The class Mammalia has three unique characteristics: hair, milk production in the mammary glands and three bones in the middle ear. Characteristics shared by other classes of animals include a warm-blooded metabolism, four-chambered heart and four limbs. Members of class Mammalia are called mammals.
Most mammals give birth to live young; the exceptions to this are five species of egg-laying mammals called monotremes. The platypus and four species of echidna are the only living monotremes. Marsupials are a group of mammal species that give birth to less-developed live young that cannot survive away from the mother. Newborn marsupials continue to grow in an pouch outside of the mother's body. Kangaroos, koalas, opossums, and wallabies are all examples of marsupials.
Mammals live on every continent and range in size from the 3-centimeter bumblebee bat to the 108-foot blue whale. They vary greatly in diet and habitat. There are herbivorous, carnivorous and omnivorous mammals, and mammals that live on land, underwater or both. Bats are the only mammals with the true ability to fly, but some other mammals glide between trees. Cetaceans, the group that includes dolphins and whales, are the only mammals that live exclusively underwater, but mammals such as seals and otters are able to move between land and water easily.