Breeding strategies, mate choice, and reproductive success in American bison

The objectives of this study were to assess the adaptive significance of various reproductive strategies in American bison bulls and cows. The study was conducted with a herd of 260 to 360 animals at the Fort Niobrara National Wildlife Refuge near Valentine, Nebraska, USA. Dominance (or fighting ability) was not correlated with age for 7- to 13-yr-old bulls or with body mass for animals > 750 kg. Reproductive success among 6 + yr-old bulls ranged from 0 to 16 young sired per bull in 3 yr and was positively correlated with dominance rank. Dominant bulls bred during the first 2-3 weeks of rut and lower ranking bulls conserved energy and achieved a significant number of matings during the late rut. Challenging bulls engaged tending bulls in bellowing contests, probably to assess their stamina and elicit the attention of cows. Tending bulls bellowed defensively and bellowed at the same time and attempted to "outbellow" challengers. Cows apparently assessed variance in male "quality" and approached high-ranking bulls and ran away from low-ranking bulls. Bulls did not discriminate between cows based on their age or prior breeding performance, but tended and mated with any cows that were in estrus. Cows exhibited flehmen behavior toward each other, probably to stimulate the onset of estrus and synchronize time of copulation. All cows were equally likely to have a son or a daughter and did not exhibit a facultative adjustment in sex ratio based on prior breeding performance. The results are discussed relative to other studies on bison and other ungulates.