The whales are a large group of aquatic animals that very in size from about 4 feet in length to nearly 100 feet. During the evolution of mammals tens of millions of years ago one group returned to live in water. These mammals became streamlined in shape, developed a powerful tail and front limbs for use as steering fins, and lost the use of the back limbs. Whales, descendants of this group, are mammals because they breathe air, are warm-blooded and suckle their young with milk. They have lost the mammalian coat of hair in order to help them swim, but they have replaced it with an insulating coat of thick blubber.