Mammals are animals. Yes but so are Insects, Reptiles, Spiders, Sponges and Slugs. We need to do better than that. Mammals have a back-bone.Yes but so do Fishes and Frogs and they are not mammals. Mammals are warm-blooded. Yes but so are birds, and birds are not mammals either.

So how do we describe a mammal??

Taxonomy is one of the more important concepts in biology, and one that is actually not too hard to understand. This gives some ideas about how it can be taught at a basic level.

Taxonomy is about the groups that different kinds of living organisms fall into.

Mammals are a bit harder than birds, for example, because "mammal" is not such a common word. Very rarely do we say something like, "Look at that mammal over there !". Our natural level of identification is usually more precise, like "Look at that cat !" or "Look at that kangaroo !", or "Look at that elephant !". If we don't have a clue what something is, it will probably be called an "animal".

So we have to define what a mammal is. Traditionally this is taught with a definition similar to the following:

Mammals are animals that have hair, feed milk to their babies, and are warm-blooded.

The intention of this definition is to name features that are found in all mammals and which clearly distinguish them from those organisms that are not mammals. At this point the student learns useful facts like even elephants and dolphins have hair, so we can know that they are mammals.

This type of definition is consistent with the idea that taxonomy is about classification - a set of criteria for each taxon, and if you satisfy them all you belong and if you don't then you don't belong.

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.