The following extracted from park service information papers

Yellowstone is the only place in the lower 48 states where a population of wild bison has persisted since prehistoric times, although fewer than 50 remained here in 1902 (Meagher 1973). tearing extinction, the park then imported 22 bison from privately-owned semi-domesticated herds, as foundation stock for a bison ranching project that 'spanned 50 years in Yellowstone's Lamar Valley.. Ranching activities there included irrigation, hay-feeding, roundups culling, and predator control, to artificially ensure herd survival. By the 1920s, some intermingling of the introduced and wild bison had begun.

All ranching and herd reduction activities were phased out by 1966, again allowing natural processes to determine bison numbers and distribution. Although winterkill takes a toll, by 1991 bison numbers had increased to about 3,000 in three herds. Park visitors sometimes fear that the large numbers of hoofed animals in northern Yellowstone may be destroying the vegetation, but studies show that these sagebrush grasslands have not been overgrazed.

For the last 50 years, almost every year a few bison have moved seasonally beyond park boundaries. In the early 1980s, larger numbers of bison began attempting to recolonize wintering areas outside the park, where they may sometimes conflict with private interests. Between 1985 and 1989 a total of 688 bison were shot by warden-supervised public hunters outside the park, to reduce these conflicts. Public hunting was discontinued in 1990 but shooting by State of Montana.wardens continues, as needed.

Bison management now includes close monitoring of herd movements. Bison use the roads now groomed for oversnow travel, making it easier for them to move outside the park. Hazing, herding, physical barriers, and scare devices have met with only limited success in preventing bison from leaving Yellowstone.

Bison shot outside the park are being examined to determine incidence of disease; research is being conducted on the possibility of disease transmission to domestic cattle, although transmission has not been demonstrated to occur in the wild.

Park managers are developing a long-range bison management plan, in cooperation with the State of Montana and the U.S. Forest Service. The objectives are to maintain a free ranging bison population in Yellowstone and to reduce potential conflicts with private property interests outside the park.

The Yellowstone bison are unique in the United States. Only here have wild bison survived since primitive times, long before establishment of the park in 1872, to provide particular cultural, scenic, educational, and scientific values which cannot be duplicated with bison herds elsewhere in the United States. The present population derives from two bloodlines; the original population of native bison, and bison from two fenced herds introduced in 1902.

Yellowstone bison are wild, free ranging, unrestricted by boundary fences, and subject to 'minimal interference within the park by man. The population numbers 3500-3600 plus at present.

Most calves are born on the wintering areas in late April and May. The calves are reddish-tan at birth and begin to darken about 2 1/2-3 months of age. Both sexes are horned. Adult bulls may weigh up to 2000 pounds; adult cows usually weigh half as much.

There is no apparent predation on bison at any time; however as carrion, bison are an important source of food for scavengers such as coyotes, ravens, and bears emerging from hibernation. Disease is not an important population influence. Brucellosis is present, but has no apparent effects; potential of transmission to livestock is low.

Bison numbers have increased dramatically in recent years as the population adapted to using energy efficient winter roads in the park to access additional foraging areas, and in some cases to facilitate exiting the park'. When bison leave the park conflicts exist with other land use objectives outside. The modern world has limited space for a truly nomadic, highly gregarious, large mammal. Long range planning efforts are attempting to address this interagency problem.

Where Found: Bison winter in the Lamar area, Pelican Valley, Hayden Valley, Firehole, and the west side complex. In summer Hayden Valley is one of the best places to see bison and often in the Lamar area. Scattered bulls are widely distributed. None of the herds are distinct throughout the year.

WARNING: Bison are very stolid by temperament, but large and have horns. They may be dangerous it approached. Treat them with great respect