Mammals are not extraordinarily diverse in terms of species number – the roughly 5,400 species of mammals distributed around the globe is much less than the number of global bird species or fish species or certainly insect species. But mammals make up for their lack of species diversity with an incredible diversity of size, form, behaviour, and occupied habitats. From deep ocean whales, to flying bats, to tree dwelling monkeys, to underground moles that never see the light of day, mammals literally have the globe covered. The smallest flying bats weigh only a few grams and the largest of the whales weigh hundreds of thousands of kilograms. The shared features that connect all of this mammal diversity include milk producing mammary glands, specialized teeth, and a body covered in hair. Mammals are similar to birds in maintaining a warm and relatively constant body temperature, and thus mammals and birds are often called warm-blooded or endotherms (literally heat from within) or homeotherms (constant temperature). Mammals are similar to fish, birds, reptiles, and amphibians in possessing a backbone and thus are classified with them as vertebrates. So a formal definition of a mammal is an endothermic vertebrate that possesses mammary glands, differentiated teeth, and hair. In fact, some modern mammals have lost their teeth (e.g., anteaters and baleen whales) and most of their hair (like whales and some seals), but all mammals have mammary glands. Major groups of North American mammals include land mammals, like hoofed mammals, furbearers, and rodents, and aquatic mammals, like whales and seals.

Reference

Hickman CP, Roberts LS, Larson A: Animal Diversity. Boston: McGraw-Hill; 2000.