Melanism in salamanders: effects of insularity, sex and size on dorsal colouration in <i>Salamandra salamandra</i>

Abstract Insular environments modify the nature, direction, intensity and complexity of both biotic and abiotic conditions, thereby shaping species' genotypes and phenotypes. Many of these changes intersect with functions commonly associated with melanin‐based colourations such as thermoregulation, camouflage and predator deterrence, suggesting a potential link between insularity and melanism. However, although the effects of insularity on traits such as body size are well documented, its influence on other phenotypic traits, including colouration, remains largely unexplored across most taxa, leaving general patterns poorly understood. Here, we address this gap by quantitatively assessing whether insular populations of the amphibian fire salamander ( Salamandra salamandra ) exhibit a higher proportion of melanic colouration than mainland populations, while accounting for local variation, sex and body size. We show that salamanders from one of the three studied islands exhibit a higher proportion of dorsal black colouration, indicating the role of local pressures in shaping colour patterns. In contrast, insular populations exhibit reduced variation in the proportion of melanic colouration relative to their mainland counterparts, suggesting constraints on phenotypic diversity in insular environments. Additionally, body size significantly influences colouration, and partial sexual dimorphism is evident, with females tending to present more black proportion than males. Overall, this study advances our understanding of colour variation in insular amphibians and provides a foundation for further work investigating the evolutionary mechanisms and functional significance of melanism in this system.