Milk, hair and a warm inside
What Makes an Animal a Mammal?
A whale, a bat and you are all in the same group. Three things settle it, and milk is the one that never fails.
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Step 1 of 5
Milk is the rule with no exceptions.
Every mammal feeds its young on milk it makes itself. No other animal group does this at all, so it is the one test that always gives the right answer.
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Step 2 of 5
Hair is the second sign, even when you cannot see it.
A dolphin's whisker pits are still on its snout after the whiskers are gone A whale looks bare, but it is born with a few bristles on its snout. Every mammal grows hair at some point, even if it loses it almost at once.
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Step 3 of 5
And a mammal heats itself from the inside.
The bill is about ten times what a reptile of the same size gets by on. It burns food to hold a steady temperature whatever the weather. That is what lets mammals be active on a freezing night, and it is why they eat so much more than reptiles do.
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Step 4 of 5
The group is stranger than it looks.
A bat flies, a whale never touches land, a mole lives underground and you read books. All of them make milk, grow hair, and keep themselves warm.
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Step 5 of 5
Even the awkward one keeps to the rule.
The platypus lays eggs, which sounds like a disqualification. But it still makes milk for its young — it just sweats it out through the skin, having no teats.
There are around 6,500 kinds of mammal. Over a fifth of them are bats, and another fifth are rats and mice.
The short version
A mammal is an animal that feeds its young on milk, grows hair at some stage, and makes its own body heat — even when it flies, swims or lays eggs.
Try it yourself
Look for the hair on an animal you think has none. A dolphin has whisker pits along its snout where the bristles were before it was born.