Low familiarity and similar ‘group strength’ between opponents increase the intensity of intergroup interactions in mountain gorillas (Gorilla beringei beringei)
Intergroup interactions in social animals can vary from hostile to affiliative and may be influenced by factors such as competitive ability, resource values and existing intergroup relationships. Despite the potential for intergroup interactions to affect individual fitness and group stability, few studies have comprehensively tested how social, demographic and ecological factors may simultaneously influence intergroup interactions. Using 13 years of continuous data on intergroup interactions (n = 464), group composition, range use and diet, we investigated the factors that influenced the initiation and escalation of intergroup interactions in a fully habituated subpopulation of mountain gorillas in Volcanoes National Park, Rwanda. The majority of interactions were non-physically agonistic (57%), while peaceful exchanges and physical aggression were less common (18% and 25% of interactions respectively). Solitary males and young dominant silverbacks were the most likely to initiate an interaction, presumably because these males have the highest incentive to attract mates. Aggressive interactions between a group and a solitary male involved a high number of participating group members, reflecting the incentive to avoid injury and infanticide associated with solitary male encounters. Aggression between social groups escalated when groups were similarly sized, perhaps because these groups have similar competitive abilities. Peaceful intergroup interactions most commonly involved opponents that contained familiar and related individuals, suggesting that the short dispersal distance of gorillas may facilitate kin-selected intergroup tolerance. Variation and plasticity in gorilla behaviour during intergroup interactions are therefore dependent on the opponent’s familiarity and threat level.
Intergroup interactions can vary from aggressive physical disputes to tolerant intermingling among extra-group individuals. Competition for access to limiting resources can often influence the occurrence of aggressive intergroup competition, but may not be sufficient to describe peaceful mingling between groups in social species. We found that although the main driver of intergroup interactions may be mating competition between males, interactions between familiar social groups were significantly more peaceful than interactions involving unfamiliar groups. These findings suggest that maintaining social and kin relationships with neighbouring groups may act as a strategy for reducing conflict in group-structured social species.
KeywordsMountain gorilla Intergroup encounter Mate competition Familiarity
We wish to thank the Rwanda Development Board and Ministry of Education, Rwanda, for the permission to conduct research in the Volcanoes National Park. We are very appreciative to all the trackers and field assistants who conducted much of the data collection used in this project. We thank DFGFI for the research collaboration and the ongoing support. Special thanks to Didier Abavandimwe for the creation of the interaction database specifically for this project and Damien Caillaud, Marc Tennant and Bruno Buzatto for statistical support. Thank you to Martha Robbins, the BEAS reviewers and the handling editor for the constructive and insightful feedback that improved the manuscript greatly. This project was funded by CCG’s project grant (School of Human Sciences, University of Western Australia).
This study was funded by the School of Human Sciences, University of Western Australia.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable interactional, national and/or institutional guidelines for the care and use of animals were followed. All procedures performed (i.e. observational data collection only) involving animals were in accordance with the ethical standards of the institution or practice at which the studies were conducted. Ethical approval for observational studies of animals was received by the University of Western Australia’s Animal Ethics Committee (RA/3/600/32).
- Brown JL (1964) The evolution of diversity in avian territorial systems. Wilson Bull 76:160–169Google Scholar
- Burnham KP, Anderson DR (2002) Model selection and multimodel inference: a practical information-theoretical approach, 2nd edn. Springer-Verlag, New YorkGoogle Scholar
- Falls JB (1982) Individual recognition by sounds in birds. In: Kroodsma DE, Miller EH (eds) Acoustic communication in birds. Academic Press, New York, pp 237–278Google Scholar
- Greenway K (2015) Threat and display: reproductive competition in wild male Western Gorillas (Gorilla gorilla). PhD thesis, School of Anthropology and Conservation. University of Kent, CanterburyGoogle Scholar
- Hill J, Gelman A (2007) Data analysis using regression and multilevel/hierarchical models. Cambridge University Press, CambridgeGoogle Scholar
- Milinski M, Parker GA (1991) Competition for resources. In: Krebs JR, Davies NB (eds) Behavioural ecology: an evolutionary approach. Blackwell Scientific Publications, Oxford, pp 137–168Google Scholar
- Schaller G (1963) The mountain gorilla. In: Chicago University Press. Chicago, IllinoisGoogle Scholar
- Stewart K, Harcourt A (1987) Gorillas: variation in female relationships. In: Smuts B, Cheney D, Seyfarth R, Wrangham W, Struhsaker T (eds) Primate societies. University of Chicago Press, ChicagoGoogle Scholar
- Trivers RL (1972) Parental investment and sexual selection. In: Campbell B (ed) Sexual selection and the descent of man. Aldine, Chicago, pp 136–179Google Scholar
- van Belle S, Scarry CJ (2015) Individual participation in intergroup contests is mediated by numerical assessment strategies in black howler and tufted capuchin monkeys. Proc R Soc B 370:20150007Google Scholar