Bison bison
American Bison
Description
Bison are North America’s largest land mammal (Kays and Wilson, 2002). Bison
mass ranges from 544 - 907 kg in males and 318 – 545 kg in females. The maximum
recorded mass for a bison was a semi-domestic male at 1,724 kg. Length of bison ranges
from 3.04 – 3.8 m in males and 2.13 – 3.18 m in females, males standing 1.67 – 1.86 m
at the shoulder and females standing 1.52 – 1.57 m at the shoulder. Bison are massive
animals, accentuated by a large shoulder hump formed by the musculature of the shoulder
and the thoracic girdle. The distribution of mass on the bison is somewhat
disproportionate, favoring the anterior, rather than the posterior as in many other bovines.
The front quarters weigh more than the hind quarters, and this disproportionate
distribution of muscle mass is accentuated by pelage differences between the front and
the rear. Pelage is brown, long over the forehead, neck, hump, and front quarters and
relatively short over the rear and tail. Bison shed their thicker winter coat in the spring for
their summer coat; the thicker winter coat begins to grow back in the fall. Calves appear a
reddish tan color. Darker hair appears at about 2.5 months, with no apparent differences
between males and females at birth. Both males and females have black horns at maturity
that point upward and inward with a sharp tip, with males having slightly more curvature
and girth (Kays and Wilson, 2002) (Meagher 1986).
Distribution in Wisconsin
No wild populations of bison are known to occur in Wisconsin, but the former
range of the bison covers nearly all of Wisconsin (Wilson and Ruff, 1999). There are
several private domestic herds of bison in Wisconsin. Bison first appeared in the fossil
record south of Beringia in North America in the early late Pleistocene. Prehistoric
distribution covered the central grasslands and northern parklands of North America, but
habitats used ranged from semidesert to boreal forest where suitable grazing was
1
available (Meagher, 1986). An estimated 30 million bison inhabited North America when
modern man arrived (Meagher, 1986), but some estimates are as high as 90 million
individuals prior to extirpation (Macdonald, 2001). Bison survived near extinction as a
wild species in two areas; Yellowstone National Park, Wyoming, and Wood Buffalo
National Park, Canada (Meagher, 1986). Jennings and Hebbring estimated the population
in 1983 at about 75,000 individuals, including public reserves and private herds.
Current Distribution of Bison bison in North America (Burt and Grossenheider, 1976)
Ontogeny and Reproduction
Few female bison conceive as yearlings, but sexual maturity usually occurs at 2 –
4 years in both female and male bison. Two year old bulls may be capable of breeding
successfully in the absence of older bulls, but generally they do not breed until 6 years of
age. The breeding season usually extends from late June or early July through September
but the actual breeding has been shown to be more strongly seasonal. 90% of copulations
are shown to occur during a two week period centered around 31 July and 1 August
(Meagher, 1986). Cows have an estrous cycle of about 21 days and an estrus of about 928 hours (Nowok, 1999). Older bulls are normally solitary, but congregate with cows
during the breeding season. At this time bulls fight for cows, usually through head-tohead ramming. The winner stays with a cow for several days to prevent other bulls from
encroaching (Nowok, 1999). At which time some cows may break away and run past
2
other bulls in an attempt to stimulate competition and provide herself with a mate of
highest rank. Bison may be classified as promiscuous, but mating occurs on a one to one
basis and is called tending. This is initiated by the bull (Meagher, 1986).
Cows have 1 calf, with twins being very rare. At birth, calves usually weigh 15 –
25 kg. Cows usually nurse their calves for 7 – 8 months and most calves are weaned by
the time they reach 1 year of age. Bulls appear to reach maximum mass by age 5 -6 and
cows reach maximum mass by about age 3 (Meagher, 1986).
Bison have been documented reaching 40 + years of age in the wild, the onset of
old age has been shown to be about 10-12 years, with very few reaching 20 years of age
(Meagher, 1986).
Ecology and Behavior
Bison are highly gregarious and typically only adult males can be found isolated
from the herd outside of breeding season (Macdonald, 2001). Formerly such aggregations
consisted of many thousands and even millions of animals (Nowak, 1999). They are
traditionally associated with prairies, but have also occurred in mountainous areas and
open forests (Nowak, 1999). Bison are grazers during all seasons, utilizing mostly grasses
and sedges. Up to 90% of their diet has been recorded as cool season grasses and sedges,
making them primarily a grassland species. Bison have a mean dry matter daily intake of
1.6 % of their body weight in both the winter and the summer months (Meagher, 1986).
Most free ranging bison seasonally migrate distances of 14-40 km in montane
habitats and up to 240 km in boreal forest-parkland habitats. Historically, free ranging
bison generally migrated southward hundreds of kilometers for better winter foraging.
(Meagher, 1986; Nowak, 1999). Bison are mostly diurnal and have an average daily
movement of about 3 km (Nowak, 1999).
Most mortality in bison herds is from commercial or sport harvest by humans.
The gray wolf (Canis lupus) and grizzly bear (Ursus arctos) (Macdonald, 2001) are
predators of free ranging bison, but usually have no significant impacts on bison
populations. Periodically, severe winters are a significant mortality factor in bison
3
populations, varying with individual age classes. Anthrax outbreaks and brucellosis are
also known mortality factors in free ranging bison herds (Meagher, 1986).
Bison frequently wallow in dust or mud and then rub against boulders, tree trunks,
and other objects to rid themselves of parasites. Bulls wallow more frequently at the
onset of rut. They have acute senses of hearing and smell. Bison can run at speeds of up
to 60 km/hr and regularly swim distances of 1 km (Nowak, 1999). Physiologically, bison
are very well adapted to enduring cold environments. Bison have been observed to reduce
metabolic rates as the temperature fell to -30 degrees Celsius (in contrast to yak and cattle
which both showed an increase of metabolic rates in response to cold below critical
temperatures of -15 degrees Celsius) (Christopherson and Hudson, 1978).
Remarks
There are two sub-species of Bison in North America; Bison bison bison to the
south and Bison bison athabascae to the north, with the dividing line extending across
west-central Canada, not reaching the western United States (Nowak, 1999), but remains
controversial. As of Meagher’s account in 1986, studies of chromosomes and blood
characteristics have not resolved the issue.
The bison is classified as a low risk, conservation dependent species by the IUCN
(2002). Only the wood bison, Bison bison athabascae, is listed on Appendix II of CITES
(CITES, 2003) and Endangered in the U.S. ESA.
Numbering possibly as high as 90 million at the height of their existence, bison
were a key component of the pre-European Great Plains ecosystem of North America. A
deliberate strategy of extirpating indigenous hunting cultures dependant on the bison in
order to make room for ranches and farms was the wholesale slaughter of bison. So
effective was the policy that by 1900 only a few isolated herds existed in the wild,
totaling barely 1000 individuals (Macdonald, 2001). The near extermination of this
megafaunal relic is to many an unfortunate chapter in the history of North America.
Literature Cited
4
Burt, W. H., and R. P. Grossenheider. 1976. A Field Guide to the Mammals of North
America North of Mexico, Third Edition. A Peterson Field Guide. Boston: Houghton
Mifflin Company.
Christopherson, R.J. and Hudson, R.J. 1978. Effects of temperature and wind on cattle
and bison. 57th Annual feeder’s Day Report. 57: 40-41.
Kays R.W. and Wilson. D.E. 2002. Mammals of North America. Princeton University
Press, Princeton, New Jersey, USA.
Macdonald, D., editor. 2001. The Encyclopedia of Mammals. Barnes and Noble, New
York, New York, USA.
Meagher, M. 1986. Bison bison. Mammalian Species 266: 1-8.
Nowak, R.M. 1999. Walker’s Mammals of the World. John Hopkins University Press,
Baltimore, Maryland, USA.
Wilson, D.E. and Ruff, S. 1999. The Smithsonian Book of North American Mammals.
Smithsonian Institution Press in association with The American Society of
Mammologists, Washington, USA.
Reference written by Wes Gibbs, Biology 378 student. Edited by Christopher Yahnke.
Page last updated.
5