But, as time passed this part of the integument became advantageous for nocturnal organisms feeding in the cooler nights. Hair is only one part of a complex integument the covers mammals and a variety of different glands in the skin provide protection as the body covering, thermoregulation, chemical communication, and even nutrition. Homeothermy requires a mechanism to prevent heat loss, insulative hair, as well as cooling which is accomplished by the sweat glands. Sebaceous glands keep the hairs soft, pliable or waterproof and secretions of the apocrine glands produce distinctive scents that are usesd to mark territory and permit communicate between animals. At some point secretions of the skin glands were also nutritive and developing young lapped up their secretions from what were to ultimately form the mammary glands that give this class of animals their name.
Mammals also adopt a strategy for their developing young that involves increased parental care.
Young are nourished from the mammary glands and protected by the parent for the early part of their lives. The strategy of investing parental effort and time into only a few young goes one step further by maintaining the embryo internally and the transition from the egg laying montreme, to the marsupial and finally placental mammals exemplifies this transition. Only in the placental mammals is the developing embryo no longer seen as an invading parasite. Mammal taxonomy is based on teeth and often fossil mammals can be identified to the level of species by only a jaw and a few teeth. The reason for this is that the shapes of the teeth are closely related to the diet that the animal feeds on.