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Mammals are a class of vertebrate animals characterized by the production of milk in females for the nourishment of young, from mammary glands present on most species and specialized skin glands in monotremes that seep or ooze milk. There is also presence of hair or fur, specialized teeth, three small bones within the ear, the presence of a neocortex region in the brain, endothermic or "warm-blooded" bodies, and, in most cases, the existence of a placenta in the ontogeny. The 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 orders,[1] though this varies with the classification scheme.