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

Abstract

Iridescent coloration plays an important role in the visual communication system of many animal taxa. It is known that iridescent structural colours result from layers of materials with different refractive indexes, which in feathers usually are keratin, melanin and air. However, the role of these materials in the production of structural iridescent coloration is still poorly documented. Despite the great interspecific variation in the organization of such structures in bird plumage, melanin layers are usually considered too opaque, suggesting its main role is to delineate the outermost keratin layer and absorb incoherently scattered stray light. We combined spectrometry, electron microscopy and thin-film optical modelling to describe the UV-reflecting iridescent colour of feather barbules of male blue-black grassquits (Volatinia jacarina), characterized by a keratin layer overlying a single melanin layer. Our models indicate that both the keratin and the melanin layers are essential for production of the observed colour, influencing the coherent scattering of light. The melanin layer in some barbules may be thin enough to allow interaction with the underlying keratin; however, individuals usually have, on an average, the minimum number of granules that optimizes absorbance by this layer. Also, we show that altering optical properties of the materials resulted in better-fitting models relative to the empirically measured spectra. These results add to previous findings concerning the influence of melanin in single-layer iridescence, and stress the importance of considering natural variation when characterizing such photonic structures.

Nanostructural self-assembly of iridescent feather barbules through depletion attraction of melanosomes during keratinization

“…Developing feathers were fixed overnight in 4 per cent glutaraldehyde in 0.1 M phosphate buffer solution, and then rinsed in phosphate buffer and post-fixed in 2 per cent OsO4 in buffer for 90 min. Samples were then rinsed three times in distilled water, dehydrated and embedded in Spurr resin following standard protocol (outlined in [24]). …”

“…These birds are ideal for this study because (i) they are sexually dichromatic, allowing us to directly compare the development of the two colours in the same species, (ii) the mechanism-producing iridescent colour in males has been previously described [24], and (iii) the iridescent nuptial plumage appears to be sexually selected [25,26], and conditiondependent [27,28]. By comparing male and female blue-black grassquit feather development, we hypothesize an ontogenetic pathway for the formation of the organized nanostructures responsible for iridescent colour production.…”

Male achromatic wing colouration is related to body condition and female reproductive investment in a dichromatic species, the upland goose

“…Iridescent colours change in appearance with the angle of observation or illumination, so results from spectrophotometric measurements of iridescent plumage can be very sensitive to changes in the measurement geometry (e.g. Cuthill et al 1999;Santos et al 2007;Maia et al 2009), which influence both the hue and the chroma of a colour. As we were not able to guarantee exactly the same measurement geometry under fieldwork conditions, we decided to only include speculum brightness as one aspect of colouration in our study.…”

Iridescent colour production in hairs of blind golden moles ( Chrysochloridae )

“…To identify the optical basis of iridescent colour production, we used standard multi-layer thin-film optical modelling [7]. We simulated light interacting with stacks of sequential light and dark layers, with the number of layers and their thicknesses equal to the average values measured from the electron micrographs, to predict the theoretical reflective curves produced by the structures.…”

“…We simulated light interacting with stacks of sequential light and dark layers, with the number of layers and their thicknesses equal to the average values measured from the electron micrographs, to predict the theoretical reflective curves produced by the structures. We used the transfer matrix method as implemented in the multi-layer function [7] in R [8]. Hair keratin has an estimated RI of 1.55 [9], which was used as a first approximation in all models.…”