Coloration strategies in peacock feathers

Abstract

We report the mechanism of color production in peacock feathers. We find that the cortex in differently colored barbules, which contains a 2D photonic-crystal structure, is responsible for coloration. Simulations reveal that the photonic-crystal structure possesses a partial photonic bandgap along the direction normal to the cortex surface, for frequencies within which light is strongly reflected. Coloration strategies in peacock feathers are very ingenious and simple: controlling the lattice constant and the number of periods in the photonic-crystal structure. Varying the lattice constant produces diversified colors. The reduction of the number of periods brings additional colors, causing mixed coloration. C olor production in nature takes advantage of either structural coloration (1, 2) or pigmentation. Structural colors result from the interaction of light waves with a featured structure having the same order of size as the light wavelength. Structural colors in avian feathers have been usually qualitatively understood by thin-film interference (3Ϫ5) or the scattering from a spongy matrix structure incoherently (6, 7) or coherently (8, 9). Although the structural colors of avian feathers have been studied for a long time (10Ϫ14), many questions remain to be answered. In particular, the precise physical mechanism that produces the diversified colors in peacock tail feathers has not been established. Materials and MethodsThe male peacock tail contains spectacular beauty because of the brilliant, iridescent, diversified colors and the intricate, colorful eye patterns. Peacock feathers serve as an excellent canonical example for investigating structural colors in avian feathers. The structures of the blue, green, yellow, and brown barbules in the eye pattern of a male green peacock (Pavo muticus) feather were characterized by using an optical microscope and a scanning electron microscope. The peacock tail feather has a central stem with an array of barbs on each side. On each side of a barb there is an array of flat barbules. Each barbule has round indentations of typically Ϸ20-30 m, which disperse the incident light, causing coloration. The round indentation has a smoothly curved crescent-like profile in transverse cross section (14).To understand the detailed mechanisms of color production in peacock feathers, a plane-wave expansion method (15) was used to calculate the photonic band structure of the periodic photonic structures. A transfer matrix method (16) was adopted to compute the reflectance spectra to compare with experimental results. Fig. 1 shows the submicron structures of barbules. The transverse cross sections reveal that a barbule consists of a medullar core of Ϸ3 m enclosed by a cortex layer. Interestingly, the cortex of all differently colored barbules contains a 2D photonic-crystal structure (14, 17Ϫ19) made up of melanin rods connected by keratin. The longitudinal cross section shows that the melanin rod length is Ϸ0.7 m. Melanin is created by melanocyte cells, deposited in developing feathers,...

Bio‐Inspired, Smart, Multiscale Interfacial Materials

“…However, recently some less complex, but nonetheless special features in biology received intense attention (Fig. 2), such as, the selfcleaning effect of lotus leaves and duck feathers, [34,35] the non-fogging, superhydrophobic compound eyes of mosquitoes, [36] the locomotion of geckos and octopuses via highly adhesive feet and suckers, [37,38] the non-wetting phenomenon of water striders walking on water, [39] the color of peacock feathers, butterfly wings, and beetle shells which is caused by a periodic microstructure, [40][41][42] the special nanostructures causing anti-reflectivity in cicada's wings and moth's compound eyes, [43,44] and lastly the special photonic reflectivity of sponge spurs due to their unique microstructure. [45] All these features are suitable for bio-inspiration.…”

“…The color of the feathers of a peacock, the wings of a butterfly, or the shell of a beetle, [40][41][42] is caused by microstructures that exhibit periodic variations in dielectric constant in one, two, or three dimensions (SEM images in Fig. 2c), with the period being in the order of the wavelength of the corresponding color.…”

Physical methods for investigating structural colours in biological systems

“…In numerous studies, the real part of the RI of melanincontaining material is taken to be 2.0 (e.g. Durrer & Villiger 1970;Schultz & Rankin 1985;Zi et al 2003;Brink & van der Berg 2004). This value, however, has no solid experimental basis, and thus urgently needs reassessment.…”

Iridescent structural colour production in male blue-black grassquit feather barbules: the role of keratin and melanin

“…thin film) and bidimensional structures have been described (e.g. Greenwalt et al 1960;Prum et al 1998;Andersson 1999;Zi et al 2003), and recent techniques have provided new insights indicating that three-dimensional photonic structures may also be found in the medullar cortex of feather barbs (Shawkey et al 2009). Structures organized in one and two dimensions have the additional feature of being iridescent, i.e.…”