Bison Pictures Close-Up of a Bison With their large, sharp horns, bison are formidable foes. During mating season, bulls fight for the right to breed with harems of cows, but rarely duel to the death. Bison and Her Calf Bison cows give birth after nine months and nurse their calves for about a year. Calves are able to stand shortly after birth, but remain vulnerable to wolves and bears until they reach maturity at the age of about three years. Two Male Bison Fighting Two male bison spar in a field. Males battle for mating primacy, but such contests rarely turn dangerous. Herd of Bison Conservation programs at parks and zoos have combined forces with farmers, working to increase bison populations. Today there are about 200,000 bison in ranches and reserves.