Yellowstone is the only place in the United States where bison have lived continuously since prehistoric times. A number of Native American tribes especially revere Yellowstone’s bison as pure descendants of the vast herds that once roamed the grasslands of the United States. The largest bison population in the country on public land resides in Yellowstone. It is one of the few herds free of cattle genes.
A bull’s head is wider and shaped more like a triangle than the female bison; its “forehead” fur is much thicker, as is the fur on its forelegs; and its beard is thicker. A cow’s horns are slightly more curved and slender than a bull’s. In addition, a cow’s shoulders are narrower than its hips while a male’s shoulders are broader than its hips.
Length: 10-12 feet (3-3.6 m). Height: 6-6.5 feet (1.8-1.9 m) at the shoulder.
Males (bulls) weigh up to 2,000 pounds (900 kg), females (cows) weigh about 1,000 pounds (500 kg).
18-22 years in the wild; over 30 years in captivity.
Yellowstone bison historically occupied approximately 7,720 square miles (20,000 km2) in the headwaters of the Yellowstone and Madison rivers. Today, this range is restricted to primarily Yellowstone National Park and some adjacent areas of Montana. The bison population is subdivided into the central and northern breeding herds. The northern breeding herd congregates in the Lamar Valley and on adjacent plateaus for the breeding season. During the remainder of the year, these bison use grasslands, wet meadows, and sage-steppe habitats in the Yellowstone River drainage, which extends 62 miles (100 km) between Cooke City and the Paradise Valley north of Gardiner, Montana. The northern range is drier and warmer than the rest of the park, and generally has shallower snow than in the interior of the park.
The central breeding herd occupies the central plateau of the park, from the Pelican and Hayden valleys with a maximum elevation of 7,875 feet (2,400 m) in the east to the lower elevation and thermally influenced Madison headwaters area in the west. Winters are often severe, with deep snows and temperatures reaching -44°F (-42°C). This area contains a high proportion of moist meadows comprised of grasses, sedges, and willows, with upland grasses in drier areas. Bison from the central herd congregate in the Hayden Valley for breeding. Most of these bison move between the Madison, Firehole, Hayden, and Pelican valleys during the rest of the year. However, some bison travel to the northern portion of the park and mix with the northern herd before most return to the Hayden Valley for the subsequent breeding season. In addition, there is some evidence numerous females recently switching breeding ranges and successfully breeding and rearing young on their new range.
Yellowstone bison feed primarily on grasses, sedges, and other grass-like plants (more than 90% of their diets) in open grassland and meadow communities throughout the year. They also eat forbs (weeds and herbaceous, broad-leafed plants) and browse (the leaevs, stems and twigs of woody plants) through the year, but those usually comprise less than 5% of the diet. They typically forage for 9 to 11 hours daily. Bison are ruminants with a multiple-chambered stomach that includes microorganisms such as bacteria and protozoa to enable them to effectively digest plant material. Bison alternate between eating and ruminating, which is regurgitating partially digested food and chewing it again, to allow microorganisms to further break down plant material into volatile fatty acids and other compounds. Their large digestive tract allows them to digest lower quality foods with greater efficiency than other ungulates such as cattle, deer, or elk.
SIGNS OF PRESENCE:
Wolves and grizzly bears are the only large predators of adult bison. However, predation has little effect on the bison population. Bison usually face their attackers and defend themselves as a group, making them more difficult to kill than animals like elk that run from predators. The size of bison also plays a role in persuading predators to look for an easier meal. When they die, bison become an important source of food for scavengers and other carnivores. Bison leave their mark on Yellowstone’s landscape by rubbing against trees, rocks, or in dirt wallows in an attempt to get rid of insect pests. Birds such as the magpie perch on a bison to feed on insects in its coat. Cowbirds will also follow close behind a bison, feeding on insects disturbed by its steps.
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