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Mammals (formally Mammalia) are a class of vertebrate animals whose name is derived from their distinctive feature, mammary glands, which they use in feeding their young. They are also characterized by the possession of sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain. Except for the five species of monotremes (which lay eggs), all mammals give birth to live young. Most mammals also possess specialized teeth, and the largest group of mammals, the placentals, use a placenta during gestation. The mammalian brain regulates endothermic and circulatory systems, including a four-chambered heart. There are approximately 5,400 species of mammals, ranging in size from the 30–40-millimetre (1.2–1.6&_160;in) Bumblebee Bat to the 33-metre (110&_160;ft) Blue Whale, distributed in about 1,200 genera, 153 families, and 29 orders,[1] though this varies by classification scheme. Most mammals belong to the placental group. The four largest orders within the placental mammals are Rodentia (mice, rats, and other small, gnawing mammals), Chiroptera (bats), Carnivora (dogs, cats, bears, and other mammals that primarily eat meat), and Cetartiodactyla (including numerous herbivore species, such as deer, sheep, goats, and buffalos, plus whales). The human species is also a placental mammal, a member of the order Primates.
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