From groups to communities in western lowland gorillas

Abstract

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 5 years, and non-invasively genotyped a larger population over four months. Both approaches revealed a social system much more dynamic than anticipated, with non-aggressive 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 not fathered by the silverback of the group where they were found. This resulted in a community composed of groups that interacted frequently and not-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.

Western gorilla space use suggests territoriality

“…Whilst our findings suggest that gorilla groups may have regions of priority or even exclusive use close to their home range centre, groups are known to overlap and even peacefully co-exist in other regions of their ranges 46 . The smaller central home range regions could feasibly be defended by physical aggression, using olfactory cues 47 or through chest-beating 48 , a commonly observed form of between-group communication in gorillas 26 .…”

“…The flexible system of defending and sharing space suggested here, combined with growing evidence for long term social bonds between gorilla groups 26,53,54 and a far more dynamic social system than previously thought 46,55 , implies the presence of a complex social structure in gorillas where between-group interactions are influenced by many factors including social affiliations, kinship and the defence of space. Gorillas may therefore represent a valuable model system for investigating how elements of territoriality can occur simultaneously with affiliative between-group interactions.…”

Long-term inference of population size and habitat use in a socially dynamic population of wild western lowland gorillas

“…This could be due to spatial proximity and a higher chance of intergroup encounters facilitating female transfers or immediate involuntary female transfers following the sudden dissolution of a group due to the death of the silverback (Stokes et al 2003). This impression of connectedness fits the observation that infectious disease outbreaks, like Ebola, have severe consequences on western lowland populations due to their changing group membership and incorporation of new individuals (Bermejo et al 2006;Genton et al 2015;Le Gouar et al 2009;Forcina et al 2019).…”

“…The fraction of MCP that did not overlap with any other group was typically less than 20%. This is consistent with other studies on western lowland gorillas revealing variable extent of home range overlap between groups and frequent non-aggressive intergroup encounters (Seiler et al in prep., Bermejo 2004;Doran-Sheehy et al 2004;Doran and McNeilage 1998;Tutin 1996;Forcina et al 2019). In addition to the inherent problems of the MCP method (Burgman and Fox 2003;Börger et al 2006) a caveat of our study is that the MCPs encompass areas that are accessible for gorillas but also areas that are not accessible for gorillas (like water surfaces) and are constructed over a time frame of up to 2.8 years thus they do not necessarily represent home ranges.…”

“…This in turn facilitates disease transmission between groups, especially after the death of a silverback which in this uni-male species leads to the dissolution of the group and females moving to other groups (Nunn et al 2008). A study at Odzala-Kokoua National Park, Republic of Congo, based on the observation of three habituated groups over 5 years plus non-invasive genetics over 4 month postulated a highly dynamic social system with tolerant intergroup encounters and interactions like social play between members of different groups (Forcina et al 2019). Accordingly, the outbreak of infectious diseases like Ebola have led to documented population declines of 90% from October 2002 to January 2003 in Lossi Sanctuary, Republic of Congo or 95% from December 2003 to July 2004 at Lokoué clearing, Odzala-Kokoua National Park, Republic of Congo (Bermejo et al 2006;Caillaud et al 2006).…”