How do chameleons change colour?

A chameleon changes colour by retuning tiny crystals in its skin. Beneath a layer of yellow pigment cells, its skin holds cells called iridophores, packed with transparent nanocrystals of guanine arranged in a lattice. Those crystals don't contain colour: they create it, by making light waves interfere, so that only certain colours bounce back.

At rest, the crystals sit close together and reflect blue light. Filtered through the yellow layer above, blue becomes the chameleon's ordinary green. When the animal gets excited, the spacing between the crystals changes, and the reflected light can quickly shift to yellow, orange or red. A panther chameleon can switch from one vivid colour to another within a minute or two, to court a female or face a rival. How exactly the chameleon moves them apart is still not fully known.

For a long time, scientists knew how chameleons darken but not how they jump from one bright hue to another. In 2015, a team of physicists and biologists at the University of Geneva showed that the answer is physics: chameleons tune the spacing of their crystals like an adjustable filter.

And the famous reason, camouflage? It is real, but it isn't the main one. Chameleons most often change colour to send social signals, flashing bright when aggressive and going dark when they give up, and to control their temperature, darkening to absorb heat and lightening to reflect it. The old image of a chameleon matching its background, recorded by Pliny the Elder two thousand years ago, is only part of the truth.