Reeve's (1989) 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. By examining changes in intraspecific aggression among colonies of Argentine ants (Linepithema humile) in various discrimination contexts, we determined that aggression occurs at higher rates when either nestmates or familiar territory indicate nest proximity, but is greatly diminished where social context is 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 non-kin. Starks's et al. (1998) nest indicator and the cost minimizer hypotheses explained to some degree the change in the action component of Argentine ant nestmate discrimination, however, isolated nest referents (familiar territory, conspecific brood, or single familiar nestmates) had no effect on aggression thresholds. We provide mixed support for Tsutsui et al. (2003) proposition that workers from genetically less diverse colonies attack workers from more diverse colonies. We found that in the context of nest defense genetically more diverse colonies will initiate 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.
Species 1: Hymenoptera Formicidae Linepithema humile (Argentine ant)
Keywords: nestmate recognition, aggression assay
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