Scientists say they finally understand why the glass frog has see-through skin: it's camouflage—just not in the way most other tree frogs do it. An international team of scientists wondered why the glass frogs found in Central and South America have partially translucent skin—with intestines and a heart visible through the underbelly—rather than fully translucent skin or "the opaque camouflaged patterns of other tree frog species," James Barnett, lead author of the study published in PNAS, tells the Guardian. First, they used computer models to compare pairs of photos of 55 glass frogs, one with a white background and one with a background of leaves. "We found that the color of the frogs' bodies did not change much between backgrounds, but the legs did change significantly," says Barnett. This pointed to a new kind of camouflage, known as edge diffusion.
"By having translucent legs and resting with the legs surrounding the body, the frog's edge is transformed into a softer, less contrasting gradient from the leaf to the legs, and again from the legs to the body," Barnett explains, noting this makes the frog less recognizable to predators. While the frogs always appear green, per a release, human participants had a harder time identifying the frogs in photos when they showed a natural pattern of translucency compared to when they appeared more opaque. And in an experiment in which fake frogs were placed in vegetation in Ecuador, birds attacked opaque versions more than twice as often as translucent versions. In other words, "being translucent does help glass frogs camouflage themselves from predators, but not necessarily in the way expected by comparison to fully transparent species," says Barnett. (Antarctica once had frogs.)