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, 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 can interbreed.
The American bison and the European wisent are the largest terrestrial mammals 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, due to their necks being set differently. Compared to the American bison, the nose of the European species is set further forward than the forehead when the neck is in a neutral position. The body of the American bison is 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 charging. American bison are more easily tamed than their European cousins, and breed with domestic cattle more readily.
Bison are born without their trademark hump and horns and live for approximately twenty years. They grow to maturity at two to three years, although males continue to grow until about their seventh year. Adult bulls express a high degree of dominance competitiveness during mating season. Male bison fight for females and these fights often result in injury or death. After the bison mate, the herd splits up into smaller herds. Calves are born nine months after mating. The mothers take care of and nurse their young for a year.
Male bison grow to as much as 11.5 ft (3.5 m) long, and 6.5 ft (2.0 m) tall at the shoulder and can weigh up to 2,200 lb (1,000 kg).
Wallowing is a common behavior of bison. A bison wallow is a shallow depression in the soil, either wet or dry. Bison roll in these depressions, covering themselves with mud or dust. Possible explanations suggested for wallowing behavior include grooming behavior associated with moulting, male-male interaction (typically rutting behavior), social behavior for group cohesion, play behavior, relief from skin irritation due to biting insects, reduction of ectoparasite load (ticks and lice), and thermoregulation. In the process of wallowing bison may become infected by the fatal disease anthrax, which may occur naturally in the soil.
Bison have a fairly simple diet. The bison's main food is grass. Bison also eat the low lying shrubbery that is available. In the winter, bison forage in the snow looking for grass. If there is little grass available, bison have to resort to eating the twigs of shrubs.
Due to their large size few predators attack bison. Wolf packs, but not single wolves, could take down a bison. Brown bears will also prey on calves, and have been observed driving off wolves to take over their kills.