mammal

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mammal Any member of the class Mammalia of vertebrate animals characterized by mammary glands in the female and full, partial or vestigial hair covering. Mammals are warm-blooded. They have a four-chambered heart with circulation to the lungs separate from the rest of the body. As a group, mammals are active, alert, and intelligent. They usually bear fewer young than other animals, and give them longer and better parental care. Most mammals before birth grow inside the mother's body and are nourished from her by means of a placenta. When born, they continue to feed on milk from the mother's mammary glands. There is a wide range of features, shapes, and sizes among mammals. Mammals include 17 orders of placentals, one marsupial order – all live-bearing – and an order of egg-laying monotremes. They probably evolved c.180 million years ago from a group of warm-blooded reptiles. Today, mammals range in size from shrews weighing a few grams to the blue whale, which can weigh as much as 150 tonnes.

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