The effect of dominance rank on female reproductive success in social mammals

Life in social groups, while potentially providing social benefits, inevitably leads to conflict among group members. In many social mammals, such conflicts lead to the formation of dominance hierarchies, where high-ranking individuals consistently outcompete other group members. Given that competition is a fundamental tenet of the theory of natural selection, it is generally assumed that high-ranking individuals have higher reproductive success than lower-ranking individuals. Previous reviews have indicated large variation across populations on the potential effect of dominance rank on reproductive success in female mammals. Here, we perform a meta-analysis based on 444 effect sizes from 187 studies on 86 mammal species to investigate how life-history, ecology and sociality modulate the relationship between female dominance rank and fitness. As predicted, we found that (1) dominance rank is generally positively associated with reproductive success, independent of the approach different studies have taken to answer this question; and that (2) the relationship between rank and reproductive success is conditional on life-history mechanisms, with higher effects of dominance rank on reproductive output than on survival, particularly in species with high reproductive investment. Contrary to our predictions, (3) the fitness benefits to high-ranking females appear consistent across ecological conditions rather than increasing when resources decrease. Instead, we found that the social environment consistently mitigates rank differences on reproductive success by modulating female competition, with, as predicted, (4) dominant females showing higher reproductive success than subordinates in two different types of societies: first, effect sizes are highest when females live in cooperatively breeding groups composed of a single dominant female and one or more subordinate females; second, they are also elevated when females form differentiated relationships which occurs when groups are composed of unrelated females. Our findings indicate that obtaining a high ranking position in a social group consistently provides female mammals with fitness benefits, even though future studies might show lower effects given various biases in the literature we were able to access, including, but not restricted to, a publication bias. They further draw a complex landscape of the level of social inequality across mammalian societies, reflected by variation in the benefits of social dominance, which appears to be shaped by reproductive and social competition more than by ecological competition.