All mammals are vertebrates. A vertebrate is any living being with a backbone or a spine.

Though some live on land and some in water, mammals have some other things in common. They all have body hair and are warm blooded; they can live in most environments because they are able to regulate their body heat. Adult female mammals produce milk to feed their young.

More than 4,000 species of mammals exist. The largest of them is the blue whale. It can grow up to 100 feet long and weigh up to 300,000 pounds. In stark contrast, the hog-nosed bat weighs a mere 0.05 ounces.