Mammal, common name applied to any warm-blooded animal belonging to the class that includes humans and all other animals that nourish their young with milk, that are covered with varying amounts of hair, and that possess a muscular diaphragm. Mammals have the most highly developed nervous systems of all animals. Most members of the group have four appendages, usually legs. These may be adapted for use as swimming appendages, as in seals, or as wings, as in bats. Some Types,, however, have two limbs that have been reduced to small vestiges beneath the skin, as in whales, or have been lost altogether, as in se