Glass frogs have a previously unknown form of camouflage

Glass frogs have a fascinating transparent appearance, but why the frogs evolved to have see-through skin has remained a mystery. 

In a new study from the University of Bristol, experts set out to find the ecological significance of translucent skin in glass frogs and identified a previously unknown form of camouflage.

The research combined behavioral studies with computational visual modeling and a computer-based detection experiment. 

The investigation revealed that glass frog translucency serves as a type of camouflage, but the mechanism differs from that of true transparency.

“The frogs are always green but appear to brighten and darken depending on the background. This change in brightness makes the frogs a closer match to their immediate surroundings, which are predominantly made up of green leaves,” explained study lead author Dr. James Barnett.

“We also found that the legs are more translucent than the body and so when the legs are held tucked to the frog’s sides at rest, this creates a diffuse gradient from leaf color to frog color rather than a more salient sharp edge.”

Dr. Barnett said this finding represents a new form of camouflage that the researchers refer to as “edge diffusion.”

According to Dr. Barnett, scientists have often been skeptical of the degree to which glass frogs can be called transparent.

“Transparency is, at face value, the perfect camouflage. It is relatively common in aquatic species where animal tissue shares a similar refractive index to the surrounding water.”

“However, air and tissue are quite different in their refractive indices, so transparency is predicted to be less effective in terrestrial species. Indeed, terrestrial examples are rare.” 

“Although glass frogs are one commonly cited example of terrestrial transparency, their sparse green pigmentation means they are better described as translucent,” said Dr. Barnett.

The research was supervised by Professor Nick Scott-Samuel and Professor Innes Cuthill when Dr. Barnett was a PhD student at the University of Bristol.

“Animal camouflage has long been a textbook example of the power of Darwinian natural selection,” said Professor Cuthill. “However, in truth, we are only beginning to unravel how different forms of camouflage actually work. Glass frogs illustrate a new mechanism that we hadn’t really considered before.”

Professor Scott-Samuel said the study addresses a question that has been the topic of much speculation, both among the public and the scientific community. “We now have good evidence that the frogs’ glass-like appearance is, indeed, a form of camouflage,” he concluded. 

The study is published in the journal Proceedings of the National Academy of Sciences.

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By Chrissy Sexton, Earth.com Staff Writer