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...