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Mammals are members of class Mammalia, air-breathing
vertebrate animals characterized by the possession of endothermic, hair, three
middle ear bones, and mammary glands functional in mothers with young. Most
mammals also possess sweat glands and specialized teeth.
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The largest group of
mammals, the placentals, have a placenta which feeds the offspring during
gestation. The mammalian brain, with its characteristic neocortex, regulates
endothermic and circulatory systems, the latter featuring red blood cells
lacking nuclei and a large, four-chambered heart maintaining the very high
metabolism rate they have. Mammals range in size from the 30–40 millimeter bumblebee bat to the 33-meter (108-foot) blue whale.
The word "mammal" is modern, from the
scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the
Latin mamma ("teat, pap"). All female mammals nurse their young with
milk, which is secreted from special glands, the mammary glands.
According to
Mammal Species of the World, which is updated through periodic editions, 5,676
species were known in 2005. These were distributed in 1,229 genera, 153
families and 29 orders.
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