That many frogs and toads are greenish or brownish in color does not seem surprising – this color makes them almost invisible in their natural habitat. The majority of these amphibians owe their camouflage to pigment cells in their skin. The pigments it contains refract incident light in a certain way and make the skin appear green.
But this does not apply to the glass frog of the genus Espadarana shown here: its skin hardly contains any pigment cells and is almost transparent. Instead, its organs and even bones are colored green. Researchers at Duke University have found out why. According to this, the tissues of these special frog species contain very high levels of a bile pigment called biliverdin, which is a by-product of breaking down old red blood cells. This pigment is usually thought of as a toxin that is filtered out in the liver and excreted as quickly as possible.
However, it has been found that these frogs carry four times as much biliverdin as people who are severely affected by liver disease and 200 times as much as the frog species colored by green skin pigments. The fact that this high dose of biliverdin does not harm the glass frog is thanks to a special protein that binds the biliverdin. As a result, the bile pigment loses its highly toxic effect and at the same time takes on its special green tone.
The occurrence of the protein BBS (biliverdin-binding serpin) turns out to be an evolutionary advantage for frog species, as the resulting green coloration in bones and blood ensures good camouflage on the leaves – even with infrared light.
As the scientists found, this is a convergent development that has occurred independently more than 40 times in eleven different frog families. Most of these adapted frog species are tree frogs found in Madagascar, South America and Southeast Asia. Their camouflage serves the animals living in the trees as protection from predators and at the same time for effective hunt for prey.