To be considered a mammal, an animal must produce milk to feed its young, have hair on its body, be a vertebrate and be warm blooded. Animals not falling into these categories are not mammals.
In the world today, there are more than 4,000 species of mammals on land and in the water. The largest known mammal is a blue whale, which can weigh more than 150 tons and grow to be over 100 feet long. The smallest mammal is a hog-nosed bat that weighs half an ounce.
Mammals are different from other animals in that they are vertebrates. This means they have a backbone. Animals lacking a backbone are called invertebrates.
Mammals produce milk to feed their young. The babies feed from glands located somewhere on the mother's body.
Mammals are warm blooded, or endothermic. This means mammals can regulate their own body temperature, and it gives them the ability to adapt to many different types of climate.
Finally, all mammals have hair on their bodies. This can range from the body being fully covered, like that of an orangutan, or having little hair like humans.
Paleontologists and other scientists have discovered proof that mammals have been on Earth since the late Triassic period, some 200 million years ago. During the event at the end of the Cretaceous period that caused the dinosaurs to become extinct, a few small mammals survived and are the ancestors to today's modern mammals, including humans.