Bison are the largest grazing mammals in Yellowstone National Park. They are obligate herbivores, a grazer of grasslands and sedges in the meadows, the foothills, and even the high-elevation, forested plateaus of Yellowstone. Bison males, called bulls, can weigh upwards of 1,800 pounds. Females (cows) average about 1,300 pounds. Both stand approximately six feet tall at the shoulder and can move with surprising speed to defend their young or when approached too closely by people. Yellowstone is the only place in the lower 48 states where a population of wild American bison has persisted since prehistoric times.

Source: Wikipedia