How Do Chameleons Change Color?

There is a misconception about chameleons that they change their color in order to blend in with their environment. That is actually not the case. When a chameleon is calm, it is green, and so it naturally blends in with its leafy surroundings. But male chameleons change color when they become excited in the presence of a female or a rival male, as shown here.

I will put the first frame in the bottom right for comparison. As you can see, as he becomes more excited, his color changes to be more yellow, orange, and red. So instead of blending in, this chameleon will now stand out. But how do chameleons actually change their color?

For years, we thought that there was a simple, well-established mechanism for this color change. In their skin, chameleons have different cells containing colored pigments. Some are yellow, others are red, and some contain the dark melanin pigment. Many animals, including chameleons, are known to turn a darker shade by causing the melanin to spread out along the fingery extensions of the cell. They turn lighter again by condensing the pigment back into one spot.

It was assumed that chameleons must turn yellow, orange, and red in the same way, by causing those pigments to spread out in those cells. But new research from collaborating groups of physicists and biologists at the University of Geneva shows that this is incorrect. Now the first clue should come from the fact that there is actually no green pigment in a chameleon.

The green color is actually created by two distinct mechanisms: pigment color and structural color. Beneath the yellow pigment cells, there are cells containing tiny 130 nm crystals regularly arranged in a lattice. Now light diffracts off these crystals, and due to the spacing between them, blue light constructively interferes and is therefore strongly reflected, whereas the other colors are not reflected.

This is very similar to how a Morpho butterfly wing creates an iridescent blue. There is no blue pigment, just the periodic nanoscale structure that acts as a strong reflector for only one color. So a chameleon looks green due to the combination of the yellow light from the pigment plus the blue light reflected off the crystal structure beneath it.

So how do chameleons change this green into yellow, orange, and red? Well, previously it was assumed that the color change was achieved by dispersing pigments in the colored cells. But now scientists have found it's actually the crystals underneath which are changing. By increasing the spacing between the crystals, the chameleons can change which color is selectively reflected.

Bigger gaps between the crystals are better at reflecting longer wavelengths. So as the spacing increases, the color changes from blue to green to yellow, and then orange and red. Scientists compared skin samples from calm green chameleons and excited yellow chameleons and found that indeed, in the excited chameleon, the crystal spacing was much further apart.

Here you can see a single cell enlarged, and the color it reflects changing from blue to green to yellow, orange, and red. Plus, a computer simulation of the light reflected off crystals while their spacing is decreasing also provides excellent agreement with these observations.

Furthermore, applying physical pressure to the skin causes the crystals to compress, and so they reflect more blue. When that pressure is removed, the crystals expand again, reflecting more green light. The structure of a chameleon’s skin gets even more fascinating, as there is another layer of cells underneath which also contain crystals.

But these crystals are larger, more spaced out, and more disorganized. This means they reflect longer wavelengths in the infrared part of the spectrum, and they reflect a broader range of these wavelengths. It’s thought that the function of this layer could be to reflect light from the sun since these chameleons live in bright full-sun habitats.

So male chameleons don't change their color by spreading out red and yellow pigments. Instead, they do something much more remarkable. They actively tune the spacing between nanoscale crystals in order to create structural colors that span the whole length of the visible spectrum.

Now what I find amazing is that in this day and age, we thought we had the answer to how chameleons change color, but it actually took the combined expertise of physicists and biologists working together to figure out what's really going on.

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