To be a perfectly “invisible” animal is the ultimate goal in the camouflage game. Many aquatic animals achieve this complete transparency because light travels at roughly the same speed through water and tissue (i.e. they have a similar refractive index), meaning that light can be reflected less throughout their bodies. On land, this feat is tougher to achieve, as air and tissue have quite different refractive indices, scattering the incoming light and making the animal more visible.
One group of terrestrial animals that have a good go at transparency are glass frogs. Found across central and southern America, grape-sized glass frogs are famed for their “see-through” bellies that give us a peek at their insides. Yet, despite their name, glass frogs aren’t completely transparent, as sparse green pigments mean that when viewed from above they are actually more translucent.
Therefore, instead of acting as a window to their surroundings (such as in full transparency), the imperfectly transparent glass frogs maintain their green hue and block background patterning. However, a new study by an international team of researchers has examined whether glass frogs can still effectively camouflage themselves even with translucent skin.
By applying “predator vision” to over 50 photographs of two species of glass frog: the Emerald glass frog (Espadarana prosoblepon) from Ecuador and the Santa Cecilia Cochran glass frog (Teratohyla midas) from French Guiana, the team determined that glass frogs seem to change the brightness of their green skin to match their leafy surroundings. This differs from other mechanisms, such as color-changing, where an animal produces different pigments to match their backgrounds.
“The frogs are always green but appear to brighten and darken depending on the background,” Dr James Barnett, lead author of the study published in PNAS, who began the research while a PhD student at the University of Bristol, UK, and is now based at McMaster University, Canada, said in a statement. “This change in brightness makes the frogs a closer match to their immediate surroundings, which are predominantly made up of green leaves.”
But that is not the only trick up the frog’s sleeve.
“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 colour to frog colour rather than a more salient sharp edge,” Barnett explained. “This suggests a novel form of camouflage: 'edge diffusion.'”
To test whether this is a successful form of camouflage, Barnett and his colleagues carried out several trials. The first involved a computer-based detection experiment with human participants who were timed trying to pick out opaque and translucent frogs hidden amongst leaves. The second, carried out in Ecuador, tested how long it would take for wild predators to eat the team’s translucent models of glass frogs, made out of gelatine. In both instances, glass frogs took longer to spot (and eat) when compared to the time taken to locate opaque frogs.
The team’s results suggest that glass frogs’ unique approach to “invisibility” does indeed keep them hidden from plain sight.
“Our study addresses a question that has been the topic of much speculation, both among the public and the scientific community,” Professor Nick Scott-Samuel, co-author and expert in visual perception from the University of Bristol, said in a statement. “We now have good evidence that the frogs' glass-like appearance is, indeed, a form of camouflage."