Sex Allocation in a Monogamous Bird: Advantaged Matrilines and Sons with Silver Spoons

AbstractThe ability to secure food for offspring and withstand the cost of reproduction favors high-quality mothers that overproduce the larger sex, typically sons, only if they will receive adequate food, as this should enhance these sons' fitness returns. However, high-quality daughters ensure that grandoffspring receive quality parental care and may possess greater reproductive value than their brothers, favoring daughters also from high-quality mothers. Using a mixed cross-fostering approach, we investigated effects of early rearing conditions, covariance between breeders and their genetic parents in parental quality, and primary offspring sex ratios in Carolina wrens. In this socially and genetically monogamous bird, sons grew larger than daughters, paternal food provisioning impacted the condition and recruitment of sons but not daughters, and females overproduced sons when paired with males that provisioned at a high rate, reflecting females' anticipation of the quality of the rearing environment. Components of reproductive potential, including total fecundity, were inherited matrilineally, and all else being equal, females in better condition produced larger-than-average clutches biased toward daughters, who eventually produced larger-than-average clutches themselves. Sex ratios therefore varied with components of parental investment but in opposing directions for matrilineally inherited and environmental effects, suggesting that multiple countervailing selective forces shape sex ratio variation.