Mammals can be identified by the presence in females of mammary glands that produce milk for offspring. Mammals are warm-blooded vertebrates with sweat glands, hair, three middle ear bones, and a neocortex region in the brain. Their gradual evolution from mammal-like “reptiles” called “synapsids” spanned about 70 million years. The first clear evidence of fully mammalian jaw joints and middle ears was found about 200 million years ago; mid-Jurassic fossils show early evidence of hair or fur; and lactation occurred in monotremes, egg-layers that urinated, defecated and reproduced through a single hole, though not at the same time. They are believed to have secreted milk not from nipples but through a hairy patch on their bellies. (The platypus and four echidna species are the sole surviving mammalian egg-layers.) Mammals now encompass approximately 5,400 species, including humans.

Globally, 1,094 species of mammals, or about 20 percent of the total 5,416 described mammal species, were deemed endangered or vulnerable to extinction by IUCN's 2007 Red List. Forty-one of 457 species risk extinction in the United States, or about nine percent of the total.