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.

Keywords: herding; mate defence; offspring protection; reciprocal punishment; social incentive.

Figure 1.

Proportion of observed cases of male aggression during intergroup fights (n = 37) in which the targeted individual(s) were affiliating with (black) versus behaving aggressively towards (grey) members of the opposing group. Light grey signifies cases where the targeted individual had recently exhibited intergroup aggression; dark grey signifies cases where the targeted individual was currently trying to instigate intergroup aggression. The behaviour of the targeted individual was unconfirmed in n = 7 cases (white).

Figure 2.

(a) The proportion of cases in which targeted females either participated in an episode of intergroup aggression, or attempted to instigate intergroup aggression, before versus after receiving intragroup aggression from a male group member (note that each dot represents one female in the population (n = 16 females)). (b) The participation of targeted females after receiving intragroup aggression versus their baseline likelihood of participating in two consecutive instigating/aggressive episodes, as determined by their typical participation in the absence of male intragroup aggression (n = 15 females).