Chameleons are one of the most famous champions of quick color change, alternating between stealthy camouflage and flashy displays within minutes. And now researchers have revealed their rapid color-change secrets: Chameleons have light-reflecting nanocrystals in their skin that can be rearranged. The findings are published in Nature Communications this week.
In Madagascar, male and female panther chameleons (Furcifer pardalis) of all ages can alter the brightness of their skin, but adult males demonstrate a much bigger range, with various combinations of white, fiery reds, and calming hues. When encountering a rival or a potential mate, a mature male panther chameleon can shift the background color of his skin from green to yellow or orange, while blue patches turn whitish, and reds becomes brighter. This all happens within a couple of minutes, and it's totally reversible.
Many animals can change their colors fairly rapidly—like octopuses and flounders—though these changes typically depend on the accumulation or dispersion of pigments (like the melanin in our skin and eyes). Chameleons, on the other hand, rely on structural changes to affect how their skin reflects light. Structural colors are those that are “generated without pigments, via a physical phenomenon of optical interference,” Michel Milinkovitch from the University of Geneva explains in a news release. “They result from interactions between certain wavelengths and nanoscopic structures.”
Now, by combining microscope work, videography, and modeling, Milinkovitch’s team has discovered that chameleons accomplish these color-changing feats by actively tuning a lattice of light-reflecting nanocrystals within a top layer of skin cells called iridophores.
These photonic nanocrystals are made of guanine, one of the building blocks of DNA, and chameleons can change the structural arrangement of the cells they're in just by relaxing or exciting (that is, stretching) the skin. When the animal is calm, the nanocrystals are organized into a dense network reflecting blue and green wavelengths. When excited, the nanocrystal lattice loosens to allow the reflection of other colors, like yellows and reds.
Additionally, the team found that chameleons have evolved not one, but two superimposed layers of light-reflective cells with different shapes. A second, deeper population of iridophores with larger, less-organized crystals reflect a substantial portion of sunlight in the near-infrared range. This layer likely plays a role in passive thermal protection—helping the chameleon stay cool and guarding against intense sun exposure.
In the video below, you can see the colors change in an adult male panther chameleon "under excitation" when presented with another adult male in his vision field. The original video is accelerated eight times, and the first frame of the movie is shown in the lower-right to demonstrate the extent of color change. You can watch another awesome video of a male relaxing after combat here.
Images: male panther chameleons by Michel C. Milinkovitch, www.lanevol.org