Tail. Most vertebrates (animals with backbones) have tails; many invertebrates (animals without backbones), such as spiders and certain insects, have no tails. All vertebrates have tails during at least one stage of their life histories. The frog, for example, has a tail as a tadpole, and the human embryo has a tail during its fifth to eighth week. A few human beings are born with small, imperfectly developed tails, which can be removed by surgery.

Tails vary widely as to size, shape, and use. The cow's tail is flexible and has a tuft of hair at the end, making it useful in whisking away flies and other insects. American monkeys and Old World chameleons use their long, slender tails for grasping. The tails of animals that live in water, such as fish and whales, have strong muscles and serve as aids to moving and steering. Birds also use their tails for steering, in flight. The male pheasant makes a colorful display of its tail feathers to attract the female. In some kinds of lizards, the tail breaks off easily, often allowing the animal to escape from the grasp of a predator. A new tail is then grown.