Transparency, the ultimate camouflage? Background dependency and influence of background lightness

Transparency reduces prey detectability by predators and occurs in diverse aquatic and terrestrial organisms. Previous studies have shown that in open waters, transparency level decreases as depth increases, suggesting that intermediate transparency may be more protective in dimmer conditions, an untested prediction. Another widespread camouflage strategy is background matching: background-matching specialists are better protected than generalists on their specific background, but this creates stronger background dependency. This raises the question of whether transparency could outperform background matching, thereby reducing background dependency. In a citizen-science computer-based predation experiment, we asked human participants to locate a butterfly in different backgrounds (vegetation, soil, trunk). We manipulated butterfly coloration (uniform coloration specialist or generalist), transparency level (opaque, semi-opaque, transparent), background lightness, apparent butterfly size and distance to the predator's attention point to test their effect on protection. We found that intermediate transparency protects better in dimmer conditions, as predicted. Butterflies appearing smaller (in size or viewed from a greater distance), or creating stronger edge disruption were better protected, particularly when transparent. Butterflies closer to the attention point or on less complex backgrounds were less protected, unless more transparent. Finally, transparency outperformed specialist and generalist uniform coloration, suggesting transparency can free prey from background dependency.