Mammals (formally Mammalia) are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain. There are approximately 5,400 species of mammals. Except for the five species of monotremes (which lay eggs), all mammal species give birth to live young. Most mammals also possess specialized teeth. The mammalian brain regulates endothermic (Warm blood in constant temperature) and circulatory systems, including a four-chambered heart. Though most mammals live on land, there are mammals that live beneath the ground, glide and fly in the air, and even mammals that spend their entire life at sea.