Data from: Better-surviving barn swallow mothers produce more and better-surviving sons

Sex allocation theory predicts that parents are selected to bias their progeny sex ratio (SR) towards the sex that will benefit the most from parental quality. Because parental quality may differentially affect survival of sons and daughters, a pivotal test of the adaptive value of SR adjustment is whether parents overproduce offspring of the sex that accrue larger fitness advantages from high parental quality. However, this crucial test of the long-term fitness consequences of sex allocation decisions has seldom been performed. In this study of the barn swallow (Hirundo rustica), we showed a positive correlation between the proportion of sons and maternal annual survival. We then experimentally demonstrated that this association did not depend on the differential costs of rearing offspring of either sex. Finally, we showed that maternal lifespan positively predicted lifespan of sons but not of daughters. Because in barn swallows lifespan is a strong determinant of lifetime reproductive success, the results suggest that mothers overproduce offspring of the sex that benefits the most from maternal quality. Hence, irrespective of mechanisms causing the SR bias and mother-son covariation in lifespan, we provide strong evidence that sex allocation decisions of mothers can highly impact on their lifetime fitness.

性分配理论（Sex allocation theory）预测，自然选择会驱使亲本将后代性别比例（SR）偏向于能从亲本质量中获益最多的性别。由于亲本质量对雌雄后代的存活可能存在差异化影响，检验性别比例调整适应性价值的核心判据，便是亲本是否会过量产生那些能从高亲本质量中获得更大适合度优势的后代。然而，这项针对性分配决策长期适合度后果的关键检验，迄今仍极少开展。在本项针对家燕（Hirundo rustica）的研究中，我们首先证实了雄性后代比例与母本年存活率之间存在显著正相关；随后通过实验证明，这一关联并不受抚育不同性别后代的差异化成本影响；最后我们发现，母本寿命能够正向预测其子代雄性的寿命，但对子代雌性寿命并无此正向预测效应。鉴于家燕的寿命是其终生繁殖成功率的重要决定因素，本研究结果表明，母本确实会过量产生那些能从母本质量中获益最多的性别后代。因此，无论导致性别比例偏差以及母本与子代雄性寿命共变的具体机制为何，我们均提供了强有力的证据，证明母本的性分配决策能够对其终生适合度产生显著影响。