Mammals are a diverse group of animals that range in size from the blue whale weighing over 100 metric tons to a Kitti’s bat at 1.5 grams. Mammals are thought to have evolved from a group of mammal-like reptiles called the therapsids.

Mammals have large brains and a larger skull in relation to body size. The well developed nervous system supports many specialized sensory functions. The most distinguishing characteristics of the mammals are their milk-producing mammary glands and the presence of hair. The mammary glands exude milk to nourish the young, while hair provides warmth and coloration, and can also act as a sensing device.

A complete list of characteristics follows:

Mammals are divided into three major groups based on how they give birth. There are the Monotremes, Marsupials, and Eutherians.

Monotremes
Monotremes reproduce via hard-shelled, amniotic eggs. The platypuses lay eggs in a nest, while echidnas tuck their eggs into a belly pouch. These mammals have elongated, modified snouts and no teeth.

Monotreme means "one-holed," a reference to their possession of a cloaca, a single opening that serves as a joint outlet for the urinary, intestinal, and reproductive tracts.

Marsupials
Marsupials are a diverse group known commonly as the pouched mammals. They are viviparous, meaning they give birth very early in development. The newborns crawl into a pouch on the maternal belly, where they attach to a nipple to and continue to mature for weeks or months, depending on the species. This behavior is riskier for the offspring but easier on the mother than carrying a fetus long-term. Marsupials have a cloaca with a separate genital tract.

Modern marsupials include the opossum, koala, and kangaroo. Except for the opossum, they are largely confined to Australia.

Eutherians
Eutherian mammals are the placental mammals. They are viviparous, and in this case they retain young for a long period of time within the mother’s uterus. The fetus is nourished by a placenta, which shuttles nutrients and wastes between the maternal and fetal bloodstreams.

This reproductive strategy allows for a newborn to be large and somewhat independent at birth – some are able to stand and even run within moments. Eutherians also have separate urinary, intestinal, and reproductive openings, and modifications to the ankle and pelvic bones not shared with other mammals.

Eutherians are the most diverse, abundant, and widespread mammals, and there are more than 5,000 species. This group includes burrowing mice, flying bats, swimming whales, and walking humans. It populates all of the continents with the exception of Antarctica.