Data from: Are there indirect fitness benefits of female extra-pair reproduction? Lifetime reproductive success of within-pair and extra-pair offspring

The forces driving extra-pair reproduction by socially monogamous females, and the resulting genetic polyandry, remain unclear. A testable prediction of the hypothesis that extra-pair reproduction partly reflects indirect selection on females is that extra-pair young (EPY) will be fitter than their within-pair young (WPY) maternal half-siblings. This prediction has not been comprehensively tested in a wild population, requiring data on the lifetime reproductive success (LRS) of maternal half-sib EPY and WPY. We used 17 years of genetic parentage data from song sparrows, Melospiza melodia, to compare the LRS of hatched EPY and WPY maternal half-siblings measured as their lifetime number of hatched offspring, recruited offspring and hatched grandoffspring. EPY hatchlings were not significantly fitter than WPY hatchlings for any of three measures of LRS. Furthermore, opposite to prediction, EPY hatchlings tended to have lower LRS than their maternal half-sibling WPY hatchlings on average. EPY also tended to be less likely to survive to hatch than their maternal half-sibling WPY. Taken together, these results fail to support one key hypothesis explaining the evolution of genetic polyandry by socially monogamous females and suggest there may be weak indirect selection against female extra-pair reproduction in song sparrows.社会单配制雌性的婚外配行为（extra-pair reproduction）及其所引发的遗传一妻多夫制（genetic polyandry）背后的驱动机制，目前仍不明确。针对“婚外配行为在一定程度上反映了对雌性的间接选择”这一假说，可检验的预测是：婚外配后代（extra-pair young, EPY）的适合度将高于其同母异父的婚内后代（within-pair young, WPY）。该预测尚未在野生种群中得到全面检验，这需要获取同母异父的EPY与WPY的终生繁殖成功率（lifetime reproductive success, LRS）数据。我们使用了17年间采集的歌带鹀（*Melospiza melodia*）遗传亲权数据，以孵化出的EPY与同母异父WPY的终生繁殖成功率为研究指标，比较了二者的终生繁殖成功率，具体测量指标包括其终生孵化的后代数、招募的后代数以及孵化的孙代数。在三项终生繁殖成功率衡量指标中，EPY雏鸟的适合度均未显著高于WPY雏鸟。此外，与预测结果相反，平均而言EPY雏鸟的终生繁殖成功率相较于其同母异父的WPY雏鸟更低。同时，EPY雏鸟的孵化存活率也往往低于其同母异父的WPY个体。综合来看，这些结果并不支持“社会单配制雌性演化出遗传一妻多夫制”的关键假说之一，并表明在歌带鹀中，可能存在针对雌性婚外配行为的微弱间接选择压力。