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.

诸多合作繁殖社群均以高度的繁殖偏斜（reproductive skew）为典型特征：部分社会优势个体享有繁殖权，而社会从属个体则承担协助育幼的职责。在诸多高繁殖偏斜的社群中，近交回避是催生繁殖偏斜的重要因素之一，但目前鲜有实证研究同时探讨与近交回避共同发挥作用的偏斜驱动因子，或是将个体的繁殖尝试（即繁殖竞争）与个体繁殖成功率进行对比分析。本研究借助长期遗传与观测数据，针对高繁殖偏斜的合作繁殖物种南部斑鸫鹛（Turdoides bicolor），探讨影响其繁殖偏斜的各类因子。当从属个体拥有繁殖机会时，种群仍维持较高的繁殖偏斜，这表明除近交回避外，还存在其他驱动繁殖偏斜的因素。从属雌性个体在年龄更大、或是扩散受到较强生态限制时，更倾向于参与繁殖竞争；而体型较重的从属雌性个体则更易成功完成繁殖。从属雄性个体则会在年龄更大、面临较强生态限制，或是与优势雄性存在亲缘关系时更易参与繁殖竞争；但唯有种群内存在无亲缘关系的从属雌性时，才能有效预测从属雄性的繁殖成功率。繁殖偏斜并非由繁殖投入所驱动，而是受多种力量调控：对于雌性而言，驱动因子包括内在生理限制与性内冲突；对于雄性而言，则包括雌性配偶选择、雄性配偶守护，以及潜在的繁殖克制行为。生态条件或“外部选择权”会影响繁殖冲突的发生，这一结果验证了近期综合偏斜模型的相关预测。在动物社群中，近交回避与繁殖机会竞争的共同作用或许会催生高度的繁殖偏斜；而同一物种种群内，维持雄性与雌性繁殖偏斜的调控过程可能存在显著差异。