Mammals are the class of vertebrate animals characterized by the presence of glands, which in females produce milk for the nourishment of young; the presence of hair or fur; and being warm blooded. Mammals encompass some 5,500 spehcies, distributed in about 1,200 genera, 152 families and up to 46 orders, though this varies with the classification scheme. They range in size from the 2 inch shrew to the 120 foot whale. Human beings are members of the Mammal Class, as well as dogs, cats, deer, mice, squirrels, raccoons, bats, opossums, and others.