Ritualised fighting in Iridomyrmex purpureus
The meat ant species Iridomyrmex purpureus is one the most abundant and obvious ants across much of south-eastern Australia. They form large pebble-covered nests, with individual colonies consisting of numerous individual nests, these nests often connected by well-worn "highways". Colonies normally consist of about 7 individual nests but this can vary from only a single nest up to over a dozen nests.
Workers of the meat ant establish territorial boundaries with adjoining colonies by prolonged confrontations involving ritualised fighting. The stereotyped sequence of events in the ritual is described. It is suggested that steps in this sequence are derived from solicitation for food. The energetic and evolutionary significance of ritualised fighting is discussed.
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.
In polydomous ants, individuals belonging to a single colony occupy a variable number of neighbouring nests. Polydomy is frequently associated with polygyny and species are often both facultatively polydomous and facultatively polygynous. In this study we test the generality of this association by investigating the genetic and spatial structure of polydomous colonies of Iridomyrmex purpureus in New South Wales, Australia. Genetic analysis of 15 colonies revealed high relatedness within all but one of the colonies, indicating that the workers are mostly produced by one, singly inseminated queen. Polydomy in this population therefore is not associated with polygyny. Intriguingly, our behavioural data suggests that the colony with low within-colony relatedness had been recently formed by colony fusion. While genotypes were not distributed homogenously throughout this newly formed colony, there was an obvious exchange of genotypes between the nests of the two former colonies. During 2 years of field observations in which we observed 140 colonies comprising over 1000 nests, we observed colony fusion only twice. We discuss these findings in relation to the current theories on the relationship between polydomy and polygyny.
- van Wilgenburg, E., Mulder, R.A., Elgar, M.A. 2006. Intracolony relatedness and polydomy in the Australian meat ant, Iridomyrmex purpureus. Australian Journal of Zoology, 54, 117–122.
Ettershank, G., Ettershank, J.A. 1982. Ritualised fighting in the meat ant Iridomyrmex purpureus (Smith) (Hymenoptera: Formicidae). Journal of the Australian Entomological Society, 21, 97-102.
van Wilgenburg, E., van Lieshout, E., Elgar, M.A. 2005. Conflict resolution strategies in meat ants (Iridomyrmex purpureus): ritualised displays versus lethal fighting. Behaviour, 142, 701-716.