Mammals are warm-blooded, have coats of hair or fur, and give birth to live young. All mammals feed their babies with milk and care for them until they are strong enough to look after themselves. There are more than 5,000 species of mammals, including the largest animal that has ever lived on Earth-the blue whale-and our own species.

Up to 90 percent of the food that a mammal eats is turned into the energy it uses to keep its body warm. The less heat it loses, the less it has to eat, so good insulation can be necessary, especially in cold climates. This is why mammals have hair. It's a unique feature of mammals, and ofter grows densely to form thick fur that traps layers of warm air.