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 cognition, 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 its organization into regions 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 be predicted by home range size. On the other hand, diet, expressed as a shift from folivory toward frugivory, was shown to not be related to hippocampal volume. Interestingly, CA2 was shown to exhibit phylogenetic signal (Pagel’s λ) only against certain measures of group size but not with ecological factors. We also found that in primates 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.

Supplementary filesFolder data - phylogenetic trees and all data used, Folder PGLS results - full results from PGLS and ANOVA tables, dredging.txt has all models dredged with weights above 0.05, full model.txt includes a full model with hippocampus as dependent variable. Additionally included are the R code and summary table of the missing data.Supplementary.zip