Members of the genus Bison are large, even-toed ungulates within the subfamily Bovinae. Two extant and four extinct species are recognized. The surviving species are the American bison, also known as the American buffalo (although it is only distantly related to the true buffalo), Bison bison, found in North America, and the European bison, or wisent (Bison bonasus), found in Europe and the Caucasus. The North American species is composed of two subspecies, the plains bison, Bison bison bison, and the wood bison, Bison bison athabascae. While all bison 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 good swimmers and can cross rivers over half a mile (1 km) wide.