American Bison (Bison bison): Facts, Size and History

Appearance: hump, horns and shaggy coat

A bison is easy to recognize. Its huge head hangs low, and a tall hump rises over the shoulders. The hump is not fat but muscle, supported by long spines on the backbone, and it powers the swinging head movements that bison use to sweep snow aside to reach grass in winter.

The front half of the body is covered in long, dark brown hair, with a thick mane, a “beard” under the chin and long hair on the forelegs. The back half has short, dense fur. In spring bison shed their heavy winter coat in ragged patches. Both sexes carry short, black, upward-curving horns, which can be up to about 60 cm (2 ft) long in big bulls. Newborn calves are a reddish tan color and start turning brown at about two and a half months old.

There are two forms, usually treated as subspecies: the plains bison (B. b. bison) of the Great Plains, and the wood bison (B. b. athabascae) of northern Canada and Alaska. Wood bison are taller and heavier, with a squarer hump and a less shaggy mane and beard.

How big is an American bison?

Bison are the largest land mammals in North America by weight. Males (bulls) are much larger than females (cows).

The heaviest reliably weighed wild plains bull reached about 1,270 kg (2,800 lb), and large wood bison bulls can stand about 2 m (6.6 ft) tall at the hump. Calves weigh only 15–25 kg (33–55 lb) at birth. For more detail on bulls, cows and calves, see how big is a bison?

Habitat and range

Before the 1800s, bison lived across a huge area, from Alaska and northern Canada south to northern Mexico, and from the Rocky Mountains east almost to the Atlantic coast. Their stronghold was the Great Plains, where enormous herds moved across the shortgrass and mixed-grass prairies.

Today wild bison live only in scattered, managed herds. Plains bison roam parks and reserves such as Yellowstone National Park, Wind Cave and Theodore Roosevelt national parks, Custer State Park in South Dakota, Grasslands National Park in Saskatchewan and Banff National Park in Alberta, as well as many tribal lands. Wood bison live mainly in northern Alberta, the Northwest Territories, Yukon and British Columbia, and a herd was reintroduced to western Alaska in 2015. Bison use open grasslands, river valleys, sagebrush, mountain meadows above 2,400 m (8,000 ft) and, in the north, sedge meadows within the boreal forest.

Yellowstone is special: it is the only place in the lower 48 states where bison have lived continuously in the wild since prehistoric times. Its herd, which has ranged from about 3,500 to nearly 6,000 animals in recent years, is the largest on public land in the United States. Visitors see bison most easily in the Lamar and Hayden valleys, and in winter around the warm ground of the geyser basins and along the Madison River.

Outside North America, wood bison from Elk Island National Park in Alberta have been released in Yakutia, in eastern Siberia, since 2006, as part of a project to restore large grazers to the northern steppe. The closely related European bison, or wisent, is a separate species.

What do bison eat?

Bison are grazers. In Yellowstone, grasses, sedges and other grass-like plants make up more than 90% of their diet all year, while broad-leaved herbs and the twigs and leaves of shrubs usually form less than 5%. They graze for about 9–11 hours a day.

Like cattle, bison are ruminants. Their stomach has several chambers full of microbes that ferment tough plant material, and they regurgitate and rechew their food (“chewing the cud”) between feeding bouts. In winter they swing their heads from side to side to clear snow, and they eat snow for water when rivers freeze. Their grazing, trampling and dust bathing in shallow hollows called wallows create a patchwork of short and tall vegetation that benefits many other prairie plants and animals.

Herds and behavior

Bison are social animals. Cows, calves and young animals live together in herds that are often led by older females, while adult bulls live alone or in small bachelor groups for most of the year. In Yellowstone, groups average about 20 bison in winter and about 200 in summer, with gatherings of up to about 1,000 during the breeding season.

Bison communicate with grunts, snorts and, during the rut, deep bellows from bulls that can carry a long way. They roll in wallows to shed hair and ease insect bites, and rub against trees and rocks. Despite their bulk, bison are agile: they can run at about 55 km/h (35 mph), jump about 1.5 m (5 ft) and swim across rivers. Some Yellowstone bison migrate up to about 110 km (70 mi) between summer and winter ranges. Their calm appearance is misleading; read are bison dangerous? before visiting a park.

Reproduction and life cycle

The rut (breeding season) runs from about late June to September, peaking in July and August. Bulls rejoin the herds, bellow, wallow and fight head to head for access to cows. A bull “tends” a cow that is about to come into heat, staying by her side and keeping rivals away until she is ready to mate.

Pregnancy lasts about 285 days (around nine and a half months), so most calves are born from mid-April to May. A cow has a single calf, which can stand within an hour or so and follow the herd within hours. Calves nurse for seven to eight months and are usually weaned before their first birthday. Females can breed at two or three years old, but bulls rarely win mates until they are about six or older.

Lifespan, predators and threats

Wild bison typically live about 12–20 years, and some reach their mid-20s. Gray wolves and grizzly bears are the only predators that kill bison regularly, and they mostly take calves and weakened animals, often in late winter. Healthy adult bulls are rarely killed. In Yellowstone, some wolf packs have learned to specialize in hunting bison, a dangerous task that can take hours.

The main problems for bison today are human ones. Most herds are small and fenced, so they cannot move freely or grow large. Many herds carry traces of cattle DNA from crossbreeding in the late 1800s and early 1900s. Diseases such as bovine brucellosis and tuberculosis affect some wild herds, including Yellowstone (brucellosis) and Wood Buffalo National Park (both), and fear of brucellosis spreading to cattle limits where bison are allowed to roam outside parks.

Conservation status

The IUCN Red List classifies the American bison as Near Threatened (2017 assessment). Although there are several hundred thousand bison in total, the great majority are raised as livestock. Only roughly 20,000–30,000 live in conservation herds, and far fewer roam freely in large landscapes, so the wild population depends on continued management. In Canada the wood bison is listed as a species at risk, and in the US it is listed as threatened under the Endangered Species Act.

The story of how bison fell from tens of millions to near extinction, and how a few people saved them, is told in why did bison almost go extinct?. For current numbers by herd, see how many bison are left?

Bison and people

For thousands of years, bison were the center of life for many Indigenous nations of the Great Plains, providing food, robes, tipi covers, tools and sinew, and holding deep spiritual meaning. The Lakota and other nations consider a white bison calf especially sacred. Many tribes now run their own herds; the InterTribal Buffalo Council, founded in 1990, brings dozens of member tribes together to restore bison to tribal lands.

The bison also appears on state flags and seals, on the old “buffalo nickel” (1913–1938) and on the flag of Manitoba. In 2016 it became the national mammal of the United States. Bison meat is produced on ranches in both the US and Canada. In everyday American English the animal is often called a “buffalo”, which is why scientists sometimes need to explain the difference; see bison vs buffalo.

Myths and facts

Myth: bison are slow and docile. Fact: bison can sprint at about 55 km/h (35 mph) and are unpredictable, especially bulls in the rut and cows with calves. In Yellowstone they have injured more visitors than any other animal.

Myth: the hump is a store of fat or water. Fact: the hump is mostly muscle attached to long spines on the vertebrae, which support the heavy head.

Myth: all bison are pure wild animals. Fact: genetic tests have found small amounts of cattle DNA in most herds. Yellowstone, Wind Cave and a few other public herds show no evidence of it, which makes them especially valuable for conservation.