Mammals

Mammals are a group of warm-blooded animals with fur or hair, who feed their babies on milk produced by the mother's body. There are around 6,500 known species of mammal, including humans. Mammals come in an enormous range of shapes and sizes, from the tiny bumblebee bat (the size of a thumbnail) to the blue whale (the largest animal that has ever lived). What makes a mammal a mammal is not just fur or warm blood; it is the combination of several unique features that no other animal group has.

• Known mammal speciesapprox. 6,500Far fewer than birds or insects
• Largest mammalBlue whaleUp to 30 m, 200 tonnes
• Smallest mammalBumblebee batJust 2 cm long
• Fastest land animalCheetahapprox. 100 km/h
• First mammalsapprox. 225 million years agoTiny shrew-like creatures
• Three main groupsMonotremes, marsupials, placentalsEgg-layers, pouch-rearers, womb-rearers

What makes a mammal?

Mammals share several unique features that, taken together, are not found in any other animal group:

• Fur or hair: all mammals have at least some fur, even apparently smooth ones like whales (which have a few stiff hairs around their snouts) and rhinos (whose horn is made of hair).
• Warm-blooded: mammals maintain a steady internal body temperature even in cold conditions.
• Mammary glands: female mammals produce milk to feed their babies. This is the feature that gives the whole group its name.
• Vertebrates: they have a backbone.
• Live birth: most give birth to live young (except for monotremes, see below).
• Diaphragm: a sheet of muscle below the lungs that helps them breathe.
• Three middle-ear bones: a feature found only in mammals.

The three main groups

• Monotremes: the strangest mammals. They lay eggs instead of giving birth. Only 5 species: the duck-billed platypus and four species of echidna (spiny anteaters), all from Australia and New Guinea.
• Marsupials: give birth to tiny underdeveloped babies that crawl into a pouch to finish growing. Includes kangaroos, koalas, wombats, opossums and the extinct Tasmanian tiger. Mostly found in Australia and South America.
• Placental mammals: the largest group, including over 5,000 species. The young develop inside the mother's body, nourished through a placenta, and are born more developed. Includes almost every mammal you can think of: dogs, cats, whales, bats, elephants, humans.

Where mammals live

Mammals have colonised almost every habitat on Earth.

• On land: cats, dogs, deer, bears, mice, elephants, kangaroos, monkeys, humans.
• In the air: bats are the only mammals capable of true powered flight (over 1,400 species).
• In water: whales, dolphins, porpoises, seals, sea lions, walruses, manatees, otters.
• Underground: moles, naked mole rats, badgers, wombats.
• In trees: monkeys, squirrels, sloths, opossums, koalas.
• In polar ice: polar bears, Arctic foxes, musk oxen, seals.
• In deserts: camels, fennec foxes, kangaroo rats, jerboas.

The world's biggest and smallest mammals

The biggest mammal (and the biggest animal that has ever lived) is the blue whale. A full-grown blue whale can reach 30 metres long and weigh 200 tonnes. Its heart alone weighs as much as a small car, and a human could swim through its largest blood vessels.

At the other end of the scale is the bumblebee bat (also called Kitti's hog-nosed bat), found in Thailand and Myanmar. It is just 3 cm long and weighs about 2 grams: less than a typical coin. The Etruscan shrew, the smallest land mammal, is similar in size.

Fact Whales are mammals, not fish. Their closest living relatives are hippos, of all things. Fossil evidence shows that whales evolved from small four-legged land mammals around 50 million years ago, gradually moving back into the sea over millions of years. Some early whales still had small back legs. Modern whales have tiny vestigial hip bones buried inside their bodies as evidence of this land-mammal past.

The mammal advantage

For the first 160 million years that mammals existed, they were small, mostly nocturnal creatures, living in the shadows of the dinosaurs. When the dinosaurs died out 66 million years ago, mammals suddenly had the run of the planet, and they exploded in number and variety to fill the empty niches. Within 10 million years, mammals had diversified into almost all the modern groups: bats, whales, primates, hoofed animals, carnivores. Without the asteroid 66 million years ago, mammals might still be small night creatures.

Did you know? The platypus may be the strangest mammal in the world. It is a mammal (it has fur and produces milk), but it also has a duck-like bill, lays eggs, has webbed feet, and the males have venomous spurs on their back legs. When the first platypus skin was sent to Europe in 1798, scientists thought it was a hoax and tried to find the stitches where someone had sewn a duck's bill onto a beaver. It was real all along.

Deeper dive: why mammals took over from the dinosaurs

For most of the Mesozoic Era (250 to 66 million years ago), the world was dominated by dinosaurs. They occupied almost every major habitat and ate almost every major food. Mammals, by contrast, were a marginal group: small, mostly nocturnal, mostly insect-eating, rarely growing larger than a modern house cat. For 160 million years, mammals were the support cast in the dinosaur show.

That changed in a single catastrophic day around 66 million years ago, when a 10-km asteroid hit what is now Mexico. The impact triggered a global firestorm, blocked out the Sun for years, and caused the End-Cretaceous mass extinction. Roughly 75% of all species died. All the non-bird dinosaurs were among them.

Why did mammals survive while almost all dinosaurs died? Several reasons:

• Mammals were small, so they needed less food during the long dark winter caused by the impact dust.
• Many were nocturnal and used to surviving in low-light conditions.
• Their fur kept them warm when temperatures crashed.
• Many could hibernate or burrow underground to wait out bad times.
• Their diet of insects meant they did not depend on big plants that had been killed by the lack of sunlight.

Once the dust settled (literally), the surviving mammals had the planet to themselves. Within just 10 million years, they had diversified into all the modern groups: bats started flying, whales went back into the sea, primates climbed up into the trees, and big plant-eaters and predators evolved to fill the empty niches. The asteroid that killed the dinosaurs made the modern mammal-dominated world possible. Without it, you would not be here to read this.

Pick a different group: birds, reptiles or fish. For the basics, see what is an animal.