Forewarned is forearmed: aggression and information use determine fitness costs of slave raids

Many animals use reliable indicators of upcoming events such as antagonistic interactions to prepare themselves. In group-living animals, not only the cue perceiving individuals are involved in mobilization, but the entire group can use this information. In this study, we analyze whether social insects, which perceive reliable information on an upcoming social parasite attack, can use this knowledge to better defend their colony. We focus on the interaction between the ant Temnothorax longispinosus and the slave-making ant Protomognathus americanus, which conducts destructive raids on host colonies to steal their brood. As a behavioral defense, host colonies show aggression, which has a constitutive and inducible component: earlier studies demonstrated both behavioral consistency and an increase in aggression after slavemaker contact. Here, we analyze the fitness benefits of aggression in the context of slave raids. Aggression only facilitated host colony defense if the hosts were informed about a pending slave raid: slavemaker scouts were not only less likely to enter forewarned host colonies, but more aggressive colonies who previously encountered a slavemaker saved a higher fraction of their brood. We thus demonstrate that the fitness costs of raids depend on an interaction between the collective defense abilities and reliable information on impending dangers.