They aren’t radioactive mutants, they’re not plastic toys, and they haven’t been injected with glowing jellyfish DNA, but these South American tree frogs give off a striking blue-green light!

Image: Julián Faivovich and Carlos Taboada (Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”—CONICET).

It's called fluorescence, and while animals that emit this glow are pretty common under the sea – from coral to fish to sea turtles – they're extremely rare on land. In fact, out of more than 7,000 species of amphibians worldwide, these polka-dot tree frogs are the first and only known example among their kind.

The discovery wasn't made out in the frogs' scenic forest home. Instead, researchers at the Bernardino Rivadavia Natural Sciences Museum in Argentina detected the glow while studying another peculiar feature of the frogs in the lab: their semi-transparent skin. To the scientists' surprise, the skin lit up with a vivid fluorescent radiance when exposed to the right light. 

“The observations were shocking,” said researchers Julián Faivovich and Carlos Taboada in an email. They were expecting to see very limited fluorescence, if any at all.

To back up their findings, the team headed out to the outskirts of Santa Fe, Argentina to see if they could find the fluorescing frogs in their natural habitat – and they weren’t disappointed. Males, females, young and adult ... they all gave off the same glow. “[W]e observed it in the field every time we went looking for this species,” the researchers said.

Image: Julián Faivovich and Carlos Taboada (Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”—CONICET).

Fluorescence occurs when a substance absorbs one kind of light, such as invisible ultraviolet, and re-emits it as a colour we can see (not to be confused with bioluminescence, where living things give off their own light). It's what happens when desert scorpions (or certain toys and clothing) glow brightly under an ultraviolet black light. Exactly why some animals do this, however, isn't fully understood yet.

“[T]he relevance of the phenomenon in most cases is unknown and probably meaningless,” the researchers said. Many animals may light up simply because they happen to have common, naturally fluorescent molecules in their bodies, not because there's a particular benefit they get from glowing. 

In some cases, however, fluorescence might help animals spot each other in their environment. Under the ocean, for example, light is quickly filtered down to the blue part of the spectrum, so everything starts to look a bit monochromatic. In this world of blue, turning invisible UV light into bright greens or reds can be useful for catching the eye of your fellow swimmers.

It looks like the frogs may be doing something very similar up on land. The species tends to be most active in the night and twilight hours, when light is limited. Under these conditions, the added glow from their fluorescent bodies makes the frogs much more visible than they would normally be in their natural habitats.

So is the glow just a quirk of the frogs’ body chemistry, or does it serve a particular purpose? To find out, the researchers identified the wavelengths of light being emitted, and compared this with studies on the eyesight of other tree frog species. As it turns out, the fluorescent frogs give off wavelengths of light that closely match the wavelengths that tree frog eyes pick up best. That doesn’t seem like a coincidence – these amphibians may be shining beacons for each other!

Close examination revealed where the fluorescence was coming from: in their skin and underlying tissues, the frogs have unique chemical compounds. The team named these new compounds "hyloins" – the molecules that give the species its glow.

The polka-dot tree frogs’ light show might be related to unique features of their anatomy. Besides their translucent skin, the amphibians also have unusual molecular crystals in certain tissues, as well as especially high amounts of bile pigments throughout their bodies. These may not sound like the most exciting chemical novelties, but they might be the secret to finding other fluorescent frogs.

“There are species having these characters distributed in seven [frog] families, and it is very likely that other frog species are fluorescent as well," the researchers said,

For now, the polka-dot tree frog remains the only known fluorescent frog species, but there may be many more out there, just waiting for someone to see them in the right light.