Female gorillas form highly stable dominance relationships.

Animals commonly form dominance relationships that determine the priority of access to resources and influence fitness. Dominance relationships based on age, immigration order or nepotism (alliances with kin) conventions are usually more stable than those based on intrinsic characteristics such as physical strength. Unlike most mammals, female gorillas disperse from their groups, typically more than once in their lifetimes, disrupting their group tenures and/or any alliances. Thus, we predicted that they form unstable dominance relationships that are not based on conventions. Contrarily, using a 24-year dataset on five groups of both gorilla species, we found that females form strikingly stable dominance relationships, maintained over their whole co-residence in a group (mean dyadic co-residence = 4.8, max = 17.3 years). Specifically, we observed rank reversals in only two out of 92 female dyads, and all other rank improvements resulted from emigration or death of higher-ranking females (passive mobility). These results mirror observations in chimpanzees, suggesting that dominance dynamics might have deep roots in hominid evolution. Our study challenges a hypothesized link between hierarchy instability and dispersal, particularly among animals in which fitness consequences of rank improvement may not be great enough to counterbalance the potentially high costs of challenging higher-ranking individuals.