Under normal light, the South American polka dot tree frog (Hypsiboas punctatus) sports a muted palette of greens, yellows and reds. But dim the lights and switch on ultraviolet illumination, and this little amphibian gives off a bright blue and green glow.

The ability to absorb light at short wavelengths and re-emit it at longer wavelengths is called fluorescence, and is rare in terrestrial animals. Until now, it was unheard of in amphibians. Researchers also report that the polka dot tree frog uses fluorescent molecules totally unlike those found in other animals. The team published the find on 13 March in Proceedings of the National Academy of Sciences1.

Because fluorescence requires the absorption of light, it doesn’t happen in total darkness. That makes it distinct from bioluminescence, in which organisms give off their own light generated through chemical reactions. Many ocean creatures fluoresce, including corals, fish, sharks and one species of sea turtle (the hawksbill turtle, Eretmochelys imbricata). On land, fluorescence was previously known only in parrots and some scorpions. It is unclear why animals have this ability, although explanations include communication, camouflage and mate attraction.

The researchers first thought that they might find red fluorescence in these frogs, because they contain a pigment called biliverdin. Normally, biliverdin turns the amphibian's tissues and bones green. However, in some insects, says Carlos Taboada, a herpetologist at the University of Buenos Aires in Argentina, proteins bound to biliverdin emit a faint red fluorescence. But in the polka dot tree frog, biliverdin turned out to be a red herring.

Dressed to impress

When Taboada and his colleagues trained a UVA flashlight (or black light) on polka dot tree frogs collected near Santa Fe, Argentina, they were astonished to find the animals gave off an intense greenish-blue glow instead of a faint red. “We couldn’t believe it,” says study co-author Julián Faivovich, a herpetologist who is also at the University of Buenos Aires.

Three molecules — hyloin-L1, hyloin-L2 and hyloin-G1 — in the animals’ lymph tissue, skin and glandular secretions were responsible for the green fluorescence. The molecules contain a ring structure and a chain of hydrocarbons, and are unique among known fluorescent molecules in animals. The closest similar molecules are found in plants, says study co-author Norberto Peporine Lopes, a chemist at the University of São Paulo in Brazil.

The newly described fluorescent molecules emit a surprising amount of light, providing about 18% as much visible light as a full Moon — enough for a related species of frog to see by. Almost nothing is known about the polka dot tree frog’s visual system or photoreceptors, so Taboada plans to study these to determine whether the frogs can see their own fluorescence.

“I think it’s exciting,” says marine biologist David Gruber of Baruch College, part of the City University of New York, who with his colleague discovered fluorescence in hawksbill sea turtles in 2015 (ref. 2). “It opens up many more questions than are answered,” he says — including the ecological and behavioural function of fluorescence.

Faivovich wants to look for fluorescence in the 250 other tree frog species that have translucent skin like the polka dot tree frog. He hopes he’s not the only one: “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 says.