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.