Mammal

Mammals are a class of vertebrate animals in the class Mammalia.

Mammals are a class of vertebrate animals in the class Mammalia, defined by traits including mammary glands that produce milk, hair or fur, and three middle ear bones. They include monotremes, marsupials, and placental mammals. The study of mammals is called mammalogy.

Characteristics

Mammals are typically identified by mammary glands, hair or fur, and three middle ear bones. Other commonly cited features include a neocortex, endothermy, and internal fertilization, although exceptions exist across mammal groups.

Classification

Living mammals belong to the class Mammalia and are usually grouped into monotremes, marsupials, and placental mammals. Sources differ on the exact number of species and higher-level orders, reflecting ongoing taxonomic revision.

Evolution

Mammals are part of Synapsida and diverged from the lineage leading to reptiles and birds hundreds of millions of years ago. They diversified strongly after the end-Cretaceous extinction and became the dominant terrestrial animal group in the Cenozoic.

Diversity and ecology

Mammals occupy habitats on land, in water, underground, in trees, and in the air. They include the smallest shrews and some bats, as well as the blue whale, the largest known animal.