Abstract
In humans and chimpanzees, most intraspecific killing occurs during coalitionary intergroup conflict. In the closely related genus Gorilla, such behavior has not been described. We report three cases of multi-male, multi-female wild mountain gorilla (G. beringei) groups attacking extra-group males. The behavior was strikingly similar to reports in chimpanzees, but was never observed in gorillas until after a demographic transition left ~25% of the population living in large social groups with multiple (3+) males. Resource competition is generally considered a motivator of great apes’ (including humans) violent intergroup conflict, but mountain gorillas are non-territorial herbivores with low feeding competition. While adult male gorillas have a defensible resource (i.e. females) and nursing/pregnant females are likely motivated to drive off potentially infanticidal intruders, the participation of others (e.g. juveniles, sub-adults, cycling females) is harder to explain. We speculate that the potential for severe group disruption when current alpha males are severely injured or killed may provide sufficient motivation when the costs to participants are low. These observations suggest that the gorilla population’s recent increase in multi-male groups facilitated the emergence of such behavior, and indicates social structure is a key predictor of coalitionary aggression even in the absence of meaningful resource stress.
Highlights
Mountain gorilla groups maintain overlapping home ranges, and social units frequently encounter one another in the forest[30]
Involvement of animals other than young adult and fully adult males is usually limited to vocal aggression, if they participate at all (Karisoke Research Center long-term records, pers. obs.)
Despite morphological and behavioral evidence suggesting a long history of single male groups, from the mid 1990s onward a sizable proportion of the gorilla population (~25%; Karisoke Research Center (KRC) long-term records42) resided in groups that bore more structural similarity to chimpanzee groups than to harems, but without chimpanzees’ fission-fusion dynamics[47]
Summary
Started in 1967 by Dr Dian Fossey, KRC operates one of the world’s longest-running field sites. The observers followed the gorillas in their view toward the screams, and encountered an unidentified solitary male surrounded by the rest of the group members (Table 1). On May 18 2013, tracking staff contacted group Titus, a mixed-sex group of nine animals (Tables 2 and 3) and found them with a two-member group consisting of adult male Inshuti (Table 1; the victim of the 2004 Beetsme group attack) and an adult female, Shangaza. An hour after observers arrived, Titus group’s alpha male, followed by all eight of his group members, ran after Inshuti and held him to the ground. One minute after the attack started, Inshuti escaped and rejoined Shangaza, and Titus’ group moved out of view of the observers
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