Arms races and the evolution of big fierce societies

Graeme Boswell, Nigel R Franks, Nicholsa F Britton

Research output: Contribution to journalArticlepeer-review


The causes of biological gigantism have received much attention, but only for individual organisms. What selection pressures might favour the evolution of gigantic societies? Here we consider the largest single-queen insect societies, those of the Old World army ant, Dorylus; single colonies of which can have 20 million workers. We propose that colony gigantism in Dorylus arises as a result of an arms race and test this prediction by developing a size-structured mathematical model. We use this model to explore and potentially explain differences in colony size, colony aggression and colony propagation strategies in populations of New World army ants, Eciton and Old World army ants Dorylus. The model shows by determining Evolutionarily Stable Strategies that differences in the trophic levels at which these army ants live feed forwards into differences in their densities and collision rates and hence into different strategies of growth, aggression and propagation. The model predicts large colony size and the occurrence 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 are in excellent agreement with field observations, and demonstrate that gargantuan societies can arise through arms races.
Original languageEnglish
Pages (from-to)1723 - 1730
Number of pages7
JournalProceedings of the Royal Society of London B: Biological Sciences
Issue number1477
Publication statusPublished - 22 Aug 2001


  • arms race
  • mathematical model
  • dorylus
  • eciton
  • evolutionarily stable strategies
  • gigantism


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