Citizen Observations Shed New Light on Geographic Variation in Colour Polymorphism of a Widespread Reptile

ABSTRACT Aim Colour polymorphism within ectothermic species and populations is shaped by multiple selection pressures that can vary geographically. Here, we tested different hypotheses to better understand this variation in colour polymorphism. The thermal melanism hypothesis predicts that darker colouration is beneficial in colder regions, due to enhanced heat absorption, while the predation pressure hypothesis predicts that melanistic individuals are exposed to a higher predation risk because they are more visible to predators. Additionally, temperature, land cover and predation pressure could interact to influence colour morph frequencies due to trade‐offs regarding thermoregulation and predation risk. Location Eurasia. Taxon European adder ( Vipera berus ). Methods We compiled a dataset of > 7000 citizen observations across the entire distribution of the European adder, scoring adder colouration to test the above‐mentioned hypotheses concerning geographic variation in colour polymorphism, using Bayesian generalised nonlinear regression models. Results We found support for the thermal melanism hypothesis, with the frequency of melanism increasing in colder regions. Moreover, in colder areas, the proportion of melanistic snakes increased with avian predator density, whereas this pattern was weaker in warmer areas, potentially because melanistic snakes spend less time basking and therefore experience reduced predation rates compared to brown and grey snakes. Finally, we found the proportion of melanistic individuals to decline at higher elevations (> 1000 m), potentially due to increased access to basking opportunities or because higher elevations facilitate easier detection of melanistic snakes by predators. Main Conclusions Combined, our results emphasise the large potential of citizen observations to obtain novel insights concerning biogeographic patterns of morphological divergence in colouration. Our findings increase the understanding of the underlying mechanisms affecting colour polymorphism in ectothermic animals, providing valuable information to predict how species might respond to global change.