Citizen Science helps to clarify phenotypic variation within and between two subspecies of Natrix helvetica (Squamata: Natricidae)

We examined whether phenotypic differences exist within and between two genetically delimited subspecies of the barred grass snake (Natrix helvetica helvetica and N. h. sicula) using georeferenced photographic records of approximately 3800 individuals and provide morphological diagnoses for each taxon. The nominotypical subspecies is less variable in coloration and pattern compared to the southern subspecies N. h. sicula, which typically has an extensive dark back pattern in addition to the side bars. Individuals with a dorsal pattern resembling asp vipers (Vipera aspis) and tessellated individuals are frequent in N. h. sicula, whereas most N. h. helvetica have a plain-colored back. Rare unique phenotypes occur particularly in the more southern parts of the distribution range of N. h. sicula. This variation matches its complicated mitochondrial phylogeography with multiple glacial refuges in Italy. The similarity of the dorsal pattern of barred grass snakes and asp vipers represents a case of Batesian mimicry. Barred grass snakes with such a back pattern are largely restricted to N. h. sicula, which is distributed in a region where asp vipers are broadly sympatric. In contrast, barred grass snakes with an asp-like pattern are very rare in many regions of France where the two species are sympatric as well. This suggests that the mimicry system had significantly more time to evolve in the Italian peninsula due to a longer shared distribution history of the two species.