The Eight Main Mammal Characteristics

Hair and Fur

All mammals have hair growing from some parts of their bodies during at least some stage of their life cycle. 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 with the sensitive whiskers the 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, such as whales? Many species, including ​whales and dolphins, 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.

Mammary Glands

Unlike other vertebrates, mammals nurse their young with milk produced by mammary glands, which are modified and enlarged sweat glands consisting of ducts and glandular tissues that secrete milk through nipples. This milk provides young with much-needed proteins, sugars, fats, vitamins, and salts. Not all mammals have nipples, however. Monotremes such as the platypus, which diverged from other mammals early in evolutionary history, secrete milk through ducts located in their abdomens.

Though present in both males and females, in most mammal species, mammary glands fully develop only in females, hence the presence of smaller nipples on males (including human males). The exceptions to this rule are the Dayak fruit bat and the Bismarck masked flying fox. Males of these species have the ability to lactate, and they sometimes help to nurse infants.

Single-Boned Lower Jaws

The lower jawbone of mammals is composed of a single piece that attaches directly to the skull. This bone is called the dentary because it holds the teeth of the lower jaw. In other vertebrates, the dentary is only one of several bones in the lower jaw and does not attach directly to the skull. Why is this important? The single-pieced lower jaw and the muscles that control it endows mammals with a powerful bite. It also allows them to use their teeth to either cut and chew their prey (like wolves and lions), or grind down tough vegetable matter (like elephants and gazelles).

One-Time Tooth Replacement

Diphyodonty is a trait common to most mammals in which teeth are replaced only once throughout an animal's lifetime. The teeth of newborn and young mammals are smaller and weaker than those of adults. This first set, known as deciduous teeth, fall out before adulthood and are gradually replaced by a set of larger, permanent teeth. Animals that replace their teeth continuously over the course of their lifetimes—such as sharks, geckos, alligators, and crocodiles—are known as polyphyodonts. (Polyphyodonts don't have tooth fairies. They'd go broke.) Some notable mammals that are not diphyodonts are elephants, kangaroos, and manatees.

Three Bones in the Middle Ear

The three inner ear bones, the incus, the malleus, and the stapes—commonly referred to as the hammer, the anvil and the stirrup—are unique to mammals. These tiny bones transmit sound vibrations from the tympanic membrane (a.k.a. the eardrum) to the inner ear and transform the vibrations into neural impulses that are then processed by the brain. Interestingly, the malleus and incus of modern mammals evolved from the lower jaw bone of the immediate predecessors of mammals, the "mammal-like reptiles" of the Paleozoic Era known as therapsids.

Warm-Blooded Metabolisms

Mammals aren't the only vertebrates to have endothermic (warm-blooded) metabolisms. It's a trait that's shared by modern birds and their ancestors, the theropod (meat-eating) dinosaurs of the Mesozoic Era, however, one can argue that mammals have made better use of their endothermic physiologies than any other vertebrate order. It's the reason cheetahs can run so fast, goats can climb the sides of mountains, and humans can write books. As a rule, cold-blooded animals like reptiles have much more sluggish metabolisms since they must rely on external weather conditions to maintain their internal body temperatures. (Most cold-blooded species can barely write poetry, although some of them are allegedly lawyers.)

Diaphragm

As with some of the other traits on this list, mammals aren't the only vertebrates to possess a diaphragm, a muscle in the chest that expands and contracts the lungs. However, the diaphragms of mammals are arguably more advanced than those of birds, and definitely more advanced than those of reptiles. What this means is that mammals can breathe and utilize oxygen more efficiently than other vertebrate orders, which, combined with their warm-blooded metabolisms allows for a wider range of activity and the fuller exploitation of available ecosystems.

Four-Chambered Hearts

Like all vertebrates, mammals have muscular hearts that contract repeatedly to pump blood, which in turn, delivers oxygen and nutrients throughout the body while removing waste products such as carbon dioxide. However, only mammals and birds possess four-chambered hearts, which are more efficient than the two-chambered hearts of fish or the three-chambered hearts of amphibians and reptiles. 

A four-chambered heart separates oxygenated blood coming from the lungs from the partially deoxygenated blood that heading back to the lungs to be re-oxygenated. This ensures that mammalian tissues only receive oxygen-rich blood, allowing for more sustained physical activity with fewer intervals of rest.