The social structure and behaviour of Western Lowland gorillas (Gorilla gorilla gorilla) at Mbeli Bai, Republic of Congo

Abstract

Most of what we know of the socioecology and behaviour of gorillas comes from studies of mountain gorillas (Gorilla beringei beringei), despite their representing less than 1% of all gorillas in the wild. Western lowland gorillas (Gorilla gorilla gorilla) have received far less research effort, and difficulties in habituation have led previous studies to rely heavily on indirect trail evidence. This has prevented any in-depth comparison of social structure and behaviour between species. The discovery of swampy clearings frequented by western lowland gorillas in northern Congo has offered the first opportunity to obtain accurate demographic and behavioural data. This thesis reports on 6273 hours of observer presence at Mbeli Bai, which yielded 1681 hours of direct gorilla observation. Most data come from a population of 13 groups and 7 solitary silverbacks. Group size ranged from 2 to 16 (mean = 8.4, SD 4.3) which does not differ significantly from data published on most other populations. Female natal and secondary transfer were recorded, and male emigration from the natal group appeared universal (no multi-silverback groups were recorded). No evidence of sub-grouping or a fission-fusion grouping pattern was found. During intra-group interactions, no evidence was recorded of female philopatry, and silverbacks herded and intimidated females (especially new immigrants) to prevent emigration. Inter-unit interactions took place on only 42% of shared bai-use occasions, and levels of silverback agonism were much lower than in mountain gorillas, with peaceful mingling of groups recorded. Display behaviour was common, and two previously unrecorded agonistic displays (the splash display and the crest display) are described. No contact aggression between silverbacks was observed but evidence of wounding sustained in the forest suggests that the bai environment may inhibit such aggression. High visibility and the ease of silverback 'policing' are thought to create an unusually tolerant social dynamic, and as such, frequencies of certain social behaviours should probably be viewed as site-specific. The thesis offers the fullest account of western gorilla social structure and behaviour to date. Long-term monitoring of life history variables, if continued, will provide an unparalleled opportunity to understand the effects of habitat and food availability and more stochastic influences on western gorilla social structure, fitness, and survival.