Sex-Specific Morphological Responses to Urbanisation Modulate Geographic Clines in Pterostichus melanarius (Coleoptera, Carabidae)

Urbanization and macroclimatic geographic gradients exert distinct selective pressures on the morphology of widespread insect species. However, the combined effects of these factors on sexual dimorphism remain under-explored. This study investigated variations in body size and shape, and sexual dimorphism, in the ground beetle Pterostichus melanarius (Illiger, 1798) across a three-level urbanization gradient (rural, suburban, and urban). Linear morphological traits were extracted and analyzed for 4,616 specimens collected across nine regions spanning Europe and Russia. Morphological trajectories were established using a Random Forest machine learning approach for sex prediction, alongside Principal Component Analysis to separate general size from size-independent shape. We used a Random Forest machine learning approach to establish morphological trajectories for sex prediction, alongside Principal Component Analysis to separate general size from size-independent shape. We observed persistent female-biased dimorphism in both size and shape, with scaling occurring isometrically between sexes across populations. Overall body size increased significantly as populations transitioned from rural to urban habitats. However, this size increase was severely dampened in males within the most urbanized areas, resulting in wider sexual size dimorphism. Furthermore, human-altered habitats caused a widespread shift in population shape towards wider, larger-headed phenotypes, a transformation driven disproportionately by females. These statistically significant differences suggest that, while broad geographic clines establish baseline morphology, localized urbanization gradients drive rapid, sex-specific phenotypic adaptations. Ultimately, females exhibit greater morphological plasticity in response to urbanization pressures.