Bison

In the early summer, bulls and cows gather for the rut. During the rut, bulls challenge each other for the right to mate with cows. Cows first mate when about two years old, but bulls do not do so until they are older and strong enough to challenge dominant males. A few loud roars and an enthusiastic demonstration of kicking and wallowing is usually enough to convince a lower-ranked bull to look elsewhere for a mate. Only rarely does an actual fight occur, when bulls lock horns and charge each other.

When a dominant bull selects a mate, he bonds with her by grazing side by side for some hours, away from the rest of the herd. After a brief nocturnal mating, the companionship may continue for a brief time, but then the male departs, looking for another female. The females usually mate only once.

The gestation period lasts nine-and-a-half months, and new calves are born in the spring. This begins about mid-April, though births may continue into the early autumn. Newborn calves are cinnamon-colored, weigh about 50 lb (23 kg), and can walk and nurse within 2-3 hours of birth. The calves start to eat grass within about 15 days, and at about two months of age their hump and horns begin to show and their coat darkens to the adult color. Bison typically live to an average age of about 20 years in the wild, but can reach 40 years in captivity.