Structural Inequities and Chronic Disease in Fresno County: A Narrative Review and Multi-sectoral Framework.

Because obtaining food resources is essential for survival, the diet and morphological phenotype of snakes are closely linked (; ; ). Prey characteristics can induce morphological differences that often reflect phenotypic plasticity rather than underlying genetic divergence (; ; ). Such plastic responses in feeding-related morphology have been widely documented across animal taxa, frequently in relation to prey size, shape, defenses, and other ecological attributes (; ; ). Snakes represent an ideal system for studying the relationship between morphology and feeding ecology because, as gape-limited predators, the head plays a central role in prey capture and ingestion (; ; ; ). Since snakes consume their prey whole, head dimensions and cranial structure are expected to be closely associated with prey characteristics such as size, shape, and consistency (; ; ). Indeed, several studies have reported direct relationships between head morphology and diet in snakes (; ; ). In addition to diet, intraspecific geographic variation has been recognized as an important source of morphological diversity in snakes (; ). Variation in head dimensions among populations may reflect phenotypic plasticity in response to feeding experience but may also be driven by broader ecological differences among habitats (). For example, natricine snakes consume a wide range of soft-bodied prey, including earthworms, amphibians, and fish (; ; ), and head morphology in these taxa has been linked to feeding specialization (; ; ; ). In particular, elongated and slender head shapes have been proposed as adaptations for consuming soft or elongated prey such as fish or slugs (; ; ). Piscivory, in particular, imposes distinct functional demands compared to feeding on amphibians or other prey types. Capturing fish in aquatic environments requires overcoming hydrodynamic constraints in a dense and viscous medium, which may influence the evolution of cranial morphology (; ; ). For instance, piscivorous snakes often exhibit morphological traits such as elongated skull elements or modified quadrate orientations, which are thought to enhance prey capture efficiency and ingestion performance (; ). Similarly, increases in head length or related structures may be associated with greater gape and improved prey handling capabilities (). These patterns suggest that diet, particularly piscivory, could play a significant role in shaping head morphology. Sexual dimorphism is another factor frequently associated with variation in head morphology in snakes. In many species, females tend to have relatively larger heads than males do, which has been linked to differences in prey size and feeding ecology (; ; ; ). However, the extent to which sexual dimorphism interacts with diet and geographic variation in shaping head morphology remains unclear and may vary among species. Although diet has traditionally been considered a primary driver of morphological variation, populations inhabiting different environments often exhibit distinct morphologies that reflect a combination of ecological factors, including habitat structure, resource availability, and predator–prey interactions (; ). Consequently, disentangling the relative contributions of diet and geographic variation is essential for understanding the drivers of morphological diversification. In this study, we examined head morphology in the Mexican black-bellied garter snake (), a species that exhibits marked geographic variation in diet. Some populations are predominantly piscivorous, whereas others consume a broader range of prey, including amphibians, leeches, and crayfish (; ). Previous work has demonstrated that this species can exhibit morphological plasticity in response to dietary shifts, particularly in relation to novel prey such as crayfish (; ), making it an ideal system for evaluating the relationship between diet and morphology. To disentangle the relative roles of diet, geography, and sex in shaping head morphology, we analyzed the variation in head shape and size across four populations of with contrasting diets (piscivorous non-piscivorous). Specifically, we tested whether head morphology is primarily structured by dietary differences or by geographic variation among populations. We predicted that, if diet is the dominant factor, piscivorous populations would exhibit distinct head shapes compared to non-piscivorous populations. Alternatively, if geographic factors play a stronger role, morphological variation would be structured primarily by locality rather than diet.