Female monkeys use both the carrot and the stick to promote male participation in intergroup fights

Abstract

Group-level cooperation often poses a social dilemma in which joint action may be difficult to achieve. Theoretical models and experimental work on humans show that social incentives, such as punishment of defectors and rewarding of cooperators, can promote cooperation in groups of unrelated individuals. Here, we demonstrate that these processes can operate in a non-human animal species, and be used to effectively promote the production of a public good. We took advantage of the fact that intergroup fights in vervet monkeys ( Chlorocebus aethiops pygerythrus ) are characterized by episodes of intergroup aggression with pauses in-between. During pauses, females selectively groomed males that had participated in the previous aggressive episode, but aggressed male group members that had not. In subsequent (i.e. future) episodes, males who had received either aggression or grooming participated above their personal base-line level. Therefore, female–male aggression and grooming both appear to function as social incentives that effectively promote male participation in intergroup fights. Importantly, females stood to gain much from recruiting males as the probability of winning intergroup fights was dependent on the number of active participants, relative to the number of fighters in the opposing group. Furthermore, females appear to maximize the benefits gained from recruiting males as they primarily used social incentives where and when high-quality food resources, which are the resources primarily limiting to female fitness, were at stake.

Group‐level competition influences urinary steroid hormones among wild red–tailed monkeys, indicating energetic costs

“…More speculatively, depressed T could also aid with intragroup bonding following IGEs, as has been proposed for low T levels found during chimpanzee meat sharing (Sobolewski et al, ). Several species exhibit increased intragroup affiliation following IGEs (Arseneau‐Robar et al, ; Radford, ; Radford, Majolo, & Aureli, ) including other guenons (Cords, ; Chism & Rogers, ; Rowell, Wilson, & Cords, ) and redtails often engage in grooming frenzies (after 19/25 IGEs reported here). It is unclear whether depressed T would be required for intragroup affiliation though; while many studies indicate that high T can impede cooperative behavior in humans (Bos, Terburg, & van Honk, ; Crespi, ; Eisenegger, Haushofer, & Fehr, ; Zak et al, ), others find that T can also support cooperative behavior, especially with group members (Reimers & Diekhof, ), and/or in conjunction with other hormones like oxytocin (Jaeggi, Trumble, Kaplan, & Gurven, ).…”

Punishment: one tool, many uses

“…Although punishment is undoubtedly more common among humans than non-human species (Raihani et al 2012a), it has been documented in a handful of species, such as among cleaner fish and their reef-fish clients, among male-female pairs of cleaner fish and in vervet monkeys (Arseneau-Robar et al 2016Bshary and Grutter 2002;Raihani et al 2010). In the cleaner fish mutualism, clients may use punishment to make cleaners eat their ectoparasites rather than their protective mucus, which is preferred by cleaners (Grutter and Bshary 2003).…”

“…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.…”

Intergroup Competition Enhances Chimpanzee (Pan troglodytes verus) In-group Cohesion

“…In addition, collective action may be maintained through intragroup aggression, for instance as a herding mechanism (Cooper et al 2004) or punishment of free-riders (Radford et al 2016). Such a mechanism is suggested in vervet monkeys (Chlorocebus pygerythrus), as increased female aggression toward in-group males during intergroup encounters enhanced future participation of these males in encounters (Arseneau-Robar et al 2016). However, intragroup aggression in chimpanzees can disrupt social ties, thus reducing the predictability of coalitionary support and cooperation among group members (Aureli et al 2002;de Waal 2000;Wittig and Boesch 2005), during intra-and intergroup interactions.…”