Citizen science data reveals widespread sexual dichromatism in the European fire salamander (Salamandra salamandra)

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 such as intraspecific communication. To study these other functions, we can investigate the influence of intrinsic (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-to-black ratio of the dorsal coloration based on citizen science data available in a new online database.

Our results suggest a widespread sexual dichromatism in fire salamanders in Germany with males displaying a larger yellow-to-black ratio, i.e. are more 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 yellow-to-black ratio. Among six investigated putative drivers of this color variation, the proportion of the aposematic yellow coloration increased with latitude but showed no association with other environmental variables such as temperature or the productivity of the habitat.

Integrating citizen science data in this study enabled a comparison of fire salamander populations across large parts of their distribution and highlights widespread sexual dichromatism. Future studies should further investigate potential mechanisms of mate choice in fire salamanders and other selective forces on coloration such as differential predation pressure between both sexes or the role in crypsis. Lastly, multi-national studies supported by citizen science data could fully unravel the extent of color variation in this species.