Animal Field Guide

Bison

Bos bison
(Bovidae)

Montana Species of Concern
Global Rank: G4
State Rank: S2

Agency Status
USFWS:
USFS: none
BLM: none

American bison are the largest North American land animals with weights over 1000 kilograms recorded (Foresman 2001). They have a massive dark head with short black horns curving upward and inward from the base. A large mop of long dark hair covers the top of the head. The body is tall and narrow (1.8 meters at the shoulder) (Foresman 2001), with a distinctive large shoulder hump tapering toward the hindquarters. The tail is short and tufted at the end. The legs are relatively short. Adult bison have heavy light brown hair covering their shoulders and forequarters blending to shorter darker hair from their shoulders back. The head, neck and front legs have dark hair as well. In summer, much of the hair on the hindquarters is lost. Male bison are proportionally larger and more robust than females. Calves are reddish in color but darken to adult pelage by their first fall.

American bison are unmistakable. The combination of large size, shoulder hump, and short, dark, curved horns on both sexes eliminates any other large ungulate.

American bison migrate out of Yellowstone National Park during the winter and these movements are more frequent and involve greater numbers of animals during years of heavy snow when populations are high, generally over 3000 individuals (Chevelle et al. 1998). Recently, (1985-1986) harvest has resumed in response to Montana movements out of Yellowstone National Park. American bison at the National Bison Range are confined to the range and no migration is possible. This species previously made mass migrations across the prairie in spring and fall, with mountain populations moving to lower elevations in valleys.

Because of restrictions, currently occupied habitat does not reflect the full natural range for American bison. Habitat consists of Palouse prairie and montane forest on the National Bison Range; the Yellowstone Park range is unavoidably at higher elevations with grassland interspersed with forest. Throughout their range, American bison inhabit open plains and grasslands. Woodlands and openings in boreal forest, meadows, and river valleys are used in the northern parts of their range. Like other large grazers, they are attracted to burned areas the next growing season (Shaw and Carter 1990). During the growing season at the Konza Prairie in northeastern Kansas, they preferred areas that had been burned in spring. Summer grazing was concentrated in large watershed area (79 to 119 hectares) dominated by warm-season, perennial C4 grasses. In fall and winter, they grazed both burned and unburned watersheds more uniformly, but grazed most intensively in areas with large stands of cool-season, C3 grasses (Vinton et al. 1993).

American bison are grazers and feed on grasses, forbs, and sedges. The massive head is used to sweep snow away from forage. They possess a greater digestive capacity than cattle. In Yellowstone National Park, sedges are most important in all seasons, grasses second in importance. Forbs and browse are minor components in the diet. Preferences may be related to plant phenology.

American bison are gregarious and often forms herds of 11 to 12 animals. Cows and young remain in herds throughout the year. Bulls are solitary or in small groups until summer when they begin to mix with cow-calf herds. Home ranges in the Northwest Territories averaged several hundred square kilometers (Larter and Gates 1990). The life span of an American bison is 18 to 20 years with winterkill being the primary mortality factor in Yellowstone Park. More severe winters result in increased winterkill (Podruzny and Gunther 1999). Wolf predation of American bison has increased since their reintroduction into Yellowstone National Park (Smith et al. 1999). They may be becoming a regular prey item for some wolves, particularly in late winter and spring. The primary reason for bison harvest around Gardiner is the perceived threat of brucellosis transmission to cattle.

Most cows breed at 2 to 4 years, whereas males usually mature at 3 years. Older (6+ years) males do most of the breeding. The majority of mating occurs in July and August. Gestation lasts about 9.5 months. Normally, 1 calf is born mid-April to early June, with most births occurring in May. Cows usually give birth in isolation where vegetation provides cover. Isolation during birth is infrequent where cover is lacking (Meagher 1986). Brucellosis causes abortion and temporary sterility in cattle, but in Yellowstone apparently does not effect pregnancy rates to any significant degree. Most calves are weaned by late fall or by the end of the first year and remain with their mother until spring or later if she does not conceive. The life span of an American bison is 18 to 22 years.

Management of free-ranging American bison in Montana has been controversial. The presence of brucellosis in these animals and their migration out of Yellowstone National Park into adjacent public and private lands has led to conflicts between private landowners, citizens, public administrative agencies and public land management agencies. Free-ranging herds in Montana are currently managed under the Interagency Bison Management Plan. Please consult this management plan for details concerning American bison management in Montana.