Color without pigments: What we are learning from avian optical nanostructures

Understanding the mechanisms and development of complex morphological traits can elucidate their evolution and spark advances in the biomimetic design of new materials. Thus, we take a multidisciplinary approach to (I) identify mechanisms, (II) chart evolutionary patterns and (III) synthetically mimic color-producing nanostructures built from keratin and melanin in bird feathers. Nanostructures in feathers are highly organized composites of materials with varying refractive indices (keratin, melanin and air) that produce some of the brightest colors in nature through coherent light scattering. We have identified previously undescribed ways of producing these colors, including quasi-ordered nanofibers in blue penguins and antireflection multilayers in peacock pheasants. Our phylogenetic analyses show that these traits, once evolved, enable the production of new colors and increase speciation rates. Furthermore, our work on colors in fossils suggests that they were present, and potentially used for signaling, even in early stages of feather evolution. The early origin and frequent evolution of these complex nanostructures may in part be explained because they form through low-cost self-assembly mechanisms. We examine developmental patterns and then replicate them using the same materials, enabling us to test our hypothesized developmental pathways while taking steps towards developing new manufacturing techniques for pigment-free coatings.