Colony dispersal and the evolution of queen morphology in social Hymenoptera

Social Hymenoptera show two contrasting strategies of colony reproduction. A reproductive female can raise the first generation of brood alone (independent foundation), or a colony can divide into autonomous parts in which the reproductive female is helped by sterile relatives (fission, budding, swarming). In independent-founding ants, queens can histolize their flight muscles after dispersal; in many species, large flight muscles and metabolic reserves reduce or eliminate the need for risky foraging trips during the vulnerable solitary stage. Colony division is a derived strategy, and we review the selective pressures leading to its occurrence in the different social taxa. In various ants, fission coexists with independent foundation, and alate queens are retained. However, in ants exhibiting obligate fission (e.g. all army ants and many Ponerinae), queens are permanently wingless (ergatoid), or the queen caste is missing altogether. When reproductive females are flightless, dispersal distances and colonization ability are reduced, and there are extensive modifications in mating behavior and resource allocation. We focus on the characteristics of fission in the phylogenetically primitive ants Ponerinae in which both ergatoid queens and gamergates occur. The ground-living habits of ants have permitted extensive changes in the phenotypes of their reproductive females, unlike in wasps and bees.