Mammals are vertebrates. That means they have backbones. They are warm-blooded. That means their bodies stay the same temperature no matter what temperature it is outside. Mammals have a few features that make them different from other animals.

Most mammals give birth to live young. (The duck-billed platypus is a mammal that lays eggs.) Most young, like human babies, grow inside the mother. Some young, like kangaroo babies, come out early. They crawl into the mother’s pouch and grow some more there. All mammal young are fed milk from their mothers.

All mammals have hair. Whales and dolphins are mammals. They only have hair before they are born, though. Mammals have a special jaw. It is attached right to the skull. Other animals have another bone in between. Mammal ears are different, too. There are three tiny bones inside that help carry sound.

Mammals have a special way of breathing. They have a muscle under the heart and lungs. It is called the diaphragm. When it goes down, air is pulled into the lungs. When it goes up, air is forced out.

The smallest mammal known is the bumblebee bat. It is less than an inch and a half long. The lightest mammal is the pygmy shrew. It weighs only about 0.07 ounces. The largest mammal is the blue whale. It can be over 100 feet long. There are over 5,000 species of mammals in the world. The first mammals appeared 220 million years ago.