Imperfect transparency and camouflage in glass frogs

Significance Transparency is one of the most intuitive forms of camouflage, where predators see straight through their prey as if it were not there. Glass frogs are a classic example of animal transparency and are well known for their transparent ventral skin, through which the internal organs can be clearly seen. The efficacy of transparency on land, however, has been controversial. We found that under natural conditions these frogs are better described as translucent and that translucency acts as modifiable camouflage. Differences in the degree of translucency over the frog act to disguise the frog’s outline and highlight the potential of “edge diffusion” as a form of camouflage, making glass frog camouflage distinct from both transparency and active color change. Camouflage patterns prevent detection and/or recognition by matching the background, disrupting edges, or mimicking particular background features. In variable habitats, however, a single pattern cannot match all available sites all of the time, and efficacy may therefore be reduced. Active color change provides an alternative where coloration can be altered to match local conditions, but again efficacy may be limited by the speed of change and range of patterns available. Transparency, on the other hand, creates high-fidelity camouflage that changes instantaneously to match any substrate but is potentially compromised in terrestrial environments where image distortion may be more obvious than in water. Glass frogs are one example of terrestrial transparency and are well known for their transparent ventral skin through which their bones, intestines, and beating hearts can be seen. However, sparse dorsal pigmentation means that these frogs are better described as translucent. To investigate whether this imperfect transparency acts as camouflage, we used in situ behavioral trials, visual modeling, and laboratory psychophysics. We found that the perceived luminance of the frogs changed depending on the immediate background, lowering detectability and increasing survival when compared to opaque frogs. Moreover, this change was greatest for the legs, which surround the body at rest and create a diffuse transition from background to frog luminance rather than a sharp, highly salient edge. This passive change in luminance, without significant modification of hue, suggests a camouflage strategy, “edge diffusion,” distinct from both transparency and active color change.