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, 1,199 species of mammals, or about 22 percent of the total 5,513 described mammal species, were deemed endangered or vulnerable to extinction by the International Union for Conservation of Nature's Red List. In the United States, 35 mammals are at risk of extinction.