American bison

Bison bison

TAXONOMY

Bison bison (Linnaeus, 1758), Canadian River valley, New Mexico, United States.

OTHER COMMON NAMES

French: Bison américain; German: Bison.

PHYSICAL CHARACTERISTICS

Body length males 95.2-125.2 in (242-318 cm), females 68.1-109.4 in (173-278 cm); shoulder height males 65.7-73.2 in (167-186 cm), females 59.8-61.8 in (152-157 cm); tail length males 12.9-35.8 in (33-91 cm), females 11.8-20 in (30-51 cm); weight males 1,199-1,999.5 lb (544-907 kg), fe males 701-1,201 lb (318-545 kg). High sexual dimorphism, with females 20% shorter in length, but 40% less in weight than males; the largest mammal in North America, it has a broad head that seems to be held low because it is followed by a prominent hump over the shoulders created by elongated spines on the thoracic vertebrae. The hindquarters are much smaller than the forequarters. The legs are short and the tail is medium length, terminating with tuft of black hairs. Pelage is brown to dark brown, with longer hairs on the front and top of the head, along the neck, and onto the shoulders and fore-quarters, including the forelegs. The hair on the rest of the body is much shorter. The ears are partly hidden among the long fur on the head. Although larger in males, both sexes have a beard of long hairs that extend below the chin. Both sexes also have a fringe of long, dark hair running along the lower margin of the neck as far as the chest to form a noticeable mane. Males have a relatively larger shoulder hump than females and also have longer hair on their heads. Males have short black horns that extend to the side and curve upward, then inward near their sharply pointed tips. Females have horns, but they are more slender, and often shorter and more curved than those of males.

DISTRIBUTION

Historically, they occurred from northwestern Canada to central Canada and then south through much of the United States to northern Mexico. Although there are numerous herds privately managed on ranches and game farms, wild populations are reduced to a few remnant populations and confined to a few parks and reserves in North America.

HABITAT

Inhabited a variety of ecosystems as long as there was sufficient grassland and meadows on which to graze. In northern areas, they inhabited mixed wood forests and parklands as well as ex tending into the boreal forests where wet sedge meadows occurred. In more southern regions, they inhabited the long and short grass prairies and parklands across central plains and into the foothills of the Rocky Mountains. Primarily grazers, they relied on grasslands as their primary habitat type, even in northern regions where grasslands and meadows were patchily distributed. Present-day, free-ranging populations occur in areas representative of these primary historic habitats.

BEHAVIOR

Females, calves, and 1-3-year-old males form mixed groups that may contain one or two older males. Additional adult males may join these groups during the rut. Mixed groups tend to be quite cohesive and with strong hierarchies. The three-year-old and older males form small all-male groups of up to 30 animals, although many adult males also occur alone or in pairs. During the rut, these male groups join with female groups. Herds can grow even larger during seasonal migrations. During these migrations bison may travel over 124 mi (200 km) as the animals move to ranges where there is greater forage and shallower snow. Males are polygynous, but an adult male associates with a single female within the larger group. A typical behavior, both sexes wallow; the animals paw a shallow depression in the ground and roll in it.

FEEDING ECOLOGY AND DIET

Bulk feeders, they are typically not highly selective in their food habits. They rely on obtaining large amounts of low-quality forage, rather than small amounts of high-quality forage. Feed almost exclusively on grasses and sedges. Occasionally, they consume forbs and browse, but these food types are minor components of their diet. This dependence on grasses tied bison closely to short and long grass prairies characteristic of the central part of the North American continent.

REPRODUCTIVE BIOLOGY

Mating varies across their geographic range, but mostly takes place during July and August in the southern regions, but extending into September in northern regions. Polygynous. Gestation is on average 285 days, after which they give birth to single calf in spring; twins are very rare. Adult females produce calves usually each year, and generally give birth alone, preferring areas with some taller vegetation for concealment. The calf can run within three hours of birth, and are weaned at 7-12 months. For the first three months of life, they are a reddish brown color that is in marked contrast to the darker pelage of the adults.

CONSERVATION STATUS

Classified as Lower Risk/Conservation Dependent. Historically, they ranged across half of the North American and numbered in the millions. Their range and abundance has been as severely diminished. Wild American bison occur only in several national parks and wildlife reserves in Canada and the United States. However, they also occur on numerous ranches and privately held herds; in fact, there are more bison in private ownership than on public protected areas, thus as a species they are not at risk of extinction. Threats to wild bison come mainly from diseases and parasites transmitted from domestic livestock.

SIGNIFICANCE TO HUMANS

At one time, American bison were the most important game animal for indigenous people across the plains of central and western North America. They provided all manner of uses and products from meat, bones for tools, hides for blankets, leather for garments, and sinews for twine. Skins and horns were also used in ceremonies by Aboriginal peoples. Bison raised on ranches provide a select market for wild game meat and in some places they continue to be desired trophies for hunters. ♦

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