The common adder (Vipera berus) is a polymorphic species characterized by specimens having a contrasting zigzag pattern as well as completely black (melanistic) specimens. Previous research has shown that melanistic coloration conferes a thermoregulatoty advantage and a higher risk of predation, so it is often the dominant phenotype in environments with suboptimal thermoregulatory conditions, such as higher altitudes. The color polymorphism of the common adder in Slovenia has not yet been systematically studied. In this work, we analyzed the spatial distribution of color phenotypes of the common adder in relation to ecological conditions in nature. We assumed that in Slovenia, in open space with low and poorly covering vegetation, there is stronger pressure from predators, especially birds. We hypothesized that in such an environment, more covert individuals with the contrast zigzag pattern would be more successful. In such environments, melanistic individuals may be present but rare. In contrast, in a shaded, more covered habitat, such as a forest, we expected more melanistic individuals or their dominance. In a shaded area, they have an advantage over patterned phenotypes because of better thermoregulation, and furthermore, there are often fewer opportunistic predatorsin such habitats, so there is less selection pressure from predators. We analyzed 76 animals from 5 localities and found support for our predictions.
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