Intra- and inter-group interactions of an all-male group of virunga mountain gorillas (Gorilla gorilla beringei)

Six unrelated male gorillas formed an all-male group within the Virunga mountain gorilla population. Frequent homosexual interactions characterized the high cohesiveness of this group. Such homosexual behavior reduced the inter-individual distances and increased the social tension between the two silverbacks in the group. The silverbacks retained “ownership” of the homosexual partners, but competed and fought with each other violently when the partners avoided or ignored their courtship. Neither submissive nor reassurance behavior was noted between the silverbacks. Thus, their relationships may not be explained in terms of dominance and subordinancy. However, the loser-support and mediating behavior observed in the group prevented them from engaging in severe fights. Aggression was always directed from the elder and dominant males to the younger and subordinate males, while supporting interactions occurred in the opposite direction. The blackbacks frequently supported the subadult aggressees by attacking the silverbacks, and the younger males displayed mediating behavior in violent fights between the silverbacks. The group's ranging was influenced by encounters with neighboring social units. The members avoided contact with other units and shifted their range after several encounters. On the other hand, when and after a subadult male had immigrated into their group, they frequently encountered other units and did not move away from the encounter site. An all-male group may not be a favorable unit for females to transfer to, but may be profitable for maturing males to associate with. Its formation is probably related to recent social change in the Virunga gorilla population.