Intraspecific competition influences the symmetry and intensity of aggression in the Argentine ant

Cooperative social groups rely on the ability to distinguish members from nonmembers. Accordingly, social insects have evolved a variety of systems that allow discrimination of nest mates from non–nest mates. In this study, we show that experience can modify patterns of intraspecific aggression in Argentine ants (Linepithema humile). In laboratory experiments, we found that aggression between colonies was often asymmetrical, but in all five cases, this asymmetry shifted to symmetrical aggression after contact with a hostile colony. Moreover, in the field, aggression between workers collected from colony borders was symmetrical, whereas polarized aggression occurred between workers collected 500 m away from colony borders. Coinciding with this shift in aggression symmetry, we also observed an increase in both the overall level of aggression and the frequency of aggression in both the field and laboratory bioassays. We found little evidence for colony-level competitive asymmetries stemming from polarities in aggression at the worker level, either in the laboratory or in the field. These results illustrate that recognition systems in Argentine ants are surprisingly dynamic and provide experimental evidence for how recognition can be adjusted in response to specific circumstances—in this case the presence of intraspecific competitors.