Mammals

A mouse and a whale do not look as if they belong together, yet they share more with each other than either does with a shark or a crocodile. Both are warm-blooded, both have hair at some point in their lives, both feed their young on milk, and both breathe air with lungs. These traits define the mammals, a group of about 6,500 species that includes the biggest animal ever to live, the only flying mammals, and us.

What makes a mammal

The defining feature is milk. Every female mammal has glands that produce it, and every mammal begins life drinking it. Milk lets a mother feed her young without them having to find food of their own, which allows a long childhood and a great deal of learning. It is why mammals are, on the whole, the most intelligent animals.

Mammals are also warm-blooded, keeping their body temperature steady, usually between 35 and 40 degrees Celsius, whatever the weather. That takes a lot of fuel; a mouse must eat a quarter of its weight a day. In return, mammals can be active on cold nights and in cold places where reptiles must lie still. Fur holds the heat in, and nearly all mammals have it, even whales, which are born with a few whiskers.

Less obvious features matter to biologists: a single lower jawbone, three tiny bones in the middle ear that evolved from reptile jaw bones, a diaphragm for breathing, and a large brain relative to body size.

Three ways to be born

Most mammals, including humans, dogs and whales, are placental: the young develop inside the mother, fed through a placenta, and are born relatively well formed. Marsupials, such as kangaroos and koalas, give birth to tiny, almost embryonic young that crawl into a pouch and finish growing there attached to a teat; a newborn kangaroo is the size of a jellybean. Strangest of all are the monotremes, the platypus and the echidnas of Australia and New Guinea, which lay eggs. Their young hatch and then lick milk that oozes from patches on the mother's skin. They are a glimpse of what the earliest mammals were like.

From the shadow of dinosaurs

Mammals are old. Their ancestors split from the reptile line more than 300 million years ago, and the first true mammals appeared about 200 million years ago, around the same time as the first dinosaurs. For the next 135 million years they stayed small, mostly nocturnal and mostly out of the way, shrew-like creatures hunting insects in the dark while dinosaurs dominated the day. Their good night vision, keen hearing and sense of smell, and warm blood date from that long apprenticeship.

When the asteroid struck 66 million years ago and the dinosaurs died, the mammals were ready. Within ten million years they had produced the first horses, whales, primates and bats, and filled every niche the dinosaurs had left.

Extremes

The range of mammal bodies is staggering. The bumblebee bat of Thailand weighs two grams, less than a coin. The blue whale weighs up to 150 tonnes, with a heart the size of a small car and a tongue heavier than an elephant. Bats, a fifth of all mammal species, are the only mammals that fly, using wings of skin stretched over elongated finger bones, and most of them hunt in total darkness by listening to the echoes of their own ultrasonic shouts. Whales and dolphins returned to the sea 50 million years ago, their front legs becoming flippers and their hind legs vanishing except for a few internal bones. Elephants have trunks with 40,000 muscles, giraffes have necks two metres long with the same seven neck bones as a human, and naked mole-rats live in underground colonies like termites, with a single breeding queen.

Minds

Mammals think. Chimpanzees make tools and plan ahead. Dolphins recognise themselves in mirrors and call each other by name. Elephants mourn their dead, returning to touch the bones of relatives. Rats laugh when tickled, in ultrasound. Dogs read human faces better than our closest ape relatives do. The long childhood that milk makes possible, and the social lives that many mammals lead, have selected for brains that learn, remember and cooperate.

Why it matters

Humans are mammals, and nearly everything about us, from our warm blood and long childhoods to our large brains and social instincts, is a mammal trait rather than a uniquely human one. Mammals also feed and carry us: cattle, sheep, pigs, horses and dogs shaped civilisation. Yet a quarter of mammal species are now threatened with extinction, mostly from habitat loss and hunting. The group that inherited the Earth from the dinosaurs is being thinned by one of its own members.