Cooperative fighting in the termite Nasutitermes corniger: a test of Lanchester’s models of combat
Elizabeth Clifton, Elizabeth Clifton , Eldridge Adams
Department of Ecology and Evolutionary Biology, University of Connecticut, USA; Department of Ecology and Evolutionary Biology, University of Connecticut, USA ; University of Connecticut, USA
Competition among groups is one of the major selective forces shaping the behavior of social insects. We tested the applicability of Lanchester’s models of combat to territorial battles of the termite Nasutitermes corniger, a species that naturally engages in large inter-group conflicts. Lanchester’s square law, which states that the fighting ability of a group is proportional to the square of its numbers, has been proposed to apply broadly to social animals that fight in the open where the larger group can concentrate attacks on the smaller group. To test this model, marked termites from three pairs of colonies were placed together in different initial ratios of numbers, ranging from 10:40 to 40:10. Replicate encounters were staged with workers and soldiers separately and in combination. We recorded mortality at frequent intervals and scored rates and positions of attacks from video recordings. Workers were far more effective than soldiers at killing opponents in these encounters. In worker-worker battles, group size had a disproportionate effect on mortality rates: when a focal colony’s workers constituted a lower fraction of the termites present in the battle, they suffered a higher fraction of the deaths (glmm; P < 0.001). This effect was not due to a progressive increase in the concentration of attacks, as assumed by Lanchester’s model. Instead, when a colony was in the minority role, they received a higher rate of attacks directed towards the sides and rear of the body, where they are most vulnerable. Workers in the majority role were more often able to meet their opponents head-on. Thus, the killing power of an individual is enhanced when its nestmates simultaneously attack the same opponent from different directions. We term this the “synergistic attack hypothesis” and propose that similar effects underlie battle tactics and outcomes in other aggressive social animals.