IN

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 wit