Two subspecies or ecotypes have been described: the plains bison (B. b. bison), smaller in size and with a more rounded hump, and the wood bison the larger of the two and having a taller, square hump. Furthermore, the plains bison has been suggested to consist of a northern (B. b. montanae) and a southern subspecies, bringing the total to three. However, this is generally not supported. The wood bison is one of the largest wild species of bovid in the world, surpassed by only the Asian gaur and wild water buffalo. It is the largest extant land animal in the Americas.
A bison has a shaggy, long, dark-brown winter coat, and a lighter-weight, lighter-brown summer coat. As is typical in ungulates, the male bison is slightly larger than the female and, in some cases, can be considerably heavier. Plains bison are often in the smaller range of sizes, and wood bison in the larger range. Head-and-body lengths range from 2 to 3.5 m (6.6 to 11.5 ft) long, the tail adding 30 to 91 cm (12 to 36 in). Shoulder heights in the species can range from 152 to 186 cm (60 to 73 in). Weights can range from 318 to 1,000 kg (701 to 2,205 lb) The heaviest wild bull ever recorded weighed 1,270 kg (2,800 lb). When raised in captivity and farmed for meat, the bison can grow unnaturally heavy and the largest semidomestic bison weighed 1,724 kg (3,801 lb). The heads and forequarters are massive, and both sexes have short, curved horns that can grow up to 2 ft (61 cm) long, which they use in fighting for status within the herd and for defense.
Bison are herbivores, grazing on the grasses and sedges of the North American prairies. Their daily schedule involves two-hour periods of grazing, resting, and cud chewing, then moving to a new location to graze again. Bison mate in August and September; gestation is 285 days. A single reddish-brown calf nurses until the next calf is born. If the cow is not pregnant, a calf will nurse for 18 months. At three years of age, bison cows are mature enough to produce a calf. Bison bulls of that age may try to mate with cows, but if more mature bulls are present, they may not be able to compete until they reach five years of age. Bison have a life expectancy around 15 years in the wild and up to 25 years in captivity. For the first two months of life, calves are lighter in color than mature bison. One very rare condition is the white buffalo, in which the calf turns entirely white.
Differences from European Bison EditAlthough they are superficially similar, the American and European bison exhibit a number of physical and behavioral differences. Adult American bison are slightly heavier on average because of their less rangy build, and have shorter legs, which render them slightly shorter at the shoulder. American bison tend to graze more, and browse less than their European relatives, because their necks are set differently. Compared to the nose of the American bison, that of the European species is set farther 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 forward through the plane of its face, making it more adept at fighting through the interlocking of horns in the same manner as domestic cattle, unlike the American bison which favors charging. American bison are more easily tamed than the European, and breed more readily with domestic cattle.
American bison live in river valleys, and on prairies and plains. Typical habitat is open or semiopen grasslands, as well as sagebrush, semiarid lands, and scrublands. Some lightly wooded areas are also known historically to have supported bison. Bison also graze in hilly or mountainous areas where the slopes are not steep. Though not particularly known as high-altitude animals, bison in the Yellowstone Park bison herd are frequently found at elevations above 8,000 feet and the Henry Mountains bison herd is found on the plains around the Henry Mountains, Utah, as well as in mountain valleys of the Henry Mountains to an altitude of 10,000 feet.