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

Abstract

Convergence, defined as lineages evolving to be more similar to one another than their ancestors due to selective pressures is a hallmark of adaptive evolution. Yet, the extent of convergent evolution across different taxa and in different ecological contexts 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 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 habitats. Terrestrial species display the highest morphological diversity in contrast to other habitats which appear morphologically more 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 similarity remains limited to arboreal, semi-arboreal, and terrestrial habitat use. Thus, despite strong functional constraints, evolutionary similarity in fossorial and aquatic species is weak, indicating multiple adaptive solutions rather than a single morphological trajectory, possibly due to the relatively homogenous body plan of snakes. 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.