Collective displays as signals of relative colony size: meat ants, Iridomyrmex purpureus, are economical with the truth

Abstract

Signals of resource-holding potential in dyadic (one-on-one) contests are relatively straightforward, typically reflected in the attributes of the individual, including body size and resource ownership. However, conveying this information is considerably more complex in social species: the outcome of collective contests will be influenced by both individual and group level effects, and the signal must be subsequently transmitted to the colony in order to elicit a colony response. Australian meat ants engage in collective displays where aggregations of many hundreds of pairs of workers from neighbouring colonies antennate one another while adopting a tip-toe posture with the abdomen raised. These displays rarely escalate into physical contests with their typically fatal consequences. A display between two workers may cease by one worker leaving or if one displaying worker is replaced by a nestmate. We asked whether these collective displays act to convey reliable information about relative colony size. Our field experiments and observations reveal that these displays do not necessarily convey precise information about relative colony size, and that it is possible for colonies to signal an exaggerated colony size, at least in the short term.