Coherent light scattering by nanostructured collagen arrays in the caruncles of the Malagasy asities (Eurylaimidae : Aves)

We investigated the anatomy, nanostructure and biophysics of the structural ly coloured facial caruncles of three species in a clade of birds endemic t o Madagascar (Philepittinae, Eurylaimidae: Aves), Caruncle tissues of all s pecies had reflectance spectra with prominent, peak hues between 403 and 52 8 nm, Dark blue Neodrepanis tissues had substantial reflectance in the near ultraviolet (320-400 nm), which is visible to birds but not to humans, pro viding the first evidence of ultraviolet skin colours in birds and the firs t indications of the possible function of ultraviolet skin colours in avian communication, These structural colours are produced by coherent scatterin g from arrays of parallel collagen fibres in the dermis, Tissues of Philepi tta castanea were organized into hexagonal, crystal-like arrays, whereas Ne odrepanis tissues were quasiordered, Predictions of the peak hues of reflec tance (lambda(max)) using Bragg's law were relatively accurate, but Bragg's law requires physical assumptions that are obviously violated by these str uctures. A two-dimensional discrete Fourier analysis of the spatial variati on in refractive index within the tissues documented that all the tissues a re substantially nanostructured at the appropriate spatial scale to scatter visible light coherently. Predicted reflectance spectra based on the two-d imensional Fourier power spectra are relatively accurate at predicting the hue and shape of the reflectance spectra of the tissues. These results conf irm that the nanostructure of the collagen arrays determines the colours th at are coherently scattered by these tissues, The evolution of the anatomy and nanostructure of asity caruncles is discussed.