In the Atlantic Forest of Brazil, it can take a lot to get noticed by a mate.
Pumpkin toadlets (Brachycephalus ephippium and B. pitanga) appear to be at a particular disadvantage. They are the size of a fingernail and deaf to their own mating call.
The discovery of this auditory disadvantage had researchers looking for clues about how the amphibians attracted a mate.
When researchers shone ultraviolet light on the frog, they found their answer: the toadlets have bones that glow through skin.
“In nature, if they were visible to other animals, they could be used as intraspecific communication signals or as reinforcement of their aposematic colouration, warning potential predators of their toxicity,” said Sandra Goutte, the NYU Abu Dhabi postdoctoral associate who led the research.
“However, more research on the behaviour of these frogs and their predators is needed to pinpoint the potential function of this unique luminescence.”
Research on the toadlets was published on March 29 in Scientific Reports, an open access journal by the publishers of Nature.
Scorpions, coral and most recently chameleons have all been found to glow in the dark from florescence, a rarity in terrestrial species with backbones.
In bioluminescence, chemical reactions cause animals to glow. But florescence in the pumpkin toadlet is caused when molecules absorb light and then reflect it at longer wavelengths. Pumpkin toadlets have exceptionally thin skin that is about seven micrometers thick so the glow from the frog’s back and skull can easily shine.
Younger toadlets have a bluish glow that eventually becomes yellow as their skin thickens with age.
The secret could be in their bones. Bone fluorescence is associated with collagen and these frogs may have particularly collagen rich bones.
It is unknown why the frogs glow. It could be to warn predators like spiders or birds, who can see UV florescence even in natural light.
Their less colourful counterpart, the brown species B. hermogenesi, has good hearing and calls out to mates while hidden under leaves. It does not have dermal ossification, which supports the theory that florescence attracts mates when sound does not.
The toadlets are threatened by habitat loss. More than 85 per cent of the Atlantic rainforest has been destroyed.