A mammal is an animal in the vertebrate group within the class of Mammalia, humans are mammals. There are several distinct characteristics of mammals that distinguish them from the other animal classes (reptiles, birds, fish and amphibians). All mammals have a backbone, hair, warm blood, a four chambered heart and sweat glands. Female mammals give birth to live young and have mammary glands to feed their offspring with milk. The largest living mammal, and also largest living animal, is the Blue Whale.