Behavioural response of workers to repeated intergroup encounters in the harvester ant Messor barbarus

Abstract

The evolution of cooperation in animal societies is often associated with the evolution of hostility towards members of other groups. It is usually predicted that groups under attack from outsiders should respond by becoming more cohesive or cooperative. However, the responses of individuals to real or simulated intergroup encounters vary widely, for reasons that are poorly understood. We tested how groups of workers of the harvester ant, Messor barbarus, responded to exposure to members of a different colony versus members of their own colony, and how previous exposure to an intruder affected the intensity of the within-group response. We found that workers increased in activity and had more contact with one another immediately following exposure to an ant from a different colony, but also showed a similar behavioural response to presentations involving an ant from their own colony. However, exposure to an intruder from a different colony resulted in much stronger behavioural responses to a second intruder, encountered shortly afterwards. Our results are consistent with studies of social vertebrates which suggest that exposure to intruders results in increased social cohesion. Our results also show that exposure to an intruder primes group members to respond more strongly to future intrusions. Our findings highlight a disconnect between the assumptions of theoretical models which study the effect of intergroup conflict on social evolution over many generations, and the short-term behavioural responses that are the usual focus of studies of intergroup conflict in insects and vertebrates.