Yolk steroids in great tit Parus major eggs: variation and covariation between hormones and with environmental and parental factors.

C M Lessells,S Ruuskanen,H Schwabl

doi:10.1007/s00265-016-2107-1

Abstract

Avian mothers can potentially alter the phenotypes of their offspring by varying the concentration of steroid hormones in their eggs. We explored variation in androstenedione (A4), testosterone (T), 5α-dihydrotestosterone (DHT), 17β-estradiol (E2), and corticosterone (CORT) in the yolks of 12 free-living great tit Parus major clutches. We analyzed variation and covariation in greater detail than previous studies, using models for variation with laying sequence that take into account variable clutch size and comparing correlations between pairs of hormones at the within- and between-clutch levels. We also investigated relationships between hormone levels and various environmental, life history, and parental traits. For three of the five steroids, we found no significant correlates, but based on individual statistical tests (a) DHT varied between clutches with male age (1 year old vs older); (b) DHT and CORT were negatively correlated within clutches with the average temperature on the day (DHT and CORT) or 3 days (DHT only) preceding laying; and (c) DHT in the last egg of the clutch relative to the clutch mean was positively correlated with the interval between clutch completion and the onset of incubation (incubation delay). Relationships with ambient temperature and incubation delay have not previously been reported for any yolk hormone in birds. Intriguingly, the three relationships for DHT are consistent with more DHT being transferred to eggs in situations that could be more energetically challenging for the female. More research is needed to determine the generality of the patterns we found and to understand their functional significance.Significance statementThe yolks of birds’ eggs contain steroid hormones produced by the mother which can affect the development and behavior of the resultant chicks. We analyzed five steroid hormones in the yolks of wild great tits and show for the first time that yolk hormone levels are related to ambient temperature in the day(s) just before laying and, in the last-laid egg, with the day it is laid relative to the onset of incubation, and that the concentrations of pairs of yolk hormones can vary with each other in a different way between and within clutches. These results contribute insights into the ways in which yolk hormones may adaptively modify the chicks or may reflect physiological processes occurring in the mother.Electronic supplementary materialThe online version of this article (doi:10.1007/s00265-016-2107-1) contains supplementary material, which is available to authorized users.

Highlights

The discovery that avian egg yolks contain maternally derived steroid hormones (Schwabl 1993) has ignited interest in the significance and function of variation in the amount ofBehav Ecol Sociobiol (2016) 70:843–856 maternal hormones transferred to eggs (Groothuis et al 2005; Gil 2008)
We explore within- and between-clutch variation in the concentration of five steroids-androstenedione (A4), testosterone (T), 5α-dihydrotestosterone (DHT), 17βestradiol (E2), and corticosterone (CORT)-in the egg yolks of free-living great tits Parus major
We found correlations between the concentrations of different hormones both within and between clutches and that all hormones except DHT showed significant variation within the laying sequence: A4 increased significantly across the laying sequence, whereas T and E of last eggs and CORT of the first egg differed from the other eggs of the clutch

Summary

Introduction

The discovery that avian egg yolks contain maternally derived steroid hormones (Schwabl 1993) has ignited interest in the significance and function of variation in the amount ofBehav Ecol Sociobiol (2016) 70:843–856 maternal hormones transferred to eggs (Groothuis et al 2005; Gil 2008). Yolk hormones interest evolutionary and behavioral ecologists and physiologists for several reasons: first, hormone-mediated maternal effects provide a means for the female to modify offspring phenotype (from growth up to adult phenotype and behavior) in anticipation of environmental conditions (Schwabl et al 1997; Groothuis et al 2005), making them a good model to study adaptive transgenerational phenotypic plasticity. Conceptualized in this way, variation in yolk hormone levels impacts offspring fitness and is in the interests of both mother and offspring. By offering a means of manipulating sibling competition and begging behavior in chicks, maternally derived yolk hormones might enable the mother to impose her individual optimum in terms of the chicks’ behavior and development or to induce higher levels of investment by the male parent and win out in parent-offspring and sexual conflict, respectively (Lessells 2006; Müller et al 2007; Horváthová et al 2012)

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