Mammals (class Mammalia) are warm-blooded, vertebrate animals characterized by the presence of sweat glands, including milk producing sweat glands, and by the presence of: hair, three middle ear bones used in hearing, and a neocortex region in the brain. Also all mammals, other than the monotremes, give birth to live young instead of laying eggs. Most mammals also possess specialized teeth and utilize a placenta in the ontogeny. The mammalian brain regulates endothermic and circulatory systems, including a four-chambered heart. Mammals encompass approximately 5,400 species (including humans), distributed in about 1,200 genera, 153 families, and 29 order, 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 oth