Many defended species use conspicuous visual warning signals to deter potential predators from attacking. Traditional theory holds that these signals should converge on similar forms, yet variation in visual traits and the levels of defensive chemicals is common, both within and between species. It is currently unclear how the strength of signals and potency of defences might be related: conflicting theories suggest that aposematic signals should be quantitatively honest, or, in contrast, that investment in one component should be prioritised over the other, while empirical tests have yielded contrasting results. Here, we advance this debate by examining the relationship between defensive chemicals and signal properties in a family of aposematic Lepidoptera, accounting for phylogenetic relationships and quantifying coloration from the perspective of relevant predators. We test for correlations between toxin levels and measures of wing colour across 14 species of day-flying burnet and forester moths (Lepidoptera: Zygaenidae), protected by highly aversive cyanogenic glucosides, and find no clear evidence of quantitative signal honesty. Significant relationships between toxin levels and coloration vary between sexes and sampling years, and several trends run contrary to expectations for signal honesty. Although toxin concentration is positively correlated with increasing luminance contrast in forewing pattern in one year, higher toxin levels are also associated with paler and less chromatically salient markings, at least in females, in another year. Our study also serves to highlight important factors, including sex-specific trends and seasonal variation, that should be accounted for in future work on signal honesty in aposematic species.

Forewing marking coloration and cyanogenic glucoside data for Zygaenidae in this studyThis file contains all the data for the main analyses in this study: specimen info (ID, species, collection year and locality, sex, mass), coloration metrics (% forewing area, luminance, saturation, hue, internal contrasts with wing ground colour), and cyanogenic glucoside data (total amounts and concentration of linamarin, lotaustralin and total cyanogenic glucosides). Note: Hue 1 corresponds to the hue channel used in the paper; Hue 2 is based on the second principal component generated by PCA, and was not used in the final analyses. FW=Forewing.2015-2016_data_Zygaenidae_fwm_2017newdata_forpaper.csvCyanogenic glucoside levels - original data and 2017 re-runThis file presents data for the 20 specimens for which LC-MS quantification of cyanogenic glucosides was re-run in 2017. It contains specimen information and cyanogenic glucoside data for the original run (data analysed in 2015 and 2016, when the samples were collected) and for the new run in 2017, when samples from 2015 and 2016 were analysed together.machineruns.csv