Group living confers many fitness advantages, but can result in within-group competition for access to resources. When some individuals are better able to monopolize resources than others, this can lead to the formation of dominance hierarchies. Whether high rank confers more benefits or costs than low rank remains an open question. In some studies, low-status individuals exhibit worse health than high-status individuals, while in other studies; high-status individuals exhibit worse health than low-status individuals. These differences are puzzling, and similar ambiguities are evident when researchers have attempted to discern rank-related differences in parasitism and fitness. To address this puzzle, I conducted meta-analyses of vertebrates and longitudinal observational research of a wild population of male savanna baboons (Papio cynocephalus) in the well-studied Amboseli population in Kenya.
Multivariate meta-analyses revealed that, across vertebrates, high-ranking animals face higher parasite risk than low-ranking animals. The overall pattern – higher parasitism in high-status individuals – was most evident in male mammals, in linear hierarchies, in mating systems where rank predicts mating effort, and for contact- and environmentally-transmitted parasites. My research revealed that high parasite risk may be an unappreciated cost of high rank while reduced parasite risk might be a benefit of social subordination.
Longitudinal analyses of male baboons revealed that contrary to the meta-analyses, male dominance rank was not a predictor of parasitism once I controlled for variation in male age; instead, age was a strong predictor of parasitism. Further, males had higher risk of parasitism during periods of drought, in more competitive environments, and if they had strong social bonds with females. Research on the highest ranking males revealed that among the “best of the best”, males with longer tenures at high rank sire more offspring than males with shorter tenures at high rank; dominant males in less competitive social groups are more successful than dominant males in more competitive groups; and social connectedness to females is associated with reproductive success. Collectively, these finding are important for advancing our understanding of the costs and benefits of social dominance, the evolution of male mating strategies, and the forces that influence male fitness.