About 200 million years ago, the first undisputed mammals occurred. Examination of fossil remains have led scientists to believe that mammals evolved from mammal-like reptiles, therapsids. Prior to the end of the age of large reptiles, mammals survived in forms no larger than rabbits. About 65 million years ago, with the fall of the dinosaurs, mammals quickly evolved into a wide variety of creatures.

Mammals comprise only a small percentage of species in the animal world. Only about 4,500 of the one million plus species are mammals. They vary in size from less than half an ounce to over 150 tons. The blue whale, the largest animal ever to have lived on the planet, weights at lease five times more than the largest dinosaur. Mammals inhabit almost all ecological niches on earth. To be classified as a mammal, an animal must exhibit all of the following characteristics that set it apart from other animals. They must be warm blooded vertebrates, have a coat of hair and females must have the capability to nourish their young with milk secreted from specialized mammary glands.

The ability to maintain a constant body temperature has allowed mammals to adapt and become more diverse in relation to their external environment. Mammals maintain a body temperature between 99 and 104 degrees. Some mammals may hibernate in winter, a condition in which respiration rate, body temperature and metabolism fall drastically, allowing the animal to live on stored fat for months at a time. Other mammals are capable of entering a state of conserving energy called torpor and will do so for shorter periods of time.

Mammals have a covering of hair that protects and insulates the body. Some species still have body parts covered with reptile-like scales behind which are few attached hairs. Hair varies in thickness and length and some species exhibit three different lengths of hair: a thick wooly undercoat, their normal hair length, and additional long guard hairs. Other species have sensory hairs growing from cheeks or lips called vibrissa or whiskers, capable of sensing vibrations or feeling in the dark. The color of hair often serves as camouflage.

Mammals have a group of skin glands found in no other class of animals. The most important are mammary glands. Egg laying mammals have several glands which discharge into depressions in the abdominal region. All other mammals have nipples or teats. The number of nipples seems to be related to the number of average young produced per species. Other glands found in mammals are sweat and sebaceous glands. Sweat glands help regulate temperature and sebaceous glands secret oil that protectes skin and hair from moisture.

The simple jaw and skull of mammals consist of one piece each, as compared to their reptilian ancestors whose jaw and skull contained many bones. The teeth are embedded in a tooth socket in the jaw. Most are rooted teeth, however rootless teeth do occur. Most species have two sets of teeth during their lifetime. The first set are present in infancy and with maturity are replaced by permanent teeth.

Pulmonary circulation and the rest of the body are totally separated by veins and arteries. The heart has two separate ventricles and auricles. The basic stomach does not differ greatly when compared to most other vertebrates. However, some larger herbivorous mammals do have a highly complex stomach. Mammals have well developed sense organs, and a highly developed central nervous system. All the senses are present in all mammals, but at different levels of development depending on the species.

Mammals have highly developed sex organs, relatively large gestation periods and a long postnatal period. Regardless of the method of reproduction, most baby mammals stay with their parents, or only their mother, for a fairly long period of time after birth. During this time the adults bring food for the baby, teach it how to hunt and escape predators, and in the case of social animals, how to behave with others of its kind. Primitive mammals care for their young for only a few weeks, whereas advanced species tend to them for several years.