Observations of a wild colony of macaques over three years show same-sex sexual behavior among males is widespread and may be beneficial

Jackson Clive, Ewan Flintham, Vincent Savolainen. Same-sex sociosexual behaviour is widespread and heritable in male rhesus macaques. Nature Ecology & Evolution, 2023; DOI: 10.1038/s41559-023-02111-y

Abstract: Numerous reports have documented the occurrence of same-sex sociosexual behaviour (SSB) across animal species. However, the distribution of the behaviour within a species needs to be studied to test hypotheses describing its evolution and maintenance, in particular whether the behaviour is heritable and can therefore evolve by natural selection. Here we collected detailed observations across 3 yr of social and mounting behaviour of 236 male semi-wild rhesus macaques, which we combined with a pedigree dating back to 1938, to show that SSB is both repeatable (19.35%) and heritable (6.4%). Demographic factors (age and group structure) explained SSB variation only marginally. Furthermore, we found a positive genetic correlation between same-sex mounter and mountee activities, indicating a common basis to different forms of SSB. Finally, we found no evidence of fitness costs to SSB, but show instead that the behaviour mediated coalitionary partnerships that have been linked to improved reproductive success. Together, our results demonstrate that SSB is frequent in rhesus macaques, can evolve, and is not costly, indicating that SSB may be a common feature of primate reproductive ecology.

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Popular version: Imperial College London. "Study shows same-sex sexual behavior is widespread and heritable in macaque monkeys." ScienceDaily, July 10 2023. www.sciencedaily.com/releases/2023/07/230710113924.htm

The team investigated several of these theories with their data, finding that, for this colony of macaques, SSB in males was strongly correlated with 'coalitionary bonds'. This means male pairs that regularly engage in SSB were more likely to back each other up in conflicts, providing them with an advantage in the group.

Heritable behaviours

The researchers also investigated whether SSB led to any fitness cost -- a reduction in the amount of offspring they have. In fact, they found the opposite -- males that engaged in SSB may be more successful in reproducing, potentially due to the benefits provided by more coalitionary bonds.

Why military elites fight in civil wars and on what side: In addition to home state, economic and professional interests were a major influence on West Pointers

Rebel, Remain, or Resign? Military Elites’ Decision-Making at the Onset of the American Civil War. Peter B. White. Journal of Conflict Resolution, June 23, 2023. https://doi.org/10.1177/00220027231185575

Abstract: A critical element in civil wars is military fragmentation. Yet, we have a limited understanding of why military elites fight in civil wars and on what side. In this article I develop a theory of the economic and professional motivations of military elites. I test this theory using the case of West Point graduates in the American Civil War. I argue that in addition to home state, economic and professional interests were a major influence on West Pointers. Graduates with connections to Southern cash crops were less likely to fight for the Union and more likely to fight for the Confederacy. Higher ranking graduates were more likely to fight for both sides, as they were better positioned to compete for promotion. I test this argument using a new dataset of more than 1000 West Point graduates’ wartime allegiances and antebellum careers and find strong evidence in support of my expectations.