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.

Mammals come in an astonishing range of sizes. The blue whale is the largest animal ever to have lived on Earth — up to 30 metres long and weighing as much as 24 elephants. At the other end, the bumblebee bat from Thailand is one of the smallest mammals, not much bigger than your thumb and lighter than a coin.

Mammals have spread into almost every habitat on the planet, and their bodies are adapted to suit. Dolphins are powerful swimmers with smooth bodies and flippers. Cheetahs are the fastest runners on land, reaching 100 kilometres per hour in short bursts. And bats are the only mammals that can truly fly, flapping wings made of skin stretched over long finger bones.