Nanostructural determinants of angle-dependent iridescent coloration in two peafowl species

Structural colors arise from light interacting with nanoscale architectures, yet the relationships between multiple nanostructural features and their optical properties remain poorly understood. Peafowl feathers, iconic exemplars of iridescent coloration, feature photonic-crystal architectures that exhibit diverse hues. Here, we analyzed angle-dependent optical properties and seven key nanostructural parameters in different body feathers of male Indian Peafowl (Pavo cristatus) and Green Peafowl (P. muticus). Our findings revealed position-specific divergences in melanosome arrangement among neck, back, and wing feathers, resulting in distinct spectral signatures critical for speciation. Conversely, eyespots on upper tail coverts showed conserved optical properties in both species, achieved through distinct nanoscale organizations and illustrating optical degeneracy—different architectures result in the same appearance. Using a piecewise structural equation model, we established a predictive network linking seven parameters to four color properties—hue and brightness—across two illumination angles. Results elucidated a hierarchical framework where nanostructural parameters influence angle-dependent coloration via variation in melanosome layers, medullary width, and lattice spacing. Finally, color traits were influenced by angle-dependent changes in hue and brightness, as well as by layer number, melanosome rod length, and radius. Collectively, our findings provide a robust predictive framework linking nanostructure to optical phenotype and highlight the evolutionary significance of optical degeneracy for maintaining reliable sexual signals in birds.