Members of the class Mammalia have hair and nourish their young with milk produced by mammary glands. The presence of body hair or fur helps maintain a constant body temperature in the homeothermic mammals.

Several types of mammals exist: the monotremes, marsupials, and placentals. Monotremes are egg-laying mammals that produce milk. The duck-billed platypus and the spiny anteater are examples. They are both found in Australia and probably developed during the geographic isolation of that continent.

Marsupials are mammals whose embryos develop within the mother’s uterus for a short period of time before birth. After birth, the immature babies crawl into the mother’s abdominal pouch, where they complete their development. Animals such as the kangaroo, opossum, and koala bear are marsupials.

The placental mammals include many familiar animals, such as rabbits, deer, dogs, cats, bats, whales, monkeys, and humans. These mammals have a placenta: a nutritive connection between the embryo and the mother’s uterine wall. Embryos are attached to the placenta, and they complete their development within the mother’s uterus.

Mammals have spread to virtually all environments on Earth, ranging from the oceans to the deserts. They live underground, on the ground surface, in trees, and in the air. Mammals have a highly developed nervous system, and many have acute senses of smell, hearing, taste, vision, and touch. Mammals rely on memory and learning to guide their activities. They have been able to develop numerous appropriate responses to different environmental situations. They are considered the most successful animals on Earth today.