Arms races and the evolution of big fierce societies

The causes of biological gigantism have received much attention, but only f or individual organisms. What selection pressures might favour the evolutio n of gigantic societies? Here we consider the largest single-queen insect s ocieties, those of the Old World army ant Dorylus, single colonies of which can have 20 million workers. We propose that colony gigantism in Dorylus a rises as a result of an arms race and test this prediction by developing a size-structured mathematical model. We use this model for exploring and pot entially explaining differences in colony size, colony aggression and colon y propagation strategies in populations of New World army ants Eciton and O ld World army ants Dorylus. The model shows that, by determining evolutiona rily stable strategies (ESSs), differences in the trophic levels at which t hese army ants live feed forwards into differences in their densities and c ollision rates and. hence, into different strategies of growth, aggression and propagation. The model predicts large colony size and the occurrence, p ropagation strategy of battles and a colony-propagation strategy involving highly asymmetrical divisions in Dorylus and that Eciton colonies should be smaller, non-combative and exhibit equitable binary fission. These ESSs ar e in excellent agreement with field observations and demonstrate that garga ntuan societies can arise through arms races.