Interpopulation morphological differences and sexual dimorphism of Dekay’s brownsnake (Storeria dekayi) along a rural–urban gradient

In response to the surge of urbanization in the modern era, many organisms have undergone various changes, such as the shift of their morphological traits to face the challenges brought by this drastic environmental transformation. Rapid adaptive evolution in the morphology of urban-dwelling organisms has been documented in a broad array of taxa, such as lizards and birds, by comparing urban populations with their nonurban counterparts. However, relevant studies concerning more elusive and secretive organisms that also occur in both natural and urbanized habitats (e.g., snakes), are still lacking. Snakes lack appendages, which are often the trait of interest in other morphological studies, but factors such as head shape play a critical role in snakes, as it determines the prey size of these gape-limited predators. In this study, we apply both linear and geometric morphometric analyses to examine interpopulation morphological differences and sexual dimorphism in a small, semi-fossorial snake, Dekay’s brownsnake (Storeria dekayi). We focus on head shape in six different populations across the rural-urban gradient in New Jersey and New York, USA. We find evidence of increased morphological divergence and decreased sexual dimorphism in populations inhabiting more urbanized areas. Our study suggests the occurrence of an adaptive morphological shift in this common species in the urban environments, and lays the path for further investigation of urban adaptation in snakes and similar secretive species.