From groups to communities in western lowland gorillas

Social networks are the result of interactions between individuals at different temporal scales. Thus, sporadic intergroup encounters and individual forays play a central role in defining the dynamics of populations in social species. We assessed the rate of intergroup encounters for three western lowland gorilla (Gorilla gorilla gorilla) groups with daily observations over five years, and noninvasively genotyped a larger population over four months. Both approaches revealed a social system much more dynamic than anticipated, with nonaggressive intergroup encounters that involved social play by immature individuals, exchanges of members between groups likely modulated by kinship, and absence of infanticide evidenced by infants non fathered by the silverback of the group where they were found. This resulted in a community composed of groups that interacted frequently and non-aggressively, contrasting with the more fragmented and aggressive mountain gorilla (G. beringei beringei) societies. Such extended sociality can promote the sharing of behavioural and cultural traits, but might also increase the susceptibility of western lowland gorillas to infectious diseases that have decimated their populations in recent times. This is the accepted version of the article published in Proceedings of the Royal Society B: Biological Sciences (2019). The final published version is available at DOI: 10.1098/rspb.2018.2019 Additional funding for this project was provided by ARC‑SPAC Sabine Plattner African Charities (Switzerland).