Mammals (class Mammalia) are a class of vertebrate animals characterized by the presence of sweat glands, including sweat glands modified for milk production, hair, three middle ear bones used in hearing, and a neocortex region in the brain.

All mammals (except for the five species of monotremes) give birth to live young instead of laying eggs. Most mammals also possess specialized teeth, and the largest group of mammals, the placentals, use a placenta during gestation. The mammalian brain regulates endothermic and circulatory systems, including a four-chambered heart.

There are approximately 5,400 species of mammals, ranging in size from the Bumblebee Bat to the Blue Whale, distributed in about 1,200 genera, 153 families, and 29 orders, though this varies by classification scheme.

Most mammals belong to the placental group. The four largest orders within the placental mammals are Rodentia (mice, rats, and other small, gnawing mammals), Chiroptera (bats), Carnivora (dogs, cats, bears, and other mammals that primarily eat meat), and Cetartiodactyla (including numerous herbivore species, such as deer, sheep, goats, and buffalos, plus whales). The human species is also a placental mammal, a member of the order Primates.

Phylogenetically, Mammalia is defined as all descendants of the most recent common ancestor of monotremes (e.g., echidnas and platypuses) and therein mammals (marsupials and placentals). This means that some extinct groups of "mammals" are not members of the crown group Mammalia, even though most of them have all the characteristics that traditionally would have classified them as mammals. These "mammals" are now usually placed in the unranked clade Mammaliaformes.

The mammalian line of descent diverged from the sauropsid line at the end of the Carboniferous period. The sauropsids would evolve into modern-day reptiles and birds, while the synapsid branch led to mammals. The first true mammals appeared in the Jurassic period. Modern mammalian orders appeared in the Palaeocene and Eocene epochs of the Palaeogene period.

Distinguishing features

Living mammal species can be identified by the presence of sweat glands, including those that are specialized to produce milk.

However, other features are required when classifying fossils, since soft tissue glands and some other features are not visible in fossils. Paleontologists use a distinguishing feature that is shared by all living mammals (including monotremes), but is not present in any of the early Triassic synapsids: mammals use two bones for hearing that were used for eating by their ancestors. The earliest synapsids had a jaw joint composed of the articular (a small bone at the back of the lower jaw) and the quadrate (a small bone at the back of the upper jaw). Most reptiles and non-mammalian synapsids use this system including lizards, crocodilians, dinosaurs (and their descendants the birds), and therapsids (mammal-like "reptiles"). Mammals have a different jaw joint, however, composed only of the dentary (the lower jaw bone which carries the teeth) and the squamosal (another small skull bone). In mammals the quadrate and articular bones have become the incus and malleus bones in the middle ear. Note: "non-mammalian synapsids" above implies that mammals are a sub-group of synapsids, and that is exactly what cladistics says they are.