Socioecological Factors Influencing Intergroup Encounters in Western Lowland Gorillas (Gorilla gorilla gorilla)

Socioecological variables influence the rate and nature of encounters between conspecific primate groups. Both social (i.e., type of encounter) and ecological factors (i.e., fruit availability) affect rates and outcomes of intergroup encounters in eastern gorillas. However, the roles of individual factors underlying these events among western lowland gorillas (Gorilla gorilla gorilla) remain relatively understudied. We assessed whether group composition (the number of subordinate males and females in the group) and fruit availability influenced the rate of intergroup encounters in three groups of western lowland gorillas in the Republic of Congo. From April 2014 to October 2018, we monitored focal gorilla groups daily in the Goualougo and Djéké Triangles. To assess the ecological similarity between sites, we compared botanical surveys of trees and herbs across them and found overall herb densities were similar between sites. We observed 294 intergroup encounters over 2797 observation days, with a mean rate of 2.20 ± SD 0.93 encounters/month (range: 1.59–3.94) across groups, and these interactions ranged from tolerant to aggressive. The number of subordinate males in the group correlated negatively with the occurrence of intergroup encounters, and groups were more likely to avoid interaction or react aggressively when they encountered a solitary male than when they encountered another group. Some differences between the groups may be attributed to familiarity with extragroup members and the status of the dominant silverback. We found no relationship between the rate of encounters and fruit availability. Our findings indicate that defense of mates, rather than of food resources, may be the most important driver of between-group competition in western lowland gorillas. These findings, which are similar to those for mountain gorillas, suggest that primate social systems may be more variable than previously anticipated within taxa. Therefore, further longitudinal studies are needed to determine the generality of our findings and assess the direct impacts of intergroup encounters on fitness.