Territoriality in ants revisited: iconic collective displays reflect resource, not territorial defense in meat ants Iridomyrmex purpureus

Abstract

Non-injurious, collective ritualized displays may have evolved in some species of ants as a means of resolving contests for key resources, without causing a drain in worker numbers through injury. Colonies of the Australian meat ant, Iridomyrmex purpureus, deploy numerous workers to engage in collective displays, which are widely understood to be involved in maintaining exclusive territories. A combination of field surveys and behavioral assays revealed that display grounds do not delimit borders that define exclusive territories. Rather, the proportion of workers from a focal colony found in a quadrat declines monotonically with distance from the nest. In addition, we documented collective displays around food trees, where workers congregated in greater densities and engaged in more aggressive behavior. These results refute the assumption that colonies of I. purpureus establish territorial boundaries by collective displays. Rather these collective displays may be related to the defense of specific resources, including food trees and nest sites. The difference in the level of aggression among displaying workers at different locations may reflect a balance between the benefits of defending a particular resource and an unappreciated cost of escalation.