Abstract
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 prioritized 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 1 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.
Highlights
We found little evidence of quantitative signal honesty across the sampled species of Zygaenidae
Most colour metrics were not correlated with the concentration of defensive cyanogenic glucosides, whether male or female specimens were considered, and irrespective of the value of λ in phylogenetically controlled analyses (Table 3, Supporting Information Data S7)
Chemical defences are generally assessed by measuring toxin levels, but these may vary across body parts, total toxin amounts may be more relevant if prey are swallowed whole, and distastefulness, inducing taste rejection by predators (Skelhorn & Rowe, 2009, 2010) may not covary with toxicity: in nudibranchs, distasteful red-spotted species were shown to vary widely in their chemical profiles and lethality to brine shrimp (Winters et al, 2018)
Summary
Aposematic animals use conspicuous colours and patterns to warn potential predators of their unprofitability, linked to physical or. Work on ladybird beetles (Coccinellidae), combining toxin bioassays and field predation experiments with ladybird models presented to birds, has explicitly linked more conspicuous coloration and higher defence levels to greater survival in the wild (Arenas et al, 2015) These studies are restricted in taxonomic scope, primarily focusing on poison frogs (Dendrobatidae), ladybird beetles and to a lesser extent marine opisthobranchs (Cortesi & Cheney, 2010; Winters et al, 2018), so research in a wider range of taxa is needed before more general conclusions can be drawn (Stevens, 2015; Summers et al, 2015). We test the idea of quantitative signal honesty in a new study system, using relevant and meaningful measures of signals and defences, to contribute to the debate over signal honesty across aposematic species
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