Mammal

A mammal is a member of the class Mammalia, the group of backboned animals that includes humans, whales, bats, mice and kangaroos. Two features mark every mammal: hair, made of the protein keratin, at some stage of life, and mammary glands, which secrete milk to nourish newborns. Mammals are also endotherms, keeping a warm, fairly steady body temperature with heat from their own metabolism.

The skeleton identifies mammals even as fossils. The lower jaw is a single bone, the dentary, and two small bones that formed the jaw joint in their ancestors became the malleus and incus of the middle ear. Most mammals also have teeth of several kinds (incisors, canines, premolars and molars) that are replaced only once, a muscular diaphragm that drives breathing, a four-chambered heart and red blood cells that lack a nucleus. Mammals evolved from cynodonts, a branch of earlier synapsids, and the first true mammals appear in rocks about 200 million years old.

Living mammals fall into three groups: monotremes such as the platypus and echidnas, which lay eggs; marsupials such as kangaroos and opossums, whose tiny young usually finish growing in a pouch; and placental mammals, which nourish the fetus through a placenta and make up the great majority of species.