Geographic variation and allometry shape head morphology more than diet in the Mexican black-bellied garter snake ( Thamnophis melanogaster )

In snakes, head morphology is closely linked to feeding ecology, particularly in aquatic systems where prey capture imposes strong functional constraints. Here, we examined head shape variation in the Mexican black-bellied garter snake ( Thamnophis melanogaster ) across four populations with contrasting diets (piscivorous vs. non-piscivorous) using geometric and linear morphometric approaches. Contrary to our initial hypothesis, geometric morphometric analyses revealed that head shape variation is primarily structured by geographic differences among populations rather than diet. Significant differences in head shape were detected among localities ( p < 0.001), with Chapala representing the most distinct population, while no significant effect of sex was detected. Allometry had a significant effect on head shape, but the allometric trajectories were consistent across populations. In contrast, linear morphometric analyses showed that although head size scales similarly to body size across dietary groups, relative head size differs significantly between piscivorous and non-piscivorous individuals ( p < 0.01), with non-piscivorous snakes exhibiting proportionally larger heads. These results indicate that diet does not drive major differences in head shape but does influence relative head proportions. Overall, our findings highlight the complex interplay between geography, allometry, and diet in shaping morphological variation in T. melanogaster , suggesting that multiple ecological factors beyond prey type contribute to head morphology in this species.