Mammals (class Mammalia) are a class of vertebrate animals characterized by the presence of sweat glands, including sweat glands modified for milk production, hair, three middle ear bones used in hearing, and a neocortex region in the brain.All mammals other than the monotremes give birth to live young instead of laying eggs. 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. Mammals encompass approximately 5,400 species, ranging in size from the Bumblebee Bat (30-40mm) to the Blue Whale (33 m), distributed in about 1,200 genera, 153 families, and 29 orders, though this varies by classification scheme. Reference Wikipedia.
