Mammals are a class of vertebrates characterised by warm-blood, hairy bodies, a four-chambered heart, a single jaw bone, sweat glands and - in females - mammary glands for suckling young. Some mammals, such as naked mole rats and whales have lost some or all of their hair. Almost all mammals give birth to live young, but there are a few that lay eggs. These are known as monotremes, and include the platypus.
Scientific name: Mammalia
The shading illustrates the diversity of this group - the darker the colour the greater the number of species. Data provided by WWF's Wildfinder.
A comparison of elephant size in relation to humans - from the 5m tall, 17 tonne Songhua River mammoth to the 2m tall, 900kg pygmy mammoth.
Take a trip through the natural world with our themed collections of video clips from the natural history archive.
Frozen Planet is Sir David Attenborough's latest exploration into the remote and isolated polar environments.