Conflict resolution strategies in meat ants (Iridomyrmex purpureus): ritualised displays versus lethal fighting

Our understanding of the evolution of display as a conflict strategy derives from extensive theoretical and empirical studies of non-social animals. Surprisingly, such an approach has seldom been adopted for the study of social insects. In this study, we explored the circumstances under which meat ants (Iridomyrmex purpureus) display or engage in potentially lethal fights. Colonies of meat ants establish territories in which the boundaries are lined by workers engaged in ritual displays. Although more common, ritual displays are not the only means of resolving conflicts in meat ants; sometimes fights are fierce and result in injury or death. We show, using field experiments, that when two ants engage in fierce fighting, it is most often the territory-defending ant which initiated the behaviour, while ritual displays are more often initiated by the intruding ant. The fighting behaviour of the ants was not influenced by the absolute or relative size of the combatants, nor by the presence or absence of large, temporary food sources. Mandibles of fighting ants collected on conflict zones had a higher degree of wear than those of foragers, suggesting that fighting ants are the older, more dispensable individuals of the colony. Our data indicate that factors in addition to nestmate recognition may mediate the outcome of confrontations in social insects.