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Bison are social animals and live in herds that change in size and composition throughout the year. In winter, herds are much smaller, typically only 20 to 30 in number, with older bulls completely isolating themselves. During the rut (mating) season in summer, bison gather in very large herds. Herds are definitely matriarchal with cows usually leading herd movement.

Bulls weigh from 1,600 to 2,000 pounds, stand between 5.5 and 6.5 feet high at the shoulder, and measure from 9.5 to 11.5 feet in total length (including the tail). Bulls seldom live longer than 20 years but, in rare cases, may live to be 40 years old. The cow is smaller, weighing up to 1,100 pounds, although most weigh about 1000 pounds. They stand 4.5 to 5.5 feet in height at the shoulder, and are less than 10 feet in length. Less shaggy on the head and chin, she has a smaller hump and her horns are more slender and curved than the bull's.

A bison's coat attains prime condition during the winter months, then sloughs off in clumps during the annual molt in the spring. The biggest chunks come from the hump and shoulders, where fur is two to five times thicker than the hair on the hindquarters. This difference in thickness accentuates the hunchback shape. The hair on forelegs, throat, chin, crown, and forehead reaches surprising lengths, especially on older animals. The longest masses, dangling from between the horns and upper forehead, have been known to grow to 22 inches.

Bison are subject to the same diseases as cattle but in the wild seem to be amazingly free of disease. No serious epidemics have been reported in present-day animals. Some animal breeders have tried to develop a hardy, useful kind of domestic animal by crossing American bison with ordinary domestic cattle. The cattaloes or beefaloes that result have not proven satisfactory, primarily due to infertility problems.