‘Mammal’ is derived from the word ‘mammary’. The young ones of mammals suckle milk from the teats of the mother. This milk contains all the essential nutrients for growth. Except for the monotremes, all mammals are born as young ones and are not hatched from eggs. Contrary to almost all other animal species, mammals nurture and train their young ones for a considerable time. Mammals have a constant body temperature of 36-40°C, so they need a hide (like an elephant) or a thick layer of fat (like a whale) as protection against extreme heat or cold.
