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. 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. Mammals feed their newborn young with milk , a substance rich in fats and protein that is produced by modified sweat glands called mammary glands .