There are about 5400 species of mammals alive today. These species are subdivided into about 21 groups such as bats, carnivores, cetaceans, aardvark, anteaters and their relatives, elephants, hares and their relatives, hoofed mammals, insectivores, marsupials and many others.
Mammals live in a wide variety of habitats. They occur in deserts, grasslands, wetlands, seas and oceans, below ground, forests, mountaintops, polar regions and scrublands. They also are found on every continent in a wide variety of climates from tropical to arctic regions.
Although mammals are diverse in form, they do share some unique characteristics. The defining characteristic of mammals is that their lower jaw bone which carries the teeth (and is thus referred to as the dentary bone) attaches directly to the skull. In other vertebrates, the dentary is one of multiple bones in the jaw and does not attach directly to the skull.
Mammals also have a unique arrangement of three bones (the incus, malleus and stapes) in the middle ear. These bones transform sound vibrations into neural impulses. Two of these bones, the incus and malleus, originated as bones within the jaw.
Another skeletal characteristic unique to mammals is that they have two knobs on the base of the skull (known as a double occipital condyle) which serve to seat the skull in the top neck vertebra. In other vertebrates, the base of the skull has a single knob.
Hair is also unique to mammals—no other animals possess true hair and all mammals have hair covering at least part of their body at some time during their life. Hair grows from skin cells known as follicles. The hair shaft is made of a protein called keratin. Hair serves numerous functions.
When it is present as a thick covering all of the animal's body (also called a pelage), hair gives the animal superb insulation. The pelage of most mammals is a mosaic of different kinds of individual hairs such as guard hairs, barbs, bristles, awns, underfur, wool, fur, and velli. Each of type of hair has a different structure and serves a slightly different purpose.
The adaptive benefits of mammalian hair are numerous. Hair provides insulation from extreme cold, enabling mammals to inhabit some of the harshest habitats on Earth. It also provides mammals with protection for their skin from abrasions and damaging UV rays. Many mammals have pelage that is colored or patterned in a way that helps to conceal them from predators or, in some cases, prey.
Mammals nurse their young with milk produced by mammary glands. Mammary glands, like hair, is a uniquely mammalian trait. Though present in both males and females, mammary glands only fully develop in females. Mammary glands 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.
Mammals having a diaphram, a four-chambered heart and a large cereberal cortex. Additionally, some mammals hibernate during time periods when resources are scarce, such as during the winter season.