Meta-analysis questions assumptions about egg hormones

Thirty years after the discovery of maternal hormones in bird eggs, their evolutionary consequences remain unresolved. In a large-scale pre-registered meta-analysis recently published in the journal Ecology Letters, an international research team sought to shed light on this complex field of study. The last author of the study, Dr. Alfredo Sánchez-Tójar from the Faculty of Biology at Bielefeld University and associate member of the Collaborative Research Centre TRR 212 (‘NC³’), together with his co-authors, systematically analyzed 438 effect sizes from 57 studies on 19 wild bird species to assess whether higher concentrations of maternal hormones in eggs are indeed associated with fitness benefits for offspring and parents.

The study’s findings offer an unexpected insight: Contrary to the widely held assumption that maternal hormones are strategically used to optimize reproductive success, the meta-analysis revealed only a very weak and highly variable effect on fitness proxies. Surprisingly, neither the type of hormone, the age of the studied birds, nor methodological differences between studies could sufficiently explain this variability. Instead, the heterogeneity in results appears to be influenced by phylogenetic history and differences within individual studies.

This comprehensive synthesis challenges many existing hypotheses about maternal effects while also highlighting key open research questions. In this interview, Dr. Sánchez-Tójar discusses the central findings of the study, addresses methodological challenges, and describes promising future research directions.

Interview: ‘Future research should take a more holistic approach’

Your meta-analysis shows that the average effect of egg hormones on fitness in wild bird populations is surprisingly small. What do these findings mean for our current understanding of maternal effects — and should research in this field be reconsidered?

Our finding challenges the often-held view that egg hormones play a consistently strong and directional role in mediating adaptive maternal effects. Future research should shift towards understanding possible context-dependencies, exploring alternative mechanisms of maternal effects, and refining methodologies to advance our understanding of this complex field. In addition, given the particularly low reporting quality of studies in this field, which precluded us from including an additional 25% of studies, efforts should be put into better adoption of open science practices such as data-sharing as well as avoiding selective reporting.

You found a high degree of variability in effects that could not be explained by obvious factors such as hormone type or age. What biological or ecological mechanisms might still contribute to this heterogeneity?

The unexplained heterogeneity likely arises from the complex interplay of environmental conditions, the interaction of multiple maternal inputs into the egg, the active role of the developing embryo in processing these inputs, and fundamental differences in life history and physiology across species. In addition, given that most of the variation observed was at the study level, inaccurate methodology could be an important factor leading to high variation. Future research needs to adopt a more holistic approach to disentangle these interacting factors and understand the context-dependent nature of egg hormone effects on fitness

Your study highlights many open questions, including the role of environmental influences and sex-specific effects. Which research approaches do you consider particularly promising for addressing these gaps in the future?

First, studying more, particularly phylogenetically distant species, as well as investigating the environmental context experienced by both mothers and offspring, could reveal different patterns and explain context dependencies.
Second, clarifying the mechanisms involved in regulating hormone deposition into eggs and the extent to which this is a controlled process is crucial. In addition, since embryos are not passive recipients of maternal hormones and can metabolise them, investigating the embryonic metabolism of hormones is also crucial to understanding the mechanisms so that we can better formulate our hypotheses.
Finally, our study suggests that there is high variability within studies, which points to the possibility that inaccurate methodology, e.g., for extracting hormones, might explain a substantial amount of variation. More efforts should be made to standardise and validate methodologies.

Dr Alfredo Sánchez-Tójar from the Faculty of Biology at Bielefeld University and associate member of the Collaborative Research Centre TRR 212 (‘NC³’) is last author of the study.