Chameleons are color-changing, tongue-whipping, eye-rolling lizards. But did you know they also glow in the dark?

A new study published this week in the journal Scientific Reports revealed just that. It's the first time researchers have reported bone-based fluorescence in vertebrates.

Glowing Up

The proteins, pigments, and other materials that make up bones help them to glow under ultraviolet light—just think of how your florescent smile lights up under a black light. We've known that 75 percent of deep-sea creatures can glow in the dark, so this light-emitting characteristic is common in marine species.

Photograph by David Prötzel

But biogenic fluorescence is rare in terrestrial vertebrates. Only in March was the first fluorescent frog discovered in the Amazon. (These animals glow for science.)

On January 15, a team of German researchers published a paper showing that the bones of chameleons glow under UV light. They tested the light rays on 160 specimens that spanned 31 species of Calumma chameleons, which are endemic to Madagascar. Micro-CT scans revealed that a bright blue glow emanated from the lizards' skeletons and shined through their skin.

Tubercles, or small, rounded, bony projections, dot the faces of chameleons. The lizards also have four layers of thin skin, which have different pigments to help them change color. When the scientists shined a UV light over the chameleons, the glowing tubercles shined out from their skulls and through their skin.

"We could hardly believe our eyes when we illuminated the chameleons in our collection with a UV lamp," lead author David Prötzel, a doctoral student at the Bavarian State Collection of Zoology, says in a statement. "And almost all species showed blue, previously invisible patterns on the head, some even over the whole body."

The scientists said the gaps where the tubercles peaked through the skin were "windows" that helped UV rays reach the bone and get absorbed.

For chameleons, the glow likely shines brighter than it does for humans. Blue is a rare color in the lizards' forested habitats, so glowing in the dark would make them stand out from their green and brown backdrop.

Shade Shifters

Chameleons can change color depending on a lot of factors. A shift in mood, perhaps triggered by fear or anger, can cause them to shift shades, as well as varying temperature, humidity, and amounts of light. Males will sometimes make themselves brighter in order to attract females and establish dominance; more submissive shades are brown and gray. A change in color can also show if females are accepting or rejecting male partners, or it can indicate pregnancy.

But chameleons can't change to any color they want, and they can't exactly mimic their environment. A lizard on a striped or polka dotted background won't be able to adopt that funky pattern. (Read: "Inside the Secretive World of Florida's Chameleon Catchers")

Photograph by David Prötzel

Instead, species have a certain array of patterns and colors they can take on. Nerve impulses and hormone shifts can cause their skin to expand or shrink and blend different layers for different colors and patterns. (Read: "The Colorful Language of Chameleons")

Scientists aren't quite sure yet why chameleon bones glow in the dark. Since glowing tubercle patterns vary among species, the trait could help them recognize members of the same species. The researchers also found that males have more tubercles than females, a sexually dimorphic characteristic that could indicate when chameleons are ready to mate.