Mammals are a vertebrate class (Mammalia) characterized by endothermy, hair or fur, and the presence of mammary glands that produce milk to nourish offspring. They possess a neocortex, three middle ear ossicles, and a single-boned lower jaw articulating with the squamosal bone. Most exhibit viviparity with complex placentation (except monotremes, which are oviparous) and show determinate growth with advanced parental care and social behaviors. Mammalian physiology includes high metabolic rates, a four-chambered heart, and differentiated dentition, supporting ecological diversification into terrestrial, aerial, and aquatic niches across nearly all biomes.
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