For many years scientists considered Yellowstone’s bison to be a subspecies known as the mountain bison. Today, most scientists consider all bison to be one species, Bison bison.

In North America, both “bison” and “buffalo” refer to the American bison (Bison bison). Generally, “buffalo” is used informally; “bison” is preferred for more formal or scientific purposes.

The bison is the largest land mammal in North America. Bulls are more massive in appearance than cows, and more bearded. For their size, bison are agile and quick, capable of speeds in excess of 30 mph. Each year, bison injure park visitors who approach too closely.

Bison are sexually mature at age 2. Although female bison may breed at younger ages, older males (>7 years) participate in most of the breeding. In Yellowstone, their life span averages 12–15 years; a few individuals live as long as 20 years. Both sexes have horns, those of the cow being slightly more curved and slender than the bull’s. Bison are animals of the grasslands; they eat primarily grasses and sedges. Their massive hump supports strong muscles that allow the bison to use its head as a snowplow in winter, swinging side to side to sweep aside the snow.

Cows, calves, and some younger bulls comprise a herd. Mature bulls, however, spend most of the year alone or with other bulls. The exception is during the rut, or mating season. At this time, in late July and August, bulls seek out females. They display their dominance by bellowing, wallowing, and engaging in fights with other bulls. Once a bull has found a female who is close to estrus, he will stay by her side until she is ready to mate. Then he moves on to another female.

After a gestation period of 9 to 9 1 ⁄2 months, single reddish-brown calves are born in late April and May. Calves can keep up with the herds about 2–3 hours after birth and they are well protected by their mothers and other members of the herd. However, wolves and grizzly bears have killed bison calves.

Wolves are the only large predator of adult bison. Scientists have also recently seen grizzly bears hunting bison successfully. Dead bison provide an important source of food for scavengers and other carnivores.

Many insects feed upon the bison, and bison will rub against trees, rocks, or in dirt wallows in an attempt to get rid of insect pests. Birds such as the magpie “ride” a bison to feed on insects in its coat. The cowbird will also follow close behind a bison, feeding on insects disturbed by its steps.


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Like most other ungulates of the greater Yellowstone area, bison will move from their summer ranges to lower winter ranges as snow accumulates and dense snowpacks develop. When and where they migrate depends on a complex relationship between abundance of bison, quality of summer forage, and winter snow pack. However, observations of bison movement patterns show that a large number of the central herd migrate north in winter. Bison remain along the west boundary well into birthing season. Research also shows that although bison travel on groomed roads, they select these routes because they follow stream courses that connect larger patches of habitat.


From 30 to 60 million bison may have roamed North America before the mid 1800s. Their historic range spread from the Pacific to the Appalachians, but their main habitat was the Great Plains where Plains tribes developed a culture that depended on bison. Almost all parts of the bison provided something for the Native American way of life—food, tools, shelter, or clothing; even the dung was burned for fuel. Hunting bison required skill and cooperation to herd and capture the animals. After tribes acquired horses in the 1600s, they could travel farther to find bison and hunt the animals more easily.

European American settlers moving west during the 1800s changed the balance. Market hunting, sport hunting, and a U.S. Army campaign in the late 1800s nearly eliminated bison. Yellowstone was the only place in the contiguous 48 states where wild, free-ranging bison persisted. The U.S. Army, which administered Yellowstone at that time, protected these few dozen bison from poaching as best they could. The protection and recovery of bison in Yellowstone is one of the great triumphs of American conservation.

Managing Bison

Despite protection, Yellowstone’s bison were reduced by poaching to less than two dozen animals in 1902. Fearing the demise of the wild herd, the U.S. Army brought 21 bison from ranches to Yellowstone. In 1906–07, the Buffalo Ranch in Lamar Valley was constructed to manage these bison and increase their numbers. This herd grew to more than 1,000 animals; the park’s small native bison herd in Pelican Valley also slowly increased. In the 1930s, the introduced bison were allowed to move freely and intermingle with the native bison. The park’s bison popluation was close to 1,500 in 1954, and managers became concerned that bison would overgraze their habitat—so they began culling the animals. By March 1967, the herd was down to 400.

In 1968, managers stopped manipulating bison populations and allowed natural ecological processes to operate. As their population grew, bison began to move outside the park. Conflicts with humans began to occur. Bison can be a threat to human safety and can damage fences, crops, landscaping, and other private property. And, of significant concern to livestock producers, some Yellowstone bison are infected with the disease brucellosis.

Because of brucellosis, bison generally are not welcome outside the park even though other ungulates that may also harbor the brucellosis organism are. Since the 1980s, this issue has grown steadily into one of the most heated and complex of Yellowstone’s resource issues.

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