What is a Mammal?

Mammals are a group of animals distinguished from other vertebrate animals, or animals that have backbones, by a set of defining characteristics. These characteristics, however, may not be immediately apparent. Many mammals, in fact, differ from each other in areas that one may suspect define a class of life, such as their size or habitat; mammals range from the size of a Kitti's hog-nosed bat, or a "bumblebee bat," to that of a blue whale, the largest animal ever to have lived, and live in a wide range of habitats all over the world, including the Arctic, the tropics, rainforests, deserts, and the ocean.

In terms of taxonomy, mammals belong to the class mammalia, of the phylum chordata and kingdom animalia. Scientists disagree on the way mammals should be categorized, but generally recognize either twenty or twenty-one distinct taxonomic orders of living mammals, the largest of which consists of rodents.

Mammals can be defined by the presence of mammary glands, organs exclusive to mammals. Mammary glands produce the milk with which mammals feed their young. This is almost always done by a female, although males may assist in nursing infants in some species, most notably the dayak fruit bat. Almost all mammals give live birth to their young. The only exceptions are platypuses and spiny anteaters, which reproduce by laying eggs.

Mammals are also unique in that they all have hair, which may occur in the form of fur, wool, bristles, spines, or quills, at some point in their development. Mammals have several unique anatomical processes and skeletal structures as well. These qualities include a tooth replacement pattern in which each mammal's teeth are replaced only once in its lifetime, three unique inner ear bones, and a single lower jaw bone that connects directly to the animal's skull. Mammals also tend to have more highly developed brains than other animals, and therefore tend to be more intelligent than other animals.

There are also several traits that characterize mammals and are not exclusive to mammals. Tough they are not the only tetrapods, or vertebrates with four limbs, most mammals have four limbs; the only exceptions are whales and sirenians, or "sea cows." All mammals also breathe by means of lungs, inhaling oxygen and exhaling the waste product carbon dioxide. Mammals are all warm-blooded as well, meaning that they maintain an almost constant body temperature even when the temperature of their environment changes, though other animals such as birds may also be warm-blooded.

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