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The more than 4,000 species of living mammals belong to the vertebrate class Mammalia. This diverse group of animals has certain common features: all have four legs, bodies covered by hair, a high and constant body temperature, a muscular diaphragm used in respiration, a lower jaw consisting of a single bone, a left systemic aortic arch leaving the left ventricle of the heart, and three bones in the middle ear. In addition, all female mammals have milk-producing glands. There are three living subclasses of mammals: the Monotremata (egg-laying mammals), the Marsupialia (pouched mammals), and the Placentalia (placental mammals).

Mammals range in size from bats, some of which weigh less than 1 oz (28.4 g), to the blue whale, which weighs more than 200,000 lb (90,800 kg). Mammals are found in cold arctic climates, in hot deserts, and in every terrain in between. Marine mammals, such as whales and seals, spend most of their time in the ocean. While mammals are not as numerous and diverse as, for example, birds or insects, mammals have a tremendous impact on the environment, particularly due to the use of Earth's natural resources by one species of mammal: humans.

Species of mammals have developed varying adaptations in response to the different environments in which they live. Mammals in cold climates have insulating layers—a thick coat of fur, or a thick layer of fat (blubber)—that help retain body heat and keep the animal's body temperature constant. Some mammals that live in deserts survive by special adaptations in their kidneys and sweat glands, as well as by their ability to avoid heat by behavioral means. Other adaptations for survival in extreme climates include hibernation (a state of winter dormancy) or estivation (summer dormancy). These responses make it possible for the animal to conserve energy when food supplies become scarce.

The care of the young (parental care) is notable among mammals. Born at an average of 10% of its mother's weight, mammalian young grow rapidly. The protection the young receive from one or both parents during the early stages of their lives enables mammals to maintain a strong survival rate in the animal kingdom.

The subclass Placentalia contains the majority of living mammals. The embryo of placentals develops in the mother's uterus, is nourished by blood from the placenta, and is retained until it reaches an advanced state of development. The Marsupialia are found in Australia and in North and South America. Their young develop inside the uterus of the mother, usually with a placenta connected to a yolk sac. Young marsupials are born in a very undeveloped state and are sheltered in a pouch (the marsupium) which contains the nipples of the milk glands. Kangaroos, wallabies, and most Australian mammals are marsupials, as is the opossum of the New World. The Monotremata of Australia include the duck-billed platypus and two species of spiny anteaters. Monotremes lay eggs, but have hair and secrete milk like other mammals.

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