A mammal is a vertebrate that feeds off its mother's milk when it is first born. There are 4000 mammals in the world living pretty much everywhere even in extremely cold regions; which are the ones we will be talking about. Such as polar bears, whales, foxes, and seals. There are five main things that make a mammal a mammal: 1) All mammals nurse their babies feeding on their mother's milk. 2) Usually mammals give their young more care than other animals rather than deserting them to live on their own. 3) The only type of animal that has hair is a mammal. Usually their hair keeps them warm. All mammals have hair even whales, though sometimes it is before birth. 4) All mammals are warm-blooded. Other than birds mammals are the only warm-blooded -animals, which means their temperature is always about the same even if their surrounding temperature changes. 5) Most mammals have a larger and more developed brain than that of other animals.