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Yellowstone preserves the most important bison herd in the United States. Learn more about the near extinction and recovery of these remarkable animals, how they make it through harsh winters, and what their survival says about our ability to share the landscape with another species.
Yellowstone is the only place in the United States where bison (Bison bison) have lived continuously since prehistoric times. Yellowstone bison are exceptional because they comprise the nation’s largest bison population on public land. Unlike most other herds, this population has thousands of individuals that are allowed to roam relatively freely over the expansive landscape of Yellowstone National Park and some nearby areas of Montana. They also exhibit wild behavior like their ancient ancestors, congregating during the breeding season to compete for mates, as well as migration and exploration that result in the use of new habitat areas. These behaviors have enabled the successful restoration of a population that was on the brink of extinction just over a century ago.
However, some Yellowstone bison are infected with brucellosis, a livestock disease that can be transmitted to wild bison and elk as well as to cattle through contact with infected fetal tissue. To prevent conflicts with ranching and other activities outside the park, the National Park Service (NPS) works with other federal, state, and tribal agencies to manage and develop policies for bison access to habitat in Montana. Conservation of wild bison is one of the most complex of Yellowstone’s resource issues. All of the interested parties bring their own wide-ranging values and objectives to the debate.
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 9-½ months of gestation. They are reddish-tan at birth and begin turning brown after 2-½ 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.
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 leaves, 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
Wolves and grizzly bears are the only large predators of adult bison. Dead bison provide an important source of food for scavengers and other carnivores. Bison will rub against trees, rocks, or in dirt wallows in an attempt to get rid of insect pests. Birds such as the magpie perch on a bison to feed on insects in its coat. The cowbird will also follow close behind a bison, feeding on insects disturbed by its steps.
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Most other ungulates of the Greater Yellowstone Ecosystem follow the “green wave” of sprouting plants and grasses during spring because young plants are highly nutritious. Bison begin spring by leaving winter ranges in sync with the green wave but let it pass them by as they migrate and graze, not reaching summer ranges until weeks after green-up. Bison can move out-of-sync with forage green-up because they engineer the ecosystem. Rather than just moving to find the best foods, bison create high-quality foods by how they move and graze. When bison let the green wave pass them by, they spend their time returning to graze the same areas repeatedly at high intensity. The behavior keeps plants growing, although the plants never appear more than a few inches tall, and allows bison to keep getting highly nutritious foods. In winter, bison will move from their summer ranges to lower elevation as snow accumulates and dense snowpack develops. Bison migrate up to 70 miles between summer and winter ranges. Most animals travel about 1,000 miles over the course of the year by repeatedly leaving and returning to the same areas. This means bison travel a greater distance than any other ungulate in the Greater Yellowstone Ecosystem.
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 primarily restricted to Yellowstone National Park and some adjacent areas of Montana. The bison population lives and breeds in the central and northern regions of the park. 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 composed 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 among the Madison, Firehole, Hayden, and Pelican valleys during the rest of the year. However, increasing numbers of bison are travelling to the northern portion of the park and mixing with the northern herd. Some of these bison do not return to the Hayden Valley for the subsequent breeding season, and instead breed and rear 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.
Last updated: February 23, 2021