Data from: Superiority of extra-pair offspring: maternal but not genetic effects as revealed by a mixed cross-fostering design

Extra-pair copulations (EPC) are a rule rather than an exception in socially monogamous birds. Despite widespread occurrences, the benefits of female infidelity remain illusive. Most attention has been paid to the possibility that females gain genetic benefits from EPC. Fitness comparison between maternal half-siblings is considered to be a defining test of this hypothesis. Recently it was shown that these comparisons may be confounded by within-brood maternal effects. One such effect may be the distribution of half-siblings in the laying order. This possibility is difficult to study since it would be necessary to detect from which egg the chick hatched. Here we used a new approach for egg-chick assignment. We cross-fostered eggs on an individual basis among a set of nests of the collared flycatcher Ficedula albicollis. After hatching, chicks were ascribed to mothers and therefore to individual eggs by molecular-genetic methods. Extra-pair young predominated early in the laying order. Under natural conditions, this should give them a competitive advantage over their half-siblings mediated by hatching asynchrony. However, we experimentally synchronized hatching. After this treatment extra-pair young did not outperform within-pair young in any studied trait including survival up to recruitment and several indicators of reproductive success and attractiveness. We obtained only modest sample size for the last two traits and did not test for extra-pair success of male offspring. Thus we cannot exclude the possibility of slight advantages of EPY during their adult phase of life. However, our data tentatively suggest that more likely reason for females’ EPCs is the insurance against the infertility of a social mate.

婚外配（Extra-pair copulations, EPC）在社会单配制鸟类中实为常态而非特例。尽管该现象广泛存在，但雌性婚外配行为的演化收益仍未明晰。既往学界多聚焦于“雌性可通过婚外配获得遗传收益”这一假说，对比母系半同胞间的适合度差异，被视为检验该假说的经典实验范式。近期研究表明，这类对比可能受巢内母源效应干扰，其中一项潜在干扰因素即为半同胞在产蛋顺序中的分布。该议题的研究存在较大挑战，因为需要精准确定每只雏鸟孵化自哪一枚卵。本研究采用全新的卵-雏鸟匹配方法：以个体为单位，将卵跨巢寄养于领捕蝇鸟（Ficedula albicollis）的多个巢中。孵化后，研究人员通过分子遗传方法，将雏鸟匹配至对应的母体及个体卵源。研究发现，婚外配子代（extra-pair young, EPY）在产蛋早期的占比更高。自然条件下，这一分布特征可借助孵化异步性赋予其相较于半同胞的竞争优势。但本研究通过实验实现了孵化同步化，经此处理后，婚外配子代在所有检测性状上均未表现出优于婚内子代的优势，这些性状涵盖直至繁殖种群招募的存活率、多项繁殖成功率指标以及个体吸引力指标。鉴于针对后两项性状的样本量相对有限，且本研究未检测雄性子代的婚外配繁殖成功率，因此我们无法排除婚外配子代在成年阶段存在微弱竞争优势的可能性。不过本研究数据初步表明，雌性发生婚外配更可能的演化动因，是为规避社会配偶的不育风险。