What color makes tree frogs so vibrantly green? The blue beneath their skin, of course. Those are the findings of a new study, which reveal that a unique protein complex that reflects blue light is responsible for an unusual green—helping them blend in to their surroundings and evade predators.
Scientists came across the complex while trying to understand how hundreds of tree frog species can accumulate large amounts of a toxic green pigment known as biliverdin. In most animals, biliverdin is so dangerous that it is immediately broken down or excreted. In humans, it forms when red blood cells break down and causes the greenish color sometimes seen in bruises. But in these frogs, it builds up to what should be intolerable levels.
When the researchers isolated the pigment in eight species, they found it stayed stable—and innocuous—by binding with another protein called a serpine, the team reports today in the Proceedings of the National Academy of Sciences. The biliverdin-serpine complex was found throughout the body, in the lymph, muscles, and skin. And because frog skin is mostly yellow, it looks bright green wherever the protein is present.
In the back of an Aplastodiscus leucopygius tree frog (above), the green even has a dash of red, helping it blend in with surrounding vegetation. But in body parts without yellow pigment—like the tree frog’s belly—the blue shows through. This combination shifts daily. In the day, the scientists found, it spreads out to help sleeping frogs blend in; by night, the protein complex moves into the legs and the gut.