Why do chameleons change colour?

Chameleons have layers of skin cells called iridophores that contain nanocrystals of guanine. By relaxing or tensing the skin, the spacing between these crystals changes, which alters the wavelengths of light they reflect. A calm chameleon might show green; an aggressive or excited one can shift to bright yellow or red. Darker colours absorb more heat, helping the animal warm up. While colour change can aid camouflage, scientists now believe communication (signalling to rivals and mates) and thermoregulation are the primary purposes. Not all chameleon species can produce dramatic colour changes; some are limited to subtle shifts.