Hudry, D. | Search bioRxiv

bioRxiv2025

Divergent paths, convergent heads: morphological adaptation of head shape to ecological niches in snakes.

Morphological convergence--where distantly related species evolve similar traits in response to shared ecological pressures--is a hallmark of adaptive evolution. In snakes, head shape is a key functional trait linked to habitat use and diet, yet the extent and drivers of its convergence across habitat types and diet remain poorly understood. Here, we used geometric morphometric analyses to analyze dorsal head shape from photographs of over 266 snake species, representing [~]5% of global snake diversity. Our analyses reveal that head shape variation is only weakly structured by phylogeny and allometry, but shaped by ecological specialization. Morphospace patterns reflect distinct differences in species with different ecologies: fossorial species exhibit compact, posteriorly wider heads suited for burrowing; aquatic species show streamlined profiles for hydrodynamic efficiency; and arboreal snakes tend to possess elongated heads for maneuvering in complex habitats. Terrestrial and semi-fossorial snakes display broad morphospace overlap and an elevated shape disparity highlighting morphological versatility. While diet covaried significantly with head shape, species with similar diets did not exhibit strong morphological convergence. These findings underscore the dominant role of ecology over phylogeny in shaping the evolution of head shape in gape-limited predators like snakes.

evolutionary biology↗

bioRxiv2025

From head to tail: does habitat use drive morphological variation in snakes?

Convergence is a hallmark of adaptive evolution, yet its extent across different taxa remains unclear. Snakes, with over 4,100 species worldwide, provide a unique model to explore how habitat use influences morphology in a group with an at first sight uniform body plan. We here quantified body, head, and tail shape in over 400 species ([~]10% of the global snake diversity) to assess the role of habitat use in shaping morphological variation. Our results reveal significant differences across ecological groups, with habitat use acting as a key driver of phenotypic diversity. Terrestrial species display the highest morphological diversity in contrast to other habitat groups which appear morphologically specialized. For example, whereas arboreal and semi-arboreal species exhibit elongated heads and slender necks, aquatic and semi-aquatic snakes share streamlined bodies and narrow heads. Fossorial and semi-fossorial species, on the other hand, have compact bodies. Surprisingly, morphological convergence remains limited to arboreal, semi-arboreal, and terrestrial habitat groups. Thus, despite strong functional constraints, evolutionary convergence in fossorial and aquatic species is weak, indicating multiple adaptive solutions rather than a single morphological trajectory. Morphological disparity patterns show that non-specialist ecologies generally exhibit greater disparity than highly specialized ones, in accordance with the need of these species to move in different habitats. Our findings underscore the role of ecological constraints in shaping snake morphology and highlight the complexity of adaptation beyond strict convergent evolution. Lay summaryThe living environment of a species has been suggested to play a key role in shaping its morphology. Our study aimed to understand whether and how habitat use has influenced the evolution of external body and head shape in snakes. To do so, we gathered a comprehensive dataset of morphological traits from 436 snake species. Our findings reveal that species living in different habitats are morphologically distinct with species living in similar habitats displaying similar morphologies despite the conserved morphology of snakes.

evolutionary biology↗