Every so often, a distracted scientist will flash a creature with an ultraviolet (UV) light and unwittingly discover its ability to "glow in the dark". It happened to flying squirrels, it happened to puffins, and it has now happened to the pumpkin toadlet.
The latest to join this gang of glowing beasties is a species of tiny Brazilian frogs known as the pumpkin toadlets (yep, that’s their actual name). Reporting in the journal Scientific Reports, researchers from New York University discovered that this species, also known as Brachycephalus ephippium, fluoresces with an unusual pattern when put under a UV light.
"The fluorescent patterns are only visible to the human eye under a UV lamp. In nature, if they were visible to other animals, they could be used as intra-specific communication signals or as reinforcement of their aposematic coloration, warning potential predators of their toxicity," lead author Sandra Goutte, NYU Abu Dhabi Postdoctoral Associate, said in a statement.
"However, more research on the behavior of these frogs and their predators is needed to pinpoint the potential function of this unique luminescence."
Pumpkin toadlets are diurnal, meaning they carry out their business during the daytime, so their fluorescence might be detectable to certain species under natural sunlight (which contains UV light, humans just can’t see it with our naked eye).
Stranger still, they actually have a glow-in-the-dark skeleton. The fluorescence of the species emanates from their bones, not their skin. The frog’s entire skeleton is highly fluorescent, but it’s only visible where the layer of skin tissue over the bones is very thin.
Like many of these UV-related discoveries, the revelation was stumbled upon by accident. The team was out in the Brazilian Atlantic forest looking to study the acoustic communications of these miniature frogs. However, they found out the frogs could not hear their own mating calls, suggesting they must be communicating some other way. Under the glow of a UV lamp, they found the answer.
“The present study adds to the growing list of documented cases of fluorescence in terrestrial animals,” the study authors write. “Bone fluorescence within the Brachycephalus genus appears to be associated with the loss of high-frequency hearing, raising the possibility that it represents an alternate communication channel.”