How chameleons change colour: they stretch a mirror made of crystals
A chameleon doesn't change colour by mixing pigments. Under its skin sit cells packed with tiny guanine crystals, arranged in a lattice that works like a tunable mirror.
When a male panther chameleon gets excited, the crystals move about 30% further apart and the mirror starts reflecting longer wavelengths. Seen through a layer of yellow pigment, calm blue shows up as green, and the longer excited reflections as blazing yellow and orange. The flashy changes are mostly for talking to rivals and mates, not for hiding.
For a long time the standard picture of colour change in lizards was pigment on the move: tiny sacs of dark or coloured pigment spreading out through a cell to show, or bunching up to hide. Chameleons do use dark pigment to go darker. But when a male panther chameleon flushes from green to blazing yellow and orange, something stranger is happening. His skin is not mixing paint. It is retuning a mirror.
A mirror built from crystals
In 2015 a team at the University of Geneva led by Michel Milinkovitch looked at panther chameleon skin (Furcifer pardalis) under the electron microscope and published what they found in Nature Communications. Just under the surface sits a layer of cells called iridophores, packed with tiny crystals of guanine, the same molecule that is one of the letters of DNA. Each crystal is only about 130 nanometres across, several hundred times thinner than a human hair, and they are stacked in a neat triangular lattice.
A regular lattice like that behaves like the film of a soap bubble or the scales of a butterfly wing. Light bouncing off each row of crystals overlaps with light bouncing off the next. For one particular wavelength the reflections line up and reinforce; for the rest they partly cancel. Which colour wins depends on one thing: the spacing between the crystals. Tight spacing reflects short wavelengths (blue). Wider spacing reflects longer ones (green, yellow, red).
So a chameleon does not need new pigments to change colour. It needs to change a distance measured in nanometres. In calm skin the crystals sit closer together; the Geneva team found the spacing was on average about 30% smaller in resting skin than in excited skin.
Why blue turns out green
A calm panther chameleon is green, not blue. That is because the mirror is not on its own. Above it lie cells filled with yellow pigment, called xanthophores. Blue light reflected by the crystals has to pass through that yellow layer, which soaks up the shortest wavelengths. What gets out is the blue-green edge of the reflection, and the skin looks green.
When the male gets excited, the crystal lattice expands and the reflection slides toward longer wavelengths: green, then yellow, then orange and red. Those colours pass the yellow filter easily, so the skin turns vivid yellow and orange. Play with it below.
They proved it with salt water
Seeing the crystals was not enough; the team wanted to show that changing their spacing really changes the colour. So they bathed pieces of skin in solutions of different saltiness. Making the surrounding fluid more concentrated, from 236 to 1,416 milliosmoles, draws water out of the cells, shrinking them and squeezing the lattice. The reflected colour shifted toward blue, just as it does when a real chameleon calms down. Under the microscope and in the optics models, the numbers matched.
In a living animal the whole change takes a couple of minutes and is fully reversible. The chameleon is actively nudging its crystals apart and letting them fall back together.
A second mirror, for heat
Deeper in the skin the team found a second layer of iridophores, with bigger, more loosely arranged crystals. This layer does not tune colour. Instead it reflects a large share of near-infrared light, the invisible part of sunlight that carries a lot of heat. The researchers suggested it works as a built-in sunshade, helping an animal that basks in open sun avoid overheating.
Mostly for talking, not hiding
The popular idea is that chameleons change colour to match their background. Most of the time they are already green and brown, which suits leaves and branches. The dramatic flashes are a different story. A 2008 study by Devi Stuart-Fox and Adnan Moussalli compared 21 lineages of South African dwarf chameleons and found that the species with the biggest colour-change abilities were the ones whose display colours stood out most to other chameleons. Their conclusion: extreme colour change evolved mainly for social signalling, not camouflage.
Panther chameleons fit the pattern. The big switch from green to yellow and orange happens when males square up to rivals or court females. Stretching a lattice of crystals by a few dozen nanometres is how they shout.