Animals that belong to the class Mammalia are charac­terized as having bodies that are basically covered with hair, delivering live young instead of eggs (the mono* tremes are an exception), and, pertinent to this chapter, nurtur­ing their young through the use of structures called mammary glands.

The ability of mammals to nurture their young through milk secretion by mammary glands during the early part of postfetal life has given these animals survival advantages. Because the reproductive strategy of mammals involves the production of far fewer young, compared with reptiles, amphibians, and birds, mammary glands have allowed mammals to be much more efficient in the nurture of their young.

Egg-laying classes of animals, such as fish, reptiles, and amphibians, depend on favorable environmental factors for the nurture of their young; the offspring are often vulnerable to the vagaries of nature. Mammalian young do not require teeth for the suckling process and thus can be delivered with immature maxillae and mandibles, which facilitates the delivery of the head. The development of teeth coincides with the need to consume food other than milk.

More on the topic Animals that belong to the class Mammalia are charac­terized as having bodies that are basically covered with hair, delivering live young instead of eggs (the mono* tremes are an exception), and, pertinent to this chapter, nurtur­ing their young through the use of structures called mammary glands.: