In general, these studies have found that chimpanzees and bonobos develop strategies with the aim to out compete their partners and obtain the most from the social dilemma, either through monopolization of rewards, by waiting for their partner to act before them or by influencing them to change their strategy. Te 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. 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 . Vale and colleagues found that squirrel monkeys behaved similar to capuchin monkeys in cooperative scenarios such as the assurance game. In another recent study, Sánchez-Amaro and colleagues presented for the first time pairs of common marmosets with a social dilemma modeled after the snowdrift game. 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, blueberry grow bag size with the risk that if none of the two accessed the tray they would both lose the rewards. Te authors explored whether cooperative breeding marmosets would engage in more cooperative strategies than great apes due to their natural tendency to act prosocially toward others in different contexts including food sharing.

They found similarities between the strategies of marmosets, chimpanzees and bonobos to maximize benefits . They also found sex differences between the strategies of females and males, where the former were less willing to act—a strategy that allowed females to maximize rewards over males. Te marmoset results ft the natural history of this species in which males usually donate food to females in food sharing tasks, presumably to overcome the high energetic costs of female reproduction. 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 . These small apes are key species in the sense that they are closely related to both cercopithecoids and great apes. Furthermore, unlike any other ape species, gibbons primarily live in small groups mainly composed of a bonded male–female pair and their dependent kin. Thus, the study of their socio-cognitive abilities is very relevant to understand whether some cognitive traits observed in other primates are shared across anthropoids or, instead, evolved independently in diverse lineages. Up to date, the study of gibbons has mainly focused on elucidating aspects of their biology, ecology and phylogeny, 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 Many primates initiative, 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. 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 already showed, for example, that adaptations to the way gibbons could interact with an apparaThus improved their participation and performance when compared to previous studies. Furthermore, as Liebal emphasizes, 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 assumes 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. Some gibbon species have been found to engage in extra-pair copulations and a number of studies have reported different group structures in addition to pair-living. In other words, the social organization of gibbons is more flexible than originally described. Furthermore, the fact that gibbons live in reduced groups does not necessarily presuppose a lack of social complexity. According to Freeberg, pair-bonded individuals 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 relationship quality. It is thus possible that gibbons would engage in more prosocial strategies between closely bonded partners than other primates living in larger groups when conflicts of interest take place. Nevertheless, despite the lack of studies on gibbon socio-cognitive abilities in relation to other primate specie, researchers have made significant advancements on this area. For instance, it has been investigated whether gibbons recognize themselves in the mirror although the findings remain inconclusive . Furthermore, there has been extensive research on the abilities of gibbons to follow the gaze of others to discover an unseen object.

Researchers have found that gibbons are able to shift their gaze in response to an experimenter gaze shift but it remains unclear whether gibbons are taking the perspective of the experimenter into account, including her mental states. To evaluate whether gibbons take into account the perspective of others, a recent study presented gibbons 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. Te experimenters found that 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. This later result suggests that, in line with previous socio-cognitive studies in other primate species, gibbons may perform better in competitive settings compared to neutral ones. However, in previous gaze-following studies with gibbons the participants in the interaction were the human experimenter and the ape. Gibbons did not interact with conspecifics. Considering the competitive task presented by Sánchez-Amaro and colleagues 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 days of gibbons would solve conflicts of interest in a more naturalistic context. Tat is, when they need to deal with other conspecifics over access to resources such as food rewards.To answer this question, we presented days of gibbons with a simpliftied version of a social dilemma resembling a snowdrift game. In this game two individuals can decide whether to cooperate or deffect to obtain a benefit. If they engage in mutual cooperation they share the cooperation costs and both benefit. Alternatively, an individual can deffect and obtain the highest benefit if the partner cooperates. However, if they both decide to deffect they both lose. Thus, in the snowdrift game unilateral cooperation is still preferable over mutual deffection—it is better to pay the cooperation costs even if your partner deffects. Importantly, our setting maintains all core aspects of a snowdrift game with the exception that individuals cannot engage in mutual cooperation , although they could cooperate equally by taking turns across trials. In that sense, our game could also be understood as a 2-person Volunteer’s Dilemma. This is the first time a game theory model has been implemented to shed light on the nature of gibbon socio-cognitive abilities. To recreate the social dilemma, blueberry box we presented days of gibbons with the possibility to individually pull from a handle . By pulling the handle, a rope would liftt a release mechanism and rewards would fall at a distance from the handle. Te dilemma would occur if the partner that did not pull took advantage of the situation and position herself in front of the release mechanism—with the possibility to benefit more than the individual who pulled the handle. Importantly, individual roles were not assigned by the experimenter. Gibbons could decide whether to cooperate or not by pulling the handle. For the sake of clarity, we will characterize the individual who took an active role on a given trial and pulled as the actor, and the individual who took the passive role on a given trial and waited as the passive partner. days of gibbons were presented with three conditions varying in the number of available rewards: a test condition with direct food rewards in which the actor could obtain one extra reward attached to the handle while releasing five rewards at a distance from herself; a test condition with indirect food rewards in which the actor did not obtain an extra 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. Importantly, both the actor and the passive partner could benefit from the five rewards. The extra reward inside the handle was mostly accessible to the actor but the passive partner could theoretically obtain it as well. For the sake of clarity, we will refer to the three conditions succinctly as direct food test condition, indirect food test condition and no food control condition. We expected gibbons to act more often in the direct and the indirect food test conditions compared to the no food control condition. This would show that gibbons understood the contingencies of the game.

Furthermore, given that gibbons were living with close partners one possibility is that they would share the potential costs of cooperation. By cooperative costs we refer to the possibility of losing rewards due to the distance they had to cover and the time lost to return to the location where the food was released, especially if their passive partners took an advantageous position to obtain the released rewards. 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 direct food test trials given that they could easily obtain at least one direct reward from their actions.Therefore, actors were still able to obtain the majority of the five released rewards in both conditions. Furthermore, actors obtained the extra rewards in 87% of the direct food test trials in which they acted. In line with this finding, Fig. 2 shows the percentage of trials in which individuals release food against the number of rewards that each individual obtained in indirect and direct food test trials. interestingly, in only one dyad the more active individual obtained less benefits than the passive partner in both conditions. We also found significant effects of our control variables age and length of dyad suggesting that younger individuals obtain more food and that days who have been longer together distributed the five rewards more equally. Next, we evaluated whether passive gibbons acted strategically by placing themselves in front of the ramp at the moment the actor released the five food rewards and whether they would take advantage of their position in direct food test trials compared to indirect food test trials given that the actor gibbon would spend some time retrieving the food baited inside the handle. There were 287 instances in which the food was released by an actor in the direct and indirect food test conditions. In 235 of those trials the passive individual was not in front of the ramp by the time the food was released. The passive individual only placed herself in front of the ramp by the time the food was released in 52 trials and 20 indirect food test trials and there were no differences between conditions . Furthermore, in 49 of those 235 trials, the partner was within a meter from the subject by the time the subject released the food and 9 of 90 indirect food test trials. We also explored whether gibbons would take an advantageous position after the food had been released.