Scientists studying chameleon skin have discovered the secret to the lizards' color-changing prowess: Rather than relying purely on pigments, the animals use photonic nanocrystals in their skin to manipulate light with exquisite precision.
The findings, described in the journal Nature Communications, showcase the remarkable abilities of these colorful creatures in a whole new light.
The male panther chameleon (Furcifer pardalis) of Madagascar is an extremely talented shade-shifter. His thick-striped body can go from a blue-green palette to a fiery yellow-red-orange in a matter of minutes, if he wants to show off to an interesting lady lizard or a competing male.
When excited, their skin goes through some very specific color switches, the study authors wrote: Green goes to yellow or orange; blue patches turn whitish; and red becomes brighter and more uniform.
Still, they're kind of hard to actually find in the wild – their resting color scheme makes for remarkably effective camouflage.
"I assure you: In Madagascar, they are really difficult to spot," said study coauthor Michel Milinkovitch, a biophysicist at the University of Geneva.
How panther chameleons achieve this completely reversible color change has fascinated scientists, but it's not all that easy to study these reptiles, Milinkovitch said. For one thing, they're tough to raise in captivity: Their eggs take 10 months to incubate.
Many scientists assumed that the animals' speedy shade-shifting came from moving pigments around inside of cells called chromatophores, but Milinkovitch and colleagues doubted that explanation. Pigments – including the ones that we have in our own skin and hair -- typically work by absorbing most colors of visible light except one. So a red pigment absorbs most of the wavelengths of visible light and lets only the red wavelengths bounce off of its surface.
But chameleons also have iridophores, cells that manipulate color in a very different way. Instead of absorbing light, they use a phenomenon known as structural color, which harnesses a surface's nanoscale geometry to force certain wavelengths of light to bend or bounce in specific ways. Structural color is found across the animal kingdom, from the scintillating sapphire wings of the blue morpho butterfly to the iridescent shades of mother of pearl.
Could the iridophores, rather than the pigment-filled chromatophores, be responsible for the color changes? To find out, the researchers studied the panther chameleon's skin using a number of different methods, including filming the males' dramatic color changes with photometric videography and analyzing the skin's nanoscale structure using transmission electron microscopy.
The scientists identified two layers of skin with iridophores – and the ones in the top layer were filled with tiny nanocrystals of guanine, arranged into lattice formation with very precise spacing between each crystal.
The spacing, it turned out, was the key. The scientists took "excited" skin samples from male chameleons who were showing off to each other, and they found that the guanine crystals were spaced much farther apart than they had been in the "resting" state.
Each color of visible light has a different wavelength – blues are on the short end, and reds are on the long end (with the other colors arranged in between). So when the guanine crystals have short spaces between them, they reflect the bluer wavelengths. When the crystals are spaced farther apart, they reflect the longer wavelengths – more red wavelengths.
Combined with yellow pigment cells (called xanthophores) in the skin, the blues can be seen as greens and the reds as oranges (or plain old yellows). There are also red pigment cells in the skin – those areas look even redder when the chameleons turn their skin on.
There was also a second layer of skin filled with larger, less-organized crystals – these seem to be very good at reflecting near-infrared light, which in hot, dry environments, is probably a key survival mechanism to avoid overheating, the researchers said.
Milinkovitch said it would be useful to study such photonic crystals in other animals, such as zebrafish (even though they cannot adjust the color by changing the crystals' geometry).
But, he added, "we need to continue investigating the chameleons because part of the answers are there and nowhere else."