The American bison, the largest mammal in North America, once roamed the continent in vast herds and helped to shape the ecology of the Great Plains, as well as the history of the United States of America. Formidable in stature, the American bison’s massive frame is accentuated by its towering shoulder hump. Resting low on a short, stocky neck, the bulky head features a broad forehead, short, up-curving horns and a straggly beard. The head, neck, shoulders, and forelegs are covered in a long, shaggy, brownish-black coat, while the hair on the remainder of the body is considerably shorter and lighter in colour. Female bisons are on average shorter than the males, and have a smaller hump, a thinner neck and more slender horns. Two subspecies are generally recognised, the plains bison (Bison bison bison) and the wood bison (B. b. athabascae). The wood bison is typically larger and heavier than the plains bison, and usually has darker hair.
Prior to European settlement on the continent, the plains bison undertook seasonal migrations of hundreds of kilometres along the same routes year after year. Moving in vast herds, the bison was considered to be a classic ‘keystone’ species, significantly influencing grass composition, nutrient cycling, fire regimes, and the availability of habitat for a diverse assemblage of other animals. Owing to changes in land use and depopulation, the plains bison is no longer migratory (the wood bison never was), but the movements of free-roaming herds are still dictated by the availability of food. Although grasses and sedges form the mainstay of the bison diet, flowering plants, woody plant leaves and even lichens will be eaten when its staples are limited. Over winter, bison can dig through deep snow, by sweeping the muzzle from side to side, to access buried vegetation.
Adult females live with their young in hierarchical herds, led by a dominant female, whilst mature males usually move about alone, or in small bachelor groups. Joining the female herds during the mating season, male bison fight fiercely for the right to mate. This usually involves head-to-head ramming, while receptive females gallop about to stimulate competition, thus hoping to mate with the strongest male. The mating season lasts from June to September, with a peak in activity between July and August. Pregnant females usually give birth to a single calf the following spring, after a gestation period of 270 to 285 days. Within three hours, the newborn calves are able to run about, but are guarded closely by the mothers, who will charge any intruders. The young are weaned at around seven to twelve months and reach sexual maturity when they are two to four years of age, with wild individuals having a potential life expectancy of around 20 years.
Despite having such a bulky frame, bison are able to run at speeds of up to 60 kilometres an hour. Eyesight is poor, but the senses of hearing and smell are acute, and appear to be vital in detecting danger. Where present, wolves are known to be competent predators of free-roaming bison, with individuals separated from the herd being the primary targets of attack.