…. Bison Fact Sheet

Excerpted From:
Greater Yellowstone Science Learning Center

Overview **Ted's Bison

The bison was the dominant large herbivore in North America for thousands of years, and is still the biggest in size. Bison are a member of the Bovidae family, which includes cattle, goats, sheep, and true buffalo. Traditionally referred to as buffalo, bison are only distantly related to the African buffalo (Syncerus) and the Asian buffalo (Bubalus), neither of which has a shoulder hump. Although the American Society of Mammalogists continues to assign bison to a genus of its own, Bison, some experts now believe that recent evolutionary and genetic evidence supports inclusion of bison in the same genus as cattle, Bos (Boyd 2003).

Most authorities regard the European bison, called a wisent, as a separate species, B. bonasus (Wilson and Reeder 1993). Compared to the North American bison, the wisent has a less pronounced hump, smaller head and forequarters, and hairier tail, and its horns project forward rather than up. More controversial is the question of whether the wood bison of northern Canada should be considered a different subspecies (B. bison athabascae) from the far more numerous plains bison (B. bison bison) of southern Canada and the United States (Reynolds et al. 2003). The wood bison has slightly different morphology and is typically darker and larger than the plains bison.

Yellowstone is the only place in the United States where bison have lived continuously since prehistoric times; in other locations bison have been reintroduced using animals from public or private herds. Bison conservation at Yellowstone, which began in 1902, was the first federal management program to preserve a wildlife species threatened with extinction.
Distribution and Population Size

In North America

At the time of its widest distribution, which may have been about 1600, the bison’s range extended from Alaska to northern Mexico and from the Rocky Mountains to the Appalachians, with the population most concentrated on the Great Plains, where it numbered in the tens of millions. During the 1800s, bison distribution shrank as the population was reduced to scarcely 1,000 by a combination of commercial, subsistence, and sport hunting, regional drought, habitat degradation, competition with domestic livestock, and introduced bovine diseases (Isenberg 2000). Since then, the population has rebounded to more than 600,000, including about 20,000 bison on public land (Reynolds et al. 2003). Most bison today are privately owned, managed as livestock, and in herds that contain some cattle genes as a result of hybridization efforts that began in the nineteenth century (Boyd 2003).

Prior Distribution in Yellowstone

Little is known for certain about bison abundance and distribution before the park was established in 1872, but inferences have been drawn from archeological evidence and observation of contemporary movement. Prior to Euro-American settlement, bison herds that occupied the expansive grasslands around the Yellowstone Plateau may have extended their seasonal migrations into what is now Yellowstone National Park. What is referred to as “the northern range,” which previously extended from the upper Lamar Valley northward beyond what is now Livingston, Montana, appears to have been occupied continuously by bison for at least 10,000 years. Bison were also present to the south in Jackson Hole, east of the Absaroka Mountains on the park’s east boundary, and to the west and southwest of the park in intermountain valleys (Gates et al. 2005). During the spring and summer, bison would have followed advancing plant phenology to higher elevation areas; during the fall and winter, most bison would return to lower elevation ranges, but some bison wintered in the Lamar, Pelican, and Hayden Valleys (Meagher 1973). In the mid-nineteenth century, bison appear to have been abundant in lower river valleys and grasslands on the edge of and beyond Greater Yellowstone, and were widely but more sparsely distributed in higher-elevation sites within Greater Yellowstone (Schullery and Whittlesey 2006).

Current Population Size

Since Yellowstone was established, bison abundance in the park has experienced both long-term growth and substantial fluctuations, mostly as a result of management actions, but also in response to occasionally severe winters. After reaching a recorded high of about 4,900 in the summer of 2005, the bison population dropped to about 3,500 before calving the following year, primarily because of management removals at the Montana boundary to prevent bison from leaving the park.

Current distribution.

Over the years, bison distribution within the park has changed as a result of population growth and human influences. Expansion of bison range has been explained as a response to increasing density on suitable habitats, when a threshold of about four bison per square kilometer is exceeded (Gates et al. 2005).

Yellowstone bison are now usually referred to as two herds or “subpopulations”—the northern and the central. Some interbreeding may occur between the two sub-populations. The northern herd winters in an area that may extend from the Lamar Valley to the Gardiner basin, while the much larger central herd uses Pelican Valley, Hayden Valley, the Firehole Geyser Basin, the west boundary area, the Gardiner Basin, and the upper northern range.

As winter advances, bison tend to split up into smaller, more dispersed groups. Depending on the combined effects of population density, learned behavior, and environmental conditions, a variable number are inclined to move toward and beyond the park boundary, especially in Montana. In both northern and central Yellowstone, the elevation decreases from east to west, and on the northern range this descent is usually accompanied by a significant decline in snow cover. This creates a natural migration corridor from the upper Lamar Valley to the Gardiner basin, where a precipitation shadow causes drier conditions. The central bison ranges in Hayden and Pelican valleys usually accumulate much deeper and longer-lasting snow except where the presence of thermal features may allow a longer growing season and reduce snow cover. Snow melt and spring greenup occur earlier in the West Yellowstone area than in Hayden and Pelican valleys, which may draw bison toward the west boundary (Gates et al. 2005).

The bison tend to return to higher elevation areas in the summer and the population is most concentrated during the rut; by mid-July, most of the northern herd has congregated in eastern Lamar Valley, and the central herd in Hayden Valley.

Human influences on distribution

The grooming of roads for oversnow vehicles since the early 1970s may have facilitated bison winter travel between high and low elevation ranges, but it is not thought to have contributed to population growth or to winter range expansion except possibly in the movement of bison from the park interior to the northern range (Gates et al. 2005). Bison appear to travel on the road in winter where it is convenient, i.e., where the roads are aligned with corridors that bison would be likely to use because of terrain, habitat features, and bison behavior. If groomed roads have increased bison winter travel, leaving the roads ungroomed might have little or no effect now, because bison appear to retain and pass along knowledge through generations, including routes to winter range (Cheville et al. 1998).

It is not known what short- or long-term effects hazing and removals at the north and west boundary to deter bison from leaving the park may have on their distribution within the park. Bison are allowed to be on portions of the Gallatin National Forest northeast of the Yellowstone River, and in some winters more than 100 bison have gone there.
Physical Description

Bison have a massive head with a short, broad forehead, a narrowed muzzle, a beard formed by long woolly hair on the chin, and a short, thick neck. The low placement of the head and broad jaw facilitate the efficient harvesting of low-growing forage rather than browsing on shrubs and trees. Both sexes have round, curved horns that begin to emerge soon after birth, rise laterally on the side of the head with inward-curving tips, and continue to grow throughout the animal’s life. The forequarters, which bear a shoulder hump, are heavier than the hindquarters, and have longer, lighter pelage. The tail ends in a tassel of long hair. A dense wooly pelt begins appearing in the fall that reaches its prime insulating condition during the winter and is slumped off in clumps in the spring (McHugh 1972).

A bison bull may weigh upwards of 1,800 pounds and stand six feet tall at the shoulder. A typical bison cow weighs about 1,000 pounds and is less shaggy on the head and chin. Despite their size and seeming stolidity, bison are capable of leaping six-foot fences, and running up to 30 miles an hour. The bison’s disproportionately heavy forequarters enable it to pivot quickly in countering and horning predators such as wolves that typically try to disable a large animal by attacking its hindquarters.

Bison are less selective grazers than other ungulates, but they compensate for their low-quality diet by consuming large quantities of forage that other ungulates would find relatively indigestible. Food remains in their large digestive tract longer, resulting in a more efficient use of its microflora to extract nutrients. Like other ruminants, bison have four-chambered stomachs and swallow forage after only superficial chewing. The food is soaked and churned in the first chamber (rumen), then regurgitated wad by wad for more chewing before being swallowed again, when it passes from the second chamber (reticulum) to the third (omasum) and fourth chamber (abomasum), where most of the digestion takes place (Reynolds et al. 2003).

The primary habitat requirements for bison are adequate forage and water. In Yellowstone, bison are usually seen in open meadows and along river valleys, but they also traverse forested areas to reach high-elevation plateaus. They consume almost exclusively sedges and grasses. They tend to congregate in groups on large, open ranges such as those in Hayden Valley and Lamar Valley, but they are frequently on the move. Bison may contribute to new plant growth by distributing seeds, breaking up soil surfaces with their hooves and wallows, and fertilizing by recycling nutrients through their waste products. Grazing may also maintain open grassland communities by preventing accumulation of dead grass litter that would otherwise suppress growth of grasses (USDOI and USDA 2000a).

Bison habitat in Yellowstone coincides with that of other ungulates over which bison are generally dominant, but the preference of other ungulates for browsing rather than grazing avoids significant dietary overlap. Elk and bison have been observed tolerating each other’s presence within 10 meters in Yellowstone, but bison may force elk into deep snow in winter and chase them from forage in summer (Reynolds et al. 2003).

In the summer, forested areas provide bison shade and a place to avoid insect pests; during the winter they may provide shelter during storms and easier access to forage where tree cover deters snow accumulation. In Yellowstone, the snowpack is also less in thermal areas where bison can find ice-free water and heat that reduces the energy expended to keep warm in winter. Unlike ungulates that paw through the snow to reach forage, bison swing their massive heads side to side to clear the snow. This method helps them survive the winter in places where deer and elk cannot, but it can become useless when a thick crust freezes on top of the snow, as can happen after a brief thaw.

Yellowstone bison are considered seasonally migratory, but the movements of bison groups during the winter may involve traveling back and forth between ranges as well as an overall population shift from higher to lower elevations. Their movements often follow repetitive patterns that appear to indicate learned habits as well as immediate needs to locate forage (Meagher 1973).

Life Cycle

Some bison cows in Yellowstone become pregnant for the first time in the summer after their second birthday, and most by their fourth summer, but young bulls are generally prevented from breeding by older bulls until they are at least six years of age. The bison cow’s estrus cycle typically lasts 24 to 48 hours during a three-week interval that begins between mid-July and early September, a period referred to as the rut. If a cow does not conceive during her first interval, she comes back into estrus after about three weeks (Kirkpatrick et al. 1993). Many cows conceive in consecutive years and into old age, although at a lower rate (Aune et al. 1998).

Calves are born after a 9 to 9½-month gestation period. Twins are very rare. In Yellowstone, calving begins by mid-April, but most births occur during May. The number of pregnancies that end in the birth of viable calves depends in part on the severity of the preceding winter (Aune et al. 1998). Newborn calves are reddish-orange for about three months until they begin to turn brown. Calves may try to graze by five days of age, but they continue to nurse for at least seven to eight months; most calves are weaned by the end of the first year (Meagher 1986).

Occasionally a wild bison may live 20 years, but the typical life span for a Yellowstone bison that survives calfhood is 12 to 15 years.


Bison cows, calves, yearlings, and young bulls are usually found in matriarchal groups, especially during calving. Group size depends largely on forage availability. When forage is limited in availability or access because of snow or other environmental conditions, bison will move elsewhere together or split into smaller groups to search for food. In deep snow, bison often create a trench by traveling in single file, alternating leaders, to reduce energy expenditure. Although several mature bulls are often present at the edge of the cow-calf groups, most graze apart in smaller, looser associations except during the rut. The oldest bulls are often found as scattered individuals who may travel long distances to graze and may stay away from the stress of the rut (McHugh 1972).

As the rut approaches, increased competition and fights for dominance among the bulls occur. Males have a fluid hierarchy in which dominance rankings shift frequently and many males breed each year. Unlike elk, bison bulls do not dominate groups of cows as a harem. Fights among bulls during the rut usually involve head-shoving matches to establish or reinforce dominance over other bulls rather than to mate with specific cows. To avoid actual combat, bison use a lot of bluffing and bellowing to size up opponents and test the determination of competitors. An exchange of threats occasionally erupts into battle, as enraged bulls butt each other. The horns are shaped in such a way as to lock readily into a hold that impedes fatal stabbing (McHugh 1972).

As a result of their exertions and distractions from grazing, mature bulls lose an average of 200 pounds between June and October (Lott 2002). The aroused bulls course through the herd, inspecting each cow. When a bull finds a cow close to estrus, he will stay near her until she is ready to mate. Bulls guard their chosen cow, trailing closely beside her, warning off intruders with threatening postures, brief charges, and occasional attacks (McHugh 1972).

Wallows are a common feature of habitat used by bison, which typically create these craters of bare dirt where vegetation is relatively sparse to begin with. The bison’s thrashing legs and thumping torso fills their coat with dust, as the animal struggles to gain enough momentum to roll onto its hump. Bison may wallow several times a day in summer to deter biting flies, remove tufts of molted fur, scent mark, and display their strength and vigor (Lott 2002). Almost anything that juts out of the ground may be used for rubbing—boulders, earthen banks, mounds of snow, stumps, bushes, and trees. Bison rubbing sometimes debarks and kills saplings, which may also benefit the bison by inhibiting the succession of grassland to forest (Meagher 1973).

Bison as Prey

Bison provide a source of food both as carcasses for scavengers and as prey for wolves and bears. But bison are the most difficult prey in Yellowstone for these carnivores, and bison predation is largely compensatory: although killing bison may help to sustain wolf population numbers, this predation does not appear to have significantly affected the size or distribution of the bison population as a whole. Bison taken as prey are usually weak animals that may have soon died of other causes and were less likely to reproduce even without wolves present.

The largest number of documented bison kills by wolves in a single year was 29 in 2005, compared to 244 elk that same year (Smith et al. in press). Most kills are documented in the winter because summer predation on bison and elk calves is difficult to detect, but it does appear that, even though the elk population has declined since the 1990s, wolves continue to prey on elk at a much higher rate than on bison relative to the size of each species’ population. Wolves are more selective with bison because the risk of injury is much greater.

Even less is known about the extent of predation by grizzly bears, but the carcasses of bison that have been killed by wolves are often usurped by a bear.

Other Bison Mortality

Although removals to prevent bison from leaving the park have been the largest cause of Yellowstone bison mortality in some years, over the long term most bison deaths have been a direct or indirect result of the animal’s declining physiological condition during the winter. On average, about nine percent of the population dies of natural causes each winter, but few years are “average” and winterkill may be substantially more or less than that. The more severe the winter, the higher the death toll.

Bison usually expend most of their body fat in early to mid-winter. As winter progresses, some bison cannot acquire enough of the nutrients needed to maintain their body temperature and energy levels until spring. The old, the young, the sick, and the disabled are the most vulnerable (USDOI and USDA 2000a). Bulls may die as a direct result of loss of blood or an infected wound received during the rut, or because their loss of weight and weakened condition makes them more susceptible to winterkill.

As with elk, a major factor in determining a bison’s longevity is the condition of its teeth. Grass and other forage contains particles of silicon dioxide that abrade the teeth. After about 12 years, a bison’s teeth may become so worn that the animal can no longer process food efficiently. As a result of geothermal processes in the Firehole area, bison and elk that feed there are exposed to especially high levels of silica and fluoride in the vegetation, and this accelerates the rate at which the animals’ teeth are worn down (Meagher 1973).


Brucellosis is a contagious disease spread by the bacteria Brucella abortus, an intracellular pathogen which was brought to North America centuries ago in European cattle. The bacteria is spread primarily through ingestion of birthing materials or milk from an infected cow. The disease is not fatal to the cow, but can cause an infected cow’s first pregnancy to end in abortion or the birth of a non-viable calf; the likelihood of a healthy birth increases in subsequent pregnancies. Male animals may become infected by brucellosis, but they are unlikely to transmit the bacteria to other animals. If the infection becomes chronic in a herd over a period of years, many of the animals develop an immunity that enables them to clear the bacteria from their bodies (Cheville et al. 1998).

Brucellosis was probably transmitted to bison in Yellowstone early in the twentieth century when domestic cattle were still allowed to graze in the park. Although some bison calves in Yellowstone may be aborted or fail to thrive because of brucellosis, over the long term the disease does not appear to have had a significant affect on population size. Brucellosis also infects elk in Greater Yellowstone.

Human infection by Brucella abortus, which causes undulant fever, can result from the consumption of unpasteurized dairy products or direct contact with infected birthing material or carcasses, but it is now rare in the United States. As a result of a national campaign to eradicate brucellosis in domestic cattle by vaccinating calves and killing animals that test positive for exposure to Brucella abortus, the U.S. Department of Agriculture (USDA) has classified nearly all states as “brucellosis-free” for purposes of livestock commerce. This designation permits owners to ship their cattle across state lines without testing, quarantine, or other restrictions. Montana has been designated brucellosis-free since 1985. Wyoming and Idaho lost their brucellosis-free status in 2004 and 2005 respectively after the disease was detected in several vaccinated cattle herds, apparently as a result of contact with infected elk or with infected cattle that had had contact with infected elk. Both states have taken steps to regain their brucellosis-free designation as soon as possible.

Testing for brucellosis in live animals can only detect whether the animal has produced antibodies against Brucella abortus. The presence of Brucella abortus can only be determined through post-mortem examination, when it is typically found in the lymph system surrounding the reproductive organs. Based on testing of bison captured at the park boundary, it appears that about half of Yellowstone bison have been exposed to the bacteria and about 25–50% of these “seropositive” animals are actively infected (USDOI and USDA 2000a). No case of brucellosis transmission from wild bison to cattle in a natural setting has been confirmed, but that could be because the circumstances under which transmission could occur have been avoided. Vaccination of both livestock and Yellowstone bison is currently done to minimize the risk of disease transmission from the infected bison population to livestock that occupy overlapping ranges. The currently available vaccine, RB51, is a weakened form of the live bacteria and requires special caution in delivery and handling. Vaccination of experimental animals has demonstrated that most pregnant females exposed to the disease (about 80% in some studies) will not abort their pregnancy. The value of a vaccination program for bison is the reduction in quantity of bacteria shed in the environment that may be encountered by other animals naïve to the disease.
Status in the Greater Yellowstone Ecosystem

The bison populations in Yellowstone and Grand Teton national parks have flourished despite constraints on their movements outside park boundaries that prevent them from being completely “free ranging.” Although the plains bison is not listed as an endangered or threatened species in the United States or Canada, most herds outside Greater Yellowstone are intensively managed in ways that alter natural selection processes such as breeding activity, susceptibility to natural diseases and predators, and migratory tendencies. Most state and federal preserves, including the National Elk Refuge where most bison from Grand Teton spend the winter, provide supplemental feed because of insufficient range, and selectively cull older animals, especially males. Natural selection, genetic diversity, and natural population regulating factors such as weather and predation contribute to the maintenance of the wild character of a species and the traits that enable an animal to survive in its native habitat with minimum human interference (Knowles et al. 1998). The major challenge to bison as wildlife in Greater Yellowstone is posed by the need to limit their number and distribution outside of the parks and the extent to which hazing and removals may affect the bison’s natural abundance, behavior, and distribution within the parks.
Management Activities in Yellowstone National Park
Management History

The commercial slaughter of bison for their hides that peaked in herds south of the Union Pacific Railroad in the early 1870s did not finish off most of the northern herds for another ten years. In the decade following the establishment of Yellowstone National Park in 1872, its bison population was occasionally estimated to number in the hundreds. However, hunting in the park for personal consumption was not prohibited until 1887, and park managers had little means to prevent poaching. In response to a widely publicized incident of bison poaching in 1894, Congress passed the Lacey Act to establish more severe penalties, but without adequate personnel to apprehend poachers in the park, the bison population continued to decline. Although some bison may have gone undetected, the winter bison count reached a low of 21 in 1902, all of them in Pelican Valley (Meagher 1973).

With more U.S. Army troops patrolling the park, the last herd of free-roaming bison in the United States began to increase slowly. To help ensure the long-term presence of bison in Yellowstone, Congress appropriated $15,000 in 1902 to purchase bison from ranchers. These 21 animals were fenced near Mammoth Hot Springs and, with the help of winter feeding and three calves taken from the Pelican Valley herd, the captive herd soon outgrew its accommodations. A larger facility, which became known as the Buffalo Ranch, was built in Lamar Valley, where eventually more than 400 acres were cleared to cultivate hay for feed. Starting in 1914, when the captive herd numbered more than 200, it was released to graze on open range all summer. By the 1920s these bison were intermingling with the wild herd, which then numbered at least 60. Brucellosis was first detected at the Buffalo Ranch in 1917. Because the disease seemed to have little adverse effect on the bison population, no effort was made to address it until the 1940s, when a short-term program of testing and vaccinating part of the herd had no long-term effect (Franke 2005).

Winter feeding at the Buffalo Ranch continued at least sporadically until 1952, but park managers began limiting the size of the bison population in the 1920s by eliminating “outlaw” bulls, shipping live bison from the park, selling bison to meat dealers, and castrating some calves. Concern that loss of access to winter range outside the park was causing over-grazing and creating a risk of large-scale starvation led park managers to begin trapping and killing elk in 1935; bison in the park, which then numbered fewer than 1,000, were soon included in the effort.

In the early 1960s, increasing concern about the risk of bison transmitting brucellosis to livestock outside the park led to a National Park Service (NPS) effort to test bison in connection with the culling program. However, the focus on meeting herd reduction goals and the difficulty of herding bison into the traps resulted in the death of many bison that tested negative for exposure to brucellosis or had not been tested at all. By 1966, the bison winter count had been cut to 226. Increasing public opposition to the elk slaughter and a shift in NPS policy toward reducing interference with wildlife brought elk and bison culling to an end in 1967.

Brucellosis testing also ended, but bison became subject to hazing or slaughter if they approached the park boundary, usually in the Gardiner area. In 1978 the Department of the Interior issued a policy forbidding NPS personnel to shoot these bison, but few bison tried to leave the park until the winter of 1984–85, when the winter count had grown to more than 2,000, and Montana state personnel shot 88 that had crossed the boundary. (Like most other wildlife, bison come under state jurisdiction as soon as they leave the park.) The following year, Montana began issuing hunting permits for bison. This drew public opposition because of the apparent firing line at the park boundary, especially after more than 500 bison were shot by licensed hunters during the winter of 1988–89. The 14 bison harvested by hunters in the winter of 1990–91 were the last until bison hunting was reinstated under different terms in 2005.

In 1990, dissatisfaction among the various federal and state agencies responsible for wild bison and livestock regulation led to the formation of an interagency group to prepare a long-term plan for bison management. Until that plan was completed ten years later, bison were managed at the boundary by NPS and Montana state personnel under a series of interim plans that allowed for the hazing or capture of bison that tried to leave the park. In some cases, captured bison that tested negative for exposure to brucellosis were released; in other cases, bison were sent to slaughter without testing or were shot because they could not be captured. Yet the bison winter count for 1994–95 reached a record high of nearly 4,000. The winter of 1996—97 was especially lethal for bison; severe conditions in the park caused many bison to try to leave and 1,084 (nearly one third of the early winter population) were shot or sent to slaughter at the boundary. An estimated 300–400 died of natural causes (USDOI and USDA 2000a).

Conflicts with Humans

In addition to concerns about transmission of brucellosis to livestock, bison outside the park may come into conflict with property owners by injuring domestic animals, damaging fences, and consuming forage intended for livestock. In the park, the only significant property damage from bison has been inflicted on motor vehicles.

Assaults by bison have been responsible for two known human deaths in the park’s history, both of them men who approached in pursuit of a photograph, one in 1971 and the other in 1983. This compares to the five deaths known to have been caused by bears in the park. But since the 1970s, when park rangers began enforcing rules against bear feeding and the bison population grew to exceed that of bears, bison behavior has resulted in more human injuries than that of grizzly and black bears combined.

A study of park records from 2000 through 2005 found 13 reports of visitors being charged by bison, a number comparable to the preceding five-year period. In 10 of these charges the bison made contact with the visitor and caused an injury, usually a horn-inflicted puncture wound or abrasions resulting from the visitor’s being pushed over (Caslick and Caslick 2006). Unlike bear assaults, bison-caused injuries nearly always occur in developed areas or along roads, where people are apt to assume they are safe from the perils of the wilderness. Many people tend to be less cautious in the presence of bison than bears because they think of all bison as a kind of livestock, an impression reinforced by the bison’s usually impassive behavior.

Current Management

Montana boundary control. In 2000, after a decade of sometimes litigious negotiations, the NPS signed an Interagency Bison Management Plan (IBMP) with the four other participating government agencies: the Montana Department of Livestock; Montana Fish, Wildlife and Parks; the Animal and Plant Health Inspection Service (APHIS) of the USDA; and the U.S. Forest Service. The primary goals of the plan are to maintain Montana’s brucellosis-free status by minimizing the risk of bison transmitting brucellosis to cattle while conserving a viable population of at least 2,300 wild bison in Yellowstone. As part of the plan, the NPS committed to certain bison management activities as a means by which to extend the winter range available to bison on national forest lands outside the park. These activities include: bison population monitoring to enforce temporal and spatial separation of bison and livestock on public and private lands adjacent to the park, bison hazing and capture when necessary to limit bison movement out of the park, and development of a method for remote vaccination of free-roaming bison calves and yearlings.

The two areas where bison are most likely to leave the park are monitored when bison are most likely to be present there, from November until June. Yellowstone personnel have primary responsibility for the north exit area near Gardiner, using a capture facility located inside the north boundary at Stephens Creek when necessary. The state of Montana has primary responsibility for the West Yellowstone area, using capture facilities located outside the park when necessary. Bison that cannot be hazed back into the park may be captured and sent to slaughter if they test seropositive or if the late winter population estimate exceeds 3,000. Bison that cannot be hazed into the park or the capture facility may be shot. Slaughtered bison are donated to tribal or social service organizations (USDOI and USDA 2000b).

Up to 100 bison tested outside the west boundary that are seronegative may be released and allowed to remain within a designated management zone until late spring. Bison that test seronegative at the north boundary cannot be released because of the proximity of private property on which cattle are grazed. Some bison may be kept in the corral at Stephens Creek until spring growth will keep them in the park, but this is not generally done until late winter in order to avoid a long confinement during which animals are fed and brucellosis could be readily transmitted in the pen after an abortion or early calving.

Because of large shifts in the extent and location of bison movements, the results of the boundary control program since the IBMP went into effect have fluctuated widely from year to year and between exit areas. During the winter of 2004–05, for example, most of the activity was at the west boundary, where 98 seropositive bison were sent to slaughter and 73 seronegative bison were released; only one bison was removed at the north boundary. But the most intensive boundary control operations so far under the IBMP occurred the following winter, after the bison population reached a record high of 4,900. Because of the large number of bison that approached the north boundary, more than 800 were sent to slaughter in January and February without testing and about 300 were held at the Stephens Creek corral for several weeks in early spring. More than 100 bison were sent to slaughter or shot in the field from the west boundary.

Bison vaccination. Although use of the currently available vaccine is not expected to lead to eradication of brucellosis in bison, it is believed that vaccination could reduce the possibility of brucellosis transmission by reducing the number of brucellosis-caused abortions. Vaccination of bison less than two years old that are captured and test seronegative at the boundary began in 2004 using RB51. Park staff have completed an Environmental Impact Statement to assess the feasibility of implementing a park-wide program to deliver vaccine to bison without handling animals individually. They are testing the technology that could be used to remotely deliver the vaccine and field methods to determine when, where, and how young bison can be most safely approached without disturbing them.

Other management of bison that leave Yellowstone. Since the state of Montana and APHIS began the experimental program in 2005, 104 seronegative yearlings captured at the park boundary have been quarantined in a facility north of the park. Calves born to bison in the program that complete the multi-year quarantine period without testing positive for exposure to Brucella abortus may be used to start or supplement bison herds on public or tribal land.

Hunting of bison that leave Yellowstone resumed in Montana in the winter of 2005–06. Under the current terms, the hunting season lasts from November 15 to February 15 for bison that leave the park and are allowed to disburse to certain areas of the Gallatin National Forest. The first winter the hunt was in effect, 34 bison were shot by hunters who had received tags through a lottery and six by Montana Indian tribes that had been issued tags. In recognition of their treaty rights, the Nez Perce of Idaho were also given permission to shoot five bison during the hunt.

Bison also occasionally cross the park’s east boundary in Wyoming on the Shoshone National Forest. The state of Wyoming permits up to 15 bison bulls to remain there in an area where overlapping use of range with livestock is very unlikely. If any bison cows or more than 15 bulls remain there from July through January, hunting permits are issued to remove them.

For a list of projects associated with bison in Yellowstone, see current projects.


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Boyd, D. P. 2003. Conservation of North American bison: status and recommendations. M.S. thesis, Faculty of Environmental Design, University of Calgary, Alberta.

Caslick, J. and E. Caslick. 2006. Aggressive behavior and food-begging by wildlife in Yellowstone: a summary of incidents, 2000–2005. March 15, 2006. 59 pp.

Cheville, N. F., D. R. McCullough, and L. R. Paulson. 1998. Brucellosis in the Greater Yellowstone Area. Washington, D.C.: National Academy Press, Washington D.C.

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Gates, C. C., B. Stelfox, T. Muhly, T. Chowns, and R. J. Hudson. 2005. The ecology of bison movements and distribution in and beyond Yellowstone National Park. a critical review with implications for winter use and transboundary population management. Calgary, Alberta: Faculty of Environmental Design, University of Calgary. Available at http://www.nps.gov/yell/naturescience/gatesbison.htm.

Isenberg, A. C. 2000. The destruction of the bison: an environmental history, 1750–1920. Boston: Cambridge University Press.

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Knowles, C., C. Mitchell, and M. Fox. Trends in bison management. 1998. Pages 244–50 in L. R. Irby and J. R. Knight, eds., Proceedings of the International Symposium on Bison Ecology and Management. Bozeman, Mont.: Montana State University.

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McHugh, T. 1972. The time of the buffalo. New York: Knopf.

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_____. 1986. Bison bison. Mammalian Species. 266: 1–8.

Reynolds, H. W., C. C. Gates, and R. D. Glaholt. 2003. Bison. Pages 1009–60 in G. A. Feldhammer, B. C. Thompson, and J. A. Chapman, eds., Wild mammals of North America: biology, management, and conservation. 2nd edition. London: The Johns Hopkins University Press, Baltimore.

Schullery, P. and L. Whittlesey. 2006. Greater Yellowstone bison distribution and abundance in the early historical period. Pages 135–140 in A. Wondrak Biel, ed., Greater Yellowstone Public Lands: A Century of Discovery, Hard Lessons, and Bright Prospects. Proceedings of the 8th Biennial Scientific Conference on the Greater Yellowstone Ecosystem. October 17–19, 2005, Mammoth Hot Springs Hotel, Yellowstone National Park. Yellowstone National Park. Wyo.: Yellowstone Center for Resources.

Smith, D. W., D. R. Stahler, and D. S. Guernsey. In press. Yellowstone Wolf Project: annual report 2005. Yellowstone National Park. Wyo.: National Park Service, Yellowstone Center for Resources.

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_____, 2000b. Record of decision for the final environmental impact statement and bison management plan for the state of Montana and Yellowstone National Park. December 2000.

Wilson, D. E., and D. M. Reeder. eds., 1993. Mammal species of the world. Washington, D.C.: Smithsonian Institution Press.


** Funded through “Eyes On Yellowstone” – made possible by Canon.


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