The mammals are the 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; the presence of hair or fur; specialized teeth; three minute bones within the ear; the presence of a neocortex region in the brain; and endothermic or "warm-blooded" bodies. The brain regulates endothermic and circulatory systems, including a four-chambered heart. Mammals encompass some 5,500 species (including humans), distributed in about 1,200 genera, 152 families and up to 46 orders, though this varies with the classification scheme. Most mammals give birth to live young, but a few (the monotremes) lay eggs. Live birth also occurs in some non-mammalian species, such as guppies and hammerhead sharks; thus it is not a distinguishing characteristic of mammals.