Review Hormonally-mediated maternal effects in birds: Lessons from the flycatcher model system Suvi Ruuskanen Section of Ecology, Department of Biology, University of Turku, Finland article info Article history: Received 22 April 2015 Revised 9 September 2015 Accepted 17 September 2015 Available online 21 September 2015 Keywords: Maternal allocation Ficedula Steroids Parent-offspring conflict Life-history Passerine abstract Maternal effects are a crucial mechanism in many taxa in generating phenotypic variation, affecting off- spring development and fitness and thereby potentially adapting them to their expected environments. Androgen hormones in bird eggs have attracted considerable interest in past years, and it is frequently assumed that their concentrations in eggs are shaped by Darwinian selection. Currently, however, the data is scattered over species with very different life-history strategies, environments and selection pres- sures, making it difficult to draw any firm conclusions as to their functional significance for a given sys- tem. I review the evidence available as to the function, variation and potential adaptive value of yolk androgens (testosterone, T and androstenedione, A4) using one well-studied wild bird model system, the European flycatchers Ficedula hypoleuca and Ficedula albicollis. These species both show genetic and environmental variation in yolk androgen levels, along with fitness correlations for the female, suggesting the potential for selection. However, variation in yolk T and A4 seem to be differentially affected, suggest- ing that maternal constraints/costs shape the transfer of the yolk steroids differently. Most of the envi- ronmental variation is consistent with the idea of high yolk androgen levels under poor rearing conditions, although the effect sizes in relation to environmental variation are rather small in relation to genetic among-female variation. Importantly, within-clutch patterns too vary in relation to environ- mental conditions. Yolk androgens seem to have multiple short- and long-term effects on phenotype and behavior; importantly, they are also correlated with the fitness of offspring and mothers. However, the effects are often sex-dependent, and not universally beneficial for the offspring. Unfortunately, conclusive data as to the adaptive benefits of clutch mean androgen levels or within clutch-patterns in different environmental conditions is still lacking. Ó 2015 Elsevier Inc. All rights reserved. Contents 1. Hormonally-mediated maternal effects in birds: an overview ................................................................. 284 2. Motivation .......................................................................................................... 284 3. Study system and data................................................................................................. 285 4. Effects of yolk androgens on offspring development and phenotype ............................................................ 286 4.1. Short-term effects on offspring development and phenotype ............................................................ 286 4.2. Long-term effects on offspring phenotype and behavior ................................................................ 287 4.3. Fitness effects for offspring........................................................................................ 287 5. Fitness effects on parents .............................................................................................. 288 6. Environmental plasticity in yolk androgen levels in eggs and its potential adaptive value? ......................................... 288 6.1. Variation across clutches ......................................................................................... 288 6.2. Variation within clutches ......................................................................................... 290 7. Genetic variation in yolk testosterone and androstenedione concentrations ...................................................... 290 http://dx.doi.org/10.1016/j.ygcen.2015.09.016 0016-6480/Ó 2015 Elsevier Inc. All rights reserved. E-mail address: skruus@utu.fi General and Comparative Endocrinology 224 (2015) 283–293 Contents lists available at ScienceDirect General and Comparative Endocrinology journal homepage: www.elsevier.com/locate/ygcen