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In 2018, a group of scientists in Germany announced that they had seen something truly weird after shining a blacklight on a chameleon. They observed a constellation of glowing blue speckles on the chameleon’s head, without a clear source for that light. The researchers soon realized the dots were tiny bones on the chameleon’s skull that fluoresced, or glowed, brightly enough to be seen through their skin. And it turns out, all bones fluoresce when hit with blacklight, even yours!
See, objects that fluoresce aren’t actually generating their own light. What you’re seeing is one wavelength of radiation being absorbed and then rapidly reradiated at a different wavelength. Bones are a mixture of mainly collagen and calcium phosphate. By sheer coincidence, both of these tissues happen to fluoresce under ultraviolet light, especially the collagen, so all bones can fluoresce under a blacklight. But, while bones are naturally fluorescent, they’re not the strongest fluorescers. This is why the bones in our bodies are not visible through our skin, the light they radiate isn’t bright enough to fluoresce through the flesh that’s covering them.
Until recently, the only scientists who really cared about this property of bones were archaeologists and forensic scientists, because UV lights can help them find pieces of bone more easily. Outside of those circles, the fact that bones can glow hadn’t really mattered to many other biologists. That is, until those German scientists saw the light, so to speak.
And to be clear, animals using fluorescence isn’t all that rare. There’s an abundance of sometimes cute and sometimes terrifying marine and terrestrial animals that do it. However, those species usually only fluoresce from their external tissues like the cuticles, skin, or feathers. So these chameleons were the first animals that we had observed who let their bones shine through, and they do it in a pretty neat way. They have little bones called tubercles, that sit just 20-25 micrometers below the outermost layer of skin, which is thin enough for the fluorescent light to shine through. And their skin also acts as a filter to make the light glow even bluer, which suggests that the ability for other chameleons to see the bones through their skin is not an accident.
Plus, it turns out that this isn’t even that rare for chameleons! We now know there are lots of species of chameleons that have glowy tubercles. In general, chameleons are known for having big heads with bright colors and funky bones, so their light-glowing bones fit right in with their other methods of showing off to each other.
But what’s really wild is that these bright bones have since been observed in more animals besides chameleons. As we talked about on our sister channel Bizarre Beasts, the teeny, clumsy frogs called pumpkin toadlets have extra-thin sections of skin on their head and back, which allow their luminous bones to shine through.
But why would two different animal groups evolve such a weirdly specific thing? Well, the most likely hypothesis seems to be that they use it for communication, of many sorts. Territorial marking, mating, and species differentiation are all possible explanations. We don’t know how these two vertebrate groups seem to have taken advantage of this intrinsic property of their bones in a way that no other animals have, despite how common other mechanisms of fluorescence are in the animal kingdom.
That said, researchers speculate these chameleons and toadlets aren’t alone in this phenomenon, and this discovery could open up the doors to the recognition of widespread fluorescence as a communication method in many other creatures. Although we have known since the 1960s that bones are capable of fluorescing, it seems that we are only now beginning to understand the potential for this to be a widespread method of animal communication. This begs the question: what other animal adaptations are hiding in plain sight? Unfortunately, we cannot communicate with our bones like some animals can, but we can communicate with each other using Linode’s tools.
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