What is the purpose of mammalian tails?

Animal tails are used in a variety of ways. They provide a source of locomotion for fish and some other forms of marine life. Many land animals use their tails to brush away flies and other biting insects. Some species, including cats and kangaroos, use their tails for balance; and some, such as New World monkeys and opossums, have what are known as prehensile tails, which are adapted to allow them to grasp tree branches.

Tails are also used for social signaling. Some deer species flash the white underside of their tails to warn other nearby deer of possible danger, and canids (including domestic dogs) indicate emotions through the positioning and movement of their tails.

Human embryos have a tail that measures about one-sixth of the size of the embryo itself.[1] As the embryo develops into a fetus, the tail is absorbed by the growing body. The developmental tail is thus a human vestigial structure.[2][3] Infrequently, a child is born with a "soft tail", which contains no vertebrae, but only blood vessels, muscles, and nerves, although there have been several documented cases of tails containing cartilage or up to five vertebrae.[4]

Humans have a "tail bone" (the coccyx) attached to the pelvis, formed of fused vertebrae, usually four, at the bottom of the vertebral column. It does not protrude externally.