These socioecological conditions help explain differences in relative rates of both human and chimpanzee ‘warfare,’ across sites and groups, as well as its near-absence in other close relatives including bonobos 2, 3, 4 and orangutans 15. As is true for humans, rates of such encounters vary widely across sites and social groups 4, but lethal chimpanzee ‘raids’ and other forms of cooperative intergroup attacks have been regularly reported at multiple long-term field sites 2, 5, 6, 7, 8, 9, 10, 11.īetween and within species, violent intergroup conflict is most likely to occur when important resources are defensible and demographic power imbalances reduce the cost to individual participants 4, 11, 12, 13, 14. It is therefore unsurprising that the vertebrate animal behavior corollary best approximating human warfare occurs in one of humans’ closest extant relative, chimpanzees 2, 3. Intergroup coalitionary aggression is rare in the animal kingdom, but has particularly notable evolutionary and social significance in Homo sapiens 1. 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. 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. juveniles, sub-adults, cycling females) is harder to explain. females) and nursing/pregnant females are likely motivated to drive off potentially infanticidal intruders, the participation of others (e.g. While adult male gorillas have a defensible resource (i.e. 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. 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. beringei) groups attacking extra-group males. We report three cases of multi-male, multi-female wild mountain gorilla ( G. In the closely related genus Gorilla, such behavior has not been described. In humans and chimpanzees, most intraspecific killing occurs during coalitionary intergroup conflict.
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