PEACOCK TRAIN FEATHER I
This image shows the barbs and barbules that form the central “eye” portion of a peacock’s train feather. The barbs are the spines that run diagonally in this photo (1, 2, 3). They are attached to the vertical main stem of the feather (1, 3). The barbules are the segmented structures that “sprout” out from the barbs (1, 2, 3). While it is a little hard to tell, the barbules are curved in a crescent-like shape (1, 2, 3). It is curious that the colors (at least here in the yellow barbules) do not transition gradually, but instead appear to be contained in and change by segment.
Approximate Photo Location (Topside)
Field of view: ~3/8” x 1/4” (9.0mm x 6.0mm)
Images in focus stack: 34
Peacocks use their colorful train feathers in courtship displays to gain the attention of mates (4, 5, 6, 7). You can see a video clip of several of these in real-time and slow motion here. During the “train rattling” display, the peacock fans out its feathers and quickly vibrates them from side-to-side (6). It may also turn at an angle to the sun and use the light rays to enhance the iridescence of the feathers (5, 7)(i). The vibration of the feathers occurs at a frequency of 22–28Hz (Hz=cycles per second)(6). This is at or near a resonant frequency of the feather, allowing the peacock to use the minimum amount of energy needed to produce the movement (6). While the vibration moves the loose barbs near the eyespot, the eyespots themselves stay fairly still (6). This is caused by the interlocking arrangement of the barbules, as well as the frequency of the vibration, and weight of the eyespots (6)(ii). The train rattling also produces pulses of sound (like rustling) that may have some sort of signaling function for the peahen (6)(iii). The iridescence of the eyespots is associated with greater mating success, while the size of the train and number of eyespots is not (6, 7).
Like the scales on the alpine black swallowtail, the color of a peacock’s tail feather is mainly determined by its structural features rather than pigmentation (1, 3, 5). (If the color was based on pigmentation, the peacock’s feathers would be brown (8, 9)(iv).) Inside the barbule, there is a medullar core surrounded by an outer cortex layer with a thin keratin “skin” (see figures I–III below)(1). Inside the cortex, there are arrays of melanin rods that are arranged in a lattice pattern (fig. III & IV)(1, 3, 4, 5). The rods run parallel to the surface of the barbule and there are air tubes/gaps between them (fig. IV)(1, 2, 3)(v). When light reaches the rods, the waves scatter off of them and destructively interfere with each other, ultimately leaving only a relatively narrow wavelength range of light (a photonic bandgap) that is blocked and cannot move through the structure (3, 10). This light is reflected back to us as the color of the barbule (1, 3, 10)(vi). The different colors are determined by the spacing between the rods and the number of rod layers (1, 3). Zi et al. reports that the spacing and layer number is as follows: blue and green barbules=~9 to 12 layers with ~140–150nm spacing, yellow barbules=~6 layers with 165nm spacing, and brown barbules=~4 layers with more irregular 150–185nm spacing (1, 3)(vii). The colors are brighter when the diameter of the rods is larger or the rods are spaced more tightly together (5). Rods with a larger diameter are also associated with greater iridescence (5). You can see several electron microscope (SEM/TEM) images of these structures here: A, B, C. As a unit, they function as a photonic-crystal (1, 3, 4, 5).
Figure I: Front view of a barbule. Figure II: Top view, showing the crescent-like shape of the barbule. The illustration is a cross-section through the structure. Figure III: Zoomed-in view of the cross-section. The medullar core contains randomly arranged rods, while the outer cortex contains rods arranged in a square lattice pattern. As the arrays in the illustration have a depth of six (rod) layers, the barbule here would presumably have a yellow color. Figure IV: Three-dimensional view of the rods in the outer cortex. The plane at the top is the surface of the barbule, with the rods underneath running parallel to it. The illustration is somewhat simplified here. The rods are connected by keratin and there are air tubes/gaps between them (1, 3). They may also have nodules or spherical bumps along their length (3, 12). Cowley mentions that these (bumps) are thought to have an optical function (3). (Illustration references: 1, 3, 4, 5, 11.)
i. Dakin and Montgomerie observed that during the “train rattling” display, males would orient themselves with the sun on their left side (at about a 45-degree angle) and the females would be positioned directly in front of the male (7).
ii. Dakin et al. also provides an image (figure 8G) showing microhooks on a section of violet barbules (6). These help the eyespots move as a single unit (6).
iii. The pulses of sound are within the auditory range of the peahen (6). The intensity may, for example, indicate the muscular strength of the peacock (6).
iv. The pigmentation of the barbules may play a role by absorbing some of the randomly scattered light (2).
v. Medina, Diaz, and Vukusic found that the train feathers of white peacocks do not have melanin rods in the barbules (4). The white is mainly produced by the scattering of light in the keratin (4).
vi. The crescent-shaped curve of the barbule may play a role in the level of iridescence (2).
vii. There appears to be some variation in these numbers. Yoshioka and Kinoshita report that the blue barbules have 8–12 layers with ~150nm spacing and yellow barbules have 3–6 layers and 190nm spacing (2). Smyth did not find a significant difference in the number of layers in barbules with different colors (12).
1. Zi, J., Yu, X., Li, Y., Hu, X., Xu, C., Wang, X., Liu, X., & Fu, R. (2003). Coloration strategies in peacock feathers. PNAS, 100(22), pp. 12576–12578. Retrieved from PNAS.
2. Yoshioka, S., & Kinoshita, S. (2002). Effect of macroscopic structure in iridescent color of the peacock feathers. Forma, 17, pp. 169–181. Retrieved from Semantic Scholar.
3. Cowley, L. (n.d.). Optics picture of the day: Peacock feathers. Retrieved from Atmospheric Optics.
4. Medina, J.M., Díaz, J. A., Vukusic, P. (2015). Classification of peacock feather reflectance using principal component analysis similarity factors from multispectral imaging data. Optics Express, 23(8). Retrieved from ResearchGate.
5. Mitan, C. (2015). What causes variation in peacock feather colors? Honors Research Projects, 5. Retrieved from the University of Akron Idea Exchange.
6. Dakin, R., McCrossan, O., Hare, J. F., Montgomerie, R., & Kane, S. A. (2016). Biomechanics of the peacock’s display: How feather structure and resonance influence multimodal signaling. PLOS ONE, 11(4). Retrieved from PLOS ONE.
7. Dakin, R., & Montgomerie, R. (2009). Peacocks orient their courtship displays towards the sun. Behavioral Ecology and Sociobiology, 63(6), pp. 825-834. Retrieved from ResearchGate.
8. Burrows, L. (2016, November 22). News & events: A new technique for structural color, inspired by birds. Retrieved from the Harvard School of Engineering.
9. Hurst, N. (2017, May 5). A new color printing technique borrows from bird feathers. Retrieved from Smithsonian.com.
10. Bhattacharya, P. K. (2007). Editorial: Photonic crystal devices. Journal of Physics D: Applied Physics, 40. Retrieved from The University of Michigan.
11. Okazaki, T. (2018). Reflectance spectra of peacock feathers and the turning angles of melanin rods in barbules [Figure 3]. Zoological Science, 35(1), pp. 86-91. Retrieved from BioOne.
12. Smyth, S. (2007). What makes peacock feathers colorful? National Nanotechnology Infrastructure Network (NNIN), Research Experience for Undergraduates, pp. 112-113. Retrieved from NNIN.
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