Frequently Asked Questions: Bison

Cow and calf run through the sage in Lamar Valley
Despite their size, bison can run up to 35 mph (55 kph) and can pivot quickly: an advantage when fighting predators that often attack from behind.

NPS / Jacob W. Frank


What is the difference between a bison and a buffalo?
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. Early European explorers called this animal by many names. Historians believe that the term “buffalo” grew from the French word for beef, “boeuf.” Some people insist that the term “buffalo” is incorrect because the “true” buffalo exist on other continents and are only distant relatives. On this website, we use “bison.”

How many bison live in Yellowstone National Park?
The bison population fluctuates from 2,300 to 5,500 animals in two subpopulations, defined by where they gather for breeding. The northern herd breeds in the Lamar Valley and on the high plateaus around it. The central herd breeds in Hayden Valley.

Are bison an endangered species?
Bison are not listed as a threatened or endangered species. Approximately 30,000 bison live in public and private herds in North America; they are managed for conservation goals. Approximately 400,000 bison are raised as livestock however, wild bison are rare. Yellowstone bison represent the best example for preservation of wild plains bison in North America.

Why are Yellowstone bison considered special?
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.

How big is a bison?
The bison is the largest land mammal in North America. Males (bulls) weigh up to 2,000 pounds (900 kg), females (cows) weigh about 1,000 pounds (500 kg).

How fast can a bison run?
Faster than you. A bison is agile and quick, and can run up to 35 miles per hour (55 kph). A bison can also pivot quickly—an advantage when fighting predators that aim for hindquarters.

How do you tell male from female bison?
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.

How do bison stay warm in winter?
Bison have excellent winter insulation: thick skin and underfur, long guard hairs, and layers of fat.

Bison seem to be very bony in the spring; how many die during the winter?
On average, 9 out of 100 adult bison will die during the winter. Generally these animals entered winter in poor condition due to age, disease, or injuries. Young bison entering their first winter face a higher risk of dying: 20 to 40 of every 100 first-year animals may die from accidents, winter exposure, or predation.

Why do bison roll in the dirt?
This rolling, also called wallowing, deters biting flies and removes tufts of molted fur. Bison also leave their scent in the wallow. And during the rut, bull bison wallow to display their strength and vigor.

What do they eat?
Bison eat primarily grasses and sedges and will often wander far to find habitat with appropriate food.

Why do they have a big hump?
A bison’s massive hump is comprised of muscles supported by long vertebrae; this allows a bison to use its head as a snowplow in winter, swinging side to side to sweep aside the snow.

Do bison ever live alone?
Mature bulls spend most of the year alone or with other bulls—except during the rut, or mating season.

When is the rut?
The rut begins in late July and goes through August. Bulls display their dominance by bellowing, wallowing, and fighting 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.

When does a bison first breed?
Bison are sexually mature at age 2. Many female bison breed at this age, but usually not males. The older males (>7 years) participate in most of the breeding.

When are calves born?
Reddish-brown calves are born in late April and May, after a gestation period of 9 to 9½ months. Calves can keep up with the herd 2 to 3 hours after birth and they are well protected by their mothers and other members of the herd. However, wolves and grizzly bears can kill bison calves.

What predators kill adult bison?
Wolves and grizzly bears can kill adult bison, but 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 away. The size of bison also plays a role in persuading predators to look for an easier meal.

Do bison migrate?
Yes, bison are migratory animals. When and where they migrate depends on a complex relationship between abundance of bison, quality and quantity of summer forage, and winter snowpack. In the Yellowstone area, they move from their summer ranges to lower winter ranges as snow accumulates and dense snowpack develops. The central herd moves both west and north toward park boundaries in winter, and may remain along the west boundary well into birthing season. The northern herd migrates within the park most winters, but will move beyond the north boundary in severe winters.

Have bison always lived in Yellowstone?
Yellowstone is the only place in the United States where bison have lived continuously since prehistoric times. We know they lived here in centuries past due to fossils, oral histories from Indian tribes, and the stories of the earliest travelers to this region. Yellowstone bison historically occupied approximately 7,720 square miles (20,000 sq km) 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.

How many bison used to live in what is now the United States?
From 30 to 60 million bison may have roamed North America before the mid-1800s. They ranged across the continent, but the majority lived on the Great Plains.

Why are bison protected if they aren’t endangered?
Bison in the United States were hunted almost to extinction. After tribes acquired horses in the 1600s, they could travel farther to find bison and hunt the animals more easily. However, they generally did not hunt to excess. The U.S. Army held a campaign in the late 1800s to eliminate bison as a way to control tribes that depended on bison. Yellowstone was the only place in the contiguous 48 states where wild, free-ranging bison persisted into the 20th century. Ironically, the U.S. Army, which administered Yellowstone at that time, protected these few dozen bison from poaching as best they could. The Lacey Act was passed in 1894 to allow stronger punishment for poachers. Even so, Yellowstone’s bison were reduced by poaching from as many as one thousand in 1872, to approximately two dozen animals in 1902. From that small herd has grown the two large herds now occupying Yellowstone National Park. Many people consider the protection and recovery of bison in Yellowstone to be one of the great triumphs of American conservation.

Are all of Yellowstone’s bison descended from these few animals?
No. Fearing the demise of the wild herd, the U.S. Army brought 21 bison from two private herds to Yellowstone in 1902. These animals were protected and managed in Mammoth at first, and then at the Buffalo Ranch in the Lamar Valley. Meanwhile, the original herd slowly increased. In several different stages during the first half of the 20th century, the captive bison began to mix with the wild bison. The ranching operation was phased out by 1952.


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Last updated: January 27, 2021

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