Structural Colour in the Natural World

Polymer centre academics, including Dr. Andrew Parnell, Prof. Patrick Fairclough and Dr. Oleksandr Mykhaylyk, have recently been involved in a collaboration that has helped shed more light on the origin of structural colour in the natural world. Structural colour refers to cases where colour arises from the scattering of light by nanoscale structural elements, as opposed to absorption of light by pigment molecules, and it is common to many species of animal. Achieving colour in this way has a number of benefits, for example it avoids the need for constant replenishing of degraded pigment.
It has been known for a long time that birds make use of structural colour in their feathers, although the mechanism for this has also long been disputed. More recently, it was suggested that the colour arises from a number of ‘spongy’, relatively disordered networks of beta-keratin and air. The difference in refractive index between the two phases leads to coherent scattering of light, with the exact wavelengths scattered depending on the dimensions of the feather’s nanostructure. The team at the University of Sheffield were specifically interested in the origin of the non-iridescent blues and whites found on the feathers of the Eurasian Jay, as well as several other birds with similar colouration from a wide geographical range.

In the study, the feathers were imaged using both optical and electron microscopy, although the most significant results were obtained by scanning with Small Angle X-ray Scattering (SAXS). This technique allows non-destructive probing of the nanoscopic length scales relevant to the optically-active structures. The colours of the feathers could be correlated to the size of the beta-keratin domains in the scattering structures, and it was shown that the variation in colour seen along individual barbs on the feather was due to continuous variation of the dimensions of the keratin ‘sponge’. However, the exact biological mechanism which allows this degree of control during the feather’s growth is still unknown.

Their results also help explain why non-iridescent green colours in the natural world are rarely generated by structural elements alone, despite being obviously useful for camouflage. The keratin structures were found to scatter relatively broad portions of the spectrum: ‘structural blue’ can be obtained by scattering the blue and the non-visible, ultraviolet portions of the spectrum, and ‘structural white’ can obtained by scattering light across the whole of the visible spectrum, but green light occupies too narrow a set of wavelengths to be scattered selectively. However, one solution to this is to use a combination of ‘structural blue’ and yellow pigment, with the green tree frog being a good example.

Original publication: Parnell, A. J., Washington, A. L., Mykhaylyk, O. O., Hill, C. J., Bianco, A., Burg, S. L., Parker, A. R. (2015). Spatially modulated structural colour in bird feathers. Scientific Reports5, 18317.

Article by Rhys Williams; a PhD Student on the EPSRC Polymers, Soft Matter and Colloids CDT programme. For more information, please contact Dr Joe Gaunt at the Polymer Centre.