Bison Information Continued

bull bison with cow birds

Cow birds alight on a bull bison's back to scour his fur for tasty insects.



Bison are the largest land-dwelling mammal in North America. Males (1,985 lbs/900 kg) are larger than females (1,100 lbs/500 kg) and are both generally dark chocolate-brown in color, with long hair on their forelegs, head, and shoulders, but short, dense hair (3 cm/1 in) 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 between 5 and 6 feet tall. 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, though activities can occur through the night.

Bison 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 percent of their diets) in open grassland and meadow communities throughout the year. Forbs and browse are eaten through the year, but usually comprise less than 5 percent of the diet. Bison are ruminants with a multiple-chambered stomach that includes microorganisms such as bacteria and protozoa to enable them to effectively digest plant material. They typically forage for 9 to 11 hours daily. 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.

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 “ride” 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.


Like most other ungulates of the Greater Yellowstone Ecosystem, bison will move from their summer ranges to lower elevation winter ranges 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 of grasses, sedges, and riparian and shrub leaves 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 degrees Fahrenheit (-42 degrees Celsius). 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 of emigration (dispersal), with numerous females recently switching breeding ranges and successfully breeding and rearing young on their new range.


Historically, Yellowstone bison spent summer in the Absaroka Range north of Yellowstone National Park; in the Lamar Valley-Mirror Plateau area of northeastern Yellowstone; in the Hayden Valley of central Yellowstone; and in the Madison-Pitchstone plateaus of southwestern Yellowstone. Bison in northern Yellowstone spent winter in the Lamar Valley and nearby areas; bison in central Yellowstone spent winter in the Hayden and Pelican valleys; and bison in southwest Yellowstone spent winter on the Snake River plains. From 30 to 60 million bison may have roamed North America before the mid 1800s. Their historical range spread from the Pacific to the Appalachians, but their main habitat was the Great Plains where Plains tribes developed a culture that depended on bison. Almost all parts of the bison provided something for the American Indian way of life—food, tools, shelter, or clothing; even the dung was burned for fuel. Hunting bison required skill and cooperation to herd and capture the animals. After tribes acquired horses in the 1600s, they could travel farther to find bison and hunt the animals more easily.

European American settlers moving west during the 1800s changed the balance. Market hunting, sport hunting, and a US Army campaign in the late 1800s nearly eliminated bison. Yellowstone was the only place in the contiguous 48 states where wild, free ranging bison persisted. The US Army, which administered Yellowstone at the turn of the 20th century, protected these few dozen bison from poaching as best they could. The protection and recovery of bison in Yellowstone is one of the great triumphs of American conservation.

Bison were almost extirpated before 1900, leaving a remnant, indigenous herd of approximately 23 bison in the Pelican Valley of central Yellowstone. In 1902, the U.S. army administrators of Yellowstone National Park created another herd in northern Yellowstone from 18 female bison that were relocated from a ranch in northern Montana and 3 males from Texas. Protection and stewardship (husbandry), with supplemental feeding allowed these bison to propagate to more than 1,500 animals by 1954. Likewise, the relocation of 71 animals from the northern herd to central Yellowstone to form the Mary Mountain herd in 1936 contributed to an increase in abundance to about 1,300 bison in central Yellowstone by 1954.

Frequent culling by park managers limited bison numbers through 1966, but abundance rapidly increased after a moratorium on culling in the park was instituted in 1969. Bison numbers quadrupled from about 500 in 1970 to 2,000 in 1980, and then approached 3,000 by 1987. At the same time, elk numbers in northern Yellowstone increased from about 4,000 in 1968 to 12,000 by the mid-1970s and 19,000 by 1988. As herbivore (plant eaters) numbers increase in an area, the amount of forage available to sustain each individual decreases, which can eventually lead to a decrease in nutrition and body condition and, in turn, lower pregnancy and survival rates. To avoid these effects, bison and elk began to change their movement patterns and expand their winter ranges to access more food resources as their numbers increased. Only a few bull bison left the park before 1975, but thereafter, larger groups with female bison began migrating across the northern and western boundaries into Montana during winter. Also, in the 1980s bison from the central herd began moving to northern Yellowstone during winter, where some of them stayed and remained year-round. This range expansion and dispersal from the central herd to northern Yellowstone appeared to be induced by relatively high bison densities combined with deep snow pack during some winters that further limited food availability, especially in the central portion of the park.


Brucellosis, caused by the bacterium Brucella abortus, can cause pregnant cattle, elk, and bison to abort their calves. The bacteria can be transmitted between individual bison and also among bison, elk, and cattle via contact with infected birth tissues. No cure exists for brucellosis in wild animals. Cattle brought this nonnative disease to the region when pioneers settled the West. The disease was subsequently transmitted to local wildlife populations. Many bison and elk in the Greater Yellowstone Ecosystem have been exposed to the bacterium that causes brucellosis. Today, all cattle that use overlapping ranges with bison are vaccinated for brucellosis when they are calves.

Although extremely rare in the United States, humans can contract brucellosis by consuming unpasteurized, infected milk products or contacting infected birth tissue. It cannot be contracted by eating cooked meat from an infected animal. In humans, the disease is called undulant fever and is treated with antibiotics.

Presence in Yellowstone

Brucellosis was discovered in Yellowstone bison in 1917. They probably contracted the disease from domestic cattle raised in the park to provide milk and meat for visitors. Now about 50 percent of the park’s bison test positive for exposure to the Brucella organism. However, testing positive for exposure (seropositive) does not mean the animal is infectious and capable of transmitting brucellosis. For example, people who received smallpox immunization during their childhood will test positive for smallpox antibodies even though they are not infected with the disease and cannot transmit it. Research indicates about 15% of seropositive female bison are infectious at the time of testing. Male bison do not transmit the disease to other bison. Transmission between males and females during reproduction is unlikely because of the female’s protective chemistry. Bison have not been known to transmit brucellosis to cattle under natural conditions although transmission is biologically feasible and has occurred in captivity. Management strategies attempt to prevent bison from commingling with cattle. When livestock are infected, there is economic loss to producers from abortions and still births, slaughtering infected animals, increased disease testing requirements, and the potential for decreased marketability of their cattle. As a result, producers and regulators are concerned about transmission of the bacteria from wild bison or elk back to cattle. Between 1984 and 2000, more than 3,000 bison that migrated outside Yellowstone National Park and into Montana were harvested by hunters or culled from the population to prevent the possible transmission of brucellosis from bison to cattle.

Bison Management

In the year 2000 the state of Montana and the federal government developed an Interagency Bison Management Plan that prescribed collaborative actions to reduce the risk of brucellosis transmission from Yellowstone bison to cattle, including the culling of some bison near the park boundary, while conserving a viable population of bison with some migration to essential, lower-elevation winter ranges on public lands in the state. No plan was developed for elk.

Summer counts of bison in central and northern Yellowstone have varied widely under this plan. Counts of the central herd increased from about 1,900 bison in 2000 to 3,500 in 2005, and then decreased to 1,400 in 2013 due primarily to large culls of about 1,000 and 1,560 bison at the park boundary during 2006 and 2008, respectively. Conversely, counts of the northern herd increased from about 500 bison in 2000 to 3,200 in 2013. This rapid increase was enhanced by movements of bison from the central herd, and possibly, reduced competition as numbers of northern Yellowstone elk decreased from about 19,000 counted individuals in 1994 to less than 4,000 in 2013.

Yellowstone bison are migratory wildlife, not livestock. One mission of the NPS is to preserve native wildlife species and the processes that sustain them. A wild population can be defined as one that is free roaming within a defined conservation area that is large enough to sustain ecological processes such as migration and dispersal, sufficiently abundant to mitigate the loss of existing genetic variation, and subject to forces of natural selection such as competition for breeding opportunities and food, predation, and substantial environmental variability. Thousands of bison inhabit a heterogeneous, spacious landscape in and near Yellowstone National Park with a diverse association of native ungulates and predators that are subject to natural selection factors. They have high genetic diversity compared to many other populations of plains bison, and are one of a few bison populations with no evidence of potential cattle ancestry. Also, they migrate seasonally to areas where food supplies are more abundant, available, or nutritious at different times of the year. In other words, bison in Yellowstone National Park are not managed like domestic stock on a ranch and are generally allowed to move freely within the park—though some intervention occurs near the boundary and developed areas to reduce conflicts with humans and outlying jurisdictions.

The substantial recovery of free-ranging bison populations outside Yellowstone National Park and the nearby Grand Teton National Park, where there were about 800 bison in 2012, is constrained by the availability of low-elevation winter habitat where forage is relatively accessible. Much of Yellowstone and Grand Teton National Parks is mountainous, with deep snow pack that limits access to forage and increases energy expenditures during winter. Also, large portions of the original range for bison are no longer available outside these parks due to agriculture and development. Furthermore, there are political and social concerns about allowing bison outside these parks, including human safety and property damage, competition with livestock for grass, diseases such as brucellosis that can be transmitted between bison and cattle, consumption of agricultural crops, and limited funding for management. Ultimately, it is up to society to decide how they want their federal and state governments to manage bison, including how many bison should be tolerated on public lands, what should be done with “surplus” bison, and how much money should be spent on bison management and brucellosis suppression. Learn more about bison management...

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