According to a team of scientists at the University of Geneva, Switzerland, chameleons shift color through active tuning of a lattice of nanocrystals within a thick layer of dermal cells called iridophores.
The veiled chameleon, Chamaeleo calyptratus. Image credit: Russell Ligon.
Chameleons (family Chamaeleonidae) are a distinctive and highly specialized group of lizards. Most scientists identify two subfamilies, containing 4-6 genera and approximately 180 species of chameleons.
Most of them live in Madagascar and Africa. The rest are found on islands in the Indian Ocean, and one, the Indian chameleon, in India, Pakistan, and Sri Lanka.
Another species, the common chameleon, is native to Spain, Portugal, the islands in the Mediterranean Sea, and in the Near East.
Chameleons live in a variety of habitats, from rain forests and lowlands to deserts, semi-deserts, scrub savannas, and even on mountains.
Many inhabit trees, but some live in grass or on small bushes, fallen leaves, or dry branches.
Chameleons are diurnal and primarily insectivorous, although birds have been recorded in the diets of some larger species.
They can extend their tongues almost the entire length of their body, making their hunting behavior an impressive display.
Many chameleons, and panther chameleons in particular, have the remarkable ability to exhibit complex and rapid color changes during social interactions such as male contests or courtship.
In a new study published in the journal Nature Communications, Swiss researchers demonstrated that these changes take place via the active tuning of a lattice of nanocrystals present in a superficial layer of iridophores.
The scientists also reveal the existence of a deeper population of iridophores with larger and less ordered crystals that reflect the infrared light.
The organization of iridophores into two superimposed layers constitutes an evolutionary novelty and it allows the chameleons to rapidly shift between efficient camouflage and spectacular display, while providing passive thermal protection.
“Besides brown, red and yellow pigments, chameleons display so-called structural colors,” said study senior author Prof Michel Milinkovitch from the University of Geneva’s Department of Genetics and Evolution.
“These colors are generated without pigments, via a physical phenomenon of optical interference. They result from interactions between certain wavelengths and nanostructures, such as tiny crystals present in the skin of the reptiles. These crystals are arranged in layers that alternate with cytoplasm, within iridophores.”
“The structure thus formed allows a selective reflection of certain wavelengths, which contributes to the vivid colors of numerous reptiles.”
To determine how the transition from one flashy color to another one is carried out in the panther chameleon, Prof Milinkovitch and his colleagues used their expertise in both quantum physics and in evolutionary biology.
“We discovered that the animal changes its colors via the active tuning of a lattice of nanocrystals,” said co-authors Dr Jérémie Teyssier and Dr Suzanne Saenko.
“When the chameleon is calm, the latter are organized into a dense network and reflect the blue wavelengths.”
“In contrast, when excited, it loosens its lattice of nanocrystals, which allows the reflection of other colors, such as yellows or reds.”
“This constitutes a unique example of an auto-organized intracellular optical system controlled by the chameleon,” they said.
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Jérémie Teyssier et al. 2015. Photonic crystals cause active colour change in chameleons. Nature Communications 6, article number: 6368; doi: 10.1038/ncomms7368
