Genomic analysis of male reproductive skew in Bornean and Sumatran orangutans

Bornean and Sumatran orangutans exhibit pronounced differences in their social behavior, which impacts the extent to which males in these species can monopolize reproductive success. This male reproductive skew has different effects on genetic diversity on the X chromosome compared to the autosomes (X/A ratio), providing us with a genomic record documenting how variation in sociality across orangutans has shaped patterns of their reproductive behavior. Our research uses genomic analysis of sixteen published genomes from Bornean and Sumatran orangutans to compare X/A ratios between the species. After removing sites impacted by selection, correcting for male mutation bias, and fitting our estimates to a demographic model, we derive X/A ratios which show the impacts of sex-biased demographic forces across the history of both lineages. We found that Sumatran orangutans (autosomal mean: 0.0059; X/A ratio: 1.1327) and Bornean orangutans (autosomal mean: 0.0060; X/A ratio: 1.0946) had both similar autosomal diversity and X/A ratios. Our preliminary findings do not support our hypothesis of greater reproductive skew in Sumatran orangutans, though problems with our model fit indicate that more work needs to be done to improve our estimates. Furthering our understanding of differences in male reproductive skew between Bornean and Sumatran orangutans develops our knowledge of how variation in environmental productivity and sociality shape male reproductive strategies across primates more generally.