More yellow more toxic? Sex rather than alkaloid content is correlated with yellow coloration in the fire salamander

Prey species signalling their toxicity to potential predators via conspicuous coloration occur frequently in nature. This survival strategy, called aposematism, is thought to be successful when based on honest qualitative or quantitative signalling of toxicity and predator learning behaviour. However, evidence supporting these basic assumptions is absent for most species considered aposematic. Due to their conspicuous black and yellow coloration – in combination with the production of toxic steroidal alkaloids – fire salamanders (genus Salamandra) are textbook examples of aposematism; but whether this signal combination meets the requirements to be considered aposematic in a strict sense remains unclear. We therefore tested one assumption that would support aposematism in this species, that is, whether the alkaloid content is honestly correlated with their warning coloration. We studied fire salamanders (Salamandra salamandra terrestris) from different locations in the Solling, an area in Lower Saxony (Germany) known for individuals with high dorsal yellow ratios. We measured the dorsal yellow ratio of 50 individuals from each of three populations, and quantified toxicity (steroidal alkaloid content of the parotoid glands) of twelve individuals per location. We then analysed the correlation between the yellow ratio and various parameters (alkaloid content, sex, and location), as well as the interactions between these. We found no correlation between yellow ratio and toxicity; instead yellow coloration was significantly affected by sex (males displaying more yellow), and location. Our data suggest that besides possibly serving as warning coloration in a toxic species, the conspicuous colour pattern of the fire salamander might be further shaped by sexual selection and genetic drift. Thus, unravelling the effects of coloration in the context of aposematism might not be possible without disentangling the various other influences acting on coloration as a whole.