In 2017, scientists announced a natural wonder. Some frogs glow at twilight, they found, with a fluorescent compound we hadn't seen before in nature.
At the time, it was not known how many species of frogs might emit this fluorescence. New research has undertaken to find out – and it's many, many more than that initial discovery hinted at.
In a study of 151 species of South American frogs, every single individual – that's 528 frogs – exhibited some degree of fluorescence.
Moreover, the research – published at preprint server bioRxiv – suggests that fluorescence is well matched to frogs' vision and ecology, indicating it plays a role in the way the frogs signal to each other, and maybe to ward off predators.
"Via a field survey across South America, we discovered and documented patterns of biofluorescence in tropical amphibians. We more than tripled the number of species that have been tested for this trait and added representatives from previously untested anuran families," writes a team led by biologist Courtney Whitcher of Florida State University.
"With evidence of tuning to the ecology and sensory systems of frogs, our results suggest frog biofluorescence is likely functioning in anuran communication."
Many things in the animal kingdom glow, but the reason isn't always obvious.
Fluorescence is the type of glow created when light is absorbed and re-emitted at a different wavelength, and it's been seen in many species, including catsharks, chameleons, salamanders, and, oddly, Australian monotremes and marsupials. Bone fluoresces, too; yes, even yours.
The biofluorescence produced in the skin of frogs seems to be different from the fluorescence of other glowing animals, however. Whitcher and her team caught and studied their 528 frogs across South America, illuminating them with lights ranging from ultraviolet to the visible spectrum, and recording the results.
As already mentioned, every frog in the study glowed back, although for some the return glow was as small as just 2 percent. But the 'how' of the glow is perhaps even more interesting than the fact of it.
Of all the wavelengths to which the frogs were exposed, blue light that was closest to Earth's natural twilight produced the strongest fluorescence. And the fluorescence itself occurred primarily in two different visible-light peaks – one green, and one orange.
Many frogs are crepuscular; that is, they are active at twilight, in the dim light of dawn and dusk. In some species, their eyes are structured to work best in this light, dominated by rod photoreceptors sensitive to green and blue – green in particular. As the researchers note, green-sensitive rods occupy some 60 percent of the retinal area of some frogs.
This means that their green glow is brightest during the time of day at which the frogs are most active, and shining in a color the frogs' eyes are particularly sensitive to. The parts of the body that glow are those that are most involved in signaling to other frogs, such as the throat and back. This suggests that biofluorescence is part of frogs' communication toolkit.
What of the orange fluorescence, though? Well, that could be a byproduct of the evolution of biofluorescence in the frogs. Or it could be a signal to a predator, a warning to keep away. More work will need to be undertaken to determine what, if anything, the orange glow signifies.
The green glow, however, meets all the criteria for establishing ecological significance, the researchers say.
"The biofluorescence in many frog species matches the perception peak of anuran green rods but strongly differs from background colors reflected during normal frog breeding hours, making biofluorescence most visible during this time," they write in their paper.
"In sum, our results suggest that sensory drive may underlie the evolution of biofluorescence, motivating future research on its function in anuran communication."
The study can be found at bioRxiv.