Photonic crystals cause active colour change in chameleons

TLDR

Chameleons, particularly panther chameleons, rapidly change color during social interactions, a phenomenon traditionally attributed to pigment organelle dispersion within dermal chromatophores. The study investigates whether chameleons achieve color change by actively tuning a lattice of guanine nanocrystals in superficial dermal iridophores. Using microscopy, photometric videography, and photonic band‑gap modeling, the authors demonstrate that color shifts result from active tuning of guanine nanocrystal lattices in a thick superficial iridophore layer. They also discovered a deeper iridophore layer with larger crystals that reflects much sunlight, especially in the near‑infrared, and that the two‑layer iridophore organization is an evolutionary novelty enabling camouflage, display, and potential thermal protection.

Abstract

Abstract Many chameleons, and panther chameleons in particular, have the remarkable ability to exhibit complex and rapid colour 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. Here, combining microscopy, photometric videography and photonic band-gap modelling, we show that chameleons shift colour through active tuning of a lattice of guanine nanocrystals within a superficial thick layer of dermal iridophores. In addition, we show that 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.