Bison were highly gregarious, living together in large herds on rangeland and smaller herds in woodland. The core of the herd consisted of cows, calves of both sexes, and young males. Older bulls stayed on the periphery of the main herd, in small bands, or alone except during rut, when they joined the cow-subadult bands within the main herd. They usually sought water twice each day but could go several days without water during periods of drought. In spring and summer while shedding thick winter wooly hair, bison wallowed in depressions, moist or dry. The dust or caked mud may have afforded some relief from parasites, flies, and mosquitoes. Even today these basinlike depressions can be found in Kansas, formed by the thousands of wallowing bison who carried away the mud in their shaggy coats. Isolated rocks and trees also were used for rubbing. Bison herds on the Great Plains migrated seasonally, generally northward in summer and southward in winter. Unfortunately, their migratory habits were never carefully studied, and we remain ignorant of the details.
External measurements of adult males and adult females, respectively, are: total length 3,040 to 3,800, 2,130 to 3,180; length of tail 330 to 910, 3300 to 510; length of hind foot 580 to 680, 500 to 530; height at shoulder 1,670 to 1,860, 1,520 to 1,570; body mass (in kg) 544 to 907, 318 to 545. The normal maximum life span of bison is 20-30 years.
This ungulate grazes on grasses and is especially adept for feeding on shortgrass species, such as buffalograss and gramas that are widespread on the High Plains. Bison seem to tolerate dry and drought-stricken vegetation better than domesticated cattle. When grass is unpalatable, bison consume forbs or woody plants. They paw the ground or shovel snow out of the way with their heads to uncover vegetation in winter.
The only predators capable of preying on adult bison were grizzly bears, wolves, and man. Wolves in particular focused their efforts on young, aged, and infirmed individuals that became separated from the herd.
Sexual maturity commonly occurs at 2 to 4 years of age. Rut of bison is prolonged, potentially lasting from June through September, with most copulations occurring in a 2-week period in late July and early August. A single calf, or rarely twins, is born after a gestation period of about 285 days in late April or May, rarely June. Calves lack the characteristic hump and tend to be is tawny to yellowish in color. They are precocious and able to follow their mothers a short time after birth. Within a few weeks the coat darkens and the hump begins to develop. By autumn the calves are nearly as dark as adults. They remain with the females throughout their first winter.