Scattering of ultraviolet light by avian eggshells

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Abstract

Eggshells are essential for the reproduction of birds since the optical properties of shells may have an impact on biological functions such as heating and UV protection, recognition by parents or camouflage. Whereas ultraviolet reflection by some bird eggshells has been recently described, its physical origin remains poorly understood. In this study, we identified a porous structure in eggshells. Using Mie scattering modelling, we found it was most likely responsible for reflectance peaks (intensities of ca. 20-50%) observed in the near-UV range. These peaks were observed by spectrophotometric measurements from eggshells of several breeds of hen, one breed of duck and one breed of quail. This optical response was interpreted in terms of the distinct visual perception of hens and humans: eggshells appearing achromatic for humans proved to be chromatic for hens. Fluorescence emission from these eggs was also characterised and attributed to the presence of protoporphyrin IX and biliverdin IXα in the shells. Electron microscopy observations revealed the presence of pores within the so-called calcified shell part (i.e., at depths between ca. 20 μm and ca. 240 μm from the eggshell's outer surface). Mercury intrusion porosimetry allowed us to quantify the pore size distribution. Simulations of the UV response of this porous structure using Mie scattering theory as well as an effective approach accounting for multiple scattering indicate that these pores are responsible for the backscattering peaks observed in the UV range, in the case of beige hen eggshells. Due to the similarities between the pore size distributions observed for beige hen eggshells and other investigated poultry eggshells, we expect Mie backscattering to be the origin of the UV response of the eggshells of many other bird species. This journal is

Original languageEnglish
Pages (from-to)63-80
Number of pages18
JournalFaraday Discussions
Volume223
DOIs
Publication statusPublished - 1 Oct 2020

Funding

The authors thank C. Charlier (Electron Microscopy Service, UNamur) and A. Fattaccioli (Department of Biology, UNamur) for technical support during the electron microscopy observations and the spectrouorometry measurements of the extracts, respectively. D. B. Colridge (School of Physics, University of Exeter) and M. D. Shawkey (Department of Biology, Ghent University) are also thanked for reading the manuscript and for fruitful discussions, respectively. S. R. M. was supported by the Belgian National Fund for Scientic Research (FRS-FNRS) as a Postdoctoral Researcher (91400/1.B.309.18F). This research was also supported by FRS-FNRS through the Researchers’ Credit CC 1.5075.11F. This research used the resources of the Technological Platform High Performance Computing (PTCI) (http://www.ptci.unamur.be) located at UNamur and the resources of the Electron Microscopy Service (SME) of UNamur (http://www.unamur.be/en/sevmel). PTCI is supported by FNRS-FRFC, the Walloon Region and UNamur (Conventions No. 2.5020.11, GEQ U.G006.15, 1610468 and RW/GEQ2016). PTCI and SME are members of the “Consortium des Équipements de Calcul Intensif (CÉCI)” (http:// www.ceci-hpc.be) and the Technological Platform Morphology-Imaging (MORPH-IM) of UNamur, respectively.

FundersFunder number
FNRS‐FRFC
Walloon Region and UNamur1610468, 2.5020.11, RW/GEQ2016, GEQ U.G006.15
Fonds de la Recherche Scientifique F.R.S.-FNRS91400/1
Fonds de la Recherche Scientifique F.R.S.-FNRS

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