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Montana Animal Field Guide

Montana Field Guides

Bison - Bos bison

Species of Concern

Global Rank: G4
State Rank: S2

Agency Status


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Copyright Jeff Rice and the Western Soundscape Archive. Audio file courtesy of the Acoustic Atlas at Montana State University (
General Description
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.

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

Species Range
Montana Range

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Western Hemisphere Range


Summary of Observations Submitted for Montana
Number of Observations: 28

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density



(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:
    1. 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);
    2. Evaluating structural characteristics and distribution of each ecological system relative to the species' range and habitat requirements;
    3. Examining the observation records for each species in the state-wide point observation database associated with each ecological system;
    4. 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: 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.

    Literature Cited
    • 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.

Food Habits
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.

Reproductive Characteristics
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?
    • Allen, J. A. 1876. The American bisons, living and extinct. Memoirs of Comparative Zoology, Vol. 4, No. 10. Harvard College, Cambridge, MA.
    • Bison Management Plan EIS Team. 1998. Draft environmental impact statement (EIS) for the interagency bison management plan for the State of Montana and Yellowstone National Park. June 1, 1998. USDI, National Park Service, Denver, Colo. 395 pp.
    • Bork, A. M., C. M. Strobeck, F. C. Yeh, R. J. Hudson and R. K. Slamon. 1991. Genetic relationship of wood and plains bison based on restriction fragment length polymorphisms. Canadian Journal of Zoology 69: 43-48.
    • Chapman, J. A., and G. A. Feldhamer, editors. 1982. Wild mammals of North America: biology, management, and economics. Johns Hopkins University Press, Baltimore, Maryland.
    • DeSanto, J. S. 1971. Historical status of bison in Glacier National Park. Unpubt. Rep., Glacier National Park, West Glacier. 9 pp.
    • Elrod, M. J. 1908. The Flathead·buffalo range: a report to the American Bison Society of an inspection of the Flathead Indian Reservation, Montana, for the purpose of selecting a suitable location for a national buffalo range. American Bison Society.
    • Elrod, M. J. 1926. The Montana National Bison Range. J. Mammal. 7:45-48.
    • Foresman, K.R. 2001. The wild mammals of Montana. American Society of Mammologists, Special Publication Number 12. Lawrence, KS. 278 pp.
    • Frisina, Michael R., 1984, Mt. Haggin Big Game Study. Big Game Survey and Inventory, Region III. W-130-R-14, I-3.2, July 1,1982 - June 30, 1983.
    • Geist, V. 1990. Agriculture versus bison in Canada's Wood Buffalo National Park. Conservation Biology 4:345-346.
    • Geist, V. 1991. Phantom Subspecies: the Wood Bison Bison Bison "Athabascae" Rhoades 1897 Is not a Valid Taxon, but an Ecotype. Arctic 44:283-300.
    • Gertonson, Arnold., 2000, Interagency bison management plan for the state of Montana and Yellowstone National Park : final environmental impact statement
    • Hanson, J. R. 1984. Bison ecology in the northern plains and a reconstruction of bison patterns for the North Dakota region. Journal of the Plains Anthropological Society, 29(104):93-112.
    • Hoffmann, R.S. and D.L. Pattie. 1968. A guide to Montana mammals: identification, habitat, distribution, and abundance. University of Montana, Missoula. 133 pp.
    • Hornaday, W. T. 1887. Buffalo hunt.
    • Hornaday, W. T. 1889. The extermination of the American bison with a sketch of its discovery and life history. Pp. 367-548 in Part 2, Rep. U.S. Natl. Museum, 1886-87.
    • Keiter, R. B. 1997. Greater Yellowstone's bison: unraveling of an early American wildlife conservation achievement. J. Wildl. Manage. 61(1):1-11.
    • Knapp A.K., J.M. Blair, J.M. Briggs, S.L. Collins, D.C. Hartnett, L.C. Johnson, and E.G. Towne. 1999. The keystone role of bison in North American tallgrass prairie. Bioscience 49(1): 39-50.
    • McDonald, J. N. 1981. North American bison: their classification and evolution. 316 pp.
    • Meagher, M. 1986. Bison bison. Mamm. Species 266: 1-8.
    • Meagher, M. 1989. Evaluation of boundary control for bison of Yellowstone National Park. Wildl. Soc. Bull. 17:15-19.
    • Meagher, M. 1989. Range expansion by bison of Yellowstone National Park. J. Mammal. 70:670-675.
    • Meagher, M. M. 1978. Bison. Pp. 123-133 in: Schmidt, J. L. and D. L. Gilbert (eds.). Big game of North America. Stackpole Books, Harrisburg. 494 pp.
    • Meagher, M.M. 1973. The Bison of Yellowstone National Park. U.S. National Park Service Scientific Monograph Ser. 161.
    • Miyamoto, M. M., S. M. Tanhauser, and P. J. Laipis. 1989. Systematic relationships in the artiodactyl tribe Bovini (family Bovidae), as determined from mitochondrial DNA sequences. Syst. Zool. 38:342-349.
    • Montana Department of Fish, Wildlife and Parks, et al. 1990. Yellowstone bison: background and issues, a cooperatively prepared narrative of bison management history and management issues. 28 pp.
    • Montana Department of Livestock and Montana Fish, Wildlife and Parks. 1996. Interim bison management plan. 70 pp.
    • Montana Fish Wildlife and Parks, Montana Department of Livestock, 2000, Interagency Bison Management Plan-Record of Decision, December 22, 2000
    • Norland, J. E. 1985. Activity patterns of bison in Theodore Roosevelt National Park. M.S. thesis, Montana State University. 131 pp.
    • Norland, J. E., and C. Marlow. 1984. Bison impacts on woody draws. Proc. Symp. Prairie Riparian Veg.
    • Norland, J. E., C. Marlow, and L. Irby. 1984. Bison activity and habitat use in the Theodore Roosevelt National Park. Range Management Society Ann. Meeting, Rapid City, SD.
    • Norland, J. E., L. Irby, and C. Marlow. 1985. Optimum carrying capacity for bison in Theodore Roosevelt National Park. J. Environ. Manage. 21:225-239.
    • Pac, H. E., and K. Frey. 1991. Some population characteristics of the northern Yellowstone bison herd during the winter of 1988-1989. Montana Dept. of Fish, Wildl. and Parks, Helena. 29 pp.
    • Peden, D.G., G.M. Van Dyne, R.W. Rice, and R.M. Hansen. 1974. The trophic ecology of Bison bison l. on shortgrass plains. Journal Applied Ecology 11:489-498.
    • Polziehn, R. O., R. Beech, J. Sheraton, and C. Strobeck. 1996. Genetic relationships among North American bison populations. Canadian Journal of Zoology 74:738-749.
    • Reid, F. 2006. Peterson Field Guide to Mammals of North America, 4th Edition. Houghton Mifflin Company: Boston and New York, 608 pp.
    • Roe, F.G. 1951. The North American Buffalo. A Critical Study of the Species in the Wild State. Univ. Toronto Press, Toronto. xi+991 pp.
    • Rush, W. M. 1932. Bang's disease in the Yellowstone National Park buftido and elk herds. J. Mammal. 13:371-372.
    • TVX Mineral Hill Mine, Amerikanuak, Inc., Gardiner, MT., 2002, Yearly summary of wildlife observation reports. 1990-2002 Letter reports.
    • USDI National Park Service., 2000, Bison Management for the State of Montana and Yellowstone National Park. Final Environmental Impact Statement for the Interagency Bison Management Plan for the State of Montana and Yellowstone National Park. Vol. I. August 2000.
    • Van Vuren, D., and M.P. Bray. 1986. Population dynamics of bison in the Henry Mountains, Utah. J. Mammal. 67:503-511.
    • van Zyll de Jong, C. G. 1986. A systematic study of recent bison, with particular consideration of the wood bison. Nat. Mus. Nat. Sci. Publ. in Nat. Sci. No. 6. 69 pp.
    • van Zyll de Jong, C. G., C. Gates, H. Reynolds, and W. Olson. 1995. Phenotypic variation in remnant populations of North American bison. Journal of Mammalogy 76:391-405.
    • Wall, D.A., S.K. Davis, and B.M. Read. 1992. Phylogenetic relationships in the subfamily Bovinae (Mammalia: Artiodactyla) based on ribosomal DNA. Journal of Mammalogy. 73(2): 262-275.
    • Wilson, D. E., and D. M. Reeder (editors). 1993. Mammal Species of the World: a Taxonomic and Geographic Reference. Second Edition. Smithsonian Institution Press, Washington, DC. xviii + 1206 pp.
    • Winchell, J. R. 1963. Age criteria for American bison. M.S. thesis, University of Montana, Missoula.
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Citation for data on this website:
Bison — Bos bison.  Montana Field Guide.  Montana Natural Heritage Program and Montana Fish, Wildlife and Parks.  Retrieved on December 5, 2016, from