Interunit, environmental and interspecific influences on silverback-group dynamics in western lowland gorillas (Gorilla gorilla gorilla)

Abstract

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.