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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.

The behavioural ecology of western lowland gorillas was studied for 16 months in the Lope Reserve, Gabon, where gorillas are sympatric with chimpanzees in lowland tropical forest. Data were collected by direct observation, and by examination of nest-sites, feeding-sites, and trails. The nature of frugivory and the extent of seasonal variation in food selection were emphasised. Dietary composition was identified, and the contribution of fruits was evaluated from the volume of fruit ingested estimated retrospectively from seeds in the gorillas' dung. Forest structure and composition were assessed using transects, and fruit and leaf production was quantified monthly to estimate food availability. Food distribution was patchy, and many foods showed seasonal peaks in abundance. The heterogeneity of the habitat was reflected in the diverse diet: gorillas ate 139 parts of 103 species of plants, including 78 fruits. One third of dung samples contained weaver ants. Vegetative parts of Aframomum and Marantaceae formed staple foods, due to their abundance, accessibility, and year-round availability. Succulent fruit formed over 90% of fruit intake. Seasonal variation was measured in all dietary parameters. Flexible foraging strategies enabled gorillas to cope with fruit scarcity, particularly during the major dry season: when less fruit was available gorillas consumed more stems, leaves, and bark, and ate poorer-quality fibrous fruits. Ranging was influenced by the seasonal availability of particular food species: when fruit was abundant gorillas travelled large distances between sources, when scarce they adopted a low cost strategy, shifting their diet towards more abundant, but poorer quality foods, and travelling less. Differences in feeding, ranging, and climbing between lowland and mountain gorillas result from striking differences in their respective habitats, especially in the abundance and distribution of fruit sources. Lowland gorillas' home ranges were larger; they spent more time in tress, mostly feeding; yet their social structure seemed to be similar to mountain gorillas. Lope gorillas adopted strategies similar to those of other frugivorous primates: fruits were preferred foods, consumed with fibre and leaves to meet nutritional requirements. The switch in diet was facilitated by the gorillas’ large body-size, which may have enabled them to cope with succulent fruit shortages, and allowed gorillas to remain in relatively stable groups.

While a major benefit of female-male associations in gorillas is protection from infanticidal males, a silverback is also responsible for providing overall group stability and protection from predation and other environmental or interspecific risks and disturbances. A silverback’s reproductive success will be a function of his group’s survival, his females’ reproductive rates and the survival of his progeny. Here, I evaluate the western lowland silverback’s role as the protective leader of his group and provide the first detailed behavioural study of silverback-group dynamics for western lowland gorillas from a holistic perspective; in both forested and bai environments, from nest-to-nest. Behavioural data were collected from one single-male habituated western lowland gorilla group, over 12-months starting January 2007 at the Bai Hokou Primate Habituation Camp, Central African Republic. Data collection - instantaneous scans, continuous written records of all auditory signals, nesting data, and ad libitum notes on interunit interactions - focused on the silverback and those individuals in his immediate proximity. Analyses were conducted over 258 morning or afternoon sessions, on 3,252 silverback behaviour scans (plus 1,053 additional smell scans), 22,343 auditory signals and 166 nest sites. Evidence from neighbours to the silverback, group spread, progression, ranging, nesting, human directed aggression and silverback chemosignalling analyses suggest that silverback-group dynamics have developed complex, strategic spatial and social strategies to cope with perceived risk in rainforest environments, which respond to differing habitats, and differing intensities of interunit interactions and interspecific disturbance. I also show that the release of pungent extreme and high level silverback odours may function as both acute and chronic indicators of arousal designed to intimidate extragroup rival males and attract adult females by expressing dominance, strength, and health. Higher level silverback odours may also provide cues for group members to increase vigilance in risky situations, whereas low level smells may function as a baseline identification marker and provide both self and intragroup reassurance. Western lowland silverback-group relationships appear to be centred on providing a strong protective – rather than socially interactive - and stabilizing role to ensure group cohesion and safety, which ultimately increases the likelihood of male reproductive success.