Citizen Science, taxonomy and grass snakes: iNaturalist helps to clarify variation of coloration and pattern in Natrix natrix subspecies

We used a dataset of georeferenced photos of 5,751 grass snakes from iNaturalist to evaluate subspecific variation of Natrix natrix in coloration and pattern. Our results provide evidence that all four genetically delineated subspecies differ morphologically, although unstriped individuals of N. n. vulgaris are difficult to tell apart from the nominotypical subspecies. The iNaturalist dataset shows that the frequency of dark body coloration increases from south to north and from west to east. This trend is both concordant with taxonomic variation (the easternmost subspecies, N. n. scutata, being the darkest taxon) and variation within the same subspecies (in N. n. natrix and N. n. scutata more northern populations harbor more dark or melanistic individuals than more southern populations). Although available characters were limited to coloration and pattern traits, our study suggests that photo material from iNaturalist and similar platforms can be a valuable data source for studies on morphological variation. However, investigations using such databases can only supplement, but not replace, studies using museum material because only then measureable, meristic and genetic characters will be accessible.

Automated summary

Using a dataset of georeferenced photos from iNaturalist, we evaluated the subspecific variation of Natrix natrix in coloration and pattern. Our findings show that all four genetically delineated subspecies have morphological differences, although distinguishing unstriped individuals of N. n. vulgaris from the nominotypical subspecies is challenging. The dataset reveals an increasing frequency of dark body coloration from south to north and west to east, which aligns with both taxonomic variation and variation within the same subspecies. While limited to coloration and pattern traits, our study highlights the value of photo material from platforms like iNaturalist for researching morphological variation.