Male monkeys use punishment and coercion to de-escalate costly intergroup fights

Abstract

In numerous social species, males direct aggression towards female group members during intergroup fights, and this behaviour is commonly thought to function as mate guarding, even though males often target non-receptive females. In studying intergroup fights in a wild population of vervet monkeys, we found that male intragroup aggression was primarily directed towards individuals who had either just finished exhibiting, or were currently attempting to instigate intergroup aggression. Targeted females were less likely to instigate intergroup aggression in the future, indicating that male intragroup aggression functioned as coercion (when directed towards those who were currently trying to instigate a fight) and punishment (when directed towards those who had recently fought). These manipulative tactics effectively prevented intergroup encounters from escalating into fights and often de-escalated ongoing conflicts. Males who were likely sires were those most likely to use punishment/coercion, particularly when they were wounded, and, therefore, less able to protect vulnerable offspring should a risky intergroup fight erupt. This work, along with our previous finding that females use punishment and rewards to recruit males into participating in intergroup fights, highlights the inherent conflict of interest that exists between the sexes, as well as the role that social incentives can play in resolving this conflict. Furthermore, unlike other studies which have found punishment to be used asymmetrically between partners, these works represent a novel example of reciprocal punishment in a non-human animal.

Punishment: one tool, many uses

“…A second form of punishment is administered by male cleaners towards their female partners if the latter cheat a shared client (Raihani et al 2010(Raihani et al , 2012b. Finally, in vervet monkeys, males and females of the same group may punish each other during intergroup encounters (Arseneau-Robar et al 2016), with males punishing females who initiate attacks (Arseneau-Robar et al 2018) and females punishing males who do not participate. While the focus in this paper is on punishment in humans, it will be helpful to keep these examples in mind, as we shall return to the ways that punishment in humans might (or might not) differ from what we see in other species in our concluding remarks.…”

Experimental field evidence that out-group threats influence within-group behavior

“…From a proximate perspective, behavioral changes may result from conflict-induced increases in anxiety; functionally, affiliation may be used as a reward and to strengthen social bonds, while aggression may be a form of punishment (Radford et al 2016). Correlational data have indicated an influence of out-group conflict: allopreening between green woodhoopoe ( Phoeniculus purpureus ) groupmates was elevated both immediately after intergroup contests and many hours later (Radford 2008a; Radford and Fawcett 2014), while vervet monkeys ( Chlorocebus pygerythrus ) exhibited greater levels of both within-group affiliation and aggression during extended bouts of intergroup conflict (Arseneau-Robar et al 2016, 2018). In captive experiments, cichlid fish ( Neolamprologus pulcher ) increased affiliative interactions with groupmates after simulated intrusions by out-group rivals (Bruintjes et al 2016), and tufted capuchin monkeys ( Cebus apella) increased aggression levels during, but not after, out-group encounters (Polizzi di Sorrentino et al 2012).…”

Warfare in an evolutionary perspective

“…Anthropologists often highlight the importance that within‐group cooperation plays in wars in hunter‐gatherers . However, in nonhuman animals individual participation to between‐group interactions can be explained by selfish interests rather than by shared intents, even when animals apparently cooperate (e.g., when two animals are aggressive toward the same target). Understanding whether between‐group aggressive interactions in nonhuman animals are comparable to pre‐state wars in humans, and analyzing inter‐specific differences in collective action problems and in punishment of free‐riders are some of the key challenges for research on the evolutionary bases of warfare.…”

“…Anthropologists often highlight the importance that within-group cooperation plays in wars in hunter-gatherers. 37,74 However, in nonhuman animals individual participation to between-group interactions can be explained by selfish interests rather than by shared intents, 119…”