Positive and negative frequency-dependent selection acting on polymorphism in a palatable moth

Abstract

Camouflage and warning signals are contrasted prey strategies reducing predator attack, which offer an excellent opportunity to study the evolutionary forces acting on prey appearance. Edible prey are often inconspicuous and escape predation by remaining undetected. Predators learn to find the most common ones, leading to apostatic selection (advantage to rare morphs) enhancing variation in cryptic prey. By contrast, defended prey are often conspicuous and escape predation by using warning colorations identifying them as unprofitable. Predators avoid the ones they are most familiar with, leading to positive frequency-dependence and warning signal uniformity. It is less clear, however, what happens when two morphs of the same species vary strongly in conspicuousness, and how to explain the maintenance of cryptic and conspicuous morphs within populations, in the case of profitable prey. Using the white and melanic morphs of the invasive Box Tree Moth (Cydalima perspectalis) presented at three different frequencies, we investigate whether a) caterpillars and adult moths are palatable for birds, b) the less conspicuous, melanic morph experiences lower predation rates and b) whether frequency-dependence balances morph frequencies. Our results suggest that the melanic morph enjoys a survival advantage owing to a lower visibility. However, our experiments show that, unexpectedly, the two color morphs experience opposite patterns of frequency-dependent predation, despite being both fully palatable to birds. The melanic morph is under apostatic selection, whereas the conspicuous, white morph is subject to positive frequency-dependence (safety in numbers). Our experiments also show some level of unpalatability in the caterpillars. These results offer novel insight into how predation triggers contrasting evolutionary patterns in a palatable, polymorphic species within two morphs that differ markedly in conspicuousness and within two different life stages.

Reduced palatability, fast flight, and tails: Decoding the defence arsenal of Eudaminae skipper butterflies in a Neotropical locality

“…However, if host plantderived chemical defence is confirmed in other Eudaminae species, we cannot rule out that unpalatability is associated with the highly aposematic caterpillars of some Eudaminae (e.g., Telegonus fulgerator: Hebert et al, 2004;Janzen & Hallwachs, 2009 or Chioides catillus: Cock, 2016), which might indeed represent honest unprofitability signals and might be widespread in Eudaminae. This suggests the possibility that unpalatability in Eudaminae skippers may have been primarily selected during the larval, less mobile, stage, as found in other Lepidoptera species (Poloni et al, 2023), while still providing some protection during the adult stage. Therefore, while adult Eudaminae likely rely on fast flight and escape ability, a feature that may have been inherited from their common ancestor (the family Hesperiidae, in general, have highly erratic and fast flights), unpalatability may not be critically under selection in adults but may also not be strongly selected against, explaining partially its high intra-and interspecific variability.…”