The dimorphic queens of Polyrhachis australis (Hymenoptera: Formicidae)

Abstract

AbstractThe formicine weaver ant Polyrhachis australis Mayr is one among many species of ants that have queens of two more or less distinct sizes, usually referred to as microgynes and macrogynes. The current consensus explains this phenomenon in terms of alternative strategies of dispersal and reproduction: microgynes typically walk to found new nests locally, macrogynes typically fly to found new nests farther afield. This study addressed four questions bearing on that interpretation: is the dimorphism complete (without intermediate sizes) or continuous in P. australis? Does the proportion of macrogynes correlate with the number of queens in the nest? Does the microgyne/macrogyne proportion vary over time? And are brachypterous individuals always microgynes? The implications of these questions concern the fitness status of the intermediate size individuals, the relative fitnesses of the two main morphs as the number of queens diminishes, the influence of habitat conditions on the production of the morphs, and the dispersal behaviour of microgynes. The size distribution of P. australis queens was incompletely bimodal, with some individuals of intermediate size. The proportion of macrogynes in nests increased with decreasing numbers of queens in nests, suggesting that macrogynes do better in whatever contests take place between queens during the ergonomic stage. Between 2009 and 2014, the proportion of macrogynes in nests fell from about 1 in 3 or 4, to less than 1 in 11, perhaps reflecting a differential production of queen morphs in saturated and unsaturated habitats: it profits a colony to produce more microgynes and fewer macrogynes when local conditions favour the easy founding of new nests by walking. Brachyptery was centred on the smallest microgynes, consistent with their assumed mode of dispersal for which wings are not needed.