Revealing the Reasons for the Absence of Pheomelanin-Based Structural Colors in Bird Feathers

Birds are famous for their vivid and colorful feathers produced by pigment and structural colors. Melanosomes (melanin granules), composed of varying proportions of eumelanin and pheomelanin, function as absorbers of broadband visible wavelengths and as light scatterers in the generation of structural colors. While structural colors derived from eumelanosomes (eumelanin-rich melanosomes) are well-documented, structural colors driven by pheomelanosomes (pheomelanin-rich melanosomes) are rarely observed. This study examines how differences in melanosome shape and material properties influence their ability to form nanostructures necessary for structural coloration.
We found that pheomelanosomes, which predominantly exhibit spherical morphologies, exhibit weaker interparticle forces (e.g., depletion-attraction forces) compared to the rod-like shapes typical of eumelanosomes. This limits the ability of pheomelanosomes to organize into ordered nanostructures required for structural color formation. Optical simulations using finite-difference time-domain (FDTD) methods further demonstrated that differences in absorption properties between eumelanin and pheomelanin have minimal impact on structural coloration, whereas the spatial arrangement of melanosomes plays a dominant role. In experimental self-assembly studies, rod-shaped melanosomes extracted from peacock tail feathers consistently produced ordered structures, while spherical pheomelanosomes from pheasant feathers formed disorganized arrangements. These findings highlight the critical role of melanosome morphology and assembly behavior in the development of structural colors in biological systems and provide insight into why structural colors are rarely observed in pheomelanin-rich birds.