Mammals

Mammals characteristics include numerous adaptations that enable them to survive in a wide range of environments. They live in nearly every habitat around the globe, from frigid polar regions, to turbulent seas, to dense tropical forests. Modern mammals range in stature from tiny field mice to massive whales and although various species may look drastically different, all mammals still share a unifying set of characteristics.

Some mammal characteristics—such as their hair, mammary glands, and three specialized middle-ear bones—are shared by no other groups of animals. Here we'll explore key facts and information about mammals so we can better understand important mammal characteristics.

Hair Hair is one of the characteristics of a mammal that is unique to mammals—no other animals have true hair and all mammals have hair covering at least part of their body at some time during their life. An individual hair consists of a rod of cells that are reinforced by a protein known as keratin. Hair grows from skin cells called follicles. Hair can take on several different forms including thick fur, long whiskers, defensive quills or fearsome horns. Hair serves a variety of functions for mammals. It provides insulation, protects the skin, serves as camouflage and provides sensory feedback.

Some mammals have thick coats of fur that consist of two layers, a soft underfur and a coarse protective outer fur. Sea otters, for example, have this type of two-layered fur. In fact, sea otters have one the thicket coats of fur of all mammals, with more than 100,000 individual hairs per square centimeter. Since sea otters lack a layer of insulating blubber, they must compensate by having fur with superior insulation power. Cetaceans, in contrast, have a thick layer of insulating blubber and therefore have lost most of their hair over the course of their evolution. Some whales only have hair during their early development, while others retain sparse patches of hair on their chin or upper lip.

Mammary Glands Mammals nurse their young with milk produced by mammary glands. Mammary glands, like hair, are a uniquely mammalian trait. Though present in both males and females, in most mammal species mammary glands only fully develop in females. The exception to this rule is the male Dayak fruit bat, which has mammary glands that produce milk to feed its young.

Mammary glands are modified and enlarged sweat glands that consist of ducts and glandular tissues that secrete milk through nipples. Young mammals obtain milk from their mother by feeding from her nipples. The milk provides the young with much needed protein, sugars, fat, vitamins and salts.

Not all mammals have nipples. Monotremes, which include echidnas and the platypus, diverged from other mammals early in their evolution. Monotremes lack nipples and instead secrete the milk produced by their mammary glands through ducts in their abdomen.