Spotted Patterns of Glass Frogs Can Disguise Imperfections in Transparency to Improve Camouflage

Transparency could be considered the idealized version of camouflage, capable of producing effective concealment across heterogeneous habitats. However, although examples of highly transparent species do exist, ‘transparent’ species are highly variable and are often either only partially transparent or better described as translucent. Exactly how these different forms of ‘imperfect’ transparency may act as camouflage remains largely unknown. The glass frogs (Centrolenidae) have long been thought of as a classic example of transparent camouflage due to their see‐through ventral skin. Yet even in these seemingly exemplary cases, glass frogs are better described as translucent due to the presence of obligate opaque structures, such as the internal organs, within the body and opaque color patterns in their dorsal skin. These opaque patterns could be predicted to detract from transparency, with the potential to increase detectability and therefore predation risk. One potential explanation is that the addition of opaque patterning could help to conceal the internal organs from view while maintaining the cryptic effects of translucency. Here we tested whether the presence of opaque spots could reduce predation risk for translucent frogs by presenting model frogs, which differed in opacity and dorsal patterning, to wild birds and recording the rate at which these models were attacked. We found that the addition of opaque dorsal patterning reduced predation risk but there was no effect of translucency on survival. These data suggest that opaque patterning in glass frogs may help to conceal conspicuous internal structures but that partial translucency does not necessarily guarantee effective concealment. Future work is needed to explore the full range of dorsal patterning found in translucent glass frogs and to further examine how patterns comprised of both opaque and transparent elements may function as camouflage in a wider range of taxa.