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（parasitism）普遍被视为社会性行为的主要代价以及集群规模的限制因素，然而现有研究关于集群规模与寄生之间的关联却呈现出多样化的结果。依托迄今规模最大的同类数据库，我们针对69项探讨集群规模与寄生风险（parasite risk）之间关联的研究开展了元分析（meta-analysis），以寄生感染情况与免疫防御（immune defenses）能力作为寄生风险的衡量指标。我们提出如下假设：对于具备接触传播与环境传播途径的生物类群，集群规模与寄生作用呈正相关；而对于搜寻型寄生虫、寄生性天敌（parasitoids），以及可能的媒介传播寄生虫（vector-borne parasites），二者则呈负相关，这一假设基于遭遇稀释效应（encounter-dilution effect）。整体分析结果显示，集群规模存在正向效应（相关系数r=0.187），且该效应的强度随传播途径与寄生风险衡量指标的不同而存在差异，仅存在微弱的发表偏倚（publication bias）迹象。在不同宿主类群中，鸟类的集群规模与寄生风险之间的关联强度高于哺乳类，这一现象或由生态与社会因素驱动。元回归分析（meta-regression）结果表明，效应量（effect sizes）随最大集群规模的增大而提升。系统发育元分析（phylogenetic meta-analyses）结果显示，集群规模-寄生作用关联强度不存在系统发育信号（phylogenetic signal）。我们的研究结论为：除栖息于大型聚合群体中的物种（例如集群性鸟类，其效应量更大）外，集群规模仅能作为寄生风险的弱预测因子。