Transparency reduces predator detection in mimetic clearwing butterflies

1. Predation is an important selective pressure and some prey have evolved conspicuous warning signals that advertise unpalatability (i.e. aposematism) as an antipredator defence. Conspicuous colour patterns have been shown effective as warning signals, by promoting predator learning and memory. Unexpectedly, some butterfly species from the unpalatable tribe Ithomiini possess transparent wings, a feature rare on land but common in water, known to reduce predator detection. 2. 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 both wild birds and humans in behavioural experiments. Vision modelling predicted butterfly detectability to be similar for these two predator types. 3. In concordance with predictions, the most transparent species were almost never found first and were detected less often than the opaque species by both birds and humans, suggesting that transparency enhances crypsis. However, humans were able to learn to better detect the more transparent species over time. 4. Our study demonstrates for the first time that transparency on land likely decreases detectability by visual predators.