How much calcium to shell out? Eggshell calcium carbonate content is greater in birds with thinner shells, larger clutches and longer lifespans

Abstract

The avian eggshell is a bio-ceramic structure that protects the embryo. It is composed almost entirely of calcium carbonate and a small amount of organic material. An optimal amount of calcium carbonate in the eggshell is essential for the embryo's development, yet how the ratio of calcium carbonate to organic matter varies between species has not been investigated. Calcium is a limiting resource for most birds, so its investment in their eggs should be optimized for a bird's life history. We measured the relative calcium carbonate content of eggshells in 222 bird species and tested hypotheses for how this trait has evolved with the life-history strategies of these species and other traits of their respective egg physiologies. We found that (i) eggshell calcium carbonate content was positively correlated with species having thinner eggshells and smaller than expected eggs relative to incubating parental mass, (ii) species with small mean clutch sizes had lower calcium carbonate content in their eggshells, and (iii) for species with larger clutch sizes, eggshell calcium carbonate content was negatively correlated with their mean lifespan. The pattern of lower eggshell calcium carbonate in longer lived, larger clutched birds suggests that calcium provision to the eggshell has long-term costs for the individual.

The diverse terminology of reptile eggshell microstructure and its effect on phylogenetic comparative analyses

“…Furthermore, the lack of preserved proteinaceous layers in most fossil eggshells (Grellet-Tinner, 2005;Grellet-Tinner et al, 2011;Kohring, 1999;Legendre, Rubilar-Rogers, Musser, et al, 2020;Norell et al, 2020;Silyn-Roberts & Sharp, 1989;Stewart, 1997) prevents an accurate estimation of their total mineral content. Recent comparative studies have described the diversity of calcium distribution within extant reptile eggshells, supporting relative calcium proportion and mapping as key proxies to understand their evolution (Campos-Casal et al, 2020;Choi et al, 2018;D'Alba et al, 2021;McClelland et al, 2021). The use of such traits in future studies is likely to improve our understanding of reptile eggshell microstructure beyond the "hard/soft" dichotomy, and shift the focus of descriptions from the sole CL to all eggshell layers.…”

“…thickness, calcium content in the shell, crystalline structure of the cuticle, porosity) in studying the correlation of eggshell microstructure with lifestyle parameters (e.g. body mass, clutch size and mass, nest structure, water vapor conductance, geographical distribution, ambient temperature, and humidity), and/or reconstruct its evolution in both avian and non‐avian reptiles (Attard & Portugal, 2021; D'Alba et al, 2016, 2017, 2021; Kulshreshtha et al, 2022; Legendre & Clarke, 2021; McClelland et al, 2021). The characterization of additional traits (e.g.…”

“…eggshell thickness, porosity, calcium content, water vapor conductance) with life history traits in birds (Attard & Portugal, 2021; Birchard & Deeming, 2009, 2015; McClelland et al, 2021; Portugal et al, 2014), dinosaurs including birds (Legendre & Clarke, 2021; Tanaka et al, 2015), archosaurs (Tanaka & Zelenitsky, 2014), squamates (Hallmann & Griebeler, 2015), non‐avian reptiles (D'Alba et al, 2021), or across amniotes, albeit with relatively small samples (Legendre, Rubilar‐Rogers, Musser, et al, 2020; Stein et al, 2019). Some of these studies have also reconstructed ancestral states for these traits – discretized in some cases – and identified correlates that influenced these evolutionary patterns (Attard & Portugal, 2021; D'Alba et al, 2021; Legendre & Clarke, 2021; Legendre, Rubilar‐Rogers, Musser, et al, 2020; McClelland et al, 2021; Norell et al, 2020; Portugal et al, 2014; Stein et al, 2019; Tanaka et al, 2015). These studies have initiated what is perhaps the most interesting and controversial debate in this new field of quantitative eggshell research (Lindgren & Kear, 2020): what was the structure of the ancestral eggshell in dinosaurs, archosaurs, and reptiles as a whole?…”

An Egg Shell: A Nutritional Profile and Health Benefits

“…The composition of these aggregates is unclear, notwithstanding the suggestion that they include a keratan sulphate proteoglycan. The genetic makeup of each species affects the location of these nucleation sites differently (Abo El-Maaty et al, 2021;Lee et al, 2021;McClelland et al, 2021;). The separation between these locations dictates the size of the mammillary cones, the cylindrical diameter of the palisades in the compact layer of the shell, and ultimately the strength of the shell.…”

Egg Shell: An Essential Waste Product to Improve Dietary Calcium Uptake

“…Although a keratan sulphate proteoglycan has been suggested, the makeup of these aggregates is unknown. These nucleation sites' distribution is determined genetically and differs between species [12][13][14]. The distance between these sites determines the size of the mammillary cones, the cylindrical diameter of the palisades in the compact layer of the shell, and ultimately the strength of the shell.…”