How Chameleons Change Color

A new study suggests that the transformation takes place via crystals (crystals!) arranged within the reptiles' skin.

Skin that changes color according to one’s emotional status may be the stuff of human nightmares; for chameleons, though, it’s essential to survival. The lizards’ hue-changing skin, for one thing, functions as a kind of external thermostat, allowing the creatures to regulate their body temperatures: Dark skin absorbs more heat than pale. But changeable skin also allows the animals to communicate with each other. Males become bright when they’re attempting to express their dominance; they become dark during aggressive encounters with those who would question that dominance. Females use their skin color to signal a willingness—or lack of willingness—to mate. It’s skin that doubles, essentially and a little horrifically, as a flexible mood ring. “Owners of chameleons,” Wired pointed out last year, “can learn to read their pet’s mood based on the color of its skin.”

But how, actually, do the animals carry out their skin-based semaphore? A new study from the journal Nature Communication suggests an answer. Researchers used spectroscopy to examine in detail how light and matter interact within the chameleons’ skin. Their conclusion: The color-changing takes place via crystals—iridophores, they’re called—arranged under the chameleons’ skin. The iridophores are formed from guanine, one of the building blocks of DNA, and they’re “efficiently organized” across the chameleons' skin in a triangular lattice design.

When the chameleon gets aroused in some way, be the excitement be the result of a threat or of a Ryan Gosling-esque male, the latticework stretches. And that, in turn, crystals working the way they do, changes the wavelengths of light that the crystals reflect. "They are like selective mirrors," Michel Milinkovitch, a co-author of the study at the University of Geneva, told the BBC.

To reveal those “mirrors,” Milinkovitch and his colleagues focused on the panther chameleon, native to Madagascar and able to transform itself, in a matter of minutes, from green (its most common state) to yellow and red and back again. The researchers extracted the top layer of the animals’ skin, exposing it to chemicals that made the iridophores change in size. They also filmed their subjects while they were facing rival males, thus brightening and darkening themselves to demonstrate dominance and then aggression. And they saw that the colors of the chameleons being observed matched the colors of the skin that was being transformed chemically, in a petri dish. As Milinkovitch put it: "It really demonstrates that the color change is happening due to the modification of these crystals."

In other words: According to the research, it’s not pigments that change the chameleons’ color; it's the creatures’ sparkling skin. Chameleons communicate, and survive, in many ways. One of those ways, it seems, is a light show.