A nanostructural basis for gloss of avian eggshells

Birds’ eggs are some of the most colourful and variable natural products; yet, only two pigments are considered to be the major contributors to their visual appearance. Eggs of tinamou species (flighted relatives of rheas, emus, and ostriches) are best known for their beautiful mirror-like gloss. We show that a thin and extremely smooth cuticle is responsible for producing this gloss. We also discovered the presence of iridescence on Tinamus major eggshells, whereby the perceived colour changes with the angles of observation and illumination. This effect can only be produced by nanostructures that influence how light is reflected. Our findings highlight the role of nanostructure in modulating eggshell visual appearance.

HFSP Program Grant holders Tomas Grim, Mark Hauber, Matthew Shawkey and Geoffrey Waterhouse
authored on Thu, 11 December 2014

Birds’ eggs and feathers have astonished naturalists for centuries because of their beautiful and colourful appearances. Whereas it is well known that feather colors can arise through pigments and structural coloration (nanometer-scale structures that influence how materials reflect light and produce effects such as the iridescence of the peacock’s train), mostly pigments are considered responsible for producing the wide range of avian eggshell colours. Irridescence, where the perceived hue of a colour changes in relation to the angle from which it is viewed, can only be produced by structural colours and has never been documented for eggshells. As part of our HFSP grant to understand the mechanisms influencing eggshell apperance, we investigated non-pigmentary (nanostructural) mechanisms for avian eggshell coloration.

Figure 1. (left to right) Eggs of the great tinamou (Tinamus major), elegant crested tinamou (Eudromia elegans), and spotted nothura(Nothura maculosa). Photograph of great tinamou egg (UMMZ 191600) used with permission from theUniversity of Michigan Museum of Zoology. Photo credits: D. Hanley (great tinamou) and M. Hauber (elegant crested tinamou and spotted nothura).

Eggs of Tinamous (order: Tinamiformes) are well known for their beautiful ceramic-like glossy appearance. Gloss, the mirror-like component of light reflection, is a common component of animal and plant coloration. Gloss is generally produced by smooth or polished surfaces, but also influenced by the refractive index of materials (e.g. materials with higher refractive indices reflect more light). We used experimental manipulations in conjunction with angle-resolved spectrophotometry, scanning electron microscopy, atomic force microscopy, optical calculations, and chemical analyses, to examine the mechanistic basis for gloss of tinamou eggshells and presence of structural coloration.

Figure 2. Atomic force microscopy images showing the extremely smooth surface of A spotted nothura(Nothura maculosa) eggshells and rough surface of B araucana chicken (Gallus gallus) eggshells. Specular reflectance spectra of iridescent C great tinamou (Tinamus major) and non-iridescent D araucana chicken eggshells at varying angles from coincident normal.

We discovered that the glossy appearance of tinamou eggshells is produced by an extremely smooth cuticle, and we revealed the presence of weak iridescence on eggs of the great tinamou (Tinamus major). Optical calculations and experimental disruption of the surface topography corroborated surface smoothness as the main factor producing gloss. Our results open the door for further investigation into the mechanisms, functions, and evolution of non-pigmentary contributors to avian eggshell appearance.

Text by Branislav Igic.

Reference

A nanostructural basis for gloss of avian eggshells. Igic B, Fecheyr-Lippens D, Xiao M, Chan A, Hanley D, Brennan PRL, Grim T, Waterhouse GIN, Hauber ME, Shawkey MD. Journal of The Royal Society Interface. DOI: http://dx.doi.org/10.1098/rsif.2014.1210.

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