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 dinosaurs in the Cretaceous-Tertiary extinction 65 million years ago, mammals grew in size and diversified. Common mammals include rodents, bats, dogs, bears, cats, deer, sheep, goats, and humans. In all, there are about 5,400 species of mammals, distributed in about 1,200 genera, 153 families, and 29 orders. Most mammals are terrestrial, whales and dolphins being important exceptions.

Mammals are characterized by sweat glands, including variants that produce milk (mammary glands), hair all over their body, and a neocortex, a layer of brain that gives mammals superior intelligence to reptiles and birds. The success of mammals over reptiles and other animal groups in the last 65 million years has been a classic example of brain triumphing over brawn.

Mammals care for their young after birth, unlike many other species. To assist in this, female mammals have milk-producing glands called breasts. These produce nutrient-rich milk for young. Young grow up in the womb, where they are nourished through an umbilical cord which is severed at birth.

Being warm-blooded, mammals are capable of traveling through and hunting in areas where reptiles and large insects cannot. Mammals stay warm using hair as insulation, which can be quite thick in some species. In fact, humans are one of the only mammals without substantial amounts of hair — the only other examples are the whale and the naked 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. Often, mammals live in groups led by an alpha male that impregnates most of the females.