Boana punctata frog. Side, front and bottom view.

Blue-green lymph (visible on frog’s underside) and small amounts of yellow skin pigment combine to bestow a variety of green colours on the polka-dot tree frog. Credit: Santiago R. Ron

Evolution

How frogs became green — again, and again, and again

A technique for blending into green foliage arose independently in many lineages of tree frogs.

The vivid blue-green hue of hundreds of frog species allows the animals to ‘disappear’ amid green foliage — thanks to a molecular trick that arose multiple times over amphibian history.

Green vertebrates are generally thought to get their coloration from pigment-bearing cells in their skin. But many tree frogs lack these cells. These frogs are green because their translucent bodies show off blood, bones and other internal tissues that are coloured by high levels of the green pigment biliverdin.

To understand the phenomenon’s origins, Carlos Taboada at the University of Buenos Aires and his colleagues extracted lymph and other fluids from the polka-dot tree frog (Boana punctata). They traced the creatures’ blue-green coloration to a previously unknown protein that binds to and transports biliverdin. The team found similar proteins in the lymph of eight other tree frog species.

The researchers studied the plants where B. punctata rests during the day or perches at night and realized that the frog’s colour and brightness closely match that of the vegetation. The biliverdin-binding protein allowed for evolutionary fine-tuning of the frog’s coloration, causing the creature to ‘vanish’ in the forest.