A chameleon changes colour by retuning a crystal
The rapid colour shift is a change in the spacing of guanine crystals in a skin cell, which changes which wavelength they reflect. It is a lattice, not a dye being pumped.
Beneath a layer of ordinary pigment cells, a chameleon has iridophores packed with small guanine crystals in a regular array. A 2015 study showed that when the animal shifts into an excited colour, the spacing of those crystals increases, and the reflected wavelength moves from blue-green toward yellow and red. Relax the spacing and the colour shifts back. The pigment layer is real and it darkens the result. The tunable part, the part that moves in seconds, is the geometry.
Structural colour is common — a butterfly wing, a peacock feather — and it is usually fixed at growth. The chameleon’s version is adjustable in the living animal, which is why the same patch of skin can be a camouflage and then a signal. Nothing is added to the skin to make the new colour. The light was always being sorted. The sort changed.
Caveats
- Not every hue on every species is structural. Browns and blacks are pigments. The claim is about the rapid, reversible shift.
- The crystal explains the colour. It does not by itself explain how the animal chooses, or how well it matches a background.
Sources
- 1Teyssier J., Saenko S. V., van der Marel D., Milinkovitch M. C.. Photonic crystals cause active colour change in chameleons. Nature Communications 6, 2015.