Today, there are about 4,000 different species of mammals. Nearly half of all these species are rodents. The largest animals alive, such as the elephant and whales are mammals. In fact, the blue whale is the largest animal ever to have lived on earth. There also are very small mammals shrews are only one example.
All mammals are vertebrate animals, which means they have a backbone.
Mammals are the only animals that have hair. All mammals breathe air, even whales and dolphins. They are warm-blooded that is, the temperature inside their bodies stay the same, more or less, even when it is quite warm or very cold outside. Another thing that sets mammals apart from other animals is their mammary glands, which produce milk. All young mammals feed on their mothers' milk until they can find and eat food by themselves.