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.

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