Spatial Structure Explains Morphological Variation Better Than Climatic Gradients in the South American Rattlesnake...

Morphological variation in vertebrates is often shaped by geographic and climatic factors, yet the applicability of broad-scale ecogeographical rules remains debated, particularly in ectotherms. We investigated how climatic gradients and spatial structure influence body size in the South American rattlesnake (Crotalus durissus), a widely distributed species across diverse ecoregions. We analyzed morphometric data from 132 adult individuals, integrating 19 bioclimatic variables from WorldClim and geographic coordinates for each collection site. Body size variation was summarized using principal component analysis, and spatial autocorrelation was explicitly incorporated into the analytical framework. After accounting for spatial structure, climatic variables were not significant predictors of body size in either sex. Instead, males exhibited a latitudinal spatial trend, whereas females showed a structured southwestward decrease in size. These results indicate that spatial constraints and sex-specific ecological pressures, rather than contemporary climatic gradients, shape morphological variation in C. durissus. Our findings highlight the importance of incorporating spatially explicit models when evaluating climate-trait relationships and contribute to a more nuanced understanding of morphological evolution in widespread ectothermic species.