The American bison is one of two living bison species, including the European bison, which remain from about 5,000 years ago when six other bison species had become extinct. Bison are in a subfamily, the Bovinae, within the family Bovidae, which includes other cloven-hoofed ruminants like water buffalo, sheep, goats, muskoxen, domestic cattle, and several African ungulates. The head and front half of the body is covered with thick fur whereas the rear has short hair. The difference in fur coverage corresponds to the bison’s habit of facing into winter winds. The largest mammal in North America, bison males may weigh up to 2,000 pounds.
Bison forage primarily on prairie grasses but also take woody plants and herbs, as well as occasionally browsing on willows and cottonwoods. The large, historic migratory bison herds supplied the Native American plains tribes with food, shelter, clothing, tools, and a spiritual connection to the land for centuries before European settlement. Though historically numbering in the tens of millions on the Great Plains, during the late 1800s settlers reduced bison numbers to fewer than 1,000 individuals by the turn of the 20th century.
The dramatic reduction in bison numbers has created what geneticists call a population “bottleneck.” The genetic diversity contained in the original population of tens of millions has been largely eliminated and all future generations will only carry the genetic diversity present in the small remnant breeding population. That reduction in diversity results in a lessened ability to cope with environmental or physiological stresses.
Recovery efforts have resulted in a current population of up to 500,000 animals in private and public herds, and the American Bison is not now considered endangered. However, DNA testing has shown that due to interbreeding with domestic cattle, only some 15,000 to 25,000 bison are pure genetic stock; the rest being hybrids. The largest pure herds of bison are found in Yellowstone National Park, Wyoming, and Wind Cave National Park and Custer State Park in South Dakota. A small herd of 350 purebred individuals was discovered on public land in southern Utah in 2015. A herd of bison is maintained on a private ranch in northern New Mexico, where annual hunts are conducted for paying patrons.
Besides human hunters, bison are preyed on by wolves and mountain lions. Coyotes and grizzly bears take some young calves. Breeding occurs from June through September and bulls leave the herd at 2 to 3 years of age to roam in smaller male groupings. Bulls spar with each other during the rut by butting heads. Bison commonly wallow in dry or wet depressions, covering their bodies with dirt or mud. The behavior is thought to ward off parasites and biting insects, as well as helping to regulate body temperature and shed old skin and fur.
James Taulman is a semi-retired research wildlife biologist, having worked with the U.S. Forest Service research branch and taught zoology, ecology, and other courses in several university positions. He is currently living in the East Mountains, and explores natural areas observing native wildlife and conducting independent research projects.