The term "mammal" is derived from the Latin word mamma which means breast. Normally four legged animals are described as mammals. However, most reptiles are having four legs but they are not mammals. Dolphins and whales, on the other hand, are classified as mammals but they do not have legs. So mammals can be defined as a group of vertebrate animals whose young are nursed with milk which is secreted from the special secretary glands (mammae) of the mother. Mammary glands are only found in mammals. Moreover, there are many salient features by which we can distinguish mammals from other group of animals.
The body of mammal is insulated by hair (except in some whales where they lost in adult stage). Hair become water proofed by sebum, the secretion of sebaceous glands found in hair roots. Another important feature of mammal is that its lower jaw is comprised of a single bone and is directly hinged to the skull. All mammals have a four chambered heart and only the aortic arch is present. The respiration takes place through the lungs. Another important muscular structure called diaphragm, which separates the heart and the lungs from stomach and intestines, is present in the mammals. The neck has seven cervical vertebrae (except in sloth and manatee). The mammalian brain is well developed. Males posses rudimentary nipples. Mammals have two sets of teeth: the milk or deciduous teeth and permanent teeth. Mammalian feet are adapted for walking, flying, climbing, running, swimming etc .. The erythrocytes are non nucleated (the camel and theAustralian marsupial koala, however, are reported to have nucleated erythrocytes in blood). Mammals are homoiothermic (warm-blooded) animals i.e. the body temperatures do not vary with their surroundings.
All mammals (except monotremes) are viviparous i.e. live bearing animals. Fertilization takes place internally. Mammalian estrous cycles are of three types: ovulation is induced by cervical stimulation during copulation and followed by a spontaneous luteal phase (in rabbits, cats, ferrets and camels); ovulation is spontaneous but the stimulus of copulation is essential to induce the luteal phase (in rats and mice); and ovulation and the luteal phase are spontaneous (in primates, marsupial, cattle, sheep, goat etc.). In placental mammals, the young are developed within the mother's womb. Young are well developed (precocial). In pouched mammals the young, however, are less developed (altrical) at birth. Males have copulatory organs and majority of mammalian species have descended testes.