The distinctive black and red wing pattern of six-spot burnet moths (Zygaena filipendulae, L.) is a classic example of aposematism, advertising their potent cyanide-based defences. While such warning signals provide a qualitatively honest signal of unprofitability, the evidence for quantitative honesty, whereby variation in visual traits could provide accurate estimates of individual toxicity, is more equivocal. Combining sophisticated measures of cyanogenic glucoside content and wing colour from the perspective of avian predators, we investigate the relationship between coloration and defences in Z. filipendulae, to test signal honesty both within and across populations. Mean cyanogenic glucoside concentration was not correlated with wing coloration across populations in males, yet in females higher cyanogenic glucoside levels were associated with smaller and lighter red forewing markings. Trends within populations were similarly indicative of quantitative dishonesty, and consistent differences between the sexes were apparent: larger females, carrying a greater total cyanogenic glucoside load, displayed larger but less conspicuous markings than smaller males, according to several colour metrics. The overall high aversiveness of cyanogenic glucosides and fluctuations in colour and toxin levels during an individual’s lifetime may contribute to these results, highlighting generally important reasons why signal honesty should not always be expected in aposematic species.

The distinctive black and red wing pattern of six-spot burnet moths (Zygaena filipendulae, L.) is a classic example of aposematism, advertising their potent cyanide-based defences. While such warning signals provide a qualitatively honest signal of unprofitability, the evidence for quantitative honesty, whereby variation in visual traits could provide accurate estimates of individual toxicity, is more equivocal. Combining sophisticated measures of cyanogenic glucoside content and wing colour from the perspective of avian predators, we investigate the relationship between coloration and defences in Z. filipendulae, to test signal honesty both within and across populations. Mean cyanogenic glucoside concentration was not correlated with wing coloration across populations in males, yet in females higher cyanogenic glucoside levels were associated with smaller and lighter red forewing markings. Trends within populations were similarly indicative of quantitative dishonesty, and consistent differences between the sexes were apparent: larger females, carrying a greater total cyanogenic glucoside load, displayed larger but less conspicuous markings than smaller males, according to several colour metrics. The overall high aversiveness of cyanogenic glucosides and fluctuations in colour and toxin levels during an individual’s lifetime may contribute to these results, highlighting generally important reasons why signal honesty should not always be expected in aposematic species.

No description available