Scientists have discovered that chameleons are able to adjust a layer of special cells with their skin in order to change color. They found that they do not change color by accumulating or dispersing pigments in their skin cells, as the squid or octopus do, but rely on changing the structure of their skin to affect how light reflects off it, writes Laura Geggel for Scientific American.
Iridophore cells containing nanocrystals
The researchers found that chameleons have two thick layers of iridescent cells that contain pigment and reflect light, known as iridophore cells. These cells contain nanocrystals of varying sizes, shapes and distributions, and are crucial in changing color.
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The structural arrangement of the upper cell layer is changed when the chameleon relaxes or excites the skin, causing a change in color. Changes such as the presence of a rival male caused excitement in the skin and a corresponding change in color.
Researchers found that when the skin was relaxed, the nanocrystals in the iridophore cells were closely packed and the cells reflect short wavelengths, such as blue. In contrast, an excited state increases the distance between the nanocrystals and causes the cells to reflect longer wavelengths such as yellow, orange or red.
Gender and age influences color-changing ability
However chameleons do not always appear to be blue, caused by the fact that their skin contains yellow pigment, according to study senior author Michel Milinkovitch, a professor of genetics and evolution at the University of Geneva in Switzerland. When mixed with the blue light reflected by the iridophore cells, the lizards appear to be green, camouflaging them among the foliage.
A secondary layer of skin cells reflects a large amount of near-infrared sunlight. Although they do not seem to change color, these cells may help the lizards to reflect heat and regulate their body temperature.
The researchers filmed the chameleons and compared their color changes with numerical models which predicted the behavior of the nanocrystals. They also manipulated the cells using solutions of different concentrations, changing the distance between the nanocrystals.
According to the study only adult male chameleons change color, in order to scare away rivals or attract a mate. Females and young chameleons have a reduced upper layer of iridophore cells and are generally dull-colored.