Chameleons exhibit complex, rapid color changes during social interactions such as male contests or courtship.It is generally interpreted that these changes are due to dispersion/aggregation of pigment-containing organelles within dermal chromatophores. Chameleons shift color through active tuning of a lattice of guanine nanocrystals within a superficial thick layer of dermal iridophores. In addition a deeper population of iridophores with larger crystals reflects a substantial proportion of sunlight especially in the near-infrared range. The organization of iridophores into two superposed layers constitutes an evolutionary novelty for chameleons, which allows some species to combine efficient camouflage with spectacular display, while potentially providing passive thermal protection.
Many vertebrates can rapidly change color for camouflage, communication and thermoregulation, but these so-called physiological color changes are generally mediated by modifications of skin brightness (that is, diffuse and/or specular reflectivity) through dispersion/aggregation of pigment-containing organelles, especially melanosomes, within dermal chromatophores. On the other hand, rapid active tuning of skin hue has been described in only a handful of species and generally involves structural, rather than pigmentary, components, that is, multilayer nano-reflectors with alternating high and low refractive indices that generate interference of light waves. For example, some species of squid can rapidly tune skin iridescence through periodical invaginations of plasma membrane deep into specialized cells called iridophores, generating arrays of alternating cytoplasmic protein-rich lamellae and extracellular channels. In fish, amphibians and reptiles, iridophores containing transparent guanine nanocrystals generate a large variety of structural colors, and modifications of the multilayer reflector geometry has been suggested to generate color change in a few species. Finally, it must be emphasized that the color of a reptile skin patch is often the result of interactions among pigmentary and structural elements.
Chameleons have two superimposed populations of iridophores with different morphologies and functions: the upper multilayer is responsible for rapid structural color change through active tuning of guanine nanocrystal spacing in a triangular lattice, whereas the deeper population of cells broadly reflects light, especially in the near-infrared range.
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