Comparative morphology of three viper species from Northern Italy: ecological convergence and taxonomical implications of head and hemipenial morphology

The suitability of morphological characters to assess phylogenetic relationships and taxonomic identities have been much debated. However, they are often reliable to investigate patterns of variability in quantitative traits (i.e., morphometry). With this respect, head shape in snakes has often been regarded as a key morphometric character that is tightly involved in selection and evolution. In Northern Italy, species of the genus Vipera are relatively common along the Alps; more than one species can be detected, among which V. walser has been recently elevated to species rank, although its taxonomic status is still matter of debate. We used geometric morphometry to study variability in the shape of the head between Vipera walser, V. berus and V. aspis, to investigate patterns of sexual dimorphism and to verify the consistency of the morphological data with the current taxonomic identification of V. walser. Additionally, we described hemipenial morphology to discriminate the species under the perspective of reproductive isolation. We found no significant difference in head shape among the three species, with the head of male V. walser slightly matching that of V. berus more than V. aspis. On the opposite, the three species share a common pattern of sexual dimorphism in head shape, with females showing shorter and larger heads compared to males. Hemipenial morphology is consistent with GM analysis, although differences in hemipenal ornamentations and general morphology are even less marked than in head shape. V. berus and V. walser are more similar one another than to V. aspis, which is consistent with present day phylogenetic reconstructions. Our results suggest that the distinction of V. walser from V. berus cannot be robustly supported according to morphological data and that further molecular investigations are essential to determine the evolutionary history of V. walser.