Mammals are amazingly diverse animals: they live in nearly every available habitat on earth (including deep seas, tropical rainforests, and deserts), and they range in size from one-ounce shrews to 200-ton whales. What exactly is it that makes a mammal a mammal, and not a reptile, a bird or a fish? On the following slides, you'll learn about the eight main mammal characteristics, ranging from hair to four-chambered hearts.
All mammals have hair growing from some parts of their bodies during at least some stages of their life cycles. Mammalian hair can take on several different forms, including thick fur, long whiskers, defensive quills, and even horns. Hair serves a variety of functions: insulation against the cold, protection for delicate skin, camouflage against predators (as in zebras and giraffes), and sensory feedback (as witness the sensitive whiskers of your everyday house cat). Generally speaking, the presence of hair goes hand-in-hand with a warm-blooded metabolism.
What about mammals that don't have any visible body hair, like whales or Olympic swimmers? In the case of whales and dolphins, many species have sparse amounts of hair during the earliest stages of their development, while others retain wispy patches of hair on their chins or upper lips. And, of course, even completely hairless-looking humans still retain the hair follicles in their skin!
Unlike other vertebrates, mammals nurse their young with milk produced by mammary glands. Though they're present in both males and females, in most mammal species mammary glands only fully develop in the females, hence the presence of smaller nipples on males (including male humans). The exception to this rule is the male Dayak fruit bat, which nature has endowed (for better or worse) with the task of breastfeeding.
Mammary glands are modified and enlarged sweat glands consisting of ducts and glandular tissues that secrete milk through nipples; the milk provides young with much-needed proteins, sugars, fats, vitamins, and salts. However, not all mammals have nipples: monotremes like the platypus, which diverged from other mammals early in evolutionary history, instead secrete the milk produced by their mammary glands via ducts located in their abdomens.