Members of the genus Bison are large even-toed ungulates within the subfamily Bovinae. Two extant species and four extinct species are recognized. The surviving species are the American bison, also known as the American buffalo (although it isn't related to the true buffalo), Bison bison (with two subspecies, the plains bison, Bison bison bison, and the wood bison, Bison bison athabascae), found in North America, and the European bison, or wisent (Bison bonasus), found in Europe and the Caucasus. While these species are usually grouped into their own genus, they are sometimes included in the closely related genus Bos, together with cattle, gaur, kouprey and yaks, with which bison have a limited ability to interbreed.
The American bison and the European wisent are the largest terrestrial animals in North America and Europe. Bison are nomadic grazers and travel in herds, except for the non-dominant bulls, which travel alone or in small groups during most of the year. American bison are known for living in the Great Plains. Both species were hunted close to extinction during the 19th and 20th centuries but have since rebounded. The American Plains bison is no longer listed as endangered, but the Wood Bison is on the endangered species list in Canada.
Although superficially similar, there are a number of physical and behavioural differences between the American and European bison. The American species has 15 ribs, while the European bison has 14. The American bison has four lumbar vertebrae, while the European has five. Adult American bison are not as rangy in build, and have shorter legs. American bison tend to graze more, and browse less than their European cousins. Their anatomies reflect this behavioural difference; the American bison's head hangs closer to the earth than the European's. The body of the American bison is typically hairier, though its tail has less hair than that of the European bison. The horns of the European bison point through the plane of their faces, making them more adept at fighting through the interlocking of horns in the same manner as domestic cattle, unlike the American bison which favours butting. American bison are more easily tamed than their European cousins, and breed with domestic cattle more readily.
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