In the Atlantic Forest in eastern Brazil is a tiny frog that secretly glows bright blue under UV light. The discovery of the fluorescent frogs was actually just accidental.

Fluorescent Pumpkin Frogs

To the naked human eye, the poisonous pumpkin toadlets appear red, orange, or yellow. However, while researchers were studying the mating calls of the Brachycephalus ephippium and B. pitanga, they happened to shine ultraviolet (UV) light on the creatures and they unexpectedly found them to be glowing a bright blue hue on their heads, legs, and backs.

This trait of fluorescence is very rare in animals with backbones, but the bony plates on the toadlets’ head and back were found to be very fluorescent. Researchers established that the source of the fluorescence is actually the dermal bone on the back and head, and that it is visible through particularly thin skin.

Furthermore, when researchers compared it to a close relative, the Ischnocnema parva, they found that the bones of the pumpkin toadlets were exceptionally fluorescent.

Fluorescence

Scorpions, corals, and some sea turtles are known to fluoresce, and chameleons and two types of South American tree frogs have the trait as well. In the case of pumpkin toadlets, their fluorescence was found to be more like that of the chameleons, in that they originate from the creatures’ bones.

Typically, fluorescence is used by animals to change coloration or pattern for a specific purpose such as signaling. Although initially believed to be a trait specific to marine animals, as mentioned, some terrestrial creatures are now being discovered to have the trait as well.

Regarding pumpkin toadlets, researchers are still unsure as to why the toadlets have this capability. One possibility is that they use it to either attract mates or to serve as a warning to predators that their coat is toxic. It is also possible that they use it for communication since the creatures lack a middle ear and therefore the light may be serving as another means of communication with members of their own species.

However, more studies are needed to determine which of these hypotheses are more likely to be true.

The study is published in Scientific Reports.