Great Basin Naturalist


Autumn musters of bison (Bison bison) on Antelope Island State Park, Utah, conducted annually since 1987, provided data on temporal and age-specific reproductive patterns and a basis to evaluate the efficacy of management measures implemented to elevate reproductive performance in the herd. Pregnancy rates were variable and low ( = 46.2%) in comparison to other free-ranging, noncommercial bison herds in North America. Cows in the 3- and >6-yr age classes exhibited lower-than-expected pregnancy rates (P < 0.05). Annual pregnancy rates showed a significant (r = 0.64, P = 0.047) linear decline of 2.5% per annum. Variance in distribution of fetal ages observed in 6 yr indicates substantial temporal fluctuation. Long-term reproductive performance of cohorts born prior to implementation of management measures did not differ from that of cohorts born subsequent to these changes.