Scattering of ultraviolet light by avian eggshells
Eggshell is essential for the reproduction of birds since the optical properties of their shells may have an impact on biological functions such as heating and UV protection, recognition by the 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 the eggshell. Using Mie scattering modelling, we found it was most likely responsible for ca. 20-50% reflectance peak intensities observed in the near UV. These peaks were observed by spectrophotometric measurements from eggshells of several breeds of hens, one breed of duck and one breed of quail. This optical response was interpreted in terms of the distinct visual sensation 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 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 both 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 at the origin of the UV response from the eggshells of many other bird species.
- This article is part of the themed collection: Biological and bio-inspired optics