Bison - Bos bison
Bison are the largest North American land animals with weights over 1000 kilograms recorded (Foresman 2012). 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 2012), 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.
Bison are unmistakable. The combination of large size, shoulder hump, and short, dark, curved horns on both sexes eliminates any other large ungulate.
Western Hemisphere Range
Observations in Montana Natural Heritage Program Database
Number of Observations:
(Click on the following maps and charts to see full sized version)
Map Help and Descriptions
(Observations spanning multiple months or years are excluded from time charts)
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. 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 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, 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).
Ecological Systems Associated with this Species
- Details on Creation and Suggested Uses and Limitations
How Associations Were Made
We associated the use and habitat quality (common or occasional) of each of the 82 ecological systems mapped in Montana for
vertebrate animal species that regularly breed, overwinter, or migrate through the state by:
- Using personal observations and reviewing literature that summarize the breeding, overwintering, or migratory habitat requirements of each species (Dobkin 1992, Hart et al. 1998, Hutto and Young 1999, Maxell 2000, Foresman 2012, Adams 2003, and Werner et al. 2004);
- Evaluating structural characteristics and distribution of each ecological system relative to the species' range and habitat requirements;
- Examining the observation records for each species in the state-wide point observation database associated with each ecological system;
- Calculating the percentage of observations associated with each ecological system relative to the percent of Montana covered by each ecological system to get a measure of "observations versus availability of habitat".
Species that breed in Montana were only evaluated for breeding habitat use, species that only overwinter in Montana were only evaluated for overwintering habitat use, and species that only migrate through Montana were only evaluated for migratory habitat use.
In general, species were listed as associated with an ecological system if structural characteristics of used habitat documented in the literature were present in the ecological system or large numbers of point observations were associated with the ecological system.
However, species were not listed as associated with an ecological system if there was no support in the literature for use of structural characteristics in an ecological system, even if
point observations were associated with that system.
Common versus occasional association with an ecological system was assigned based on the degree to which the structural characteristics of an ecological system matched the preferred structural habitat characteristics for each species as represented in scientific literature.
The percentage of observations associated with each ecological system relative to the percent of Montana covered by each ecological system was also used to guide assignment of common versus occasional association.
If you have any questions or comments on species associations with ecological systems, please contact the Montana Natural Heritage Program's Senior Zoologist.
Suggested Uses and Limitations
Species associations with ecological systems should be used to generate potential lists of species that may occupy broader landscapes for the purposes of landscape-level planning.
These potential lists of species should not be used in place of documented occurrences of species (this information can be requested at: mtnhp.org/requests
) or systematic surveys for species and evaluations of habitat at a local site level by trained biologists.
Users of this information should be aware that the land cover data used to generate species associations is based on imagery from the late 1990s and early 2000s and was only intended to be used at broader landscape scales.
Land cover mapping accuracy is particularly problematic when the systems occur as small patches or where the land cover types have been altered over the past decade.
Thus, particular caution should be used when using the associations in assessments of smaller areas (e.g., evaluations of public land survey sections).
Finally, although a species may be associated with a particular ecological system within its known geographic range, portions of that ecological system may occur outside of the species' known geographic range.
- Adams, R.A. 2003. Bats of the Rocky Mountain West; natural history, ecology, and conservation. Boulder, CO: University Press of Colorado. 289 p.
- Dobkin, D. S. 1992. Neotropical migrant land birds in the Northern Rockies and Great Plains. USDA Forest Service, Northern Region. Publication No. R1-93-34. Missoula, MT.
- Foresman, K.R. 2012. Mammals of Montana. Second edition. Mountain Press Publishing, Missoula, Montana. 429 pp.
- Hart, M.M., W.A. Williams, P.C. Thornton, K.P. McLaughlin, C.M. Tobalske, B.A. Maxell, D.P. Hendricks, C.R. Peterson, and R.L. Redmond. 1998. Montana atlas of terrestrial vertebrates. Montana Cooperative Wildlife Research Unit, University of Montana, Missoula, MT. 1302 p.
- Hutto, R.L. and J.S. Young. 1999. Habitat relationships of landbirds in the Northern Region, USDA Forest Service, Rocky Mountain Research Station RMRS-GTR-32. 72 p.
- Maxell, B.A. 2000. Management of Montana's amphibians: a review of factors that may present a risk to population viability and accounts on the identification, distribution, taxonomy, habitat use, natural history, and the status and conservation of individual species. Report to U.S. Forest Service Region 1. Missoula, MT: Wildlife Biology Program, University of Montana. 161 p.
- Werner, J.K., B.A. Maxell, P. Hendricks, and D. Flath. 2004. Amphibians and reptiles of Montana. Missoula, MT: Mountain Press Publishing Company. 262 p.
- Commonly Associated with these Ecological Systems
Forest and Woodland Systems
Human Land Use
Recently Disturbed or Modified
Shrubland, Steppe and Savanna Systems
Wetland and Riparian Systems
- Occasionally Associated with these Ecological Systems
Recently Disturbed or Modified
Wetland and Riparian Systems
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.
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 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 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 a Bison is 18 to 22 years.
Management of free-ranging 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 the Interagency Bison Management Plan
for details concerning Bison management in Montana.
- Literature Cited AboveLegend: View Online Publication
- Cheville, N. F., D. R. McCullough, and L. R. Paulson. 1998. Brucellosis in the Greater Yellowstone Area. National Academy of Science. National Academy Press. Washington D.C.
- Foresman, K.R. 2012. Mammals of Montana. Second edition. Mountain Press Publishing, Missoula, Montana. 429 pp.
- Larter, N. C. and C. C. Gates. 1990. Home ranges of wood bison in an expanding population. Journal of Mammalogy 71:604-607.
- Meagher, M. 1986. Bison bison. Mammalian Species 266: 1-8.
- Podruzny, S.R. and K.A. Gunther. 1999. Spring ungulate availability and use by grizzly bears in Yellowstone National Park. Yellowstone grizzly bear investigations: annual report of the Interagency Grizzly Bear Study Team, 1. pp. 29-32.
- Shaw, J. A. and T. S. Carter. 1990. Bison movements in relation to fire and seasonality. Wildlife Society Bulletin 18:426-430.
- Smith, D.S., W.G. Brewster, and E.E. Bangs. 1999. Wolves in the greater Yellowstone ecosystem: restoration of a top carnivore in a complex management environment. In: Clark, T.W., Curlee, A.P., Minta, S.C., Kareiva, P.M., eds. Carnivores in Ecosystems: the Yellowstone Experience. Yale University Press, New Haven. pp. 103–125.
- Vinton, M. A., D. C. Hartnett, E. J. Finch, and J. M. Briggs. 1993. Interactive effects of fire, bison (Bison bison) grazing and plant community composition in tallgrass prairie. American Midland Naturalist 129:10-18.
- Additional ReferencesLegend: View Online Publication
Do you know of a citation we're missing?
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