Pre-existing differences in putative fertility signals give workers the upper hand in ant reproductive hierarchies

Abstract

In social groups, competition often gives rise to conflicts, which are regulated through a variety of mechanisms. In several social insect species, the conflict for male production that takes place between workers after queen loss, is regulated through the establishment of a reproductive hierarchy. A recent study of Neoponera apicalis showed that workers differ in their fertility levels in the presence of the queen and proposed that such idiosyncratic differences might influence access to the top of the hierarchy after queen loss. In this study, we therefore sought to characterize the influence of the initial heterogeneity in ovarian development and its chemical and behavioural correlates on the establishment of reproductive hierarchies among orphaned workers, which can only produce males. We monitored the chemical profile before and after hierarchy establishment in four groups of orphaned workers of N. apicalis morph 6. The analysis of the cuticular profiles showed that tricosane (n-C23) was highly correlated with ovarian development and could consequently act as a fertility signal in this ant. The relative amount of tricosane on the cuticle, both before and after the establishment of the hierarchy, was also correlated with the rank achieved within the hierarchy and with the expression of agonistic behaviours. Thus, our study experimentally shows that idiosyncratic differences in a putative fertility signal (and therefore presumably in ovarian activity) between workers in the queen's presence reliably predict the outcome of reproductive conflict after queen loss. We propose that this signal (together with an increased agonistic motivation of the more fertile workers) could play a major role in the regulation of dominance/submission behaviours, enabling the most fertile individuals to rapidly access top ranks and monopolize reproduction, thereby maximizing the global reproductive success of all colony workers while minimizing the costs associated with the expression of agonistic behaviour.