Birth Synchrony in American Bison (Bison bison): Response to Predation or Season?

Calving of American bison was observed at the National Bison Range, Montana, in 1982. Approximately 80% of the calves were born in the last week of April and first 2 weeks of May. During calving, cow-calf pairs grouped preferentially with other cow-calf pairs rather than with cows without calves. Because of their open-country habit and lack of group defense, bison are well-suited for application of Estes (1976) predator confusion and saturation hypothesis of reproductive synchrony. However, comparison shows that the length of bison calving seasons is fairly typical for temperate zone ungulates. Although bison calves are highly precocious, bison do not show the calving aggregations predicted by the anti-predator hypothesis. Thus, climatic factors appear to be the best explanation of breeding synchrony in bison.