For that we measured whether the passive individual was in front of the ramp by the time the actor arrived at the location where the food had been released. after releasing the food, the actor moved to the food location in 237 of those 287 trials . Passive individuals were in front of the ramp by the time the actor arrived in 76 occasions . We found no differences between conditions . We found that actors did not approach the released food in 50 trials in which they released it. A majority of those trials were direct food test trials . This makes sense since actors spent some time obtaining the food reward placed inside the handle. Nevertheless, in a great majority of these trials the actors ended up obtaining the food reward from the handle before the passive individual finished her last reward. Interestingly though, this behavior predominantly occurred in three of the six days. Furthermore, we only found one case in which the actor showed an intention to approach the partner by getting closer to him.The results of the study suggest that days of gibbons managed to solve a competitive food task where one dyad member had the opportunity to pull and activate a release mechanism containing five potential food rewards for both partners. When gibbons had the opportunity to obtain an extra reward from their actions , they almost always acted, avoiding mutual deffection. In contrast, in an indirect food test condition where gibbons could not obtain the extra rewards from their pulling efforts, their likelihood to pull and release potential food rewards dropped significantly in comparison to the direct food test condition. In our opinion, two primary reasons could explain this pattern of results. First, it is possible that gibbons were more motivated to pull when they could directly benefit from the extra reward inside the handle. Similarly, blueberry package gibbons would have significantly pulled more often in the indirect food test condition compared to the no food control because they could still obtain some rewards.

Clearly, gibbons showed that they did not just act for the sake of pulling the rope. Most likely, their actions were motivated by the prospect of obtaining rewards, especially when those were easier to access. While this reason is very plausible, it does not necessarily account for why gibbons did not pull for 90 s in almost 40% of the indirect food test trials. This is especially surprising if we consider that actors benefitted more than passive partners in this condition. In other words, gibbons did not seem to interpret the situation as beneficial for actors. If that were the case, we would have expected gibbons to pull more often in indirect food test trials. Thus, the second possible explanation for our results is that gibbons interpreted the situation as a conflict of interest and hesitated to pull to avoid losing rewards in favor of the passive partner. This interpretation would be in line with previous findings in chimpanzees, bonobos and common marmosets. Thus, given the two non-mutually exclusive explanations, it remains unclear whether gibbons deffected in indirect food test trials due to a reduced motivation to act because they could not access the extra reward attached to the handle or because they wanted to avoid the possibility of losing rewards to their partner. So far, we have mainly discussed gibbons’ decisions whether to pull or not. In other words, did they try to maximize their own rewards when their partners pulled? The main source of potential conflict between participants lied on the possibility that passive individuals could position themselves in front of the release mechanism during direct and indirect food test trials. In direct food test trials, this could be particularly beneficial for passive partners given that actors could lose some valuable seconds trying to obtain the extra food reward attached to the handle. However, we found that actors actually obtained most of the rewards in both conditions, with a special advantage over passive partners during indirect food test trials. In fact, in only one dyad of gibbons the passive individual obtained more rewards than the actor in both test conditions.

Furthermore, passive individuals rarely took advantage of the situation and they did not distinguish between conditions. That is, during direct food test trials passive partners did not benefit from the time that the actors spent trying to obtain the reward located inside the handle. In that sense, gibbons did not interpret the situation as a social dilemma in which they could benefit more than their partner. Yet, gibbons also did not solve this task cooperatively. For that to be the case, dyad members would have needed to benefit more or less equally on both conditions and they would have not hesitated to manipulate the handle in indirect food trials. It is also very unlikely that gibbons’ decisions were driven by proactive prosocial motivations such as releasing food rewards to favor their dyad members given that actors benefited the most from their own actions. One possible explanation to understand why actors obtained more rewards than passive partners is that both individuals approached the handle during direct food test trials to obtain the reward, although only one individual pulled the handle. This occurred in 27.6% of the direct food test trials in which an individual pulled the handle. This possibility could partially explain why passive gibbons rarely position themselves in front of the release mechanism during direct food test trials, but it cannot explain why in indirect food test trials they did not take advantage of their passive role. In fact, in indirect food test trials passive partners approached the handle in 10% of trials. An alternative explanation is that the dominant individual pulled and obtained the majority of rewards. However, there are numerous reasons to suggest that a dominance component cannot fully explain our pattern of results. First, only three of 12 individuals released rewards in less than 20% of the direct and indirect food test trials. The result suggests that although they did not solve the task cooperatively, in half of the days both individuals exchanged roles relatively often. Furthermore, our results are also in line with literature suggesting that there is no clear dominance of one sex over the other and that gibbons engage in food sharing in both captive and natural settings.

Importantly, conflict avoidance does not seem to be relevant here either. Cofeeding events occurred three times as often than displacements events, suggesting that individuals were usually tolerant with each other, as it has been observed in other populations. Relatedly, gibbons never hoarded the blueberries. They ate them as soon as they got them. Furthermore, these tolerance may be mediated by the length of time that days had spent together. In our study, the length of dyad predicted how equally gibbons divided the five food rewards. A third alternative explanation is that once a gibbon decided to manipulate the handle, the other one totally disengaged from the task, as if acknowledging a sort of property claim. This could explain why passive partners did not take an advantageous position in front of the release mechanism and why, as a consequence, actors did not face a social dilemma in many trials. after all, actors benefit more than passive partners despite the costs to pull the rope, understood as the possibility to lose rewards due to the distance they had to cover and the time lost to return to the location where the food was released. Nevertheless, this is unlikely because passive individuals still obtained a significant fraction of the food rewards . All these previous arguments cannot fully explain why passive partners rarely took advantage of their position, especially given how successful this strategy was: passive partners obtained almost 75% of rewards when they positioned themselves in front of the release mechanism by the time the actor manipulated the handle. We thus propose that passive gibbons did not always take advantage of their partner actions because of the combined processes of motivation from the side of the actor and a general high level of social tolerance towards inequities. That is, individuals that were more motivated to obtain food and more attentive were also more likely to take the actors’ role in our task. Trough participation, they could become more aware of the situation as a whole and react faster to obtain the rewards despite the potential costs of pulling the rope. This gave them an advantage over their passive partners, whom at the same time tolerated actors obtaining the majority of rewards . Importantly, tolerance towards reward inequities also came from the perspective of the actors. Indeed we found that in ffy trials individuals from five of the six days seemed to adopt a prosocial attitude towards their passive partners by letting them access all the released rewards. Future studies should improve different aspects of our setup to continue exploring gibbons’ decision-making strategies when individuals’ interests collide. Te main weakness of our design is that we were not able to separate days of gibbons before the experimental sessions. In that sense we could not train them with the different task contingencies as it is usually the case in this type of settings but see. It is thus possible that some individuals were more skillful than others, blueberry packaging and that might have affected our results. Nevertheless, all individuals approached the appara.Thus and obtained rewards and only one never pulled from the handle during the course of the study. Moreover, despite the fact that gibbons distinguished between conditions with food and the no food control, their latencies to act did not differ between the indirect food test trials and the no food control trials. This could be interpreted as strategic behavior in the indirect food test trials if the gibbons were waiting for their partners to act. It is possible that with longer trials we would have found a significant difference between gibbons’ latencies in indirect food test trials and no food control trials.

In that sense, future changes in the trial time or the food rewards can better assess whether gibbons strategize to solve conflicts of interest. Relatedly, it is possible that gibbons were more motivated to act during direct food test trials compared to indirect food test trials because there were more rewards in play . It has been found that at least great apes are able to distinguish between these two amounts when they are presented at the same time. To our knowledge, however, gibbons have never been tested with these quantities. Nevertheless, in our scenario gibbons did not necessarily need to discern between these two quantities because the two amounts were never presented at the same time. Thus, although it is possible that this difference could have played a role in their performance, it is more parsimonious to think that their motivation to pull in direct food test trials was due to the high probability to eat the extra reward while pulling the handle. Future studies may use different reward constellations varying in quantity and/or quality to continue shedding light on gibbon socio-cognitive performance. Finally, given the quasi-experimental nature of our task, we did not always capture the social dilemma scenario we envisioned. Future tasks should implement designs in which cooperative acts are clearly costly for those individuals willing to volunteer. In addition, given our restricted sample size we could not test species differences or the presence of individual biases . The present study advances our understanding of how tolerance may allow primates to solve potential conflict over food rewards. In our study gibbons exhibit high degrees of social tolerance . Passive partners tolerate that actors obtain higher benefits in a majority of trials while actors often actively forego opportunities to maximize rewards . Relatedly, gibbons engaged in cofeeding events relatively often. One possibility is that such a high degree of social tolerance towards conspecifics results from gibbons’ unique pairliving social system compared to other great apes, although future studies should inspect this relationship in more detail. Overall, the inclusion of gibbons in studies exploring the nature of primate socio-cognitive abilities is critical. It will help to elucidate the nature of our prosocial motivations and their relationship to specific socio-ecological pressures and ultimately to understand how they have evolved since the last common ancestor with all living apes.One experimenter interacted with the apes during a test session while a second experimenter recorded the session and scored the subjects behavior .