American Bison
(Bos bison)

For at least ten thousand years, from the last Ice Age until their slaughter in the nineteenth century, millions of bison (American Buffalo) once grazed the North American continent from the mountain grasslands of the West, throughout the prairies, and as far east as Georgia. In 1839 California bound traveler, John Bidwell, wrote "I have seen the plains black with them for several days' journey as far as the eye could reach." Frank Roe in his book The North American Buffalo reported of an 1850 westbound train being held up for three days for one immense migrating herd to clear the tracks.The shaggy animals are members of the sheep, goat, and cattle family. They were an important food source to Native Americans, and their hides made warm robes.

Often called buffalo (True buffaloes like the water buffalo have different characteristics and live in Asia and Africa), bison were slaughtered by hunters almost to extinction, depriving Indians of their major food source. The bison were harvested for meat to supply railroad crews. Many more were killed only for their hides, leaving the stripped carcasses laying on the ground. Later even the bones were collected for fertilizer. By 1889, only about 550 bison could be found alive in the United States.A few were protected by early conservationists and together with others bred in zoos and on ranches, they were reintroduced into parks and refuges. Today, bison are found in significant numbers in areas such as Yellowstone National Park (Wyoming), Wood Buffalo National Park (Alberta, Canada), and the National Bison Range (Montana).

Size: Bison, the largest surviving land mammal in North America, look 'front-heavy.' Their weight is concentrated in their forequarters. There are two subspecies of bison, plains bison and wood bison. The wood bison is larger than the plains bison, seven feet tall at the shoulder, compared to five to six 1/2 feet for the plains. Females are about one foot shorter. Male bison of both subspecies weigh 900 to 2,000 pounds. When full-grown, the male can be 7 to 11 1/2 feet in length. Their long tail, 20 to 26 inches, has a large ruft. The female, smaller than the male, averages a height of five feet at the shoulder and weighs about half as much as the bull, rarely exceeding 900 pounds.

Characteristics: A bison's appearance is led by its massive head that is crowned by a dark brown curly 'hairpiece' between two upwardly curving horns. The bison's foreparts are covered with this dark, woolly mane and beard, that is longer on the plains than the wood subspecies. The horns of a wood bison are longer and straighter than the plains bison. Behind the low-slung head that is supported by its heavily muscled neck, the bison's broad back rounds into a hump. Bison are mainly brownish-black in color and are covered with a thick 'blanket' section of somewhat paler rufous-brown fur from their shoulders to the forelegs. The bison has short legs with large hoofs and its hindquarters are covered with a shorter, lighter-colored hair. Young calves are a reddish brown color through their first summer.

The bison tail's position is like a 'weather vane.' A tail hanging loosely behind indicates that the animal is relaxed. If the tail is partially raised, the bison is alert. If the tail is horizontal, the bison is excited. However a tail raised upward is a warning; the bison is in a combative posture and may be ready to charge.

Food: After weaning from their mothers milk, young bison feed on a variety of green plants for the rest of their lives. Bison feed mostly on grass. They also eat a few other small plants, as well as twigs of willows and low shrubs. Bison normally feed five times a day: just before dawn, before midday, in the middle of the afternoon, one to two hours before sunset, and again around midnight. Between feedings, the herds rest, each animal regurgitating and then rechewing its 'cud' (in the same way that a domestic cow does). To forage in winter, a bison sweeps aside snow with its muzzle and digs down with its hooves as deep as four feet to get at succulent grass.

Gender: Both the bull and cow have a pair of fairly short horns that they retain from year to year. The bull's horns curve upward and are used as weapons of attack. A cow's horns curve but are more slender. Gender differences are also seen in the variety of sounds that bison make. A cow makes a series of sharp grunts, somewhat like a pig, when it wants to call her calf. The calf responds with a high-pitch grunt. However, the most distinctive bison sound is the booming bellow of the bull during their rutting season.

Habitat: Bison once lived on the open plains and rolling prairies, in the larger mountain meadows and nearby forests, and on mixed forest lands. They helped prairies by 'rubbing out' trees, treading on seeds, and creating plowed ground where new plants could grow. Bison thrived on the shortgrass prairies - the fescues, stipas, and native wheatgrasses that covered the Western plains.

All ages and both sexes of bison dig shallow depressions in the ground; wallows devoid of any vegetation that are usually eight to fourteen feet across and up to one foot deep. Here they wallow in the dirt or mud helping to rid themselves of molting hair and caking their flanks with dirt to cool and protect themselves from biting insects. In the spring, they also rub against trees and rocks to help thin out their thick winter coat.

Herding: Bison are social animals, they travel in herds. Bison bands commonly number 60 or more; occasionally they form larger herds. From late fall to spring, bison usually separate into two herds. Cows and calves gather in one, and older bulls in the other. The two groups do intermingle from time to time, drifting together, then splitting off again.

During the daytime, the herd moves from one feeding area to another or to a watering source. Herds are usually led by an older cow. In the course of a day, the herd will usually travel within a one or two mile radius, dependent on how near the water is located. In their travel, bison often follow established trails. They can cross large, swift rivers.

Locomotion: Tracks of the bison are shaped like cloven hearts and rounder, larger than cattle's footprints. The bison's walking strides are 24" to 36". Bison herds can run, stampede at speeds over 30 miles an hour.

Reproduction: Bison mate when they are three years old, but they do not become full-grown for another five years. Healthy bison cows have been known to bear calves at thirty years of age.

The bison's breeding season occurs between July and October, peaking around mid-August. During the rut, the males long, grunting calls can be heard miles away. Aggressive rival bulls sometimes clash over the possession of a chosen cow. They will paw the ground, kick up swirls of dust, then lunge at each other, butting their heads and horns together until one is defeated. A bull bison may lose three hundred pounds during the rut, from battling other bulls and constantly accompanying a cow until she is ready to mate. But fighting is dangerous and requires too much energy and time; therefore bulls have developed alternatives to demonstrate dominance in the bison herd. The bulls snort, stamp their feet, or bellow at their competition. One may turn broadside to the other, demonstrating its massive size. Signs of submission include swinging the head and neck widely to one side, lowering the head, or turning and walking away completely. Strong males in a herd may acquire harems of up to seventy females.

In spring, April or May, a cow gives birth to a single calf that is bright tawny to buff in color and weighs between thirty and seventy pounds. The newborn can suckle immediately. Within a few minutes of their birth, the calf can stand and in about three hours, it is able to run. The cow is very protective of her young calf and keeps it close by her. During the first two or three weeks, they follow their mother. After that, calves often band together in playful 'nursery' groups with the watchful mothers nearby. Calves are weaned at about seven months.

Range: Today in the United States, bison are found only in protected areas mainly in Idaho, Montana, Wyoming, North Dakota, South Dakota, Nebraska, and Colorado. The largest free-roaming herds are in Yellowstone National Park, Wyoming. Many of these are crossbreeds between plains and wood bison. Wood bison are almost extinct except for about 1,000 animals in two Canadian parks.

Predators: Healthy adults have little to fear, especially in their herds. Bison's only significant predators are wolves and grizzlies that usually have to content themselves with preying on stray calves or old, weakened adults. The bison's primary defense is to stampede, running away from predators at speeds up to 32 miles per hour, faster than a wolf. They also can use their hooves and horns as formidable weapons against intruders. 

More Information

If you want to learn more about bison, visit these websites:

  1. American Bison
  2. American Buffalo: Spirit of a Nation
  3. Bison Roundup


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Note: All photographs taken with a digital camera in Wyoming and North Dakota.

Developed by Annette Lamb and Larry Johnson, 6/99. Update4/02.