Encounters between gorillas in different social groups are a regular feature of mountain gorillas’ lives30,32. When they escalate to contact aggression, most involve only a small, predictable demographic, i.e. adult males, and the great majority end with only minor injuries. We are unsure precisely what prompted the events described here. Whatever their origin, these attacks are remarkable for several reasons.

First, the timing of these attacks suggests that multi-male, multi-female social structures are a prerequisite for such behavior. Despite the extended observation history on the population, this type of aggression was not observed until after a remarkable demographic shift that left many mountain gorillas living in social structures that both humans and chimpanzees share. A dominant theory for explaining similar behavior in chimpanzees, the imbalance-of-power hypothesis, predicts that attacks will only occur when victim(s) are outnumbered and the risk to individual attackers is low50. The demographics of the incidents were highly consistent with that prediction, facilitated by large group size and multiple adult males, who are far more powerful fighters than females due to their size and large canines. The costs to individual attackers would likely have been too high for the behavior to evolve in a population where groups contained far fewer males. Once groups were free from this constraint, coalitionary attacks occurred. However, it is important to note that in one case some of the attacking animals did sustain injuries, suggesting that the risk is not zero even when the victim is greatly outnumbered. To our knowledge, injuries to attackers have not been reported in chimpanzees.

Second, they confirm that food resource competition is not necessary for coalitionary violence to occur in great apes. Theory predicts conflict when coveted resources are defensible4; attacks on neighbors have direct benefits for individuals and groups by maintaining or increasing range size, and therefore access to preferred feeding sites11. Mountain gorillas are herbivores that eat at least 55 species of plants [ref. 51, KRC long-term records], many of which are available year-round and few of which are monopolizable27,28. There are probably few wild primate populations on earth with less food resource stress than Virunga mountain gorillas and solitary males are in no way a threat to a group’s food supply, so this is not a convincing explanation for coalitionary aggression in mountain gorillas.

The gorillas’ behavior is also consistent with the intergroup dominance hypothesis, which posits that intergroup dominance promotes fitness through a variety of mechanisms13. Male gorillas do have a defensible resource—i.e., females—and pregnant or nursing females presumably have strong incentive to drive off potentially infanticidal intruders31. Solitary males can be vicious fighters, and are dangerous to both infants and to other adult males. In the last three years, three alpha males in mixed-sex groups monitored by KRC died as a result of interactions with solitary males (KRC long-term records). Solitary males are known to “stalk” mixed-sex groups for extended periods of time as they attempt to obtain females to start their own groups52, and encounters with them are more likely to result in aggression than encounters with other groups33. For males plus nursing females and their infants, extra-group males are clearly dangerous; driving them permanently away or killing them has obvious direct benefits for these classes of individuals.

However, females who were apparently neither pregnant nor nursing, sub-adults, and juveniles also participated, and the benefits for them are less obvious. It is unclear what might have motivated their participation. If anything, cycling adult females may benefit from interactions with other males since they are a chance to evaluate potential mates. One possibility is that the potential for severe social group disruption or disintegration, which can occur when an alpha male is seriously injured or killed, creates sufficient motivation for these classes of animals to participate. Being forced to find and join a new social group (for females) or disperse before full physical maturity (for males) likely carries considerable personal risk. Alternatively, selection for participation in coalitionary aggression against outside males may be so strong for adult males and pregnant/lactating females that the associated proximate mechanisms have carry-over effects that generalize to other age or reproductive status categories. In other words, the possible net benefits of interactions with outgroup males for cycling females are not big enough to select for more condition-dependent mechanisms that motivate coalitionary aggression when pregnant or lactating.

Kin selection is believed to be an important proximate mechanism underlying similar behavior in male chimpanzees, and it is important to note that overall relatedness in this small, closed population (n = ~480 individuals42) is quite high. The mean relatedness coefficient of the participating males in each group was r = 0.25 (Table 3). However, virtually all were related paternally (Table 3), and there is currently no evidence that mountain gorillas discriminate paternal kin53. Three of the attackers in the 2004 case, plus one in 2010, were the victim’s maternal nephews, though they had never lived in the same group and thus may not have identified one another as kin (KRC long-term records). Given these facts, plus the whole group participation (some females had few or no close relatives co-resident), kin selection alone seems an unsatisfying explanation. Reciprocity is also an inadequate explanation. All group members participated so no subset incurred most of the costs, and chances for any sort of in-kind repayment are clearly limited. However, it is worth noting that their behavior is consistent with recent experimental work in humans indicating that perceived threat to the in-group causes not only retaliatory, but also preemptive aggression54.

Though these cases bore striking resemblance to reports of coalitionary violence in chimpanzees, there were two noteworthy differences. First, in both humans and other non-human primates coalitionary violence generally involves one sex (e.g. refs 7, 8, 9, 55 and 56, reviewed in ref. 11) and immature animals are most often victims rather than attackers2. To our knowledge there have been no reports of the whole-group participation observed here in chimpanzees, though its occurrence is logistically limited by chimpanzees’ fission-fusion social structure. However, chimpanzees too are regularly found in mixed-sex, multi-age parties, but the great majority of observed intergroup violence is adult males attacking other adult males (though see refs 9 and 10). The same is true in humans; most cases of intergroup violence involve primarily or exclusively adult males despite nearly universal mixed-sex and age residence patterns55.

Second, humans and chimpanzees often actively seek out victims. Male chimpanzees will patrol territory boundaries silently and appear to search for lone victims56, and humans spend considerable amounts of time planning attacks against neighbors in both industrialized and small-scale societies [e.g. refs 57 and 58]. There was no evidence of such behavior in the gorillas. In the first attack the victim approached the group, though we cannot be certain if the victim or the attackers approached in the other two cases as the initial contact was unobserved.

Both the whole-group participation and lack of victim seeking are characteristics of spontaneous group violence in humans (i.e. communal rioting or mob violence59). Human mobs are sometimes characterized by participant demographics that do not fit expected patterns, including individuals who have little or nothing to gain60. Nonetheless, the gorillas’ behavior appeared remarkably coordinated, clearly had direct benefits for some individuals, and bore important hallmarks of classic descriptions of coalitionary intergroup aggression in chimpanzees.

While group attacks on neighbors are clearly rare events in G. beringei, it is unclear how the rates might compare to (for example) lethal coalitionary aggression among chimpanzees in Gombe National Park, which contains the world’s longest-studied chimpanzee population. In the 1960’s through early 1990s, KRC staff lived in the forest and conducted all-day group follows, making it less likely that coalitionary attacks occurred but were simply missed. From 1995 on, staff no longer lived in the forest and were limited to ~6 hours per day with the animals, which increases the possibility of missing rare events. Furthermore, deaths of solitary males are nearly impossible to detect.

Observation and reporting of rare but potentially evolutionarily significant behaviors is yet another important reminder of the value of long-term monitoring of animal populations with slow life histories61,62. As data years mount at long-term field sites, new and surprising behaviors (for another recent example, see ref. 15) continue to refine our understanding of the plasticity of primate behavior and the complex origins of our own remarkable sociality.