How can a chameleon change color by moving tiny crystals?

A male panther chameleon can shift a patch of skin between vivid colors during social interactions. The change is not always explained by moving pigment alone. In specialized skin cells called superficial iridophores, an ordered arrangement of tiny guanine crystals interacts with incoming light.

Guanine crystals and the surrounding cell material have different refractive indices. Reflections and scattering from their organized structure interfere, making some wavelengths more strongly reflected than others. The result is structural color: a material can select a color through geometry even when its crystals are not painted that color.

Measurements comparing relaxed and excited skin show a change in the separation of neighboring crystals. Greater spacing shifts the structure’s reflection toward longer wavelengths. Microscopy, optical measurements and photonic modeling support the connection. The crystals need not grow larger during this rapid change; their arrangement changes. The detailed biological control of that arrangement is a separate question from how spacing affects light.

The illustration uses a simple one-dimensional optical analogy: an effective index of 1.5 and a 150-nanometer repeat spacing give a 450-nanometer reflection wavelength under λ = 2nd. Increasing spacing by one fifth shifts that illustrative wavelength to 540 nanometers. Real chameleon skin has a three-dimensional crystal lattice, multiple layers and pigments, so this equation does not predict a particular animal’s exact color. Yellow pigments can combine with structurally reflected blue to produce a green appearance; reflected wavelength and the final visible skin color are not interchangeable.