Comparisons of microstructure and elemental composition of eggshells among wild plover populations

Reproduction investment is a prominent trade-off in life-history theory and is subject to strong selection pressure. The avian eggshell, as a crucial barrier between the bird embryo and the surrounding environment, undergoes optimization under different environmental selection regimes to ensure the successful development of embryos, which can be linked to local adaptation. Therefore, understanding the variation in eggshell microstructure and composition in wild bird populations living in contrasting ambient environments is of great significance. In this study, we utilized electron microscope ultrastructure measurement and elemental analyses to measure and compare the microstructure and element composition of eggshells from three wild plover populations (Charadrius alexandrinus and C. dealbatus) residing in heterogeneous habitats across varied climatic zones. These populations include the high-altitude Qinghai Lake population, the temperate coastal Tangshan population, and the tropical coastal Zhanjiang population. Our findings revealed that the palisade layer was thinner in the Qinghai Lake population compared to its lowland populations. This difference might be attributed to hypoxia which facilitates the hatching process by allowing chicks to easily break through their shells. Additionally, the variations in the elemental composition of the eggshells among populations well reflected the distribution of element content in different geographical regions. The Qinghai Lake population displayed low zinc and low manganese levels but high calcium levels, while the Zhanjiang population exhibited high zinc, high iron, high manganese, and high phosphorus levels. Furthermore, these variations in elemental composition could also account for the observed microstructural differences among populations. Collectively, we propose that the dissimilarities in eggshell microstructure and elemental composition among populations could be attributed to adaptations to different environmental conditions. Our findings lay the groundwork for future research to explore the mechanisms behind the variations in eggshell characteristics among wild bird populations, and contribute to a broader understanding of biodiversity mechanisms.