Octopamine and cooperation: octopamine regulates the disappearance of cooperative behaviours between genetically unrelated founding queens in the ant.

We investigated whether octopamine (OA) is associated with the disappearance of cooperation in Polyrhachis moesta ant queens. Queens of P. moesta facultatively found the colony with genetically unrelated queens. The founding queens perform frequent food exchange with these non-related queens and partake in cooperative brood rearing, whereas single colony queens exclude non-related queens via aggressive behaviour. Thus, aggression is a factor that reduces cooperation. Given that aggression is generally associated with brain OA in insects, we hypothesized that OA controls the behavioural change in cooperation in the ant queen, via an increase in aggression. To test this hypothesis, we compared the amounts of OA and related substances in the brain between founding and colony queens, and observed the interaction of founding queens following oral OA administration. The brain OA levels in colony queens were significantly higher than those in founding queens. Oral administration of OA to founding queens caused significantly less trophallaxis and allogrooming behaviour than in the control founding queens, but with no significant increase in aggression. These results suggest that OA promotes the disappearance of cooperation in founding queens of P. moesta. This is the first study to reveal the neuroendocrine mechanism of cooperation in ant queens.