Manakins can produce iridescent and bright feather colours without melanosomes

Abstract

Males of many species often use colourful and conspicuous ornaments to attract females. Among these, male manakins (family: Pipridae) provide classic examples of sexual selection favouring the evolution of bright and colourful plumage coloration. The highly iridescent feather colours of birds are most commonly produced by the periodic arrangement of melanin-containing organelles (melanosomes) within barbules. Melanin increases the saturation of iridescent colours seen from optimal viewing angles by absorbing back-scattered light; however, this may reduce the wide-angle brightness of these signals, contributing to a dark background appearance. We examined the nanostructure of four manakin species (Lepidothrix isidorei, L. iris, L. nattereri and L. coeruleocapilla) to identify how they produce their bright plumage colours. Feather barbs of all four species were characterized by dense and fibrous internal spongy matrices that likely increase scattering of light within the barb. The iridescent, yet pale or whitish colours of L. iris and L. nattereri feathers were produced not by periodically arranged melanosomes within barbules, but by periodic matrices of air and β-keratin within barbs. Lepidothrix iris crown feathers were able to produce a dazzling display of colours with small shifts in viewing geometry, likely because of a periodic nanostructure, a flattened barb morphology and disorder at a microstructural level. We hypothesize that iridescent plumage ornaments of male L. iris and L. nattereri are under selection to increase brightness or luminance across wide viewing angles, which may potentially increase their detectability by females during dynamic and fast-paced courtship displays in dim light environments.

Colour formation on the wings of the butterfly Hypolimnas salmacis by scale stacking

“…Spectral alteration by scale stacking was only recently observed in small regions of European Nymphaline butterflies44. A similar colouration mechanism is also found in the feathers of the bird Steller’s Jay where the colour difference of white and blue feathers arises primarily due to the inherent melanin pigment content difference below spongy nanostructures4546.…”

Hybrid speciation leads to novel male secondary sexual ornamentation of an Amazonian bird

“…S5B). The melanin pigments in melanosomes can absorb incoherently scattered light at certain wavelengths (42)(43)(44)(45). The absence of melanin allows a brilliant white coloration, as observed in albino individuals of normally blue-or black-colored avian species (44).…”

“…The white crown color of L. nattereri is produced by incoherent scattering of light from the disordered array of air spaces in the keratin spongy layer. In contrast, in L. iris the air spaces are arranged in highly ordered layers-unique in the bird world-that produce the strong iridescent coloration in combination with a flattened barb shape, a restricted distribution of melanosomes, and the presence of vacuoles at the center of barb medullary cells (43). As expected for a hybrid lineage, L. vilasboasi is intermediate between the parental species in both the number of ordered layers and the distance of air spaces in the keratin spongy matrix (Fig.…”

Mechanisms involved in the production of differently colored feathers in the structurally-colored Swallow Tanager (Tersina viridis; Aves: Thraupidae)

“…Highly white broadband reflectance (>70%) was described in the scales of white beetles of the genus Cyphochilus, generated by interconnected and randomly arranged chitin fibers with a diameter of about 250 nm (Vukusic et al, 2007). White coloration in birds plumage has been related directly to an anisotropic spongy matrix only for the crown feathers of Lepidothrix isidorei (Igic et al, 2016). A comparative analysis of white plumage across 61 species of birds of wide taxonomic coverage characterized how the macrostructures of the plumage (quantity, shape and density of barbs and barbules and internal structure of the barbs) affect the characteristics of the reflected light (Igic et al, 2018).…”