Oct. 14, 2003 — A team of physicists has just determined the exact mechanism behind the luminescent beauty of the feathers in a peacock's tail.
The discovery could help to explain how certain hummingbirds, ducks, pheasants, insects and even objects, such as opals, achieve their brilliant colors.
According to a report published in the current Proceedings of the National Academy of Sciences, color in nature occurs due to two basic processes: pigmentation and structural coloring.
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Pigment is a substance that, like a dye, gives color to living and inanimate objects. Peacocks and other structurally colored animals and things, such as rainbows, soap bubbles and the blue sky, instead get most of their color from light reflection.
As a result, while human hair can look shiny clean, it can never achieve the brilliance and radiant coloration of a peacock's tail feather.
"In human hair, color is due to pigment," explained Jian Zi, professor of physics in the Surface Physics Laboratory of Fudan University in Shanghai, China, and lead author of the PNAS paper. "If you change the view angle, color is not altered. In peacock feathers, color is produced by the reflection of light with frequencies within the partial photonic band gap. If you change the view angle, the partial photonic band gap will shift to short wavelengths for oblique incidence."
The photonic band gap refers to the crystalline structure of barbules that Jian Zi and his colleagues analyzed using an optical microscope and a scanning electron microscope. Each peacock tail feather possesses a central stem with an array of barbs on each side. Each barb, in turn, possesses an array of smaller, flat barbules.
The outer layer of the barbules consists of a two-dimensional crystal framework made of melanin rods connected by keratin — a fibrous protein — in a lattice pattern. The number and spacing of the rods controls how light reflects which produces different colors. For peacocks, the colors are green, golden yellow, brown and a very bright blue.
Male peacocks display their iridescent feathers for prospective female mates. Females may check out the feathers of a number of different males before deciding on a suitor. The length and quality of a male peacock's feathers can indicate his age, vigor, and status.
Zi and his team did not study the mechanism behind the classic "eye" print found within the feathers but they believe it is due to genetics.
"The formation of the eye pattern must be the result of evolution," Zi said. "The coordination of individual barbules in the formation of the eye pattern must be controlled by the instruction coded in genes."
Che Ting Chan, a physicist at the Hong Kong University of Science and Technology, agrees with the new findings. He told Discovery News that Zi and his team present "a very convincing" explanation.
Chan added, "The research is very impressive because it establishes convincingly and quantitatively the mechanism of color production in peacock feathers, which is so subtle, and yet so precise and beautiful."
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Name: Common Peafowl (Pavo cristatus)
Primary Classification: Galliformes (Gamebirds) Location: Southern Asia. Habitat: Dry, open deciduous forests and open fields with lots of bushes. Diet: Mostly insects. Also snakes, including young cobras. Size: Up to 7.5 ft in length and 13 lbs in weight. Description: Males have a shiny blue body and a long train of colorful feathers with eye marks; females are comparatively drab; short, strong legs. Cool Facts: The male, commonly known as a peacock, has a loud, piercing mating call that has been described as "bloodcurdling"; it is considered a nuisance in some areas because of this call. Conservation Status: Common |
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