Data from: Do animals living in larger groups experience greater parasitism? A meta-analysis

Parasitism is widely viewed as the primary cost of sociality and a constraint on group size, yet studies report varied associations between group size and parasitism. Using the largest database of its kind, we performed a meta-analysis of 69 studies of the relationship between group size and parasite risk, as measured by parasitism and immune defenses. We predicted a positive correlation between group size and parasitism with organisms that show contagious and environmental transmission, and a negative correlation for searching parasites, parasitoids, and possibly vector-borne parasites (based on the encounter-dilution effect). Overall, we found a positive effect of group size (r=0.187) that varied in magnitude across transmission modes and measures of parasite risk, with only weak indications of publication bias. Among different groups of hosts, we found a stronger relationship between group size and parasite risk in birds than in mammals, which may be driven by ecological and social factors. A meta-regression showed that effect sizes increased with maximum group size. Phylogenetic meta-analyses revealed no evidence for phylogenetic signal in the strength of the group size-parasitism relationship. We conclude that group size is a weak predictor of parasite risk except in species that live in large aggregations, such as colonial birds, where effect sizes are larger.寄生现象普遍被视为社会性演化的主要代价，同时也是制约群体规模的关键因素，但现有相关研究对于群体规模与寄生现象间的关联所得结论并不一致。本研究依托当前同类研究中规模最大的数据库，对69项以寄生程度与免疫防御水平作为寄生虫风险衡量指标、探讨群体规模与寄生虫感染风险之间关联的研究开展了元分析。我们提出如下预设：对于存在接触传播与环境传播路径的生物类群，群体规模与寄生现象呈正相关；而对于搜寻性寄生虫、拟寄生生物以及可能的媒介传播寄生虫，二者则呈负相关（该推测基于相遇稀释效应）。整体而言，我们发现群体规模对寄生虫感染风险存在正向影响（相关系数r=0.187），且该影响的强度因传播方式与寄生虫风险衡量指标的不同而存在差异，仅存在微弱的发表偏倚迹象。在不同宿主类群中，鸟类的群体规模与寄生虫感染风险之间的关联强度显著高于哺乳类，这一差异可能由生态与社会因素所驱动。元回归分析结果显示，效应量随宿主最大群体规模的增大而提升。系统发育元分析结果表明，群体规模与寄生现象之间关联的强度不存在系统发育信号。我们最终得出结论：除了诸如集群鸟类这类生活在大规模聚集体中的物种（其效应量更大）之外，群体规模对寄生虫感染风险的预测能力相对较弱。