Fogel, A. S. | bioRxiv Science

bioRxiv2020

Genetic ancestry predicts male-female affiliation in a natural baboon hybrid zone

Opposite-sex social relationships are important predictors of fitness in many animals, including several group-living mammals. Consequently, understanding sources of variance in the tendency to form opposite-sex relationships is important for understanding social evolution. Genetic contributions are of particular interest due to their importance in long-term evolutionary change, but little is known about genetic effects on male-female relationships in social mammals, especially outside of the mating context. Here, we investigate the effects of genetic ancestry on male-female affiliative behavior in a hybrid zone between the yellow baboon (Papio cynocephalus) and the anubis baboon (P. anubis), in a population in which male-female social bonds are known predictors of lifespan. We place our analysis within the context of other social and demographic predictors of affiliative behavior in baboons. Genetic ancestry was the most consistent predictor of opposite-sex affiliative behavior we observed, with the exception of strong effects of dominance rank. Our results show that increased anubis genetic ancestry is associated with subtly, but significantly higher rates of opposite-sex affiliative behavior, in both males and females. Additionally, pairs of anubis-like males and anubis-like females were the most likely to socially affiliate, resulting in moderate assortativity in grooming and proximity behavior as a function of genetic ancestry. Our findings indicate that opposite-sex affiliative behavior partially diverged during baboon evolution to differentiate yellow and anubis baboons, despite overall similarities in their social structures and mating systems. Further, they suggest that affiliative behavior may simultaneously promote and constrain baboon admixture, through additive and assortative effects of ancestry, respectively. HIGHLIGHTSO_LIOpposite-sex social relationships can have important fitness consequences. C_LIO_LIIn hybrid baboons, genetic ancestry predicted male-female affiliative behavior. C_LIO_LIBoth an individuals genetic ancestry and that of its social partner mattered. C_LIO_LIMale-female affiliation was assortative with respect to genetic ancestry. C_LIO_LIDominance rank and group demography also influenced male-female social affiliation. C_LI

evolutionary biology↗

bioRxiv2020

Comparing proportional and ordinal dominance ranks reveals multiple competitive landscapes in an animal society

Across group-living animals, linear dominance hierarchies lead to disparities in access to resources, health outcomes, and reproductive performance. Studies of how dominance rank affects these outcomes typically employ one of several dominance rank metrics without examining the assumptions each metric makes about its underlying competitive processes. Here we compare the ability of two dominance rank metrics--ordinal rank and proportional or standardized rank--to predict 20 distinct traits in a well-studied wild baboon population in Amboseli, Kenya. We propose that ordinal rank best predicts outcomes when competition is density-dependent, while proportional rank best predicts outcomes when competition is density-independent. We found that for 75% (15/20) of the traits, one of the two rank metrics performed better than the other. Strikingly, all male traits were better predicted by ordinal than by proportional rank, while female traits were evenly split between being better predicted by proportional or ordinal rank. Hence, male and female traits are shaped by different competitive regimes: males competitive environments are largely driven by density-dependent resource access (e.g., access to estrus females), while females competitive environments are shaped by both density-independent resource access (e.g. distributed food resources) and density-dependent resource access. However, traits related to competition for social and mating partners are an exception to this sex-biased pattern: these traits were better predicted by ordinal rank than by proportional rank for both sexes. We argue that this method of comparing how different rank metrics predict traits of interest can be used as a way to distinguish between different competitive processes operating in animal societies.

animal behavior and cognition↗