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Bison bisonAmerican bison

By Toni Lynn Newell and Anna Bess Sorin

Geographic Range

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).

Habitat

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).

Physical Description

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).

  • Range mass
    318 to 900 kg
    700.44 to 1982.38 lb
  • Range length
    2.1 to 3.8 m
    6.89 to 12.47 ft

Reproduction

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).

  • Breeding interval
    Bison breed once yearly.
  • Breeding season
    Breeding occurs from late June through September.
  • Average number of offspring
    1
  • Average number of offspring
    1
    AnAge
  • Average gestation period
    285 days
  • Average gestation period
    274 days
    AnAge
  • Range weaning age
    7 to 12 months
  • Average time to independence
    1 years
  • Range age at sexual or reproductive maturity (female)
    2 to 3 years
  • Average age at sexual or reproductive maturity (male)
    3 years
  • Average age at sexual or reproductive maturity (male)
    Sex: male
    912 days
    AnAge

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.

  • Parental Investment
  • no parental involvement
  • precocial
  • pre-fertilization
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female
  • pre-independence
    • provisioning
      • female
    • protecting
      • female

Lifespan/Longevity

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.

  • Range lifespan
    Status: captivity
    40 (high) years
  • Typical lifespan
    Status: wild
    15-20 (high) years

Behavior

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).

Communication and Perception

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.

Food Habits

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).

  • Plant Foods
  • leaves
  • roots and tubers
  • wood, bark, or stems

Predation

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.

Ecosystem Roles

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.

Economic Importance for Humans: Positive

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.

  • Positive Impacts
  • food
  • body parts are source of valuable material
  • ecotourism

Economic Importance for Humans: Negative

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.

Conservation Status

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).

Contributors

Toni Lynn Newell (author), University of Michigan-Ann Arbor, Anna Bess Sorin (author), Biology Dept., University of Memphis.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

causes or carries domestic animal disease

either directly causes, or indirectly transmits, a disease to a domestic animal

chemical

uses smells or other chemicals to communicate

colonial

used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.

diurnal
  1. active during the day, 2. lasting for one day.
dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates

ecotourism

humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

folivore

an animal that mainly eats leaves.

food

A substance that provides both nutrients and energy to a living thing.

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

herbivore

An animal that eats mainly plants or parts of plants.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

nomadic

generally wanders from place to place, usually within a well-defined range.

polygynous

having more than one female as a mate at one time

scrub forest

scrub forests develop in areas that experience dry seasons.

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

sexual ornamentation

one of the sexes (usually males) has special physical structures used in courting the other sex or fighting the same sex. For example: antlers, elongated tails, special spurs.

social

associates with others of its species; forms social groups.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

young precocial

young are relatively well-developed when born

References

Honacki, J.H., ed.; Kinman, K.E., ed.; Koeppl, J.W., ed. 1982. Mammal Species of the World; A Taxonomic and Geographic Reference. Association of Systematics Collectionsz; U.S.A.

Meagher, M. (16 June 1986). "Bison bison." Mammalian Species. The American Society of Mammalogists, 266.

VanGelder, R.G. 1982. Mammals of the National Parks. The John Hopkins University Press: Baltimore and London.

National Bison Association, 2002. "BisonCentral.com" (On-line). Accessed June 1, 2003 at http://www.bisoncentral.com/.

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