Regulation of queen development through worker aggression in a predatory ant

Extreme developmental plasticity within the eusocial insects defines the separation between the queen and worker castes. The switch between these 2 developmental pathways is thought to be under tight social control. Although a large emphasis has been placed on the effect of larval nutrition on caste determination, workers of many species have no direct control over larval feeding. This may be particularly relevant to the early evolution of ants, when behaviors that allow fine control over larval nutrition, such as mouth-to-mouth food exchange between larvae and workers, were probably not yet present. We investigated larval-directed aggression as an alternative means to regulate queen development in the ant Harpegnathos saltator, a species that retains ancestral characteristics. We tested worker response toward natural queen-destined larvae and larvae induced to develop as queens using a juvenile hormone analog (JHA). Workers from colonies that were not rearing queens bit queen-destined larvae, whereas worker-destined larvae were not attacked. When colonies were naturally rearing queens, workers did not bite JHA-treated larvae, and a larger proportion of these larvae developed into queens compared with larvae that received biting. This supports the hypothesis that workers of H. saltator use biting to inhibit queen development during periods when the conditions for queen rearing are not met. We propose that mechanical stress, in addition to nutrition, could serve as a mechanism to regulate queen development in species that lack fine control over larval food consumption, and this may have played a role during the early evolution of eusociality in ants.