Bison bison
American bison

At one time, bison were widespread from Alaska to northern Mexico (Meagher, 1986). Wholesale slaughter of bison herds caused the extermination of wild bison from the major part of their former range until recently. Bison are now found on private and protected lands in areas of the western United States and Canada (National Bison Association, 2002). Most prominent of those herds are those of Yellowstone National Park in Wyoming and Wood Buffalo Park, Northwest Territory, Canada (Honacki, 1982).

Bison historically occurred throughout the grasslands and open savannas of North America. However, they were also found from boreal habitats to semi-desert habitats if grazing was suitable. Bison are now more limited in distribution and, therefore, the habitats they occupy. They are currently found in disjunct populations in protected areas throughout western North America. They occupy a large elevational range, being found at all elevations in the protected areas they occupy (Meagher, 1986).

Bison are huge animals, ranging in length from 3.6 m to 3.8 m in males to 2.13 m to 3.18 m in females. They are also tall animals, with the height at the shoulder ranging from 1.67 m to 1.86 m for males and 1.52 m to 1.57 m in females. Two distinctive features of bison are the shoulder hump and their huge head. Fur color is brown, varying slightly from the front and back of the animal. The hair is longer in the front than in the rear. The distinction between hair length is most noticeable in males. The horns are black, curving upward and inward and ending in a sharp tip. The hooves are black and circular in shape (Meagher, 1986).

Dominant bulls attempt to restrict access to a small group of females for mating. Individual bulls "tend" females until allowed to mate, following them around and chasing away rival males.

Females are sexually mature in two to three years and males reach maturity around age three. Bulls, however, do not usually breed until six years of age, when they have reached a size that makes them able to compete with other bulls for access to females. The breeding season begins in late June and lasts through September. Gestation is around 285 days, so the calving season is from mid-April through May. Any out of season births occur in the late summer.

Bison are born away from the herd in a location that has a lot of cover. Mothers protect the young from danger; males do not participate in this activity. One calf is born per season, weighing from 15 to 25 kg. Male calves are born slightly more frequently than females. Young calves are red in color. They begin turning brown in two and a half months and are entirely brown in four months. Calves are nursed for seven to eight months and are fully weaned by the end of the first year. Females are seasonally polyestrous with a cycle of approximately three weeks. Estrus may last anywhere from 9 to 28 hours (Meagher, 1986).

Female bison nurse, protect, and care for their young for up to one year. Males do not participate in caring for their young. Calves are capable of walking and running within a few hours of being born.

Bison live 15 to 20 years in the wild, although average lifespan depends on local predation and hunting pressures. Bison have been known to live up to 40 years in captivity.

Bison are gregarious animals and are arranged in groups according to sex, age, season, and habitat. Cow groups are composed of females, males under three years of age, and a few older males. More males enter these groups as the rut approaches. Males live either individually or in groups that may be as large as 30. Dominance between the bulls is linear. Bulls that have a higher rank in the society breed more often than those of a lower rank. Cows also live in a linear dominance hierarchy, which is established early in life.

Grazing takes place during several periods each day and is conducted in loose groups. When bison travel, they form a line. The traveling pattern of bison is determined by the terrain and habitat condition. An adult cow supplies the leadership. Bison are good swimmers as well as runners, capable of reaching speeds of 62 km/hr.

Copulation is initiated by the bull and is quick. During the rut, bulls fight among themselves. The amount of wallowing and tree horning also increases during the rut (Meagher, 1986).

The olfactory sense of bison is excellent and is essential in detecting danger. Bison can hear very well as well. Bison are able to distinguish large objects from a distance of 1 km and moving objects 2 km away. Bison can communicate vocally through grunts and snorts. It is likely that chemical cues are used in communicating reproductive states.

Bison are year round grazers. They feed primarly on grasses, but when food is scarce, they will eat vegetation such as sagebrush. On average, bison ingest 1.6% of their body mass per day of dry vegetation. Bison require water every day as well (Meagher, 1986).

Their large size and formidable defenses make healthy, adult bison relatively safe from predators. Elderly and ill bison and calves are preyed on by large predators such as mountain lions, wolves, and humans.

Huge herds of bison once roamed the grasslands of North America. Their grazing and dust-bathing strongly influenced the composition of plant communities and the communities of other animals. Bison can reasonably be called a keystone member of North American prairie communities, along with prairie dogs.

Bison were once a major source of meat and hides in the United States. They formed the basis of the economy of a number of groups of Native Americans. Paths made by the bison through the mountains were used by highway crews when they mapped routes for highways in the west (VanGelder, 1982). Today, bison are found in many zoos throughout the world (Meagher, 1986). Bison and hybrid cattle/bison are raised as a source of meat. Bison also attract many people to national parks in the west.

Bison are important members of functioning prairie ecosystems.

Bison can carry and transmit diseases that also infect domestic cattle, such as Brucellosis. However, authorities argue whether transmission of such diseases between bison and cattle is likely in field settings.

Bison are listed under CITES - Appendix I, and the sub species B. b. athabascae is listed as endangered by the U.S. Endangered Species Act (Honacki, 1982).

The pre-Columbian population of bison in North America was estimated to be around 60 million. By 1890 the number was reduced to less than 1000. The destruction of the herds was in part a result of a political and economic act. The United States government had the bison killed en masse to destroy the livelihood of Plains Indians (VanGelder, 1982).