Batesian mimicry evolves when the 'umbrella' of protection provided by resemblance to a conspicuous unpalatable model species is sufficient to overcome increased predation risk associated with greater conspicuousness. However, the shape and extent of this umbrella, that is, how the level of protection provided by mimicry changes with degree of resemblance between model and mimic, is poorly known. We investigated the response of wild predatory fishes to plastic replicas of a model-mimic species pair of tropical reef fishes, Canthigaster valentini (a toxic pufferfish, the model) and Paraluteres prionurus (the putative mimic), and additional replicas with progressively lower degrees of resemblance to the mimic species. Our results reveal a relatively broad region of protection, indicated by a reduced approach rate by piscivorous fishes, surrounding the colour pattern of the model species. Protection increased with increasing resemblance. By contrast, the response of non-piscivorous fishes was unrelated to degree of resemblance of replicas to the model. Our results suggest that piscivorous fishes on the reef are educated regarding the toxicity of C. valentini, and that avoidance of fish having the pufferfish colour pattern has generated selection favouring mimetic resemblance by the palatable P. prionurus. The relatively broad protective umbrella has probably facilitated the initial evolution of resemblance in the palatable prey species despite the potential hazards of greater conspicuousness.
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