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 the degree to which they could facilitate or constrain long-term evolution of extra-pair reproduction.

1. 现有假说提出，社会一夫一妻制的雌性可通过与配偶外雄性交配，调整配偶外子代（extra-pair offspring, EPO）相对于其与社会配对雄性所产配对内子代（within-pair offspring, WPO）的近交系数（inbreeding coefficient, f）。此类子代近交系数的调整，要求针对亲缘关系开展非随机的配偶外繁殖（extra-pair reproduction）；而这一条件通常被认为需要借助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，在随机与可获得雄性进行配偶外繁殖的前提下，即便不依赖明确的亲族识别，也可能导致雌性个体的配偶外子代与配对内子代的平均近交系数产生差异。具体而言，若社会配对的形成针对亲缘关系呈现非随机性，或社会配对的存续针对亲缘关系呈现非随机性，抑或是雌性与其潜在配偶集合间的亲缘关系分布在社会配对存续期内发生改变，那么上述差异便可能产生。
3. 我们利用来自野生歌带鹀（Melospiza melodia）的全面谱系数据与配对数据，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配偶外繁殖调整子代的平均近交系数。
4. 在所定义的可获得雄性集合内随机配对的前提下，雌性歌带鹀与无亲缘关系或亲缘关系较远的雄性形成社会配对的频率略低于预期。此外，亲缘关系越近的配对个体，其社会配对跨年度存续的概率越高。不过，此类效应的强度较弱，且雌性与其潜在配偶外雄性集合间的平均亲缘关系，在社会配对存续的年份中并未发生显著变化。
5. 我们的研究框架与分析结果表明，种群内的种群统计与社会结构，如何能让雌性通过随机的配偶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。这意味着子代近交系数的调整或许是配偶外繁殖的必然结果。未来需要开展更多的理论与实证研究，以探究此类效应的普遍强度，以及它们在多大程度上能够促进或制约配偶外繁殖的长期演化。