Slave-making ants prefer larger, better defended host colonies

Abstract

Collective decision making is important for social insects living in highly organized societies. However, often only a few individuals acquire information relevant for the entire colony. In the slave-making ant Protomognathus americanus, single scouts search for colonies of their Temnothorax hosts, which are subsequently attacked by a group of raiding workers. Scouts and raiders risk being killed by host workers defending their colony. Considering both the raiding risk and the potential benefit, that is, host pupae that could be taken to serve as slaves after eclosion, scouts must decide whether or not a discovered host colony is worth attacking. We investigated the occurrence of slave raids, host colony assessment and the final decision making by P. americanus colonies during choice trials in the laboratory. We confronted slavemaker colonies with two host colonies of different demographic composition to analyse which host colony attributes are considered during the scouts’ decision process. Slavemaker colonies showed increased raiding activities when the slave to slavemaker ratio inside the slavemaker colony was low. Slavemakers did not favour host colonies with more pupae, but preferentially attacked colonies with more workers. These represent riskier raiding targets, but as larger colonies usually contain more brood in the field, the increased benefit may necessitate fewer raids, decreasing the total risk during a raiding season. However, confronted with two host colonies that showed more distinct benefit to risk ratios, their decision shifted. Thus raiding behaviour and decision making in P. americanus are affected by a combination of external and internal stimuli.