Caring for offspring after birth is a common feature of mammals.
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 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 whales and dolphins being important exceptions.
Whales are one of the few mammals without large amounts of hair.
Animals classified as mammals often have sweat glands, including variants that produce milk (mammary glands); hair all over the body; and a neocortex, a layer of the brain that gives them intelligence superior 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 muscle.
Humans and other mammals have superior intelligence due to the neocortex in their brain.
Mammals typically care for their young after birth, unlike many other species. To help with this, women have milk-producing glands called breasts, which produce nutrient-rich milk for their young. The young develop in the womb, where they are nourished by an umbilical cord that is cut at birth.
Being warm-blooded, mammals are able to travel and hunt in areas where reptiles and large insects cannot. They stay warm using their hair as an insulator, which can be quite thick in some species. In fact, humans are one of the only mammals without large amounts of hair – the only other examples being 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. They often live in groups led by an alpha male who impregnates most of the females.