Not only are tiny Brazilian pumpkin toadlets bright orange, but they also with have fluorescent bones, which glow intense blue, no less.
Their little crania and a section of their ribs glow when illuminated with ultraviolet light, a collaboration of intrepid scientists from Brazil and NYU Abu Dhabi published in Scientific Reports this weekend. The glow seems to develop over the frog's life, and is weaker in the young ones and more intense in the older, larger individuals, most of which barely reach a couple of centimeters in size.
To be specific, the fluorescence was found in not one species of pumpkin toadlet, but two: Brachycephalus ephippium and Brachycephalus pitanga.
Why would Sandra Goutte of Universidade Estadual de Campinas, Matthew Mason of NYU Abu Dhabi and their colleagues shine UV light on the frogs anyway? Because science has serendipitously discovered that a lot of animals and plants fluoresce.
On land, fluorescence has been observed in parrots and other birds, spiders and their scorpion sisters, chameleons and frogs, they write.
Off land, the glow has been belatedly detected in sharks (yes, really), some other fish, the hawksbill turtle and other amphibians too. It turns out that some flowers have fluorescent stripes, for instance, that presumably guide bees – which can see them – to the nectar.
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We can't see the glow without shutting the creatures in the dark and shining UV lights at them. But just like how we can see red while dogs can't, other animal can see colors in ranges invisible to us. They can see the glow to which we have been oblivious until very recently.
To us it's spooky or gorgeous or just weird, but to them it's just another color.
What good does glowing do for them? We don't know specifically, but can surmise that they do exactly what other colors do for them: the fluorescence might aid in camouflage, or communication – telling fellow frogs that they are desirable, or telling non-frogs that they are toxic, for instance.
"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,” Goutte says.
This group of tiny toadlets, which actually range in color from orange to red to yellow, are the second frog-type found to fluoresce. The first, also found in South America, was only discovered in 2017: the polka dot tree frog. To us, it looks brownish and greenish and has dark spots, not so different from the rotting-leaf look so beloved of small amphibia. Stick it in a cave with UV light and it glows neon blue and green.
But here's an interesting little aside. The polka dot tree frog has the luminescent molecules in its skin, lymphatic tissue (why not?) and in glandular secretions (might as well). The Brazilian pumpkin frogs have their radiant molecules in the dermal bone of the head and back, and it is visible through particularly thin skin. The other glowing animals tend, more sensibly, to glow from their skin, cuticles or feathers. The only other being whose bones glow is the chameleon.
In fact, many chameleon species have bony tubercles protruding from the skull that are visible through their scales, and fluoresce under UV light, wrote a different team in January 2018, also in Scientific Reports. In chameleons, "Tubercles arising from bones of the skull displace all dermal layers other than a thin, transparent layer of epidermis, creating a ‘window’ onto the bone," explained David Protzel et al. Definitely sounds attractive. Indeed, that group postulated the glow was all about inter-chameleon sexual attraction.
As for our pumpkin toadlets – their fluorescence is very intense under UV illumination, suggesting that it serves for signaling. Previous research has found that while they make calls, they can't actually hear them, and have been known to resort to paw-gestures and demonstratively opening their mouths at one another. This could support another theory: The light show is a weapon, or at least a threat. In chameleons, a 2013 study found that the males use color change to intimidate one another. Imagine if while landing their chromatic punches, they also glow in the dark.