And in 2017, scientists announced a natural wonder. They found that some frogs glow at twilight using a fluorescent compound never seen before in nature.
At the time, it was unknown how many species of frogs could emit such brilliance. New research has been set out to find out – and it’s much more than that This initial discovery alluded to.
In a study of 151 species of South American frogs, each individual—that’s 528 frogs—displayed some degree of fluorescence.
Moreover, the research – Published on bioRxiv Preprint Server – indicates that the fluorescence matches well with the frogs’ vision and their environment, suggesting that it plays a role in the way the frogs signal to each other, possibly to ward off predators.
“Through a field survey across South America, we detected and documented bioluminescence patterns in tropical amphibians. We more than tripled the number of species tested for this trait and added representatives from previously untested anuran families.” Writes a team led by biologist Courtney Witcher from Florida State University.
“With evidence of environmental adaptation and frog sensory systems, our results indicate that frog bioluminescence likely functions in anuran communication.”
Many things glow in the animal kingdom, but why isn’t always clear.
Fluorescence is a type of glow produced by the absorption and re-emission of light at a different wavelength, and has been seen in many species, including sharks, chameleons, salamanderand, oddly enough, monotremes are Australian marsupials. Bones also sparkle. Yes, even you.
However, the bioluminescence produced in frog skin appears to differ from that of other glowing animals. Witcher and her team captured and studied 528 frogs across South America, illuminating them with lights ranging from ultraviolet light to the visible spectrum, and recording the results.
As mentioned before, every frog in the study glowed back, although for some the back glow was only as small as 2 percent. But the “how” of the flare is probably more interesting than the fact that it happens.
Of all the wavelengths the frogs were exposed to, the blue light closest to Earth’s natural aurora produced the strongest fluorescence. And the fluorescence itself occurred primarily in two different peaks of visible light—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 designed to work better in this light, and are dominated by rod photoreceptors that are sensitive to green and blue-green in particular. As the researchers note, green-sensitive rods occupy about 60 percent of the retinal area of some frogs.
This means that their green glow is brightest during the time the frogs are most active, and shines with a color to which frogs’ eyes are particularly sensitive. The body parts that glow are those most involved in signaling to other frogs, such as the throat and back. This indicates that bioluminescence is part of the frogs’ communication toolkit.
But what about fluorescent orange? Well, this could be a byproduct of the evolution of bioluminescence in frogs. Or it could be a signal to a predator, or a warning to stay away. More work will need to be done to determine what, if any, the orange glow means.
However, the researchers say the green glow meets all criteria for demonstrating environmental significance.
“The bioluminescence in many frog species matches the peak of perception of green anuran sticks but differs sharply from the background colors reflected during the frog’s normal reproductive hours, making the bioluminescence more noticeable during this time.” They write in their paper.
“In sum, our results suggest that a sensory drive may underlie the development of bioluminescence, motivating future research on its function in anuran communication.”
The study can be found at com. bioRxiv.