foxes, snakes, coyotes, domestic dogs and cats, and humans. Squirrels are considered game animals, and most states have established a hunting season for them. In cities, the biggest threat to a squirrel is getting hit by a car. Squirrels chatter, squeal or bark to warn other squirrels if a predator approaches. Any threat causes the squirrel to run toward its home, whether in a burrow or a tree.