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The Optimal Caste Ratio in Polymorphic Ants: Estimation and Empirical Evidence

A simple model is proposed for estimation of the optimal caste composition in polymorphic ants. The model gives the optimal caste composition as the highest point on a hypersurface that represents the total efficiency of the colony as a whole. The hypersurface is the product of all partial efficiency surfaces for all tasks. The model was applied to the dimorphic ant Colobopsis nipponicus to test its utility. The ergonomic efficiency of the colony as a whole was estimated from the quantitative ability of each caste to perform each role. When all partial functions of the major workers were included in the model, the predicted optimal ratio closely reflects the observed caste ratio in the field. In addition, the pattern of changes in brood productivity upon experimental manipulation of the caste ratio was consistent with the pattern predicted from the model. These results suggest that the model is useful for estimations of the optimal caste ratio and that the caste ratio in field colonies is tuned to maximize the ergonomic efficiency of the colony as a whole through colony-level selection.