Chameleons change color via photonic crystals in their skin

BBC : Photonic crystals, rather than colored pigments, appear to be the mechanism by which chameleons achieve their rapid and brilliant color changes. According to a new study , researchers at the University of Geneva found panther chameleons to have two layers of light-reflecting nanocrystals, called iridophores, in their skin. The size and spacing of the crystals are what determine the skin color: When the lizard is relaxed, the crystals are tightly packed and reflect light at short wavelengths, such as blue; when the animal gets excited, the spacing of the crystals widens and they reflect longer wavelengths, such as red. The deeper layer of iridophores appears to have larger crystals arranged in a more disordered fashion, such that they reflect a substantial portion of near-IR light. The researchers say that layer may serve to help keep the animal cooler, but more experiments are needed to determine exactly what function the second layer provides.