Abstract
BACKGROUND: Many animals face some form of conflict over reproductive opportunities. Queen selection in social insect colonies represents a high-stakes conflict where competition occurs among multiple queens for a few or a single reproductive role(s). The outcome of the contest is critical to the fitness of all colony individuals as most are sterile, and thus represents a conflict at multiple levels. Aphaenogaster senilis is a monogynous, monandrous, fission performing ant, in which queen selection occurs during colony fission and when replacement queens are produced to overcome orphaning. First-born queens are usually behaviourally dominant over subsequent queens, and eventually inherit the colony. We investigated the importance of physical dominance in queen selection in orphaned groups by manipulating the fighting ability of first-born queens via mandibular ablation. RESULTS: First emerged queens were heavier than second emerged queens, performed almost all aggression, were behaviourally dominant 92% of the time, and prevailed in 76% of groups after co-existing for 16 days on average. Mandibular ablation had no effect on queen behaviour or contest outcome. CONCLUSION: Aggression is probably ritualised and contests are decided by workers based on relative queen fertility. First-born queens thus have an inherent advantage over second-born queens as they have more time to develop ovaries. Subordinates never retaliated against aggression from dominants and this lack of retaliation can be interpreted as a form of bourgeois strategy as dominants were almost always first-born. However, the lack of alternative reproductive options makes not-fighting effectively a form of suicide. High relatedness between full-sister queens means that subordinates may be better off sacrificing themselves than risking injury to both queens by fighting.