Bison are the largest land-dwelling mammal in North America. Males (2,000 lbs/900 kg) are larger than females (1,100 lbs/500 kg) and both are generally dark chocolate-brown in color, with long hair on their forelegs, head, and shoulders, but short, dense hair (1 in/3 cm) on their flanks and hindquarters. Calves of the year are born after 9 to 9½ months of gestation. They are reddish-tan at birth and begin turning brown after 2½ months. Both sexes have relatively short horns that curve upward, with male’s averaging slightly longer than those of adult females.
All bison have a protruding shoulder hump. Large shoulder and neck muscles allow bison to swing their heads from side-to-side to clear snow from foraging patches, unlike other ungulates that scrape snow away with their front feet. Bison are agile, strong swimmers, and can run 35 miles per hour (55 kph). They can jump over objects about 5 feet (1.5 m) high and have excellent hearing, vision, and sense of smell.
Bison are mostly active during the day and at dusk, but may be active through the night. They are social animals that often form herds, which appear to be directed by older females. Group sizes average about 20 bison during winter, but increase in summer to an average of about 200, with a maximum of about 1,000 during the breeding season (known as the rut) in July and August. Bison are sexually mature at age two. Although female bison may breed at these younger ages, older males (>7 years) participate in most of the breeding.
During the rut mature males display their dominance by bellowing, wallowing, and engaging in fights with other bulls. The winners earn the right to mate with receptive females. Once a bull has found a female who is close to estrus, he will stay by her side until she is ready to mate. Then he moves on to another female. Following courtship, mature males separate and spend the rest of the year alone or in small groups. Group sizes decrease through autumn and into winter, reaching their lowest level of the year during March and April. Watch a bison calf take its first steps with commentary by bison ecologist Rick Wallen.
Yellowstone bison feed primarily on grasses, sedges, and other grass-like plants (more than 90% of their diets) in open grassland and meadow communities throughout the year. They also eat forbs (weeds and herbaceous, broad-leafed plants) and browse (the leaevs, stems and twigs of woody plants) through the year, but those usually comprise less than 5% of the diet. They typically forage for 9 to 11 hours daily. Bison are ruminants with a multiple-chambered stomach that includes microorganisms such as bacteria and protozoa to enable them to effectively digest plant material. Bison alternate between eating and ruminating, which is regurgitating partially digested food and chewing it again, to allow microorganisms to further break down plant material into volatile fatty acids and other compounds. Their large digestive tract allows them to digest lower quality foods with greater efficiency than other ungulates such as cattle, deer, or elk.
Interaction with Other Wildlife
Like most other ungulates of the Greater Yellowstone Ecosystem, bison will move from their summer ranges to lower elevation as snow accumulates and dense snowpack develops. Most bison alter their diets somewhat during winter, feeding in lowland meadows with concentrated sedges and grasses compared to a more diverse diet during the rest of the year. Bison appear to select foraging areas during winter based more on plant abundance than quality, and then consume the most nutritious plants available. High densities of bison can deplete forage in high quality patches, resulting in subsequent use of areas with plants of lower diet quality. Bison in central Yellowstone frequently use thermally influenced areas near geysers, hot springs, fumaroles, and rivers with less snow during winter. Forested areas are used occasionally for shade or shelter, escape from insects and other disturbances, or to travel between foraging areas or seasonal ranges.
Yellowstone bison historically occupied approximately 7,720 square miles (20,000 km2) in the headwaters of the Yellowstone and Madison rivers. Today, this range is restricted to primarily Yellowstone National Park and some adjacent areas of Montana. The bison population is subdivided into the central and northern breeding herds. The northern breeding herd congregates in the Lamar Valley and on adjacent plateaus for the breeding season. During the remainder of the year, these bison use grasslands, wet meadows, and sage-steppe habitats in the Yellowstone River drainage, which extends 62 miles (100 km) between Cooke City and the Paradise Valley north of Gardiner, Montana. The northern range is drier and warmer than the rest of the park, and generally has shallower snow than in the interior of the park.
The central breeding herd occupies the central plateau of the park, from the Pelican and Hayden valleys with a maximum elevation of 7,875 feet (2,400 m) in the east to the lower elevation and thermally influenced Madison headwaters area in the west. Winters are often severe, with deep snows and temperatures reaching -44°F (-42°C). This area contains a high proportion of moist meadows comprised of grasses, sedges, and willows, with upland grasses in drier areas. Bison from the central herd congregate in the Hayden Valley for breeding. Most of these bison move between the Madison, Firehole, Hayden, and Pelican valleys during the rest of the year. However, some bison travel to the northern portion of the park and mix with the northern herd before most return to the Hayden Valley for the subsequent breeding season. In addition, there is some evidence numerous females recently switching breeding ranges and successfully breeding and rearing young on their new range.
Yellowstone has played a key role in the conservation of wild bison in North America. If fact, we've been so successful that we now face the challenge of helping to manage a rapidly growing population of migratory bison that frequently roam beyond our borders onto private land and land managed by other agencies. Read more about the history of bison management and the challenges of maintaining a wild, migratory population of bison in a modern landscape.