This tiny, Brazilian frog may have harnessed the power of fluorescence to communicate with other animals.
During the rainy season, from mid-October to March, a small, bright yellow frog prowls the floor of the Brazilian Atlantic forest. This is the pumpkin toadlet, and as it wanders, it emits a soft, buzzing mating call.
The ritual is heartfelt, but a little wasted. In 2017, scientists discovered that the call vibrates at a frequency that its desired audience - the alluring female toadlet - cannot detect. This makes the pumpkin toadlet the only perplexing example in biology of a species deaf to its own mating call.
So how do the frogs communicate, if not by sound? Scientists think that the tiny toadlet may have come to rely on a more visual strategy. When the male catches sight of a lady-frog that tickles his fancy, he waves his arms frantically above his head and lets his mouth gape open, in a grand display which is apparently irresistibly tempting if you happen to be a female toadlet. (Evidently, unwarranted and over-enthusiastic public displays of affection are not the exclusive reserve of human males in nightclubs). As an extension of this, some researchers believe that the physical throbbing of the creature’s throat could also be an important visual cue, with the sound of the call simply being an unnecessary by-product of the movement.
Now, however, a new element of intrigue has been added to the story of the pumpkin toadlet. While studying methods of communication between the frogs, researchers based at NYU Abu Dhabi stumbled upon a fascinating observation. To their surprise, shining a UV light on the tiny amphibian revealed striking fluorescent patches decorating its head and back. The patches cannot be seen by the naked human eye, but it is possible that they could be visible to other animals.
In a paper published in Scientific reports, the researchers explain that the entire skeleton of the pumpkin toadlet is actually intensely fluorescent. The patches are visible in the back and head of the amphibian because this is where the skin is thinnest, and therefore where the bone is most exposed to the UV light. Though fluorescence has been observed before in nature, it has almost always been in some form of external tissue: the cuticle of scorpions, for example, or the skin of marine turtles. The chameleon is the only other vertebrate known to have an externally-visible fluorescent skeleton.
As for the purpose of this fluorescence, there are still several possibilities. The researchers suggest that it could play a role in communication with other members of the species, or that it could serve to ward off potential predators. In jumping spiders, scientists have shown that fluorescence is important for mating, with females that fail to fluoresce being, at worst, callously ignored by potential suitors, and at best only half-heartedly pursued (a different kind of beauty standard indeed). Though a similar scenario could be envisioned in the pumpkin toadlet, Sandra Goutte - who worked in the study - believes that it is too soon to say.
“In nature if [the fluorescent patches] 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," she says. “However, more research on the behavior of these frogs and their predators is needed to pinpoint the potential function of this unique luminescence."