Scientists Discover the Reason Behind the Glass Frog's Translucent Skin

The glass frogs of Central and South America aren’t named for their fragility; it’s because their bones, intestines and beating hearts can be seen through the skin covering their torso and limbs. The reason for their unique appearance has gone mostly uninvestigated, but new research has found the frogs’ glass-like skin helps them blend in and avoid being spotted by predators, reports Nicola Davis for the Guardian.

Being see-through seems like the ultimate form of camouflage. But, unfortunately for land animals, the trick works better underwater, where the watery backdrop keeps critters’ fluid-filled insides from standing out, says James Barnett, a frog researcher at McMaster University and lead author of the paper, in a statement. That’s one reason why on this list of ten glass-like animals created by Ella Davies for BBC Earth in 2015, the glass frog is the only land animal with a partially see-through body.

But even glass frogs are not totally transparent; they all have green backs and their glass-like tummies are usually pressed against a leaf. For this reason, Barnett says glass frogs are better described as translucent rather than transparent, and it was this puzzling mélange that Barnett and his team sought to investigate.

When glass frogs emerge during the rainy season, researchers at the Smithsonian Tropical Research Institute follow their migration out of the forest and along streams. These transparent leaf-lying frogs are mysterious to experts who are only now starting to track their mating rituals and behavior.

“If predators cannot see straight through the frogs, why do glass frogs have transparent skin at all, and not the opaque camouflaged patterns of other tree frog species?” Barnett, tells the Guardian.

To test the frogs’ camouflage, the researchers photographed 55 glass frogs on green leaves and on white backgrounds. The researchers then conducted three experiments. First, they used computer analysis to assess the color and outline of the frog in each photo, according to the Guardian.

The researchers found the frogs always looked green, but that they brightened or darkened depending on the background, per the paper. Changing brightness in this way allows the frogs to more closely match the green of the leaf they happen to be sitting on.

The researchers also report that the frogs’ legs are more translucent than their bodies. This muddles the outline of a sitting frog, creating diffuse edges that predators are less likely to recognize.

“Visual systems are very sensitive to edges where two different colors meet, and thin, highly contrasting edges are particularly conspicuous, Barnett tells Tibi Puiu of ZME Science. “By having translucent legs and resting with the legs surrounding the body, the frog’s edge, where it meets the leaf, is transformed into a softer less contrasting gradient, blending the frog and leaf together more smoothly.”

The researchers termed this novel camouflage technique, “edge diffusion.”

The team also presented 25 people with 125 computer-generated images of frogs at differing levels of translucency and asked them to spot the frogs as quickly as possible, reports the Guardian. The hardest frogs for people to spot sported the pattern of translucency typically seen on the glass frog.

And finally, the Guardian reports that the team created 360 fake frogs out of gelatin and set them loose in the Ecuadorian rainforest for three days. Half the fake frogs were opaque and half were translucent, and the researchers found the opaque frogs were attacked by birds more than twice as often, the team reports this week in the journal Proceedings of the National Academy of Sciences.

“Camouflage is very widespread and incredibly diverse... Two of the most common strategies are ‘background matching’, where a pattern replicates the surroundings, and ‘disruptive coloration’, where high contrast patterns create unrecognizable shapes that hide the animals’ outline,” explains Barnett to ZME Science. “Being transparent is usually thought of achieving camouflage in a similar manner to background matching. We found that glass frog translucency works in a different way, that is more akin to disruptive coloration, but which is conceptually distinct.”

Devi Stuart-Fox, an animal color and behavior expert at the University of Melbourne who was not involved in the research, tells the Guardian that “predators form a search image for the shape of their prey, so masking the body’s outline is a very effective strategy to enhance camouflage… The sheer diversity of camouflage strategies in nature is truly remarkable.”

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