Maybe their moms told them nobody likes a showoff. That would explain why many species of chameleon are hiding fluorescent bone bumps on their heads that scientists only just discovered. Chameleons also have independently moving eyeballs, superlative tongues and sophisticated color-changing skills. The animals might use their glowing head bumps as signals to each other. These patterns of dots are invisible to a human eye, but may light up deep blue to the eye of another chameleon in a shaded forest.
Scientists knew that chameleons have bony crests and bumps called tubercles on their skulls, and that these bone shapes vary between species. But “we were always wondering about the function of the tubercles on the head,” says David Prötzel, a herpetologist at Zoologische Staatssammlung München in Germany.
Then a photo on Flickr caught Prötzel’s attention. The picture showed a chameleon called Calumma gastroaenia. The photographer, Paul Bertner, had put a UV light over the lizard, and three tiny tubercles on the chameleon’s head were glowing.
Prötzel and his colleagues set out to shine a UV light on every chameleon they could get their hands on. The researchers checked hundreds of preserved chameleon specimens, representing dozens of species from Madagascar and mainland Africa. They also photographed a few living chameleons in the wild in Madagascar.
The fluorescent bumps were everywhere. “We found that many species fluoresce with even larger pattern on their heads” than the animal in that Flickr photo, Prötzel says.
Chameleons don’t light up like fireflies or glowing squid do, using chemical reactions. Instead, they take advantage of the fact that bone is naturally fluorescent under UV light. (Forensic researchers take advantage of this too.) Fluorescent materials absorb light of one wavelength and send it back out at a different wavelength.
Under a microscope, the researchers saw that the bony tubercles on a chameleon’s head displace most of the layers of its skin. Only a thin layer of epidermis covers each bump. This thin skin “functions as a ‘window’ through which the bone is directly visible,” the scientists write.
The fluorescent bump patterns were more common in types of chameleons that live in forests, as opposed to in open areas. These shaded forests have more ambient UV light than habitats under direct sunlight do. Males also carry more head bumps than females do in most species of the genus Calumma, which the researchers focused on. This suggests the glowing tubercle patterns are some sort of signal.
Although we humans can’t see these patterns without shining extra UV light on an animal’s head, chameleon eyes have receptors for UV light. The researchers think the chameleons can see the patterns under the natural sunlight falling through a forest, “but we cannot prove that,” Prötzel says. The patterns might even appear brighter and clearer to the lizards than they do in the scientists’ pictures.