Mammalogy is the branch of zoology that deals with the study of mammals, which are any organisms in the class Mammalia. Mammals are warm-blooded (or homoiothermic) vertebrates having mammary glands in the female, a four-chambered heart, a diaphragm, and hair or fur. Major subject areas in mammalogy include anatomy, physiology, behavior, ecology, evolution, and classification and systematics.

Mammals first evolved from an ancient group of reptiles known as the Therapsida, which were most abundant and diverse during the Triassic period (which began about 245 million years ago). The time of the evolution of first mammals is not known exactly, but probably occurred early in the Triassic. The first fossils of placental mammals are from the Jurassic period (about 210 million years ago), but mammals did not become prominent until the late Mesozoic (just before 290 million years ago). The Cenozoic era, during which numerous species of mammals evolved to fill ecological...