American bison (Bison bison)
|Also known as:||American buffalo|
|Size||Length: 2.1 - 3.5 m (2)|
Shoulder height: 1.5 – 2.0 m (2)
|Weight||350 – 1000 kg (2)|
Classified as Near Threatened (NT) on the IUCN Red List (1). Subspecies Bison bison athabascae listed on Appendix II of CITES (2).
The American bison, the largest mammal in North America, once roamed the continent in vast herds and helped to shape the ecology of the Great Plains, as well as the history of the United States of America (2) (3) (4) (5). Formidable in stature, the American bison’s massive frame is accentuated by its towering shoulder hump (3) (6). Resting low on a short, stocky neck, the bulky head features a broad forehead, short, up-curving horns and a straggly beard (2) (3). The head, neck, shoulders, and forelegs are covered in a long, shaggy, brownish-black coat, while the hair on the remainder of the body is considerably shorter and lighter in colour (2) (3) (6). Female bisons are on average shorter than the males, and have a smaller hump, a thinner neck and more slender horns (2). Two subspecies are generally recognised, the plains bison (Bison bison bison) and the wood bison (B. b. athabascae) (1) (7). The wood bison is typically larger and heavier than the plains bison, and usually has darker hair (7).
Historically, the American bison had the widest natural range of any North American herbivore, extending all the way from northern Mexico to Alaska, with central Alberta being the dividing line between the plains bison to the south and the wood bison to the north (1). Today, free-roaming bison herds occupy less than one percent of their former range, and are restricted to a few national parks and small wildlife areas (1) (4) (5).
Although the American bison is primarily known as a species of grassland and meadow communities, its historical range covered a wide range of habitats from semi-desert to boreal forest (1) (6).
Prior to European settlement on the continent, the plains bison undertook seasonal migrations of hundreds of kilometres along the same routes year after year (1) (2) (3). Moving in vast herds, the bison was considered to be a classic ‘keystone’ species, significantly influencing grass composition, nutrient cycling, fire regimes, and the availability of habitat for a diverse assemblage of other animals (4) (7) (8). Owing to changes in land use and depopulation, the plains bison is no longer migratory (the wood bison never was) (1), but the movements of free-roaming herds are still dictated by the availability of food (2) (3) (5). Although grasses and sedges form the mainstay of the bison diet, flowering plants, woody plant leaves and even lichens will be eaten when its staples are limited (1) (5). Over winter, bison can dig through deep snow, by sweeping the muzzle from side to side, to access buried vegetation (1) (4) (5).
Adult females live with their young in hierarchical herds, led by a dominant female, whilst mature males usually move about alone, or in small bachelor groups. Joining the female herds during the mating season, male bison fight fiercely for the right to mate. This usually involves head-to-head ramming, while receptive females gallop about to stimulate competition, thus hoping to mate with the strongest male (2) (3). The mating season lasts from June to September, with a peak in activity between July and August. Pregnant females usually give birth to a single calf the following spring, after a gestation period of 270 to 285 days (2) (5) (6). Within three hours, the newborn calves are able to run about, but are guarded closely by the mothers, who will charge any intruders. The young are weaned at around seven to twelve months and reach sexual maturity when they are two to four years of age, with wild individuals having a potential life expectancy of around 20 years (2).
Despite having such a bulky frame, bison are able to run at speeds of up to 60 kilometres an hour. Eyesight is poor, but the senses of hearing and smell are acute, and appear to be vital in detecting danger (2) (3) (5). Where present, wolves are known to be competent predators of free-roaming bison, with individuals separated from the herd being the primary targets of attack (6).
As recently as 200 years ago, the North American continent was home to around 40 million bison, providing a sustainable source of meat, as well as hides for shelter and clothes, for many of the continent’s native people (4) (7) (8). Sadly, overhunting during the westward expansion of European settlement in the 19th century resulted in the decimation of the bison population (1) (4) (8). By the late 1800s, the species was near-extinct, with as few as 1,000 individuals remaining either in the wild or in captivity (4) (8). Recognising that an iconic American animal was about to be lost, conservationists launched a national recovery campaign that included the implementation of protective legislation preventing the extirpation of the remaining wild herds (7) (8). Following recovery efforts, the bison population has risen to around 500,000 today, but the vast majority (over 90 percent) are held in captive stocks managed for commercial production (3) (7) (8). With only a small fraction occurring in free-ranging herds, the modern perception that the bison is secure in the wild is somewhat misguided (7). In addition, numerous threats are still putting pressure on both captive and wild populations, including habitat loss, reduction in genetic diversity, cross breeding with domestic cattle, and the culling of dispersing wild bison to prevent the spread of bovine diseases (1) (4) (7).
National parks and wildlife areas in the United States and Canada, such as Yellowstone National Park and Wood Buffalo National Park respectively, play an extremely important role in maintaining conservation herds (1) (3) (4) (5). Plans are underway for the restoration of large populations of plains bison into southern Colorado and northern Montana, as well as the reintroduction of wood bison to the state of Alaska (1). The Bison Specialist Group is also currently developing a conservation strategy which will provide support and guidance for policy development, as well as for the planning and implementation of wild bison conservation projects (1) (7). Furthermore, in 2005, the American Bison Society, which was integral in the recovery of the bison population at the beginning of the 20th century, was re-launched by the Wildlife Conservation Society. The long-term aim of the revitalised American Bison Society is to ensure the ecological future of this iconic species. This includes working on projects that promote the reintroduction of large, wild herds to more habitats across the bison’s historical range (8).
To find out more about the conservation of the American bison, visit:
- Bison Specialist Group North America:
- American Bison Society:
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- Gestation: the state of being pregnant; the period from conception to birth.
- Subspecies: a population usually restricted to a geographical area that differs from other populations of the same species, but not to the extent of being classified as a separate species.
IUCN Red List (June, 2009)
- Nowak, R.M. (1999) Walker's Mammals of the World. Johns Hopkins University Press, Baltimore, Maryland.
- Burnie, D. (2001) Animal. Dorling Kindersley, London.
- Macdonald, D. (2001) The New Encyclopedia of Mammals. Oxford University Press, Oxford.
Defenders of Wildlife (August, 2009)
- Meagher, M. (1986) Bison bison. Mammalian Species, 266: 1 - 8.
Bison Specialist Group North America (August, 2009)
American Bison Society (August, 2009)