Chameleon colour-change breakthrough

Male chameleons have a well-deserved reputation as the colour-changing kings of the natural world, quickly switching their skin colour into a wonderful array of hues. But how do they do this?

It was previously thought that chameleons performed this quick change by shuffling pigment chemicals around cells in their skin, but there wasn't any actual evidence to support this idea. Now a team at the University of Geneva have taken a closer look, using a high-powered electron microscope. They've discovered that a special layer of colour cells in the chameleon's skin - called iridophores - contain tiny nano-crystals that bend light, and give the reptiles their colour-changing ability. Kat Arney caught up with Michel Milinkovitch to find out more...

Michel - We came up with this crazy hypothesis that given we didn't see how the animal could change colour, we thought that maybe, what is happening is that this animal was able to actually tune the distance between their nanocrystals because that would of course shift the light that is reflected. The photonic crystals act like a selective mirror. All light is going through except a specific wavelength that will be reflected with 100% efficiency. So, what you get is a very bright and very pure colour. But the wavelength that is reflected specifically is a function of the distance between the successive layers of materials. So, when the distance is very short, it would be blue light and when the crystals are more distant from each other it would be more yellow or red light.

Kat - Then when you shift the organisation of these layers of crystals then it basically goes a different colour. Were you surprised when you realised that this was happening?

Michel - Yes, we were. We never thought that actually this would be a possibility, for the animal to actively modify the geometry of the photonic crystal to change the distance among the nanocrystals.

Kat - How do you think that the chameleons are doing this?

Michel - Maybe they do it in the same way as we recapitulate the phenomenon ex vivo. We take a piece of skin, we put that in a Petri dish and then we change the concentration in salt basically, of the solution of which we keep the sample of skin. And therefore, the cells will shrink or swell depending on the amount of soil that you put in the system.

Kat - That will mean that the crystals get closer or further apart.

Michel - Exactly. That's what we were hoping for, is that by changing the geometry of the whole cell, you would force the lattice of nanocrystals to also shrink or swell. That's what is happening. We see exactly that. We see really single cells going from red to blue, passing by all the other wavelength of the spectrum.

Kat - It must be wonderful to work in your lab and see all these beautiful colours changing in front of you. It must make you think that nature and chameleons are just quite amazing.

Michel - These are amazing creatures. Actually, that's probably the reason why there is so much interest for these results. People really love chameleons because they have many different features that are spectacular. They have these protruding tongues that they can project at a distance to capture a prey. They have these weird feet and then they have these spectacular abilities to change colour which is really something unique in lizards. They have an amazing toolkit in their skin.