Sexual size dimorphism in mammals is one of the root causes for diet and habitat segregation between the sexes. In ungulates, rumen size is isometric with body size and as body size increases, relative energy demands decrease. Thus, in sexually size dimorphic ungulates, differences in energy demands are expected to result in different dietary needs, and potentially, diet segregation and therefore habitat segregation. Using stable isotope values of carbon (δ¹³C) in conjunction with mesowear and fecal nitrogen analysis, the primary objective of this study is to evaluate two hypotheses of sexual segregation in bison (Bison bison) of Yellowstone National Park, WY., USA. The forage-selection hypothesis states that different dietary requirements will result in different optimal habitats and thus, sexual segregation will occur. The reproductive-strategy hypothesis claims that sexual segregation results from different anti-predator strategies between males and females. Fecal nitrogen is an index for relative diet quality, and high and low mesowear scores are associated with diets consisting largely of graze and browse, respectively. Finally, δ¹³C values in scat and body tissues of animals reflects the isotopic values of their food and lower δ¹³C values are indicative of C₃ plants, which typically show less negative values in dry open habitats and more negative values in closed forested habitats.

 Results/Conclusions

I sampled 47 carcasses and collected 60 fecal samples from the northern and central ranges (NR and CR, respectively) of Yellowstone National Park. Females had higher fecal nitrogen content and lower δ¹³C­_scat then males, with females exhibiting greater variation in both dietary indices. Additionally, CR bison had higher fecal nitrogen content and lower δ¹³C_scat values than their NR counterparts, with CR animals showing greater variation in both. Mesowear analysis revealed no significant difference between sexes. However, CR bison were found to have significantly higher mesowear scores than those in the NR. On average, female bison had higher quality diets than males, implying that the bison of Yellowstone National Park are best characterized by the forage selection hypothesis. These results suggest that CR bison have higher quality diets than those from the NR, corresponding with known variations in annual precipitation and levels of interspecific competition between bison and other resident ungulates.