Social dynamics of captive western lowland gorillas living in all-male groups

Male mammals show considerable variation in their association (e.g. single-male, multi-male, all-male groups) and relationship (e.g. affiliative versus intolerant) patterns. Although a number of primates have been observed to form all-male groups, studies of the social dynamics of these groups are limited. This study examined the social interactions of 25 male western lowland gorillas living in nine captive all-male groups. Over 1,300 hours of data were collected using group scan and all-occurrence sampling methodologies. Groups were cohesive, with males spending approximately one-third of their time within 5 meters of another individual. Although complete linear dominance hierarchies within a group were not observed, dominance relationships between individuals were evident for the majority (66%) of dyads. Social interactions varied as a function of age, with subadults engaging in more affiliative behavior and less non-contact aggression than either blackbacks or silverbacks. Visual/olfactory access to females increased non-contact aggression between males. Such results are similar to those found for all-male groups of mountain gorillas in the wild and demonstrate that all-male groups can be a cohesive social unit in this species. They also raise the questions of why all-male groups have rarely been observed in wild populations and how social, ecological and anthropogenic factors influence male sociality.