Evolution of brilliant iridescent feather nanostructures

Abstract

The brilliant iridescent plumage of birds creates some of the most stunning color displays known in the natural world. Iridescent plumage colors are produced by nanostructures in feathers and have evolved in diverse birds. The building blocks of these structures—melanosomes (melanin-filled organelles)—come in a variety of forms, yet how these different forms contribute to color production across birds remains unclear. Here, we leverage evolutionary analyses, optical simulations, and reflectance spectrophotometry to uncover general principles that govern the production of brilliant iridescence. We find that a key feature that unites all melanosome forms in brilliant iridescent structures is thin melanin layers. Birds have achieved this in multiple ways: by decreasing the size of the melanosome directly, by hollowing out the interior, or by flattening the melanosome into a platelet. The evolution of thin melanin layers unlocks color-producing possibilities, more than doubling the range of colors that can be produced with a thick melanin layer and simultaneously increasing brightness. We discuss the implications of these findings for the evolution of iridescent structures in birds and propose two evolutionary paths to brilliant iridescence.

Tailoring Iridescent Visual Appearance with Disordered Resonant Metasurfaces

“…Figure a is obtained for a reference case (no hybridization), in which the nanocube directly sits on a semi-infinite SiO 2 substrate. Consistent with earlier works on plasmonic color generation, , both the real and imaginary parts of ED eigenfrequency λ̃ ED largely vary. Notably, Re­(λ̃ ED ) spans almost the entire visible spectral domain, Re­(Δλ̃ ED ) ≈ 200 nm.…”

“…50−52 For instance, for color generation with Mie or plasmonic resonances, small size or shape variations of the constituent nanoparticles lead to significant color changes. 2,53 Despite significant size or shape dispersions and occasional particle aggregation (see the statistical analysis in the top inset in Figure 6) quantitative agreement is achieved between the measurements and our theoretical predictions based on a monodisperse approximation (Figures 5a and S5.1). This suggests that the present samples are exceptionally resilient to fabrication imperfections.…”

“…Nature offers beautiful colored appearances produced by the interaction of light with micro- and nanoscale structures . The most resplendent appearance of butterfly wings or bird plumage often comes from the iridescence of melanosome forms organized into thin layers . The micro- and nanoscale complexity studied by biologists interested in structural color biodiversity inspires engineers designing metasurfaces with angle-insensitive structural colors or prescribed angular color response. − Be they disordered, − as in Figure a,b, or minutely organized, − man-made metasurfaces composed of arrays of Mie or plasmonic nanoresonators offer several degrees of freedom.…”

How keratin cortex thickness affects iridescent feather colours

“…Some studies have suggested that the keratin cortex surrounding multilayered melanosomes affects the colours of bird feathers [16,37,38,45]. Of these, most have considered keratin cortices of different thicknesses with theoretical models [36][37][38]46], statistical analyses [33,45] and by observing barbule samples where the keratin cortex had been accidentally removed [36,37]. Specifically, based on a quantitative optical modelling of multi-layered melanosomes in mallard barbules-a very similar system to the pheasant feathers examined in this study-Stavenga et al [38] have convincingly explained the effect of changing barbule cortex thickness on the iridescent colours produced by multi-layered melanosomes on the hue.…”

“…More saturated colours can be produced by increasing the difference between the refractive indices, the degree of order or the thickness of optical nanostructures [29][30][31][32]. Moreover, the varying shapes of the melanosomes (solid rods, hollow rods, solid platelets or hollow platelets) also play a significant role in producing structural colours, as they can vary refractive index profiles [32][33][34][35].…”