Chameleons are well-known for their striking ability to quickly change colors, but scientists have just discovered a new visual trick in the creature's repertoire. Under UV light, many types of chameleons have been found to fluoresce in vibrant patterns, which they may use to communicate with others of their species.

Color-changing is more than a cool pet trick. In the wild, the ability helps chameleons hide when they don't want to be seen – say in the presence of predators – and stand out when they do – say, in the presence of a mate. They tend to turn bright colors to warn of aggression, and switch to a duller tone to signal submission. While that technique is well-studied, a team of German researchers made a new discovery simply by running an ultraviolet light over the creatures.

"We could hardly believe our eyes when we illuminated the chameleons in our collection with a UV lamp, and almost all species showed blue, previously invisible patterns on the head, some even over the whole body," says David Prötzel, lead author of the study.

Next, the team set about figuring out the how and the why of the fluorescence. First, they conducted micro-CT scans of the chameleons' heads, and found that the glowing patterns perfectly lined up with each animal's pattern of tubercles – bony bumps protruding from their skulls.

By making 3D reconstructions of the tissue cells, the researchers then found that the skin covering the tubercles is transparent and very thin, so it basically acts as a window to let UV light reach the bone, which is naturally fluorescent.

"It has long been known that bones fluoresce under UV light, but that animals use this phenomenon to fluoresce themselves has surprised us and was previously unknown," says Frank Glaw, co-author of the study.

The researchers also found that different species or species groups fluoresced in distinct patterns, and that males tended to have far more detailed patterns than females. That means these "invisible," unchanging patterns could be used as a kind of secret handshake, allowing chameleons to identify other members of their own species and possible mates. Color-changing skin might otherwise make that identification hard in the wild.

The research was published in the journal Nature Scientific Reports. Check out the fluro chameleons in action in the video below.

Source: LMU Munich