How chameleons change colour

Colour change and iridophore types in panther chameleons.

CHAMELEON colours are not just for camouflage. Many chameleons, panther chameleons (Furcifer pardalis) in particular, can remarkably exhibit complex and rapid colour changes during social interactions such as male contests or courtship. Panther chameleons have been observed to display a dazzling array of colours when they fight over territory. It is generally interpreted that these changes are because of dispersion/aggregation of pigment-containing organelles (subunits of cells). Results published in a recent issue of Nature Communications, however, suggest that the colour changes, which are completely reversible and occur within minutes, are not the result of shifts in pigments alone. They are due to quick changes in light-reflecting guanine nanocrystals, which create structural colour within chameleon skin.

Two layers of cells known as iridophores (certain types of pigment cells) contain these nanocrystals. A superficial layer, known as S-iridophores, actively alters the spacing of these crystals to cause the rapid colour changes, while a deeper layer, made up of D-iridophores, reflects a broader spectrum of light near the infrared wavelengths. This idea of the tunable photonic crystals is a new way to look at physiological colour change in animals. In addition to camouflage and flashy fights, the latter property of these cells plays a key part in keeping these lizards cool.

Michel Milinkovitch and associates at the University of Geneva in Switzerland captured the chameleons’ vivid colour displays using high-speed videography, which confirmed that the changing colours were a result of shifts in reflected wavelengths—from blue to green and then orange or red —rather than an increase or decrease in the proportions of pigments. Further, electron microscopic analysis of the skin of adult chameleons revealed that in the S-iridophores of resting animals, which were blue or green, guanine nanocrystals were approximately 30 per cent closer to one another than they were in excited males, whose skin was white or yellow.

To test whether packing guanine crystals closer together caused the colour changes, the scientists placed chameleon skin cells in a high-salt solution. The increased osmotic concentration caused cells to shrink and crystals to cluster, shifting reflected wavelengths from yellow to green and then blue, much like the animals’ colours, indicating that the distance between nanocrystals likely causes the iridophores to shift the wavelengths they reflect.

According to Milankovitch, these animals can actually actively tune the geometry of these crystals in their iridophores, and they can do that reversibly. They can choose to go from green to yellow and back to green within minutes, which is surprising.

But precisely how chameleons tune their colour displays by changing the spacing in the guanine nanocrystal lattices remains unknown. In future work, Milinkovitch and his colleagues intend to test whether cytoskeletal elements play a part in moving crystals around within S-iridophores.

Typically, light-reflecting cells are only thought to play colour-related roles. This mechanism might explain why these chameleons are able to stay out in a lot of sunlight, whereas that would be a problem for a lot of other lizards.