Transparency reduces predator detection in chemically protected clearwing butterflies

Abstract

1. Predation is an important selective pressure and some prey have evolved warning colour signals advertising unpalatability (i.e. aposematism) as an antipredator strategy. Unexpectedly, some butterfly species from the unpalatable tribe Ithomiini possess transparent wings, an adaptation rare on land but common in water where it helps avoiding predator detection.\n\n2. We tested if transparency of butterfly wings was associated with decreased detectability by predators, by comparing four butterfly species exhibiting different degrees of transparency, ranging from fully opaque to largely transparent. We tested our prediction using using both wild birds and humans in behavioural experiments. Vision modelling predicted detectability to be similar for these two predator types.\n\n3. In concordance with predictions, more transparent species were almost never the first detected items and were detected less often than the opaque species by both birds and humans, suggesting that transparency enhances crypsis. However, humans could learn to better detect the most transparent species over time. Our study demonstrates for the first time that transparency on land likely decreases detectability by visual predators.