There are over 4000 species of mammals worldwide, but all share the same defining characteristics that separate them from other members of the phylum Chordata. These characteristics include having hair, being warm-blooded and producing milk for their young. All mammals except the platypus and echidna give birth to live young.
Some scientists further divide mammals into 21 groups, including primates, carnivores, rodents, bats and marsupials. Rodents comprise the largest group of mammals, with over 1700 species known to science. Mammals range in size from Thailand and Burma's inch-long bumblebee bat to the 72-foot long blue whale.
All mammals, and only mammals, have true hair, although the amount of hair each creature has varies from species to species, as does the form that the hair takes (such as spines or whiskers). Different types of animal hair perform different functions, such as protection or sensing the environment around them. Keratin is a major component of animal hair.
Mammals feed their young milk from their own bodies. This gives the mother and her young the opportunity to bond and also allows the mother to pass on survival skills to her progeny. All female mammals produce milk, even the massive Pacific gray whale, which generates about six tons of it.