Sci. Adv. 4, eaar3219 (2018)

The characteristic shape of eggs makes them surprisingly resistant to mechanical pressure. However, to understand other remarkable properties of eggshells — whose basic structure has changed relatively little since the days of baby dinosaurs — requires taking a look at the nanoscale. Dimitra Athanasiadou and colleagues have done just that.

Credit: RubberBall / Alamy Stock Photo

Using atomic-force electron and optical microscopy, they found a hierarchical nanogranular structure that varied distinctly across the three major layers of chicken eggshells. Moreover, the nanostructure changed during egg incubation. For eggs incubated for 15 days (chicks hatch typically after some 21 days) the nanostructure of the outermost layer was the same as that of fully developed eggs, but in the innermost layer features were significantly smaller.

The resulting larger area of the inner surface might mean that minerals there dissolve more easily. This helps to deliver calcium for building up the embryo’s skeleton, but should also conveniently weaken the shell structure such that chicks can break through more easily when they hatch. Call that an evolutionary advantage.