Elastic Moduli of Avian Eggshell

SIMPLE SUMMARY: Birds have existed on Earth for over one hundred million years, and the eggshell is one of the main factors in them having survived for such a long period of time. The avian eggshell is a multifunctional thin-shelled structure that protects the developing embryo from damage and excessive water loss, provides the embryo with calcium for its skeleton, and sustains the weight of incubating birds. It must also be breakable for the hatchling to emerge. Elastic modulus (Young’s modulus) is the most fundamental mechanical property for such a load-bearing structure. Despite extensive studies on avian eggs, our understanding of the elastic moduli and structure–function relationship of avian eggshells remains incomplete—most previous works have focused on chicken or only a few species. One challenge is the availability and collection of freshly-laid egg samples. The present study is based on 700 freshly-laid eggs collected over a period of seven years from 2015 to 2021, covering a wide taxonomic scale and egg mass (from 1 g to 1459 g). With this large dataset, we may obtain a bird’s-eye view of the elasticity and structure–function relationships of the avian eggshell. ABSTRACT: We analyze 700 freshly-laid eggs from 58 species (22 families and 13 orders) across three orders of magnitude in egg mass. We study the elastic moduli using three metrics: (i) effective Young’s modulus, E(FEM), by a combined experimental and numerical method; (ii) elastic modulus, E(nano), by nanoindentation, and (iii) theoretical Young’s modulus, E(theory). We measure the mineral content by acid-base titration, and crystallographic characteristics by electron backscatter diffraction (EBSD), on representative species. We find that the mineral content ranges between 83.1% (Zebra finch) and 96.5% (ostrich) and is positively correlated with E(FEM)—23.28 GPa (Zebra finch) and 47.76 GPa (ostrich). The EBSD shows that eggshell is anisotropic and non-homogeneous, and different species have different degrees of crystal orientation and texture. Ostrich eggshell exhibits strong texture in the thickness direction, whereas chicken eggshell has little. Such anisotropy and inhomogeneity are consistent with the nanoindentation tests. However, the crystal characteristics do not appear to correlate with E(FEM), as E(FEM) represents an overall “average” elasticity of the entire shell. The experimental results are consistent with the theoretical prediction of linear elasticity. Our comprehensive investigation into the elastic moduli of avian eggshell over broad taxonomic scales provides a useful dataset for those who work on avian reproduction.