The color-changing ability of chameleons isn't down to chemistry, researchers have discovered, but rather structural changes inside their skin cells involving crystals.
Male chameleons can change their colors within seconds to attract a female or warn off a competing male.
Previously, most zoologists assumed the creatures' impressive color-swapping trick was probably a matter of moving pigment-filled bodies called melanosomes within the skin cells.
Many creatures rely on such accumulation or dispersion of pigments to modify their colors.
However, in chameleons, it's not about colors in or under the skin, researchers have found; it's about crystals and reflected light.
"These colors are generated without pigments," says Michel Milinkovitch, a professor of evolution and genetics at the University of Geneva and the senior author of a study published in Nature Communications.
At the heart of the chameleon's ability to turn from green to yellow with additional splashes or red and blue are nanocrystals of pure guanine, a component of DNA, the scientists say.
The creatures can actively "tune" these crystals in an outer layer of skin cells known as iridophores, they say.
Underneath is a deeper layer of cells with larger crystals, less well ordered, that can reflect infrared light.
The two superimposed layers are what allow chameleons to quickly switch between an efficient more-or-less green camouflage and a spectacular color display, the researchers found.
"We discovered that the animal changes its colors via the active tuning of a lattice of nanocrystals," say study co-first authors Jeremie Teyssier, a physicist, and biologist Suzanne Saenke.
"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."
Exactly how the chameleon manages and controls theses structural changes in its skin is as yet unknown, the researchers acknowledge, suggesting it may be a matter of an ability of swell or shrink the cells to transform the guanine lattice.
Every species of chameleon studied to date has displayed these two layers of specialized guanine-containing cells, Milinkovitch says.
"If this is confirmed in all chameleons, it would suggest that the ancestor of all chameleons had color-change abilities," he says.
In some species of chameleons that have lost the ability to change color, despite still possessing the dual skin layers, it is likely the guanine lattice has become less regular or less flexible, Milinkovitch suggests.
