A Guide to characteristics of Class Mammalia

The Class Mammalia is well represented in Southern Africa. There are 293 species of land mammals and 37 species of marine mammals in the Southern African subregion. That is 330 of the around 5000 mammal species found on Earth!

Class Mammalia -- all mammals share three characteristics not found in other animals: 3 middle ear bones; hair; and the production of milk by modified sweat glands called mammary glands.

Mammals hear sounds after they are transmitted from the outside world to their inner ears by a chain of three bones, the malleus, incus, and stapes. Two of these, the malleus and incus, are derived from bones involved in jaw articulation in most other vertebrates.

Mammals have hair. Adults of some species lose most of their hair, but hair is present at least during some phase of the ontogeny of all species. Mammalian hair, made of a protein called keratin, serves at least four functions. First, it slows the exchange of heat with the environment (insulation). Second, specialized hairs (whiskers or "vibrissae") have a sensory function, letting the owner know when it is in contact with an object in its external environment. These hairs are often richly innervated and well-supplied with muscles that control their position. Third, through their color and pattern, hairs affect the appearance of a mammal. They may serve to camouflage, to announce the presence of especially good defense systems (for example, the conspicuous color pattern of a skunk is a warning to predators), or to communicate social information (for example, threats, such as the erect hair on the back of a wolf; sex, such as the different colors of male and female capuchin monkeys; presence of danger, such as the white underside of the tail of a whitetailed deer). Fourth, hair provides some protection, either simply by providing an additional protective layer (against abrasion or sunburn, for example) or by taking on the form of dangerous spines that deter predators (porcupines, spiny rats, others).

Mammals feed their newborn young with milk, a substance rich in fats and protein that is produced by modified sweat glands called mammary glands. These glands, which take a variety of shapes, are usually located on the ventral surface of females along paths that run from the chest region to the groin. They vary in number from two (one right, one left, as in humans) to a dozen or more.

Other characteristics found in most mammals include highly differentiated teeth; teeth are replaced just once during an individual's life (this condition is called diphyodonty, and the first set is called "milk teeth); a lower jaw made up of a single bone, the dentary; four-chambered hearts, a secondary palate separating air and food passages in the mouth; a muscular diaphragm separating thoracic and abdominal cavities; highly developed brain; endothermy and homeothermy; separate sexes with the sex of an embryo being determined by the presence of a Y or 2 X chromosomes; and internal fertilization.

The Class Mammalia includes around 5000 species placed in 26 orders (systematists do not yet agree on the exact number or on how some orders are related to others). Mammals can be found in all continents and seas. In part because of their high metabolic rates (associated with homeothermy and endothermy), they often play an ecological role that seems disproportionately large compared to their numerical abundance.

Subclass Prototheria - Not represented in southern Africa