Buffalo. Bison are large, even-toed ungulates in the genus Bison within the subfamily Bovinae. Two extant and six extinct species are recognised. Of the six extinct species, five became extinct in the Quaternary extinction event. Bison palaeosinensis evolved in the Early Pleistocene in South Asia, and was the evolutionary ancestor of B. priscus, which was the ancestor of all other Bison species. From 2 million years ago to 6,000 BC, steppe bison ranged across the mammoth steppe, inhabiting Europe and northern Asia with B. schoetensacki, and North America with B. antiquus, B. latifrons, and B. occidentalis. The last species to go extinct, B. occidentalis, was succeeded at 3,000 BC by B. bison. Of the two surviving species, the American bison, B. bison, found only in North America, is the more numerous. Although commonly known as a buffalo in the United States and Canada, it is only distantly related to the true buffalo. The North American species is composed of two subspecies, the Plains bison, B. b. bison, and the wood bison, B. b. athabascae, which is the namesake of Wood Buffalo National Park in Canada. A third subspecies, the eastern bison is no longer considered a valid taxon, being a junior synonym of B. b. bison. References to woods bison or wood bison from the eastern United States confusingly refer to this subspecies, not B. b. athabascae, which was not found in the region. The European bison, B. bonasus, or wisent, is found in Europe and the Caucasus, reintroduced after being extinct in the wild. While all bison species are classified in their own genus, they are sometimes bred with domestic cattle and produce sometimes fertile offspring called beefalo or zubron. The American bison and the European bison are the largest surviving terrestrial animals in North America and Europe. They are typical artiodactyl ungulates, and are similar in appearance to other bovines such as cattle and true buffalo. They are broad and muscular with shaggy coats of long hair. Adults grow up to 2 metres in height and 3.5 m in length for American bison and up to 2.1 m in height and 2.9 m in length for European bison. American bison can weigh from around 400 to 1,270 kilograms and European bison can weigh from 800 to 1,000 kg. European bison tend to be taller than American bison. Bison are nomadic grazers and travel in herds. The bulls leave the herds of females at two or three years of age, and join a herd of males, which are generally smaller than female herds. Mature bulls rarely travel alone. Towards the end of the summer, for the reproductive season, the sexes necessarily commingle. American bison are known for living in the Great Plains, but formerly had a much larger range, including much of the eastern United States and parts of Mexico. Both species were hunted close to extinction during the 19th and 20th centuries, but have since rebounded. The wisent in part owes its survival to the Chernobyl disaster, as the Chernobyl Exclusion Zone has become a kind of wildlife preserve for wisent and other rare megafauna such as the Przewalski's horse, though poaching has become a threat in recent years. The American Plains bison is no longer listed as endangered, but this does not mean the species is secure. Genetically pure B. b. bison currently number only about 20,000, separated into fragmented herds, all of which require active conservation measures. The wood bison is on the endangered species list in Canada and is listed as threatened in the United States, though numerous attempts have been made by beefalo ranchers to have it entirely removed from the Endangered Species List. Although superficially similar, physical and behavioural differences exist 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 less slim in build and have shorter legs. American bison tend to graze more, and browse less than their European relatives. Their anatomies reflect this behavioural difference; the American bison's head hangs lower 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.