Mammals (formally Mammalia) are a class of vertebrate animals whose name is derived from their distinctive feature, mammary glands, which they use in feeding their young. They are also characterized by the possession of sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain. Except for the five species of monotremes (which lay eggs), all mammals give birth to live young. 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 30 to 40-millimetre (1.2 to 1.6 in) Bumblebee Bat to the 33-metre (110 ft) Blue Whale.
In intelligent mammals, such as primates, the cerebrum is larger relative to the rest of the brain. Intelligence itself is not easy to define, but indications of intelligence include the ability to learn, matched with behavioral flexibility. Rats, for example, are considered to be highly intelligent as they can learn and perform new tasks, an ability that may be important when they first colonize a fresh habitat. In some mammals, food gathering appears to be related to intelligence: a deer feeding on plants has a brain relatively smaller than a cat, who must think to outwit its prey.