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.
Highlights
Life-history theory explains what determines when, how and to what extent reproduction should occur for an organism to optimize its individual fitness [1]
All other predictors and interactions were not retained in the averaged model set of phylogenetically informed least-squares (PGLS) models, indicating that these variables neither improved the fit of the model nor were significant predictors of eggshell calcium content, and as such were not included in the phylogenetically informed multivariate mixed model (PMM)
There was an interaction between clutch size and lifespan on calcium carbonate content: among species with a clutch size over two eggs, calcium carbonate content of eggs decreased with increased lifespan; this effect was not evident in species with less than an average of 2.5 eggs per clutch
Summary
Life-history theory explains what determines when, how and to what extent reproduction should occur for an organism to optimize its individual fitness [1]. Egg production provides a critical example of life-history theory in action as the investment into an egg and/or clutch will greatly influence the quality of that offspring, but, will reduce the parent’s resources for both immediate self-maintenance and future reproductive investment [7]. This trade-off has been explored in the context of egg contents [8], such as androgen deposition in the yolk [9,10] and pigment deposition in the shell matrix [11], yet the production of the eggshell itself and its composition have not been considered within the same framework
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