How Do Chameleons Change Colour?

For decades, scientists assumed chameleons changed colour using pigment-containing cells (chromatophores), like octopuses. In 2015, researchers at the University of Geneva discovered the real mechanism: chameleons have a layer of iridophore cells containing a lattice of guanine nanocrystals. By relaxing or tensing the skin, the chameleon adjusts the spacing between these crystals, changing which wavelengths of light are constructively reflected (a process called structural colour, similar to how opals produce colour). At rest, the crystals are close together and reflect shorter wavelengths (blue), which combines with yellow pigment in overlying cells to appear green. When excited or threatened, the skin stretches, crystal spacing increases, and the reflected wavelength shifts to yellow, orange, or red. A deeper layer of iridophores reflects near-infrared light, possibly helping with thermoregulation. Colour change serves communication (territorial displays, mating signals), camouflage, and temperature regulation.