Mammals are animals with lungs, and they breath air.
Mammals are warm blooded, with a body temperature that stays constant most of the time. The babies of mammals are born alive, and the babies take milk from their mothers. Cetaceans can be enormous, like the blue whale, or tiny, like some species of porpoise. They live in all the world's oceans from the cold reaches of the arctic to the warm waters of the tropics. Illustration by REMedia.