Frequency-dependent flight activity in the colour polymorphic wood tiger moth

Predators efficiently learn to avoid one type of warning signal rather than several, making colour polymorphisms unexpected. Aposematic wood tiger moth males Parasemia plantaginis have either white or yellow hindwing coloration across Europe. Previous studies indicate that yellow males are better defended from predators, while white males have a positively frequency-dependent mating advantage. However, the potential frequency-dependent behavioural differences in flight between the morphs, as well as the role of male-male interactions in inducing flying activity, have not been previously considered. We ran an outdoor cage experiment where proportions of both male morphs were manipulated to test whether flying activity was frequencydependent and differed between morphs. The white morph was significantly more active than the yellow one across all treatments, and sustained activity for longer. Overall activity for both morphs was considerably lower in the yellow-biased environment, suggesting that higher proportions of yellow males in a population may lead to overall reduced flying activity. The activity of the yellow morph also followed a steeper, narrower curve than that of the white morph during peak female calling activity. We suggest that white males, with their presumably less costly defences, have more resources to invest in flight for predator escape and finding mates. Yellow males, which are better protected but less sexually selected, may instead compensate their lower flight activity by ‘flying smart’ during the peak female-calling periods. Thus, both morphs may be able to behaviourally balance the trade-off between warning signal selection and sexual selection. Our results emphasize the greater need to investigate animal behaviour and colour polymorphisms in natural or semi-natural environments.

To quiver or to shiver: increased melanization benefits thermoregulation, but reduces warning signal efficacy in the wood tiger moth

Melanin production is often considered costly, yet beneficial for thermoregulation. Studies of variation in melanization and the opposing selective forces that underlie its variability contribute greatly to understanding natural selection. We investigated whether melanization benefits are traded off with predation risk to promote observed local and geographical variation in the warning signal of adult male wood tiger moths ( Parasemia plantaginis ). Warning signal variation is predicted to reduce survival in aposematic species. However, in P. plantaginis , male hindwings are either yellow or white in Europe, and show continuous variation in melanized markings that cover 20 to 90 per cent of the hindwing. We found that the amount of melanization increased from 40 to 59 per cent between Estonia (58° N) and north Finland (67° N), suggesting melanization carries thermoregulatory benefits. Our thermal measurements showed that more melanic individuals warmed up more quickly on average than less melanic individuals, which probably benefits flight in cold temperatures. With extensive field experiments in central Finland and the Alpine region, we found that more melanic individuals suffered increased predation. Together, our data suggest that warning signal efficiency is constrained by thermoregulatory benefits. Differences in relative costs and benefits of melanin probably help to maintain the geographical warning signal differences.