Data from: Complex sources of variance in female dominance rank in a nepotistic society

Many mammalian societies are structured by dominance hierarchies, and an individual's position within this hierarchy can influence reproduction, behaviour, physiology and health. In nepotistic hierarchies, which are common in cercopithecine primates and also seen in spotted hyaenas, Crocuta crocuta, adult daughters are expected to rank immediately below their mother, and in reverse age order (a phenomenon known as ‘youngest ascendancy’). This pattern is well described, but few studies have systematically examined the frequency or causes of departures from the expected pattern. Using a longitudinal data set from a natural population of yellow baboons, Papio cynocephalus, we measured the influence of maternal kin, paternal kin and group size on female rank positions at two life history milestones, menarche and first live birth. At menarche, most females (73%) ranked adjacent to their family members (i.e. the female held an ordinal rank in consecutive order with other members of her maternal family); however, only 33% of females showed youngest ascendancy within their matriline at menarche. By the time they experienced their first live birth, many females had improved their dominance rank: 78% ranked adjacent to their family members and 49% showed youngest ascendancy within their matriline. The presence of mothers and maternal sisters exerted a powerful influence on rank outcomes. However, the presence of fathers, brothers and paternal siblings did not produce a clear effect on female dominance rank in our analyses, perhaps because females in our data set co-resided with variable numbers and types of paternal and male relatives. Our results also raise the possibility that female body size or competitive ability may influence dominance rank, even in this classically nepotistic species. In total, our analyses reveal that the predictors of dominance rank in nepotistic rank systems are much more complex than previously thought.许多哺乳动物社会以优势等级（dominance hierarchy）为结构框架，个体在该等级中的地位会对其繁殖、行为、生理状况与健康水平产生影响。在裙带式优势等级中——这类等级结构常见于猕猴科（Cercopithecine）灵长类，同时也存在于斑鬣狗（Crocuta crocuta）种群中——学界普遍推测，成年雌性后代的顺位理应紧邻其母亲，且按照年龄逆序排列（这一现象被称为“最年轻上位（youngest ascendancy）”。该模式已有充分描述，但鲜有研究系统检验偏离预期模式的发生频率与成因。本研究依托黄狒狒（Papio cynocephalus）自然种群的纵向数据集（longitudinal data set），分析了母系亲属、父系亲属与群体规模对雌性在两个生命史里程碑——月经初潮（menarche）与首次产仔（first live birth）——时的等级顺位的影响。在月经初潮时，多数雌性（73%）与其家族成员顺位相邻，即该雌性的序数顺位与其母系家族其他成员处于连续序列中；然而仅33%的雌性在月经初潮时的母系谱系（matriline）内呈现最年轻上位模式。待到首次产仔时，多数雌性已提升了自身优势等级：78%的个体与家族成员顺位相邻，49%的个体在母系谱系内呈现最年轻上位模式。母猴与母系姐妹的存在对等级结果产生了显著影响。但在本研究的分析中，父猴、兄弟与父系兄弟姐妹的存在并未对雌性优势等级产生明确影响，这或许是因为本研究种群中的雌性与不同数量和类别的父系亲属与雄性亲属共同栖息。本研究结果还提示，即便在这一经典的裙带式优势等级物种中，雌性体型或竞争能力或许也会对优势等级产生影响。总体而言，本研究分析表明，裙带式等级系统中优势等级的预测因子远比此前所认为的更为复杂。