Do egg hormones have fitness consequences in wild birds? A systematic review and meta-analysis

Egg-laying species are key models for understanding the adaptive significance of maternal effects, with egg hormones being identified as an important underlying mechanism. However, even thirty years after their discovery, the evolutionary consequences of hormone-mediated maternal effects remain unclear. Using an evidence synthesis approach, we tested the extent to which an increase of prenatal maternal hormone deposition into eggs relates to fitness in wild birds (19 species, 443 effect sizes, 57 studies). Egg androgens, glucocorticoids, and thyroid hormones showed an overall near-zero mean effect for both maternal and offspring fitness proxies. However, heterogeneity was high, suggesting that egg hormone effects on fitness will vary depending on context. Hormone type, offspring sex and age did not explain much of the observed variance, nor did methodological factors such as type of study or experimental design. Our heterogeneity investigation showed that differences in effect sizes were mostly driven by within-study variability and phylogenetic relationships. Our study provides the most comprehensive estimate to date of the relationship between egg hormones and fitness in vertebrates. Additionally, it identifies significant biases (e.g., taxonomic, publication) and knowledge gaps that are key to address in future research to fully understand the evolutionary relevance of maternal egg hormones.