Foxes are swift and agile runners which live in family groups. A female fox is called a "vixen", and a male is called a "dog". Foxes' tails are multi-purpose organs. Their bushy tail helps them keep warm while they are sleeping in cold weather. It is also part of the animal's food store for wintertime. Foxes' plump, bushy tail is easily seen, and is used for sending signals to its family members. The tail is also used for balance while running.