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Peacock hues a trick of the light

Over the centuries, humanity has been so impressed by the splendor of the peacock's colors that this magnificent bird has variously been a symbol of divine beauty, endless love, paradise, purity, rebirth and God's omniscience.

Now physicists in China have discovered the secret of the peacock's array of hues.

In a study published this month in the Proceedings of the National Academy of Sciences, researchers report slight variations in the arrangement of keratin and melanin are responsible for the palette of colors found in the eye of a peacock's tail feather. (Keratin is the material found in human fingernails; melanin is the substance that darkens human skin.)

"It's an ingenious and simple way to diversify colors," said Dr. Jian Zi, a physicist at Fudan University in Shanghai and lead author on the paper.

He began the study with colleagues after being struck by the sight of peacock feathers in a Chinese market. "I study optics," Zi said. "So when I looked at these peacock feathers against the sunshine, it was a fascinating experience. What can produce such diversified colors?"

In peacocks, each feather has a central rigid stem lined on both sides by a row of smaller barbs. Each barb is then lined on both sides by rows of even tinier barbules.

When scientists examined the barbules with a high-powered electron microscope, they saw the barbules had as an outer covering a very regular structure built of tiny rods of melanin connected by keratin.

But the researchers also found that differently colored barbs differed slightly in how these matrices were arranged. Depending on how closely the rods were spaced and how many layers of rods were stacked up, the barbule could appear anything from yellow to green to blue to brown.

Scientists tested the importance of these structures in creating color by dipping the feathers in glycerin, altering the structure by filling in its minuscule air holes with the clear liquid.

What they found was that the glycerin slightly altered the colors of the feather, something that would not have happened if pigments were responsible for the feather's coloration.

Most often, colors in nature are caused by pigments, like the pigments in paint. When light shines on a pigment, the pigment produces the color red, for example, by absorbing all the light except for the red light. That light is reflected back to the eye, which then sees a red color.

In contrast, in peacock feathers, it is the precise structural array of melanin rods in keratin that creates different colors, with one array reflecting back yellow light, for example, and a slightly different arrangement reflecting back blue light.

Such so-called structural colors are more akin to the way colorless water droplets reflect light to produce the colors of the rainbow, as opposed to pigments that would produce the colors in a painting of a rainbow.

Zi said he and his colleagues were not the first physicists to be entranced by the peacock. He said they were following in the footsteps of Sir Isaac Newton, who suggested about 300 years ago that the peacock feather colors might be caused by structural coloration.