Coevolutionary arms races in Batesian mimicry? A test of the chase-away hypothesis

Reciprocal selection on harmless Batesian mimics and their defended models has long been hypothesized to spawn coevolutionary arms races. Mimics are thought continuously to experience selection to resemble their models better. Models are thought continuously to experience ‘chase-away’ selection for phenotypes that let them escape from these ‘parasitic’ mimics, especially when the cost of having mimics is high. Although abundant evidence suggests that models affect the evolution of mimics, evidence that mimics affect selection of models is meager in natural systems. Here, we tested whether mimics affect selection of models in a snake mimicry complex. Specifically, we tested whether models show phenotypic signatures of chase-away selection in regions that vary in levels of mimetic parasitism and whether models experience chase-away selection in regions where there are many mimics. Contrary to the predictions of the chase-away hypothesis, we found no evidence that models exhibit phenotypic signatures of chase-away selection or that models experience either directional or disruptive selection favouring extreme phenotypes. The absence of chase-away in a mimicry complex that appears to be primed for it suggests that chase-away in natural systems might be weak, rare or of a different character than has been predicted by theoretical studies of Batesian mimicry.