Nature Commun. 6, 6368 (2015)

Chameleons change colour for camouflage, communication and regulation of body temperature. It has been thought that they change colour through the dispersion and aggregation of melanosomes — organelles in the skin that contain light-absorbing pigment. However, this process cannot easily explain the large colour variation seen in adult male panther chameleons under stress, whose background colour of the skin shifts from green to yellow or orange. Now, Michel Milinkovitch and colleagues at the University of Geneva show that male panther chameleons from Madagascar change colour by altering the spacing of guanine nanocrystals found in their skin.

Using histology and transmission electron microscopy, the researchers found that unlike other lizards, the skin of panther chameleons has two thick superimposed layers of iridophores — reflective and iridescent cells. The upper layer, which was seen to be fully developed only in the adult male, consists of iridophores containing closely packed guanine nanocrystals organized in a triangular lattice. When excited (for example during male–male contests), the distance between the nanocrystals increased. It is suggested that the shift in reflectivity from blue and green in the resting skin to yellow, orange and red seen in the excited state is due to this alteration in the geometry of the nanocrystals. Excited skins subjected to hypertonic solutions that cause the crystal lattice to shrink to the resting state resulted in a blue-shift in reflectivity, further confirming that the change in skin colour is due to the expansion and contraction of the crystal lattice. The lower layers of iridophores did not change colour. Instead, they reflected sunlight in the near-infrared range, suggesting that this layer is for thermal protection.