Data from: Sex differences in the drivers of reproductive skew in a cooperative breeder

Many cooperatively breeding societies are characterized by high reproductive skew, such that some socially dominant individuals breed, while socially subordinate individuals provide help. Inbreeding avoidance serves as a source of reproductive skew in many high-skew societies, but few empirical studies have examined sources of skew operating alongside inbreeding avoidance, or compared individual attempts to reproduce (reproductive competition) with individual reproductive success. Here we use long-term genetic and observational data to examine factors affecting reproductive skew in the high-skew cooperatively breeding southern pied babbler (Turdoides bicolor). When subordinates can breed, skew remains high, suggesting factors additional to inbreeding avoidance drive skew. Subordinate females are more likely to compete to breed when older or when ecological constraints on dispersal are high, but heavy subordinate females are more likely to successfully breed. Subordinate males are more likely to compete when they are older, during high ecological constraints, or when they are related to the dominant male, but only the presence of within-group unrelated subordinate females predicts subordinate male breeding success. Reproductive skew is not driven by reproductive effort, but by forces such as intrinsic physical limitations and intra-sexual conflict (for females), or female mate choice, male mate-guarding and potentially reproductive restraint (for males). Ecological conditions or ‘outside options’ affect the occurrence of reproductive conflict, supporting predictions of recent synthetic skew models. Inbreeding avoidance together with competition for access to reproduction may generate high skew in animal societies, and disparate processes may be operating to maintain male vs. female reproductive skew in the same species.诸多合作繁殖社会（cooperatively breeding societies）均以高繁殖偏斜（reproductive skew）为典型特征：部分处于社会支配地位的个体参与繁殖，而社会从属个体则提供抚育帮助。近亲回避（inbreeding avoidance）是诸多高偏斜社会中繁殖偏斜的成因之一，但目前鲜有实证研究探讨与近亲回避共同发挥作用的偏斜成因，或是对比个体繁殖尝试（即繁殖竞争）与个体繁殖成功率之间的差异。本研究依托长期遗传与观测数据，针对高繁殖偏斜的合作繁殖物种南非斑鸫鹛（*Turdoides bicolor*），系统剖析影响其繁殖偏斜的各类驱动因素。当从属个体具备繁殖可能性时，繁殖偏斜程度依然居高不下，这表明除近亲回避外，还存在其他驱动繁殖偏斜的因素。从属雌性个体在年龄增长，或是扩散受到较强生态限制时，更倾向于发起繁殖竞争；而体重较高的从属雌性个体则更易获得繁殖成功。从属雄性个体则在年龄更大、面临较强生态限制，或是与群体内支配雄性存在亲缘关系时，更易参与繁殖竞争；但唯有当群体内存在非亲缘从属雌性时，才能有效预测从属雄性的繁殖成功率。本次研究发现，繁殖偏斜并非由繁殖投入所驱动，而是由诸如内在生理限制与性内冲突（针对雌性），或是雌性配偶选择、雄性配偶守护以及潜在的繁殖抑制（针对雄性）等多重因素共同塑造。生态条件或"外部可选机会"会影响繁殖冲突的发生概率，这一结果验证了近期综合偏斜模型的相关预测。综上，近亲回避与繁殖机会竞争的共同作用，或可在动物社会中催生高繁殖偏斜；而同一物种内维持雄性与雌性繁殖偏斜的核心机制可能存在显著差异。