Sheep are quadrupedal, ruminant mammals typically kept as livestock. Like most ruminants, sheep are members of the order Artiodactyla, the even-toed ungulates. Although the name sheep applies to many species in the genus Ovis, in everyday usage it almost always refers to Ovis aries. Numbering a little over one billion, domestic sheep are also the most numerous species of sheep. An adult female is referred to as a ewe, an intact male as a ram, occasionally a tup, a castrated male as a wether, and a young sheep as a lamb.