The interplay of geography and urbanization shapes morphological variation in the common toad, Rhinella arenarum

Abstract

Urbanization influences responses of organisms differently compared to non-urbanized environments, but the patterns of these responses across different geographic contexts remain poorly understood. In this study, we employed a geographically replicated design across 32 sites in two countries—Argentina and Uruguay—to examine how the morphology of the common toad, Rhinella arenarum, varies in relation to spatial predictors, environmental factors, and urbanization. We characterized each sampling site using a composite urbanization index and collected 16 morphometric traits from 360 adult toads (255 males, 105 females). We used Random Forests to identify sex-discriminant traits, linear mixed models to assess the influence of urbanization, and variance partitioning to clarify the contributions of environmental, spatial, and urban components to morphological traits. We found a marked difference between the sexes in spatial patterns: male morphology was significantly spatially structured, whereas female morphology was not. Results revealed that environmental gradients, particularly water availability, were the primary correlates of morphological variation. Urbanization acted as a significant secondary correlate, exhibiting sex-specific and context-dependent influences. Males showed nonlinear changes in both snout-vent length and body weight along the urbanization gradient, whereas females did not. Both sexes, however, exhibited distinct changes in head morphology and limb dimensions. Despite geographic heterogeneity in morphological responses, male foot length and female tibia–fibula length showed consistent and parallel responses among cities. We suggest that urbanization does not override broad-scale ecological patterns but interacts with them, producing both context-dependent and parallel morphological responses. This interplay highlights the role of phenotypic plasticity in facilitating the persistence of R. arenarum in urban ecosystems.