Dogs, cats, horses, monkeys, and humans are some of the creatures called
mammals [(mam]{.smallcaps} uhls). They’re all warm-blooded and have some
hair. This is one way to tell them from fish and reptiles, which are
cold-blooded and hairless.
But there is one thing that makes a mammal truly different from any
other kind of animal. Baby mammals get milk from their mother’s body.
The milk is made inside the mother, in parts of her body that scientists
call mammae (MAM ee). It is from the word mammae that we get the word
mammal.
Most kinds of mammals live on the land. But there are a number of
kinds—seals, walruses, whales, dolphins, and others—that make their
home in the sea. Some can stay underwater for long periods of time. But,
like all mammals, they are air breathers. They must come to the surface
to breathe.
Whales and dolphins look so much like fish that many people think they
are fish. But they are mammals—because they have some hair, are
warm-blooded, and nurse their young.
sea lion