Chameleons Change Colour to Send Messages, Not Just to Hide

High on a sunny branch in Madagascar, a panther chameleon is having a peaceful morning. It is a calm leafy green, its curly tail wrapped around a twig, its eyes swivelling in two directions at once in the way that makes chameleons look permanently amused. Then another male appears on a nearby branch, and within a few minutes the first one has turned a blazing yellow and orange, as if someone had switched on a lamp inside it.

Most of us grew up with a simple picture of what is going on. The chameleon, we were told, changes colour to match whatever it is sitting on, so that it disappears. Put it on a red cushion and it turns red; put it on a chessboard and it gets very confused. It is a lovely idea, and like many lovely ideas it is only a small part of the story.

Chameleons can blend into their surroundings, and their colours do help them stay hidden. But the fast, spectacular changes that made them famous are mostly about talking to each other. When a mature male panther chameleon meets a rival, or a female he hopes to impress, he can shift the background colour of his skin from green to yellow or orange in a matter of minutes, and then back again. It is less a disguise and more a very bright announcement.

The surprise is in how the colour is made. For a long time the colour changes were put down mainly to pigments inside the skin cells spreading out and gathering in. In 2015 a team at the University of Geneva showed that the panther chameleon's big shifts come from something far more like physics. Beneath the surface of the skin lies a layer of cells packed with tiny crystals of guanine, arranged in a neat, regular lattice. Light bouncing off that lattice produces colour in the same way a soap bubble or a butterfly wing does, without any pigment at all.

When the chameleon is relaxed, the crystals sit close together and reflect blue light. Mixed with a yellow pigment in the skin above, blue looks green, which is why a calm chameleon is green. When the animal gets excited, it stretches that layer so the crystals move apart, and the reflected light shifts towards yellow, orange and red. The researchers measured the difference: in resting skin the crystals were on average about 30 per cent closer together than in excited skin. The crystals themselves do not change size. Only the spacing does.

The team also found a second, deeper layer of cells with larger crystals that reflects a good share of the sun's near-infrared light, the invisible warmth in sunshine, which may help a cold-blooded animal basking in the open avoid overheating. And the upper, colour-changing layer is fully developed only in adult males. Females and young chameleons have a reduced version, which is why the showiest displays belong to the grown-up males.

So how did anyone work this out? The Geneva researchers looked at the skin under electron microscopes, measured the crystal lattice in calm and excited states, and built computer models that predicted which colours each spacing should give. Then they tested it. In laboratory experiments, making the liquid around the crystals saltier drew them closer together, and the colour shifted back towards blue, just as the models said it would. The chameleon, it turns out, is not so much painting itself as tuning itself.