ison have a shaggy, dark brown winter coat, and a lighter weight, lighter brown summer coat. Bison can reach up to 2 metres (6,5 ft) tall, 3 metres (10 ft) long and weigh 900 to 2,000 lbs. The biggest specimens on record have weighed as much as 1140 kg (2,500 lb). The heads and forequarters are massive, and both sexes have short, curved horns, which they use in fighting for status within the herd and for defense. Bison mate in August and September; a single reddish-brown calf is born the following spring, and it nurses for a year. Bison are mature at three years of age, and have a life expectancy of 18 to 22 years in the wild and 35 to 40 years in captivity. Bison are capable of jumping up to 6 feet high and can run up to 40 mph.

Due to its size and the protection afforded by living in a herd, the bison have few enemies besides humans. Grizzly bears and wolves may attempt to attack young calves or subadults, but only in the dead of winter when the herd cannot expend the energy to protect stragglers. A wolf pack can also take down an adult bison, wolves frequently test even the largest bison for weaknesses; usually several wolves may pursue a bison and attempt to bring it down after the bison has succumbed to exhaustion or wounds from the wolves' bites. The only other threat, other than hunting by humans, that leads to the depletion of wild bison is interbreeding with domestic bovines. In fact, only a small number of bison herds found in North America today consist of pure-bred bison