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A young African wild dog feeds on its mother’s milkA young African wild dog feeds on its mother’s milk Mammals are warm-blooded vertebrates: animals with backbones. Their key characteristic, one possessed by no other kind of animal, is their ability to nurture their young with milk. Milk is produced by the mammary glands in the female, a term which gives its name to the Class to which all such animals—humans included—belong: Mammalia.

Greater white-toothed shrewGreater white-toothed shrew

A diverse group

There about 5500 different species of mammal. They range in size from tiny shrews, only a few centimetres long, to the 30-metre (100-foot) blue whale. Most mammals have four limbs and a covering of hair or fur. Sea mammals, such as whales and dolphins, have adapted to life in the water by losing their hair and forming their hind limbs into a tail.

All except five species of mammal give birth to live young. The monotremes—the platypus and four species of echidna—all lay eggs. The majority of mammals are placentals: they give birth to young that develop fully in the womb.
Platypus, a monotremePlatypus, a monotreme
Marsupials, a much smaller group than the placentals, give birth to their young that have not yet fully developed. The young then continue to develop while clinging to their mother’s body.

All except five species of mammals give birth to live young. The monotremes—four species of echidna, together with the platypus—all lay eggs.

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