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Wood bison

Bison bison athabascae

North American bison are an endemic keystone species of northern Canada as well as the dominant species of the largest biome found on the continent. Wood bison are a recovering subspecies of the North American bison. This mammal, which is the largest terrestrial animal in Canada, has faced extinction twice in the past hundred years. Since it was downlisted from the endangered category in 1988, the wood bison is currently classified as a threatened species. The latter is likely to become endangered if limiting factors leading to the threatened status are not reversed. Therefore, if we want to reduce their risk of extinction, we need to put efforts into conservation.


Habitat and foraging habits

Subspecies : Controversial status

Population distribution

Population status and conservation history

Remaining threats and problems

Recent developments


CWS & RENEW links





Physical characteristics

The wood bison is characterised by:

  • Dark brown coat
  • Massive head
  • High hump on its large shoulders
  • Long shaggy hair on its shoulders and front legs
  • Short legs with rounded hooves
  • Short black horns - curved inward on males and straight on females

Males are larger than females and can measure from 3 to 3.8 meters in length. Their height ranges from 1.67 to 1.82 meters at shoulders and their weight from 350 to 1000 kilograms.

wood bison

Damien Joly - Copyright


Wood bison are sexually mature at 2 to 4 years of age. Since the dominant male is responsible for 30 % of all copulation, most males usually mate only at 6 years of age when they are large enough to compete for females.

  • Rut occurs in August and early September
  • Gestation period lasts 270 to 300 days
  • Female gives birth to one calf, twice in three years
  • Calf's coat is red
  • Reproductive synchrony takes place in May

cow and calf

Damien Joly - Copyright


Wood bison can live up to 40 years. With the exception of humans, wolves and bears are the main predators. The predators preferably attack bison herds with calves. If a female is heavy and in good condition, her offspring has an increased chance of survival.

Group structure

Wood bison live in groups of variable size and are usually at their largest in the summer. Females tend to gather in larger groups so as to reduce the chances of predation on their calves. Vigilance varies with group size, where as the net vigilance (i.e. for the entire group) is increased and the per capita vigilance (i.e. for each individual in the group) is decreased in large groups.

In bison populations, polygyny takes place and sexual segregation occurs except when nutritional or reproductive demands need to be met.


Habitat and foraging habits

Wood bison are found throughout aspen and open boreal forests of north-western Canada. They have a strong affinity for meadows, especially wet sedge meadows which are used almost exclusively during winter. Females are found to be more selective than males, preferring habitats with relatively increased visibility and with presumed higher protein quality. Forage availability is the main factor determining both habitat selection and use, which explains the shifting of habitat at specific periods.

Bison are primarily grazers and eat a wide variety of grasses and sedges. They also feed on the leaves and bark of both trees and shrubs, such as willows, as well as feeding upon lichens during the fall.

bison herd

Damien Joly - Copyright


Wood bison versus plains bison

True subspecies?


Generally, wood bison are taller and less stocky than plains bison. They lack hair on their upper forelegs and have:

  • Larger body sizes
  • Larger horn cores
  • Darker coat colours
  • Smaller beard lengths

Wood bison are currently considered as a subspecies of the North American bison, but their taxonomic status is actually unclear and controversial. Many studies have been done, yet none have lead to a clear and definitive conclusion.

At one point, van Zyll de Jong (1986) concluded that the phenotypic discontinuity between grassland and woodland populations fully justified the recognition of the two current subspecies of bison.

On the other hand, Geist (1996) concluded that the wood bison was a phantom subspecies. He found that wood bison transformed miraculously into perfectly good "plains bison" when they were removed from Elk Island National Park and put in captivity or in the wild. He argues that wood bison is not a subspecies but rather an ecotype reflecting environmental conditions due to confinement and shortage of nutrients, stopping them from finishing growing their hair coats.

Polziehn et al. (1996) found a lack of monophyly in bison. This suggests that the bison subspecies have only been recently separated from each other, making it very difficult to have well-defined taxa for each subspecies. Thus, defining the subspecies and detecting hybridisation is less effective and hard to accomplish.


Population distribution

The historical range of the wood bison in Canada covered north-eastern British-Columbia, northern Alberta, north-western Saskatchewan, south-western Northwest Territories, and the Yukon. Nowadays, its geographical range is mainly reduced to small patches in Alberta, Manitoba, Yukon, and south-western Northwest Territories, where small herds are surviving.


historical range

North American bison historical range

Modified from van Zyll de Jong (1986)


present distribution

Recent distribution of captive and free-roaming herds of bison

Modified from van Zyll de Jong (1986)


Population status and conservation history

At one point in Canada's history, it was estimated that approximately 168 000 bison were free-roaming. By the late 1800s, only about 250 wood bison were left, a considerable decline to which the introduction of the firearm and the severity of numerous winters may have been contributing factors.

Early conservation efforts:

1877 and 1893
Protection started through legislation

Establishment of Wood Buffalo Park

1925 to 1928
Introduction of plains bison into the park

Certain persons protested; they maintained inbreeding and diseases would occur. Their protests were ignored and the introduction resulted in hybridisation of the wood bison and transmission of tuberculosis and brucellosis into the park.

Wood bison were thought to be extinct due to hybridisation

Wood Buffalo Park became a National Park

A small remnant herd was discovered in north-western Wood Buffalo National Park

As part of a recovery program in 1959, disease-free wood bison from this herd were transferred to the Mackenzie Bison Sanctuary and Elk Island National Park in order to establish breeding herds for captive breeding and subsequent transplants. In 1975, a cooperative recovery program was initiated with the objective of re-establishing free-roaming non-diseased herds of wood bison.

These efforts increased the population number of free-roaming wood bison from 200 to approximately 3000 in 1999. The latest count of non-diseased wood bison is 3536, of which 2828 are found in the wild and 708 in captivity. Six populations are found in the wild but only two exceed the minimum number for a viable population of 400 individuals. Four captive breeding herds are also present and are kept for the preservation of gene pools.


Remaining threats and problems

Risks of crossbreeding and contracting diseases

All populations of wood bison in Canada are now believed to be disease-free apart from those from Wood Buffalo National Park. The increasing number of wood bison in each of the various populations leads to an increase of required area. Obviously, this increases chances for disease-free wood bison to be in contact with diseased bison from Wood Buffalo National Park and for crossbreeding with other bison (i.e. hybrids and plains bison).


Large bodied mammals are especially vulnerable to effects of habitat fragmentation because they require lot of space. Their vulnerability is mainly a consequence of the isolation of small populations. This increases the level of inbreeding, as relatives are forced to mate with one another. When the species is characterised by polygyny, as with bison populations, only a few dominant individuals may be responsible for most offspring, leading to a further increase in the mean relatedness of the population.

Since most bison are found in small heavily managed populations, the main pressing management problems include isolation and restriction to small tracks of land. These problems result in rapid population increases and require subsequent culling. Thus, the elimination of unhealthy and/or hybrid bison is required to release healthy wood bison populations throughout their original range and keep the genetic diversity.

To try to solve the problems, a study was performed to refine techniques for the development of assisted reproductive technologies in the form of embryo collection and transfer, as well as to evaluate animal responses to these techniques. The advancement of this technology could help ensure the long-term propagation and the genetic management of wood bison by improving captive propagation efficiency and maintaining genetic diversity.

Agriculture, human development & bison

In 1985, Canadian cattle were declared free of brucellosis and almost free of tuberculosis. The bison of the Wood Buffalo National Park were and are still the last remaining focus of both diseases. Since agricultural activities had become closer to bison ranges as a result of the gradually increasing needs of human populations, the diseases were and are still a real concern for cattle.

Another consequence of the expansion of agricultural and human developments is a major loss of bison habitat, restricting bison to smaller ranges and diminishing food availability.

Home range size of bison is determined by forage requirements. When an area has a higher food accessibility (i.e. having larger, more abundant, and closer food patches), home ranges are smaller. In larger groups, since less food is available per individual in a given area, larger home ranges are required. As wood bison populations are expanding, bison are being restricted to small isolated areas, as well as being threatened by habitat loss and fragmentation, which leads to a decline in food availability. These factors necessitate an increase of the size of bison's home range, a concern with regards to bison conservation.

Other potential problems

  • Predation
  • Poaching
  • Accidental death


Most recent developments in the conservation of wood bison

The Federal Environmental Assessment Review Office investigated several possible options in 1990. They concluded that the best one was to eliminate infected hybrid populations and to repopulate them with disease-free wood bison.

Advantages of the option

  • Elimination of inbreeding and crossbreeding problems
  • Elimination of the risk of contracting the diseases for human, livestock, and disease-free bison
  • Re-establishment of wood bison over a large and important part of their former range

Disadvantages of the option

  • Decreased value of the Wood Buffalo National Park
  • Disruption of predator-prey relationships between wolf, bison, and moose populations
  • Loss of genetic diversity


After such a controversial option was given, an independent assessment of the effects of tuberculosis and brucellosis on wood bison demography was performed. The results have shown that:

  • Both diseases were prevalent in older animals but tuberculosis was more prevalent in males than in females.
  • Both diseases were not a function of bison density.
  • Both diseases affected pregnancy rates; diseased populations had a significantly lower calf:cow ratio than healthy populations.
  • Both diseases affected the annual survival rate; the mortality rate of diseased bison was 2.5 to 3.7 times higher than that of healthy populations.

Also, it was shown that two factors, disease and wolf predation, were required to happen simultaneously to result in a significant population decline. When only one factor was present, populations seemed to be more or less stable.


In the light of these results, it was concluded that the ongoing presence of two economically important diseases in northern Canada results in a conflict with respect to further bison recovery. The only way to effectively eradicate the diseases involves depopulating and repopulating, as it was suggested in 1990 by the Federal Environmental Assessment Review Office. On one hand, it brings the social concern that the disease management is too drastic and an unnecessary intrusion into the policies of a National Park. On the other hand, these diseases prevent the reintroduction of a keystone species throughout its former range in northern Canada, forty percent of this range being evaluated to be available and suitable for reintroduction. In addition, the Mackenzie Bison Sanctuary contains approximately 2000 disease-free wood bison and the migration of diseased bison presents a quantifiable risk of transmitting the diseases to healthy populations.

Finally, the need for reintroduction of bison was expressed in another study. As a conservation strategy, it was mentioned that reintroduction of bison into several independent sites of their historical range would facilitate re-colonisation and achieve a faster spread than a reintroduction into a single site and waiting for the population to spread as a result of its own density dependent responses.



To conclude, whether or not wood bison are a subspecies, a host of environmental and human factors contributed to their destruction and finding a clear solution for a successful comeback is not that easy. The solution to this problem needs a clear consensus on the long term recovery goals of this species and a transparent evaluation of whether short term costs of disease management are justified in relation to these objectives.

Bison ranching is not conservation, it is domestication!

We should therefore concentrate our conservation efforts toward free-roaming wood bison populations and apply the best solution for their reintroduction to their former range.

To find the best management solution for wood bison conservation, more advanced research on the following subjects would be useful:

  • Genetic and taxonomic identities
  • Population dynamics
  • Habitat and nutritional requirements
  • Predation by wolves
  • Adaptive behaviour
  • Bison health
  • Interspecific competition
  • Reproductive problems of captive herds

The management and study of larger species is hard because of the long generation time. However, the study of larger species more readily stimulates public awareness and education, which hopefully might greatly contribute to the preservation of the wood bison in the future.


For more information

Canadian Wildlife Service (CWS)

Recovery of Nationally Endangered Wildlife (RENEW)


Berger, J. and C. Cunningham. 1994. Bison: Mating and conservation in small populations. Columbia University Press, New York.

Carbyn, L. N. and T. Trottier. 1987. Responses of bison on their calving grounds to predation by wolves in Wood Buffalo National Park. Can. J. Zool. 65: 2072-2078.

Federal Environmental Assessment Review Office. 1990. Northern diseased bison. Report of the Environmental Assessment Panel, Hull, Canada.

Geist, V. 1996. Buffalo nation: History and legend of the North American bison. Voyageur Press, Stillwater, Minnesota.

Isenberg, A.C. 2000. The destruction of the bison: An environmental history, 1750-1920. Cambridge University Press, New York.

Komers, P. E., F. Messier and C. C. Gates. 1993. Group structure in wood bison: nutritional and reproductive determinants. Can. J. Zool. 71: 1367-1371.

Larter, N. C. and C. C. Gates. 1990. Home ranges of wood bison in an expanding population. Journal of Mammalogy 71(4): 604-607.

Larter, N. C. and C. C. Gates. 1991. Diet and habitat selection of wood bison in relation to seasonal changes in forage quantity and quality. Can. J. Zool. 69: 2677-2685.

Larter, N. C. and C. C. Gates. 1994. Home-range size of wood bison: Effects of age, sex, and forage availability. Journal of Mammalogy 75(1): 142-149.

Larter, N. C., A. R. E. Sinclair, T. Ellsworth, J. Nishi and C. C. Gates. 2000. Dynamics of reintroduction in an indigenous large ungulate: the wood bison of northern Canada. Animal Conservation 3(4): 299-309.

Othen, L. S., A. C. Bellem, C. J. Gartley, K. Auckland, W. A. King, R. M. Liptrap and K. L. Goodrowe. 1999. Hormonal control of estrous cyclicity and attempted superovulation in wood bison (Bison bison athabascae). Theriogenology 52: 313-323.

Polziehn, R.O., R. Beech, J. Sheraton and C. Strobeck. 1996. Genetic relationships among North American bison populations. Can. J. Zool. 74: 738-749.

Tessaro, S. V., C. C. Gates and L. B. Forbes. 1993. The brucellosis and tuberculosis status of wood bison in the Mackenzie Bison Sanctuary, Northwest Territories, Canada. Can. J. Vet. Res. 57: 231-235.

The Wood Bison Recovery Team. 1987. Status report on wood bison (Bison bison athabascae) in Canada, 1987.

van Zyll de Jong, C.G. 1986. A systematic study of recent bison, with particular consideration of the wood bison (Bison bison athabascae, Rhoads 1898). National Museums of Canada, Ottawa.



Reproductive synchrony

During reproduction, all the females of a given population give birth approximately at the same time.

Sexual segregation

In polygynous ungulates such as the bison, sexes form separate groups and are spatially segregated for most of the year.


System where individual males may mate with more than one female per breeding season.


The smallest taxonomic subdivision of an ecospecies, consisting of populations adapted to a particular set of environmental conditions.


A taxonomic species considered in terms of its ecological characteristics and usually including several interbreeding ecotypes.



Characterise a group or taxon that consists of an ancestor and its descendants.

Bovine Tuberculosis

Disease that results from infection with the bacterium Mycobacterium bovis and is characterised by the formation of tubercles on the lungs and other tissues of the body, often developing long after the initial infection. The disease is contagious. Infected animals pass bacteria in their secretions and excretions. It may spread from infected to uninfected animals through inhalation of droplets expelled by coughing animals, by ingestion of food or water contaminated with the bacterium by infected animals, or from mother to offspring either through the placenta or contaminated milk.

This disease is progressively debilitating and can affect the respiratory, digestive, urinary, nervous, skeletal, and reproductive systems. It can weaken animals and make them more susceptible to predator, it can reduce fertility in sexually mature animals, and in advanced cases, it can be fatal.

tuberculosis lesions
Tuberculosis lesions

Damien Joly - Copyright

Bovine Brucellosis

Disease caused by infection with the bacterium Brucella abortus. It is characterised by involvement of the reproductive organs resulting in abortion, infertility, and uterine infection in females and inflammation of the testes in males. Calves born alive from infected females can be weak and die soon after birth. It can also invade joints, causing crippling arthritis.

Transmitted by exposure to aborted fetuses, fluids passed from pregnant uteri, infected new born calves, or by consumption of food or water contaminated by these materials. Calves can be infected by ingestion of their mother milk. Infected males can transmit the disease through contaminated semen at breeding time.