Citizen science data supports sexual dichromatism but rejects thermal melanism in the European fire salamander (Salamandra salamandra)

Background Conspicuous color patterns are traditionally believed to advertise the toxicity of prey to potential predators. However, many aposematic species show drastic variation in coloration, indicating the possibility of other functions of coloration. To study these other functions, we can investigate the influence of inherent (e.g., sex) and external factors (e.g., climate) on color variation. We used the aposematic European fire salamander (Salamandra salamandra) to study drivers of variation in the yellow proportion of their dorsal black and yellow coloration based on citizen science data available in a new online database. Results Our results suggest a widespread sexual dichromatism in fire salamanders in Germany with males displaying a larger proportion of yellow than females. This dichromatism persisted even after correcting for a sex-difference in body shape that accounts for some of the variation in the proportion of yellow. Among six investigated putative drivers of this color variation, the proportion of the aposematic yellow coloration increased with latitude and longitude but showed no association with other environmental variables such as temperature or the productivity of the habitat. Conclusions Integrating citizen science data in this study enabled a comparison of fire salamander populations across large parts of their distribution and conclusively supports sexual dichromatism. Given the lack of assocations with temperature-related variables, thermal melanism appears to play no role in fire salamander coloration. Future studies should further investigate potential mechanisms of mate choice in fire salamanders and other selective factors on coloration such as differential predation pressure between both sexes or the role in crypsis. Lastly, multi-national studies supported by citizen science data should fully unravel the extent of color variation in this species.