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 dyads 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.We also explored whether gibbons would take an advantageous position after the food had been released.

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,hydroponic nft 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 dyads. Furthermore, we only found one case in which the actor showed an intention to approach the partner by getting closer to him.Te results of the study suggest that dyads 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 defection. 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, 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 defected 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. The next question concerns whether gibbons strategized when they took a passive role. 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. However, there are numerous reasons to suggest that a dominance component cannot fully explain our pattern of results. First,hydroponic channel only three of 12 individuals released rewards in less than 20% of the direct and indirect food test trials. Te result suggests that although they did not solve the task cooperatively, in half of the dyads 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 dyads 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. Tat 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 fly trials individuals from five of the six dyads 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 dyads 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, and that might have affected our results. Nevertheless, all individuals approached the apparatus 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.