What does the eggshell cuticle do? A functional comparison of avian eggshell cuticles

The avian eggshell is a highly ordered structure with several layers (mammillae, palisades, and vertical crystal layer) composed of calcium carbonate (∼96%) and minerals within an organic matrix. The cuticle is a noncalcified layer that covers the eggshells of most bird species. Eggshells are multifunctional structures that have evolved in response to diverse embryonic requirements and challenges, including protection from microbial infection, nest flooding, and exposure to solar radiation. However, experimental evidence for these functions across diverse taxa is currently limited. Here we investigated the effects of nanosphere cuticles on (1) bacterial attachment and transshell penetration, (2) eggshell wettability, (3) water vapor conductance, and (4) regulation of ultraviolet (UV) reflectance in seven ground-nesting bird species. We found considerable interspecific variation in ultrastructure and chemical composition of cuticles. Experimental removal of the cuticle confirmed that all nanospheres were highly effective at decreasing attachment of bacteria to shell surfaces and at preventing bacterial penetration. Cuticles also greatly decreased the amount of UV reflected by eggshells. In species with particularly small nanospheres, gas exchange was reduced by the presence of cuticle. Our results support the hypothesis that microbes and solar UV radiation can cause strong selection on bird eggs but also show that we need a greater understanding about the effects of specific nesting conditions (e.g., hydric and gaseousmilieu) on embryo well-being and eggshell structure variation.