The horns of the bull are larger and heavier than the horns of the cow. In late fall, the bison's coat is a rich, dark brown. As winter progresses, the coat changes color and is much paler by spring. When the weather warms, the hair loosens and hangs in patches until it is completely shed and replaced with new hair by late spring. Hair on the chin resembles a goatee.
Older animals tend to have more hair on their heads.
Newly born calves have a reddish coat. At about 6 days of age, calves start grazing. Their reddish-orange coat begins to darken at about 10 weeks, with the molt to dark brown complete about five weeks later. Cows are sexually mature at 2 years of age and give birth to single calves twice in three years. The gestation period is approximately 270 days.