Under regular beams, the polka dot tree frog, a species common across Latin America, is aesthetically arresting enough — its exterior is daubed in a subtle blend of green, yellow and red. However, when the lights are dimmed and UV shines, the amphibian emits a pulsing blue and green glow.
Fluorescence — the ability to absorb light at short wavelengths, and re-emit it at longer wavelengths — is uncommon in terrestrial animals, but prior to now entirely unheard of in amphibians.
A team of scientists at the Buenos Aires Bernardino Rivadavia Natural Sciences Museum made the serendipitous discovery while studying the pigmentation of the frogs. The group initially believed the creatures might exhibit red fluorescence, as they contain a pigment called biliverdin. Typically, biliverdin turns an amphibian's tissues and bones green, although it can emit a red fluorescent glow in some forms of insect.
When the team shone a UV flashlight on polka dot tree frogs, they found the amphibians emitted an intense blue-green glow, instead of faint red. Further research indicates the glow is created by three fluorescent molecules (hyloin-L1, hyloin-L2 and hyloin-G1) in the frog's lymph tissue, skin and glandular secretions — they contain a ring structure and a chain of hydrocarbons. These molecules are so far said to be unique among known fluorescent particles in animals — although similar molecular properties can be found in plants.
The newly described fluorescent molecules emit an unprecedented amount of light, providing about 18 percent as much visible light as a full moon — enough for a related species of frog to see by. Little is presently known about the frog's visual system or photoreceptors, so the team intend to study these in detail to determine whether the frogs can use their fluorescence to see their environments and identify each other.
Julian Faivovich, one of the researchers on the project, intends to search for fluorescence in the 250 other tree frog species that possess similar traits to the polka dot tree frog, and hopes the discovery will inspire interest in the phenomenon in his peers.
"I'm really hoping that other colleagues will be very interested in this phenomenon, and they will start carrying a UV flashlight to the field," he said.
Because fluorescence requires the absorption of light, it by definition cannot occur in total darkness — this makes fluorescence distinct from bioluminescence, which allows organisms to emit light via internal chemical reactions. Many ocean creatures fluoresce, including coral, fish, sharks and the hawksbill turtle.
In terms of land creatures, fluorescence was previously exclusively detected in parrots, and some scorpions. Scientists are yet to determine the exact purpose of this quality — although speculative explanations include attracting mates, camouflage and communication.