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Mammals evolved from reptiles 175 million years ago and diversified after the extinction of dinosaurs. They have sweat glands, hair, and a neocortex, giving them superior intelligence. Most care for their young and have a more complex social structure than other organisms.
Mammals are warm-blooded vertebrates that evolved in the Jurassic Period, about 175 million years ago. They evolved from reptiles. For over a hundred million years, mammals were small and not very diverse, but with the extinction of the dinosaurs in the Cretaceous-Tertiary extinction 65 million years ago, they grew in size and diversified. Common examples include rodents, bats, dogs, bears, cats, deer, sheep, goats, and humans. In all, there are about 5,400 species, distributed in about 1,200 genera, 153 families, and 29 orders. Most are terrestrial, with notable exceptions of whales and dolphins.
Animals classified as mammals usually have sweat glands, including milk-producing variants (mammary glands); hair all over the body; and a neocortex, a layer of the brain that gives them superior intelligence to reptiles and birds. The success of mammals over reptiles and other groups of animals over the past 65 million years has been a classic example of the triumph of brain over brawn.
Mammals typically care for their young after birth, unlike many other species. To aid in this, females have milk-producing glands called breasts that produce nutrient-rich milk for their young. The young develop in the uterus, where they are fed through an umbilical cord which is severed at birth.
Being warm-blooded, mammals are able to travel and hunt in areas where reptiles and large insects cannot. They stay warm by using their hair as insulation, which in some species can be quite thick. Indeed, humans are one of the few mammals without a significant amount of hair—the only other examples being the naked whale and mole rat. No one is quite sure why humans lost their hair.
Mammals tend to have a more complex social structure than many other organisms, with complex dominance hierarchies. They often live in groups led by an alpha male who fertilizes most of the females.
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