Fluorescence discovered in tiny Brazilian frogs

Pumpkin toadlets are colourful miniature frogs that live in the Brazilian Atlantic forest, and now researchers have discovered that these tiny frogs are fluorescent.

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation.

"The fluorescent patterns are only visible to the human eye under a ultra-violet (UV) lamp," said Sandra Goutte from New York University Abu Dhabi.

"In nature, if they were visible to other animals, they could be used as intra-specific communication signals or as reinforcement of their aposematic colouration, warning potential predators of their toxicity," Goutte said in a statement.

Pumpkin toadlets, also called Brachycephalus ephippium, are tiny, brightly-coloured, and poisonous frogs that can be found in the Brazilian Atlantic forest.

During the mating season, they can be seen by day walking around the forest and producing soft buzzing calls in search of a mate.

In the study published in the journal Scientific Reports, an international team of researchers studied the acoustic communications of these miniature frogs.

When they discovered that Brachycephalus ephippium could not hear its own mating calls, they searched for alternative visual signals the frogs could use to communicate instead.

When they shone an ultra-violet (UV) lamp on the frogs, their backs and heads glowed intensely.

The researchers said that fluorescent patterns are created by bony plates lying directly beneath a very thin skin.

The toadlet's entire skeleton is highly fluorescent, but the fluorescence is only externally visible where the layer of skin tissue over the bones is very thin (about seven micrometers thick).

The lack of dark skin pigment cells -- which block the passage of light -- and the thinness of the skin allow the ultraviolet light to pass through and excite the fluorescence of the bony plates of the skull.

The fluorescent light is then reflected back from the frog's bone, and can be seen as bluish-white markings by the observer if they have a UV lamp.

"However, more research on the behaviour of these frogs and their predators is needed to pinpoint the potential function of this unique luminescence," Goutte said.

The researchers compared the skeletons of the two species of pumpkin toadlets to closely related, non-fluorescent species.

The pumpkin toadlets' bones proved to be much more fluorescent.

Pumpkin toadlets are diurnal, and in their natural habitat, the UV or near-UV components of daylight might be able to create fluorescence at a level detectable by certain species.