Video: Chameleons use crystals to change colour
It takes just 2 minutes for panther chameleons to change colour from demure green to flashy bright yellow or red, and experiments on their skin have now revealed how they do it. And it is in a way no one expected.
Scientists always assumed that chameleons changed their appearance by making different colours flowed through their skin, but the reptiles actually have a much smarter method. They rapidly rearrange tiny crystals in their skin so that they reflect different wavelengths of light. "Essentially, the crystals act as a selective mirror," says Michel Milinkovitch of the University of Geneva in Switzerland, whose team made the discovery.
Panther chameleons normally wear green, as camouflage, but adult males rapidly change to yellow or red when approached by a rival
A milllion crystals
Milinkovitch's team examined chameleon skin under the microscope and found that the cells contain crystals of guanine - one of the four DNA bases - in highly regular grid-like arrangements. Each of these cells, called superficial iridophores, contains hundreds of thousands of crystals.
The researchers then used computer simulations to show that, in theory, simply by altering the distance between the crystals their arrangement could reflect any colour of visible light.
Close together, they reflect blue light, which has a short wavelength, and allowed the other wavelengths through. As the distance between the crystals increased, the wavelengths of yellow, and then red, were reflected instead.
To find out if this mechanism was behind the colour changes in chameleon skin, Milinkovitch's team took small skin samples and dipped them in a salt solution. By varying the concentration, they could make the skin cells swell up or shrink, which in turn would either increase or decrease the distance between the guanine crystals.
Swell cells
They found that the wavelength of light reflected changed in exactly the way predicted by the simulations. "The change of colour in the simulation matched exactly the change in the colour of real cells," says Milinkovitch. "So to our surprise, we found it's a completely different, physical mechanism."
The team also found that females and young chameleons, which are unable to change colour, lack superficial iridophore cells.
Next, Milinkovitch aims to investigate how the animals actually trigger the colour changes. He suspects it could be by making cells swell up - as in the lab experiments with salt. Alternatively they might do it by physically pushing or pulling the skin cells to alter their size.
"This is a fascinating piece of work," says Ian Manners of the University of Bristol, UK, whose team is developing similar types of reflective crystals for use in sensors and display screens.
"It's remarkable and sobering to find out that nature has already tackled the problem very efficiently," he says. "The results illustrate beautifully how nature often solves problems through clever design rather than by complex molecular synthesis."
Journal reference: Nature Communications, DOI: 10.1038/ncomms7368
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