Data from: Large brains and groups associated with high rates of agonism in primates

Animals living in social groups will almost inevitably experience competition for limited resources. One consequence of competition can be agonism, an activity that is not only costly to participate in at the individual level but potentially also at the group level due the detrimental effects that agonism can have on group stability and cohesion. Agonism rates across primate species have previously been associated with group size and terrestriality; therefore primates, particularly those in large groups, should develop strategies to mitigate or counter-act agonism. Here, we use phylogenetically controlled analyses to evaluate whether the known relationship between brain size and group size may partially reflect an association between agonism and brain size in large groups. We find strong positive associations between group level agonism and 2 measures of brain size (endocranial volume and neocortex ratio) in 45 separate populations across 23 different primate species. In contrast, dyadic (pair-wise) rates of agonism are inversely associated with group size and not with brain size. Moreover, we find a distinct absence of relationships between agonism and the prevalence of prosocial, cooperative behaviors. That overall rates of agonism increase but dyadic rates decrease with group size suggests that individuals in larger groups either can buffer aggression better or only species with low levels of dyadic conflict can maintain large groups.

群居动物几乎不可避免地会面临有限资源的竞争。竞争的后果之一便是争斗行为（agonism）：这类活动不仅会让个体付出高昂代价，由于其会对群体稳定性与凝聚力产生不利影响，同样可能对群体层面造成负面影响。此前已有研究表明，灵长类动物的争斗行为频率与群体规模、陆生习性存在关联；因此，灵长类动物，尤其是群居规模较大的类群，理应演化出缓解或抵消争斗行为的策略。
本研究借助系统发育控制分析，探讨已知的脑容量与群体规模之间的关联，是否部分反映了大型群体中争斗行为与脑容量之间的联系。我们在覆盖23个不同灵长类物种的45个独立种群中发现，群体层面的争斗行为与两项脑容量指标（颅腔容量（endocranial volume）与新皮层比例（neocortex ratio））之间存在显著的正相关关系。与之相反，二元（dyadic）争斗行为频率与群体规模呈负相关，而与脑容量无关联。此外，我们未发现争斗行为与亲社会合作行为的占比之间存在任何关联。
随着群体规模扩大，总体争斗行为频率上升而二元争斗频率下降，这一结果表明，大型群体中的个体能够更好地缓冲攻击行为，或是仅有二元冲突水平较低的物种能够维持大型群居群体。