Datasets of hippocampal subsection and brain area dimorphism plus the phylogenetic tree used from Primate hippocampus size and organization are predicted by sociality but not diet.

The hippocampus is well known for its roles in spatial navigation and memory, but it is organized into regions that have different connections and functional specializations. Notably, the region CA2 has a role in social and not spatial cognition, as is the case for the regions CA1 and CA3 that surround it. Here, we investigated the evolution of the hippocampus in terms of its size and organization in relation to the evolution of social and ecological variables in primates, namely home range, diet and different measures of group size. We found that the volumes within the whole cornu ammonis coevolve with group size, while only the volume of CA1 and subiculum can also be predicted by home range. On the other hand, diet, expressed as a shift from folivory towards frugivory, was shown to not be related to hippocampal volume. Interestingly, CA2 was shown to exhibit phylogenetic signal only against certain measures of group size but not with ecological factors. We also found that sex differences in the hippocampus are related to body size sex dimorphism. This is in line with reports of sex differences in hippocampal volume in non-primates that are related to social structure and sex differences in behaviour. Our findings support the notion that in primates, the hippocampus is a mosaic structure evolving in line with social pressures, where certain subsections evolve in line with spatial ability too.

海马体（hippocampus）素来以其在空间导航与记忆中的关键作用为人熟知，但其内部可划分为多个具有差异化连接模式与功能特化的脑区。值得注意的是，CA2脑区参与社交认知而非空间认知，其周围的CA1与CA3脑区亦是如此。本研究围绕灵长类的社交与生态变量（具体包括活动范围、饮食模式以及多项群体规模衡量指标），探究了海马体的体积与组织结构演化特征。研究发现，整个海马角的体积与群体规模协同演化；而仅CA1与下托（subiculum）的体积可通过活动范围进行预测。另一方面，以从食叶性向食果性转变为表征的饮食模式，与海马体体积并无关联。有趣的是，CA2脑区仅在部分群体规模衡量指标下呈现出系统发育信号，而与生态因素无关。本研究还发现，海马体的性别差异与身体大小的性别二态性相关。这一结果与非灵长类动物的相关研究报道相符：非灵长类动物的海马体体积存在性别差异，且该差异与社会结构及行为性别差异相关。本研究结果支持这一观点：灵长类的海马体属于镶嵌结构，其演化既受社交压力驱动，部分亚区也会随空间认知能力的变化而演化。