Ecological significance of dorsal polymorphism in a Batesian mimic snake.

Abstract

Batesian mimicry is the process in which harmless species adopt the appearance of a dangerous, aposematic species. In some prey species, both Batesian mimetic and non-Batesian morphs coexist, presupposing that both morphs have to be evolutionarily advantageous. The viperine snake, Natrix maura, exhibits a zigzag dorsal pattern and antipredatory behavior that mimics European vipers. This snake also has a striped dorsal pattern that coexists with the zigzag pattern. We have examined whether individuals belonging to different geographically structured clades were more likely to exhibit a certain dorsal pattern, and whether the zigzag pattern has a protective function by exposing artificial snakes to predation in natural environments, in addition to comparing antipredatory behavior between zigzag and striped snakes also in natural environments. Our results indicate that the striped pattern was not geographically structured, but habitat-dependent. Aerial predators less frequently attacked zigzag plasticine models than striped or unpatterned models. We detected a shift in antipredator behavior between the 2 morphs, as Batesian mimicking N. maura responded to an approaching potential predator by remaining immobile or fleeing at shorter distances than did striped ones. We conclude that Batesian mimics maintain the cryptic and aposematic value by resembling vipers, whereas in open habitats the non-Batesian mimic has altered its antipredator behavior to maintain its fitness.