Science Puzzle
How Does a Chameleon Change Colour?
A male panther chameleon can turn from green to bright yellow and orange in a couple of minutes. It does this mostly to signal to rivals and mates, not to hide.
For a long time people assumed it worked like an octopus, which changes colour by squeezing and stretching tiny sacs of coloured pigment in its skin. Experiments in 2015 showed that isn't what a chameleon does at all. So how does a chameleon change colour?
The Answer
By moving tiny crystals in its skin closer together or further apart. The colour comes from the spacing of the crystals, not from any pigment.
A layer of skin cells holds a regular, closely packed arrangement of minute crystals of guanine, each far smaller than the wavelength of light. Light reflects off every layer of crystals. For one particular colour, the reflections from all the layers line up and add together; for other colours they cancel out. Which colour wins depends on the spacing between the crystals.
When the chameleon is calm, the crystals sit close together and reflect blue. A layer of yellow pigment above them mixes with that blue, so the skin looks green. When it gets excited, it stretches those skin cells, the crystals move apart and the reflected colour shifts towards yellow, orange and red. Scientists at the University of Geneva showed this in 2015 by changing the spacing in samples of chameleon skin and watching the colour change with it.
Colour made like this is called structural colour, and it's the same trick behind the shimmer of a soap bubble, a peacock's tail and a blue morpho butterfly's wings. The chameleon's skin also has a deeper layer of larger crystals that reflects much of the Sun's infrared, which helps keep it cool.
The principle: Structural colour. Evenly spaced layers reflect one colour strongly because their reflections add together; changing the spacing changes the colour, with no pigment involved.