What makes a feather shine? A nanostructural basis for glossy black colours in feathers

Abstract

Colours in feathers are produced by pigments or by nanostructurally organized tissues that interact with light. One of the simplest nanostructures is a single layer of keratin overlying a linearly organized layer of melanosomes that create iridescent colours of feather barbules through thin-film interference. Recently, it has been hypothesized that glossy (i.e. high specular reflectance) black feathers may be evolutionarily intermediate between matte black and iridescent feathers, and thus have a smooth keratin layer that produces gloss, but not the layered organization of melanosomes needed for iridescence. However, the morphological bases of glossiness remain unknown. Here, we use a theoretical approach to generate predictions about morphological differences between matte and glossy feathers that we then empirically test. Thin-film models predicted that glossy spectra would result from a keratin layer 110-180 nm thick and a melanin layer greater than 115 nm thick. Transmission electron microscopy data show that nanostructure of glossy barbules falls well within that range, but that of matte barbules does not. Further, glossy barbules had a thinner and more regular keratin cortex, as well as a more continuous underlying melanin layer, than matte barbules. Thus, their quasi-ordered nanostructures are morphologically intermediate between matte black and iridescent feathers, and perceived gloss may be a form of weakly chromatic iridescence.

Key ornamental innovations facilitate diversification in an avian radiation

“…Although the morphology of African starling melanosomes has been described previously (34,38,42), we confirmed these by examining transmission electron micrographs of at least one species per genus, as well as species from genera or clades reported not to share the morphology of their closest relatives. Feathers were collected from specimens from The American Museum of Natural History and prepared following standard procedures (72). Table S4) across the posterior sample of trees.…”

A nanostructural basis for gloss of avian eggshells

“…Iridescence can be produced by diffraction gratings, or when light passes through multiple semi-transparent materials that differ in refractive index, causing light to phase-shift and cancel out particular wavelengths at particular viewing angles [4,10,11]. Gloss, which is loosely defined as the specular or mirror-like component of light reflection, is a common component of animal coloration and is present in invertebrates, vertebrates and plants [2,[12][13][14]. Gloss is often produced by smooth or polished surfaces.…”

“…Iridescence and gloss are often not independent because production of both is strongest when light is reflected from a smooth flat surface. As a result, most iridescent materials are glossy [17] and some glossy materials are weakly iridescent [2].…”

Manakins can produce iridescent and bright feather colours without melanosomes

“…To our knowledge, they provide the first reported example of feathers with highly directional iridescence produced independently of melanosomes or melanosome arrangement. The highly directional iridescence of other bird species' feathers studied to date is produced by the crystalline arrangement of melanosomes within barbules, which may be hollow or solid (Greenewalt et al, 1960;Stavenga et al, 2011a;Maia et al, 2011;Eliason et al, 2013). The L. iris crown feathers had a limited and ventral distribution of melanosomes that did not affect coloration when removed; their iridescent quality was produced by the periodic arrangement of air voids and β-keratin within the medullary layer of feather barbs.…”

“…Although melanins are pigments that absorb light across all wavelengths visible to birds (300-700 nm; Osorio and Vorobyev, 2005;Meng and Kaxiras, 2008), for many birds, the precise arrangement of melanosomes within their feather barbules produces iridescent colours by causing particular reflected wavelengths to be amplified, while others are attenuated, as light travels through materials that periodically vary in refractive index (i.e. air, β-keratin and melanin; Greenewalt et al, 1960;Stavenga et al, 2011a;Maia et al, 2011;Eliason et al, 2013). Independent of the nanoscale arrangement of melanosomes, some species produce weakly iridescent colours through quasi-ordered nanostructures of β-keratin and air within feather barbs (Noh et al, 2010a).…”