Data from: Demographic mechanisms of inbreeding adjustment through extra-pair reproduction

1. One hypothesis explaining extra-pair reproduction is that socially monogamous females mate with extra-pair males to adjust the coefficient of inbreeding (f) of extra-pair offspring (EPO) relative to that of within-pair offspring (WPO) they would produce with their socially paired male. Such adjustment of offspring f requires non-random extra-pair reproduction with respect to relatedness, which is in turn often assumed to require some mechanism of explicit pre-copulatory or post-copulatory kin discrimination. 2. We propose three demographic processes that could potentially cause mean f to differ between individual females' EPO and WPO given random extra-pair reproduction with available males without necessarily requiring explicit kin discrimination. Specifically, such a difference could arise if social pairings formed non-randomly with respect to relatedness or persisted non-randomly with respect to relatedness, or if the distribution of relatedness between females and their sets of potential mates changed during the period through which social pairings persisted. 3. We used comprehensive pedigree and pairing data from free-living song sparrows (Melospiza melodia) to quantify these three processes and hence investigate how individual females could adjust mean offspring f through instantaneously random extra-pair reproduction. 4. Female song sparrows tended to form social pairings with unrelated or distantly related males slightly less frequently than expected given random pairing within the defined set of available males. Furthermore, social pairings between more closely related mates tended to be more likely to persist across years than social pairings between less closely related mates. However, these effects were small and the mean relatedness between females and their sets of potential extra-pair males did not change substantially across the years through which social pairings persisted. 5. Our framework and analyses illustrate how demographic and social structuring within populations might allow females to adjust mean f of offspring through random extra-pair reproduction without necessarily requiring explicit kin discrimination, implying that adjustment of offspring f might be an inevitable consequence of extra-pair reproduction. New theoretical and empirical studies are required to explore the general magnitude of such effects and quantify the degree to which they could facilitate or constrain long-term evolution of extra-pair reproduction.

1. 用以解释婚外繁殖的一项假说提出：社会一夫一妻制的雌性会与婚外雄性交配，以此调整婚外子代（extra-pair offspring, EPO）相较于其与社会配偶雄性所产婚内子代（within-pair offspring, WPO）的近交系数（coefficient of inbreeding, f）。这类子代近交系数的调整，需要基于亲缘关系的非随机婚外繁殖，而过往研究通常认为，此类情况需依赖明确的交配前或交配后亲族识别（kin discrimination）机制。
2. 我们提出了三种种群统计过程，在可获得的雄性个体中随机发生婚外繁殖的前提下，无需明确的亲族识别，即可令雌性个体的婚外子代与婚内子代的平均近交系数产生差异。具体而言，若社会配对的形成基于亲缘关系的非随机模式，或社会配对的存续基于亲缘关系的非随机模式，抑或雌性与其潜在配偶间的亲缘关系分布在社会配对存续期内发生变化，那么两类子代的近交系数差异便可能产生。
3. 我们利用源自野生歌带鹀（Melospiza melodia）的完整家系与配对数据，量化上述三种过程，以此探究雌性个体如何通过瞬时随机的婚外繁殖调整子代的平均近交系数。
4. 在既定可获得雄性群体内随机配对的预设情境下，雌性歌带鹀与无亲缘关系或远缘亲缘雄性形成社会配对的频率略低于预期。此外，亲缘关系越近的配偶间，其社会配对跨年度存续的概率越高。不过这类效应的幅度较小，且雌性与其潜在婚外雄性间的平均亲缘关系，并未在社会配对存续的年份中发生显著变化。
5. 我们的研究框架与分析结果表明，种群内部的种群统计特征与社会结构，如何能够令雌性通过随机婚外繁殖调整子代的平均近交系数，而无需依赖明确的亲族识别机制。这意味着子代近交系数的调整或许是婚外繁殖的必然结果。未来需要开展新的理论与实证研究，以探究这类效应的普遍强度，并量化其对婚外繁殖长期演化的促进或制约程度。