Ecological determinants of chromatic diversity in temperate snakes: comparative analysis across climate types

Snakes display significant chromatic variability across the Nearctic and Western Palearctic biogeographic regions. Dominant background colours include black, orange, brown, grey, and various shades of olive to grass green. This study evaluated the environmental factors that may determine this variation, focusing on the differences between the Köppen climate types and between venomous and non-venomous species. Photographic images were used to extract colours, which were then decomposed into the Hue-Saturation-Lightness (HSL) scale to generate 14 colour categories (using human-visible colour variation). These categories were used to assess their relationship with environmental conditions and compute functional diversity indices for regional species groups. The analyses indicate that the variability in snake coloration was influenced by climatic-­macrohabitat factors (precipitation, temperature, forest cover), habitat type (semi-aquatic, terrestrial, arboreal, semi-fossorial, fossorial), prey type, foraging behaviour (ambush, active foragers), and reproductive mode (viviparous, oviparous), once phylogenetic relatedness is controlled for. Highly venomous species exhibited higher chromatic overlap compared to non-venomous and mildly venomous species. Desert snakes showed greater chromatic specialization and higher interspecific similarity compared to snakes from other climate types. In contrast, species inhabiting humid hot-temperate climates tended to display more divergent and unique coloration patterns.