Frogs flirting with fluorescence 

Sandra Goutte, of New York University Abu Dhabi’s Evolutionary Genomics Lab, in the United Arab Emirates, stumbled across the toadlets’ fluorescence back in 2016 while trying to unpick the mystery of why they have auditory mating calls despite ostensibly being deaf. “We were thinking: these frogs are brightly coloured, and we knew already that they have some visual communication signals, so maybe there’s something we don’t see,” says Goutte. 

Thinking that pumpkin toadlets may have patches that reflect UV light, Goutte went in search of a UV-sensitive camera – instead, finding a UV lamp. “I thought I might as well shine it on the frogs. And that’s when we found out that they were fluorescent.”

Recent studies have described a number of fluorescent frogs and other animals, including birds, spiders and fish. However, the fluorescent compounds in these animals have been found in external tissues, such as skin. The pumpkin toadlet has highly fluorescent bones, externally visible on the back and head where bony plates are covered by thin skin. Chameleons are the only other vertebrate ever discovered to have highly fluorescent bones. 

Biofluorescence occurs when ultraviolet light strikes molecules in an animal that absorb UV light and re-emit it at a lower wavelength. Human eyes aren’t often attuned to the weak visible signal elicited by the low levels of UV in natural light, leading us to miss a great deal of fluorescence in nature. Over the last decade, researchers have been continuously finding new examples of fluorescent animals.

Goutte is now working to reveal what makes the pumpkin toadlet’s bones so fluorescent, while her colleagues in Brazil conduct behavioural experiments to discern the fluorescence’s functions. Possibilities include being used to attract/discover mates or to signal toxicity to would-be predators.

Appreciating how fluorescence is perceived in its natural environment is key to understanding its purpose, says marine biologist David Gruber at City University of New York, USA, who was not involved in the study. After discovering fluorescence in catsharks, Gruber developed²  a ‘shark-eye camera’ in order to see them with the same sensitivity to fluorescent wavelengths as other catsharks. He suggests that Goutte and her team could create a ‘pumpkin toadlet-eye camera’.

“What I like about [Goutte’s] study,” says Gruber, “is that it adds another piece to this ever-expanding puzzle.”

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