Scientists have long thought that chameleons change color when skin cell pigments spread out along veinlike cell extensions.

But Michel Milinkovitch, an evolutionary geneticist and biophysicist, says that theory didn’t wash—there are many green chameleons but no green pigments in their skin cells.

So Milinkovitch and his University of Geneva colleagues began “doing physics and biology together,” he says.

Beneath a layer of pigmentary skin cells, they found another layer of skin cells containing nanoscale crystals arranged in a triangular lattice. (Also see "Amazing Pictures: Baby Chameleon Doesn't Know It Hatched.")

By exposing samples of chameleon skin to pressure and chemicals, the researchers discovered that these crystals can be “tuned” to alter the spacing between them. That in turn affects the color