Sexual segregation in American Bison: social or ecological underpinnings?

Sexual segregation is near ubiquitous in sexually dimorphic ruminants. Factors underpinning this phenomenon, however, continue to be debated. We conducted research on the spatial and social organization of American plains bison (Bos bison bison) on Antelope Island State Park, Utah, United States, to provide further insights into patterns and potential causes of sexual segregation. We examined 2 hypotheses associated with socially driven segregation: social factors and activity budget. We also investigated 2 ecological hypotheses: gastrocentric and predation. Our results did not support social segregation as a cause of spatial differences between sexes. Indeed, if social factors and activity patterns were driving segregation of the sexes, the spatial distribution of single-sex groups should have remained the same between seasons, and mixed-sex groups should not have been observed during parturition—outcomes contradictory to our results. Further, social hypotheses cannot explain why specific areas were used during sexual segregation, which requires an ecological explanation. Results from multi-response permutation procedure and overlap analyses illustrated significant spatial and elevational separation between male and female groups, which increased during parturition (a period of strong sexual segregation) and decreased during the mating season (a time of pronounced sexual aggregation). Male-only groups commonly used habitat on the northern end of the island, which was characterized by low elevation and heavy recreational use by humans. Before green-up of vegetation at high elevations, females used that habitat ostensibly to reduce predation risk. Later during parturition, females used southern and eastern areas of Antelope Island, where human recreational activity was low and vegetation was greenest. Those outcomes provide evidence supporting hypotheses for both predation risk and the gastrocentric model. This research can aid management of habitat and restoration of areas for bison, a species of conservation concern, and other wild ruminants by providing new insights into spatial use and social organization during 2 critical periods—parturition and rut.