Mammals are creatures.
Mammals are any vertebrates within the class Mammalia ( [ məˈmeɪli.ə ] from Latin mamma #Latin `` breast '' ) , a clade of endothermic amniotes distinguished from reptiles ( including birds ) by the possession of a neocortex ( a region of the brain ) , hair , three middle ear bones and mammary glands . Mammals include the biggest animals on the planet , the great whales . Mammals range in size from the 30 -- bumblebee bat to the 30 m blue whale .