mammal

Mammalia is a class of endothermic vertebrates within the phylum Chordata. Members are united by key features that distinguish them from other animals, including the presence of hair or fur at some life stage, mammary glands producing milk for their young, and a three-bone middle ear (the malleus, incus, and stapes). Most mammals also have a four-chambered heart, a diaphragm, and a highly developed brain, though diversity in size, form, and ecology is vast.

All mammals nurse their young with milk; in most species, mothers provide parental care after birth. Reproduction and development vary among the three major lineages. Monotremes (e.g., platypus and echidnas) lay eggs and hatchlings usually require extensive parental care. Marsupials (e.g., kangaroos, opossums) give birth to underdeveloped young that complete development in a pouch. Placental mammals (e.g., humans, whales, rodents) retain their embryos inside the uterus, nourished by a placenta until a relatively advanced stage of development.

Extant mammals comprise roughly 6,000 species, occupying a wide range of habitats across all continents and many oceans. They range from tiny bumblebee bats to the blue whale, and from fossorial moles to arboreal primates. Their diets are highly varied, including herbivory, carnivory, and omnivory, and they have adapted to freshwater, terrestrial, and marine environments.

Mammals have a fossil record dating to the late Triassic, and they diversified significantly after the extinction event at the end of the Cretaceous, leading to the broad distribution and ecological roles seen today.