Temperature-dependent resolution of conflict over rank within a size-based dominance hierarchy

In social groups, hierarchies are the fundamental organizational unit and integral to the structure of social groups. For many social fishes, rank is determined by body size and conflict over rank is resolved via aggressive threats from dominants and growth restraint by subordinates. However, this balance may be offset by an alteration of abiotic factors, such as elevated temperature expected from climate change, which could thereby disrupt the usual mechanisms of conflict resolution. Here, we determined the effect of elevated temperature on hierarchy structure, stability, and conflict resolution in the Eastern mosquitofish, Gambusia holbrooki. Body size was significantly related to dominance rank, and aggression was more commonly directed toward subordinates and was heightened between individuals of adjacent rank, demonstrating that conflict over rank occurs in size-based hierarchies. Temperature did not affect overall levels or directionality/adjacency of aggression but substantially altered subordinate growth patterns. In only the high-temperature groups, growth rates of subordinates decreased as the size ratio between themselves and their immediate dominant approached 1.0, whereas growth rates of dominants were unaffected. This unique finding suggests that only under high temperatures, subordinates may adopt growth regulation to resolve conflict, when the costs of conflict with dominants are greater. This provides the first causal link between abiotic stressors and changes to hierarchical structure and functioning, providing a springboard for further research into implications of temperature-dependent subordinate growth alteration at higher levels of ecological organization.