Phylogeographic and environmental correlates support the cryptic function of the zigzag pattern in a European viper

Abstract

The predator–prey relationship is a strong agent of natural selection on phenotype, and two evolutionary strategies derived from this antagonistic interaction are crypsis and aposematism. Although usually considered as opposites, both strategies could be ascribed to the dark zigzag pattern of European vipers (Vipera). Experiments using plasticine models demonstrated its aposematic role, and no evidence had been found regarding a possible cryptic function. We examined the possibility of a cryptic role by measuring five characters related to the zigzag size and shape in 465 Vipera latastei specimens from the Iberian Peninsula to assess geographic variation in these characters. This species shows genetic substructuring resulting from population isolation and occurs in strong environmental gradients, which allows testing whether historic and/or environmental (adaptive) factors explain this variation. Spatial interpolation of zigzag characters identified two major Iberian groups: the Western and the Eastern. The Western group was characterised by a larger zigzag extension and higher number of dorsal marks; specimens within this group were in granitic grounds and areas with higher rainfall and lower solar radiation than those of the Eastern group. The correlation of the zigzag shape and size with lithology and climatic variables suggested that dorsal pattern variation is driven by: (1) its cryptic role, as detectability might be influenced by the degree of contrast between the target and background lithology, or (2) its thermal role, as the larger zigzag may allow for faster heating in Western Iberian regions with limited thermal opportunities. A log-linear analysis using dorsal pattern groups, genetic lineages and lithological classes, showed significant interactions among the three variables. These results suggest that dorsal pattern variation of V. latastei resulted from genetic (i.e. historic) as well as environmental (i.e. adaptive) factors, first by population isolation in geographic refuges and further by local adaptation to particular environments.