Capra is a genus of mammals, the goats, composed of up to nine species, including the wild goat, the markhor, and several species known as ibexes. The domestic goat (Capra aegagrus hircus) is a domesticated subspecies of the wild goat (Capra aegagrus). Evidence of goat domestication dates back more than 8,500 years. Wild goats are animals of mountain habitats. They are very agile and hardy, able to climb on bare rock and survive on sparse vegetation. They can be distinguished from the genus Ovis, which includes sheep, by the presence of scent glands close to the feet, in the groin, and in front of the eyes, and the absence of other facial glands, and by the presence of a beard in some specimens, and of hairless calluses on the knees of the forelegs. The Rocky Mountain goat is in a separate genus, Oreamnos. Present-day genetic and phenotypic differences between the Capra species are largely related to (1) discontinuity of and impeded migration between Capra populations during glacial periods, and (2) insufficient time in the postglacial period for now-adjoining Capra populations to overcome behavioral mechanisms impeding hybridization in the wild so as to erase these differences.