What Is a Mammal?

Mammals are animals that have vertebrae or backbones. So are fish, amphibians, reptiles, and birds. What makes mammals different?

Female mammals can make milk to feed their young. No other kind of animal can do that. The name "mammal" comes from the mammary glands that make milk. Most mammals give birth to live young. Most baby mammals start their lives inside their mothers' bodies. They don't hatch from eggs like birds, amphibians, or reptiles. Mammal babies look like their parents, only they are smaller. They do not change into different forms like frogs or butterflies.

All mammals have hair or fur at some point in their lives. Mammals are the only animals to have this trait, too. Some may not keep their hair for very long. For example, dolphins and whales are born with hair around their nose. After a few weeks, the hair falls out.

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