Mammals: Warm-Blooded Wonders

Mammals are a group of animals that share some very important features. They are warm-blooded, which means their bodies make their own heat and stay at roughly the same temperature whatever the weather outside. They breathe air through lungs — even mammals that live in the sea, like whales, must come up to the surface to take a breath.

Scientists use a checklist of features to decide whether an animal is a mammal. Mammals are warm-blooded, breathe air with lungs, have a backbone, grow hair or fur at some point in their lives, and feed their babies with milk. An animal does not have to do all of these in an obvious way, but it ticks every box on the list.

Most mammals have hair or fur on their bodies. Fur traps a layer of warm air close to the skin, which helps keep heat in, and it can also provide camouflage. Even mammals that look bald have some hair: a whale's smooth skin still has a few bristles near its mouth — a clue that proves it is a mammal and not a fish.