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Video: Scientists surprised to discover tiny toadlets can glow

  • Pumpkin toadlets (Brachycephalus ephippium) inhabit Brazil’s Atlantic Forest, where they crawl through the leaf litter in search of mates.
  • However, researchers found that they can’t hear their own high-frequency mating calls.
  • While investigating how they communicate to find mates, the researchers unexpectedly discovered that the frogs fluoresce when exposed to UV light.
  • The researchers aren’t sure why they do this, but say it could be a way to avoid predation or attract mates.

The forest floor along Brazil’s Atlantic coast is home to a species of teeny-tiny frogs called pumpkin toadlets (Brachycephalus ephippium), so named because of their bright orange color that warns would-be predators that they contain a deadly toxin. Now, new research reveals they have another trick up their sleeves – they glow.

Pumpkin toadlets (Brachycephalus ephippium) are tiny, reaching only about a centimeter (half an inch) in length. Photo by Carol Manzano via Wikimedia Commons (CC BY-SA 4.0)

The discovery came as something of a surprise. A team of researchers from institutions around the world was studying the pumpkin toadlets’ acoustic abilities. During mating season, the toadlets crawl around en masse in the leaf litter, softly buzzing in their quest for procreation. But when the researchers took a closer look, they discovered that the little frogs did not have the ability to hear high-frequency sounds – they couldn’t hear their own mating calls.

Communication is a big part of frog life. Most frogs attract mates by vocalizing (think of the familiar croaking in springtime swamps), but a few species employ other means. For instance, India’s dancing frogs, which inhabit swift-moving streams that are that are too noisy for ribbiting, wave their legs to seduce potential paramours.

So just how do love song-deaf pumpkin toadlets find mates? The researchers thought the answer might lie in some sort of visual display, and in one of their experiments tried shining a UV light on them.

To their surprise, the toadlets’ backs and heads lit up in a dramatic, opalescent blue glow.

Video by NYU Abu Dhabi.

This glow comes from fluorescence, which happens when certain molecules absorb light and then re-emit it in longer wavelengths. Unlike bioluminescence, which is a chemical reaction that creates light, fluorescence is dependent on the presence of light and can’t happen in complete darkness.

When the researchers dug a little deeper, they discovered that the frogs’ entire skeletons are fluorescent, but only the skin on their heads and backs is thin and unpigmented enough to let the light through. Their findings were published today in the journal Scientific Reports.

The researchers aren’t really sure why pumpkin toadlets fluoresce under UV light. They hypothesize that it could serve as an additional warning to predators looking for a snack, or perhaps help the little frogs find mates.

“The fluorescent patterns are only visible to the human eye under a UV lamp,” said Sandra Goutte, a postdoctoral associate at NYU Abu Dhabi who led the study. “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.

“However, more research on the behavior of these frogs and their predators is needed to pinpoint the potential function of this unique luminescence.”

A pumpkin toadlet exposed to UV light (right) emits a bright blue glow. Image courtesy of NYU Abu Dhabi.

Pumpkin toadlets live in Brazil’s Atlantic Forest, which is one of the most degraded and endangered natural regions on the planet. As little as 3.5 percent of its primary forest remains, as vast tracts have been cleared to make way for soy, cattle, and sugar cane.

But the toadlets are lucky – they inhabit a portion of the Atlantic Forest along the coast that has largely escaped deforestation due to a network of protected areas. This means that there should be plenty of pumpkin toadlets – and time – for researchers to figure out just why they glow the way they do.


Banner image: A pumpkin toadlet, by Mauro Regalado Soares via Wikimedia Commons (CC BY-SA 4.0).

Citation: Sandra Goutte, Matthew J. Mason, et al.  (2019). Intense bone fluorescence reveals hidden patterns in pumpkin toadlets. Scientific reports 9, Article number: 5388 (2019)

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