Social animals constantly face conflicts of interest with other group members. In primates, decisions about different travel routes, group defense, or access to limited resources may result in divergent preferences for individuals and hence potential conflicts between group members 1–6. To resolve these conflicts, primates may engage in different sets of strategies including dominance, mutual cooperation or majority rules. For instance, Lar gibbon females(Hylobates lar) lead travel routes and access high value rewards before males despite the lack of a clear dominance of one sex over the other 7. In olive baboons (Papio anubis) instead, travel routes seem to be driven by majority rule despite their highly hierarchical social system 5. However, while these observations allow us to understand different aspects of primate behavior, it is often difficult to dissect the factors contributing to specific behavioral patterns in nature. Accordingly, experimentally controlled studies with primates can shed crucial light on the decision-making strategies underlying the observed behavior.
To that end, over the last years numerous studies have adapted game theory models to explore the strategies that different primates use to overcome social dilemmas in which their interests come into conflict 8–11. That is, the free rider benefits the most from the interaction. For instance, computerized tasks have presented primate species including chimpanzees, capuchin monkeys and rhesus macaques with different economic games borrowed from the game theory literature 12–14. These studies have found that, in general, primates can converge to a Nash Equilibrium (i.e. the optimal outcome from an interaction given the strategy of your partner) during coordination and conflict games between pairs. Using a different approach, other researchers have presented great apes, mostly chimpanzees (Pan troglodytes), with non-computerized social dilemmas in which apes had to decide between different physical actions to obtain rewards from an apparatus. In general, these studies have found that chimpanzees and bonobos develop strategies to outcompete their partners and obtain the most from the social dilemma, either through monopolization of rewards 9, by waiting for their partner to act before them 8,10,15 or by influencing them to change their strategy 11 .
The use of game theory models to explore how different primate species coordinate actions for mutual goals, as well as how they overcome conflicts of interest, is a growing field in comparative psychology 14. For example, a very recent study presented squirrel monkeys, a primate species that rarely cooperates in nature, with a set of computerized social dilemmas previously presented to more cooperative species such as the capuchin monkeys (Cebus apella). Vale and colleagues 16 found that squirrel monkeys (Saimiri boliviensis) behaved similar to capuchin monkeys in cooperative scenarios such as the stag hunt game. In another recent study, Sánchez-Amaro and colleagues (under review) presented for the first time pairs of common marmosets (Callithrix jacchus) with a social dilemma modeled after the snowdrift game 10,17. In this study, marmosets could access an unequal reward distribution in the form of a rotating tray. In the social dilemma condition, the preferred reward could only be obtained by waiting for the partner to act, with the risk that if none of the two accessed the tray they would both lose the rewards. The authors explored whether cooperative breeding marmosets would engage in more cooperative strategies due to their natural tendency to act proactively toward others in different contexts including food sharing. They found similarities between marmosets and great apes’ strategies to maximize benefits (e.g. waiting for the partner to act before them). They also found sex differences between females and males’ strategies, where the former was more willing to forego a cooperative act and maximize rewards. The results fit the natural history of this species in which males usually donate food to females in food sharing tasks. However, we still know very little about the socio-cognitive strategies that other primate species may develop to deal with similar conflicts of interest.
Perhaps surprisingly, one of the primate families we know less about in terms of their socio-cognitive abilities are the gibbons (family Hylobatidae). These small apes are key species in the sense that they are closely related to both old world monkeys and great apes 18. Furthermore, unlike any other ape species, gibbons primarily live in small groups mainly composed of a bonded breeding couple and their kin 19. Thus, the study of their socio-cognitive abilities is crucial to understand whether some cognitive traits are shared by common descent in all apes or have instead arise through convergent evolution in distinct primate species. Up to date, the study of gibbons has mainly focused on elucidating aspects of their biology, ecology and phylogeny 20,21, with little work assessing their socio-cognitive abilities in experimentally controlled settings. For instance, in a recent literature review on primate cognitive studies published by the Manyprimates initiative 22, it was found that gibbons only appeared in 2 of the 574 studies surveyed between 2014 and 2019.
Gibbons may have been excluded from cognitive studies due to difficulties securing a sufficient sample size to conduct experimental studies or due to their limited motivation to participate in cognitive tasks 23–25. While limited sample size is a problem that a majority of comparative psychologists and primatologists face when studying primate behavior, a lack of motivation is often the product of experimental designs and methods not suited to the biology of the species. In the case of gibbons, early work by Beck 26 already showed, for example, that adaptations to the way gibbons could interact with an apparatus (e.g. lifting the access to the ropes instead of leaving them on the ground on a flat surface) improved their participation and performance when compared to previous studies 27,28.
Furthermore, as Liebal emphasizes 25, it has been sometimes assumed that gibbons’ are less interesting than other primate species due to their relatively simpler social system based on pair-living, which presupposes low socio-cognitive abilities in relation to other primates. However, mounting evidence over the last 20 years has challenged the assumption that gibbons are truly monogamous 19,20,29−31. Some gibbon species have been found to engage in extra-pair copulations 32 and a number of studies have reported different group structures in addition to pair-living 31,33−36. Furthermore, the fact that gibbons live in reduced groups does not necessarily presuppose a lack of social complexity. According to Freeberg 37, pair-bonded individuals would form more complex and intense relationships than those living in large polygamous groups—possibly because they are more interdependent. In other words, social complexity is not only a matter of group size but of relation quality. It is thus possible that gibbons would engage in more prosocial strategies between closely bonded partners when conflicts of interest take place.
Nevertheless, despite the lack of studies on gibbons’ socio-cognitive abilities in relation to other primate species 22, researchers have made significant advancements on this area. For instance, it has been investigated whether gibbons recognize themselves in the mirror (see 25 for review) or whether they are able to follow others gaze to discover an unseen object 38–42. In the case of gaze-following studies, researchers have found that gibbons are able to shift their gaze in response to a previous experimenter gaze shift but it remains unclear whether gibbons are taking the perspective of the experimenter into account, including her mental states. However, a recent study found that when gibbons were presented with a competitive scenario in which they could only retrieve uncontested rewards—when the experimenter did not orient his body, head or eyes towards the rewards, gibbons avoided the contested table by paying attention to the orientation of the body and the head of the experimenter but not to his eyes 43. This later result suggests that, in line with previous socio-cognitive studies in other primate species 44–46, gibbons may perform better in competitive settings compared to neutral ones. However, in previous gaze-following studies the interaction only occurred between the human experimenter and the ape and not between conspecifics. Considering the competitive task presented by Sánchez-Amaro and colleagues 43 as an example, the experimenter and the gibbon faced a conflict of interest every time the gibbon approached the contested table since the experimenter and the gibbon competed for the same food reward. Therefore, one open question is how pairs of gibbons would solve conflicts of interest in a more naturalistic context. That is, when they need to deal with other conspecifics over access to resources such as food rewards.
To answer this question, we presented pairs of gibbons with a simplified version of a social dilemma resembling a snowdrift game 17. This is the first time a game theory model has been implemented to shed light on the nature of gibbons’ socio-cognitive abilities. In this situation, one pair member should volunteer to provide a common good that becomes accessible for both of them. The dilemma occurs when the passive individual takes advantage of her position. In other words, the costs to volunteer may hinder the actor’s chances to benefit herself in relation to the passive partner. In our task, one gibbon could pull from a handle attached to a rope. By pulling the handle, the rope would lift a release mechanism and rewards would fall at a distance to the actor, giving the passive individual the chance to position herself in front of the released rewards and hence benefit from the actors’ action (although the actor could potentially benefit from those rewards as well).
Pairs of gibbons were presented with three conditions varying in the number of available rewards: a Test condition in which the actor could obtain one reward attached to the handle while releasing five rewards at a distance from herself; an Altruistic condition in which the actor did not obtain any reward from pulling the handle but could still release the five rewards at a distance and a No Food control condition with no rewards involved.
We expected gibbons to act more often in the Test and the Altruistic conditions compared to the No Food condition. This would show that gibbons understood the contingencies of the game. Furthermore, given that gibbons were living with closely bonded partners one possibility is that they would cooperate to solve the dilemma by sharing the volunteer costs. If that were the case, we would expect pairs of gibbons to obtain similar amounts of food in both conditions. Furthermore, we would not expect pairs members to show significant differences in their rate of participation and in their latencies to release the rewards between Altruistic and Test conditions.
If, in contrast, gibbons would react competitively as other great ape species did in previous social dilemmas, we would expect them to try to maximize their food rewards by hesitating to act and by taking advantage of their passive role—placing themselves in front of the release mechanism. In that sense, we would expect passive partners to benefit from their position and obtain more rewards than the actors. Furthermore, we would also expect gibbons to be more likely to act in Test trials given that they could obtain direct benefits from their actions.