• behaviour
• conflict resolution
• intergroup conflict
• intergroup interactions
• intergroup tolerance
• multi-level society
• primate
• reconciliation
• social behaviour

Encounters between different social groups of the same species involve a complex interplay between attractive and repulsive forces. The outcome and nature of these interactions depend on several factors, including dominance, the relationship and familiarity between the groups, their demographic composition and their position within the wider interconnected network of group-group bonds. Species living in Multi-Level Societies (MLSs) represent a fertile ground for investigating this topic, as inter-unit interactions happen on a daily basis.
In this work, we examine the interunit social dynamics within the MLS of geladas (Theropithecus geladas) in their natural habitat of the Ethiopian highlands. We collected data on interunit social interactions, including proximity, play and agonistic encounters, as well as faecal samples for genetic analyses. The structure of geladas’ society was examined through Social Network Analysis, and a non-linear hierarchy was detected using the Perc framework. Generalised linear mixed models were fitted to investigate the determinants of interunit dominance and hostility, while a PC/MC protocol together with Bayesian modelling was used to assess the presence of an interunit reconciliatory mechanism.
Our analyses suggest an egalitarian and non-nepotistic society at the interunit level, as neither dominance nor kinship affected the structure of geladas’ interunit network. The presence of non-reproductive males was associated with higher dominance tier as well as with a higher aggression rate between OMUs, indicating their role as both a source of interunit tension and of agonistic support. Finally, we provide the first ever evidence of an intergroup reconciliatory mechanism in a non-human primate.
This work sets the basis for future studies on interunit interactions in MLS-living primates, suggesting the need for further research on determinants of interunit hostility and on conflict resolution in this and other species.