Science
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Humans have long been fascinated by chameleons’ changing color to dazzle mates, scare rivals and confuse predators. On Tuesday, scientists said they uncovered the mechanism of the feat and that the results of their investigation astounded them.
Rather than use pigments to switch color, nanocrystals in the lizards' skin are tuned to alter the reflection of light — channeling blue wavelengths when calm and yellow or red when excited, researchers found.
"We were surprised," Michel Milinkovitch, a biologist at the University of Geneva told AFP.
"It was thought they were changing color through ... pigments. The real mechanism is totally different and involves a physical process," he said.
Color-switching in chameleons is the preserve of males. They use it to make themselves more flamboyant to attract mates and scare off challengers or duller to evade predators.
The mature panther chameleon used in the study, for example, can change the background color of its skin from green to yellow or orange, while blue patches turn whitish and then back again.
In most other color-changing animals, the pigment melanin alters a color's brightness by dispersing or concentrating within cells called melanophores, thus changing color intensity but not hue.
This process had long been thought to explain chameleons' color change as well, the team said. But that theory turned out to be false.
Skin analysis revealed that the change is regulated by transparent nano-objects called photonic crystals found in a layer of cells dubbed iridophores, which lie just below the chameleon's pigment cells.
Iridophores are also found in other reptiles and amphibians like frogs, giving them the green and blue colors rarely found in other vertebrates.
In chameleons, however, nanocrystal lattices in the iridophores can be tuned to change the way light is reflected, the university said in a statement.
"When the chameleon is calm, the latter [crystals] are organized into a dense network and reflect the blue wavelengths" of light, it said. "In contrast, when excited, it loosens its lattice of nanocrystals, which allows the reflection of other colors such as yellows or reds."
The team used biopsies of chameleon skin, pre- and post-excitement, combined with optical microscopy and high-resolution videography to study the phenomenon.
They also discovered that chameleons have a second, deeper layer of iridophore cells. These contain larger and less ordered crystals that reflect infrared wavelengths from strong sunlight — in essence, a clever heat shield.
"The organization of iridophores in two superimposed layers constitute an evolutionary novelty," the team said.
"It allows the chameleons to rapidly shift between efficient camouflage and spectacular display while providing passive thermal protection."
Other reptiles have only one type of iridophore cell that cannot be used to change color, the researchers said.
Next scientists would like to figure out the mechanisms that allow chameleons to tune the crystal lattice.
The groundbreaking findings were published in the journal Nature Communications.
Discoveries in the animal world have spawned a field called biomimicry.
Engineers often seek to replicate wonders of evolution such as spider silk and gecko feet for products with commercial or military uses.
Agence France-Presse
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