Context-dependent nestmate discrimination and the effect of action thresholds on exogenous cue recognition in the Argentine ant

Abstract

The optimal acceptance threshold model predicts that kin/nestmate discrimination is context dependent and that, in a fluctuating environment, the action component of nestmate discrimination is plastic, rather than static. We examined changes in intraspecific aggression among colonies of Argentine ants, Linepithema humile , in various discrimination contexts, and found that aggression occurred at higher rates when either nestmates or familiar territory indicated nest proximity, but not when social context was absent, thereby providing additional support for the optimal acceptance threshold model. Context-dependent aggression in the Argentine ant appears to result from a shift in acceptance threshold in response to fitness costs associated with accepting nonkin. The change in the action component of Argentine ant nestmate discrimination was explained to some degree by the hypothesis that the presence of nestmates indicates nest proximity and denotes a fitness payoff for active defence (nest indicator hypothesis) and by the hypothesis that nestmates share the cost of nest defence in groups, but not singly (cost minimizer hypothesis). Isolated nest referents (familiar territory, conspecific brood, or single familiar nestmates), however, had no effect on aggression thresholds. We provide mixed support for the hypothesis that workers from genetically less diverse colonies attack workers from more diverse colonies. We found that, in the context of nest defence, genetically more diverse colonies initiated attacks on colonies with lower genetic diversity. Therefore, the role of asymmetrical aggression in reducing genetic diversity within introduced populations of L. humile remains unknown and other extrinsic factors such as nest status and/or colony size may affect the outcome of aggressive interactions in the field. Finally, our finding that colonies reared under uniform conditions showed diminished intraspecific aggression only when assayed in a social and/or ecological context underscores the importance of using appropriate aggression assays for testing patterns of intercolony aggression in L. humile , and that the use of different rearing regimes, source colonies and collection times may produce contradictory results.