Inferring the evolutionary history of cognitive abilities requires large and diverse samples. However, such samples are often beyond the reach of individual researchers or institutions, and studies are often limited to small numbers of species. Consequently, methodological and site-specific-differences across studies can limit comparisons between species. Here we introduce the ManyPrimates project, which addresses these challenges by providing a large-scale collaborative framework for comparative studies in primate cognition. To demonstrate the viability of the project we conducted a case study of short-term memory. In this initial study, we were able to include 176 individuals from 12 primate species housed at 11 sites across Africa, Asia, North America and Europe. All subjects were tested in a delayed-response task using consistent methodology across sites. Individuals could access food rewards by remembering the position of the hidden reward after a 0, 15, or 30-second delay. Overall, individuals performed better with shorter delays, as predicted by previous studies. Phylogenetic analysis revealed a strong phylogenetic signal for short-term memory. Although, with only 12 species, the validity of this analysis is limited, our initial results demonstrate the feasibility of a large, collaborative open-science project. We present the ManyPrimates project as an exciting opportunity to address open questions in primate cognition and behaviour with large, diverse datasets.

Social animals need to coordinate with others to reap the benefits of groupliving even when individuals' interests are misaligned. We compare how chimpanzees, bonobos and children coordinate their actions with a conspecific in a Snowdrift game, which provides a model for understanding how organisms coordinate and make decisions under conflict. In study 1, we presented pairs of chimpanzees, bonobos and children with an unequal reward distribution. In the critical condition, the preferred reward could only be obtained by waiting for the partner to act, with the risk that if no one acted, both would lose the rewards. Apes and children successfully coordinated to obtain the rewards. Children used a 'both-partner-pull' strategy and communicated during the task, while some apes relied on an 'only-one-partner-pulls' strategy to solve the task, although there were also signs of strategic behaviour as they waited for their partner to pull when that strategy led to the preferred reward. In study 2, we presented pairs of chimpanzees and bonobos with the same set-up as in study 1 with the addition of a non-social option that provided them with a secure reward. In this situation, apes had to actively decide between the unequal distribution and the alternative. In this set-up, apes maximized their rewards by taking their partners' potential actions into account. In conclusion, children and apes showed clear instances of strategic decision-making to maximize their own rewards while maintaining successful coordination.

The snowdrift game is a model for studying social coordination in the context of competing interests. We presented pairs of chimpanzees with a situation in which they could either pull a weighted tray together or pull alone to obtain food. Ultimately chimpanzees should coordinate their actions because if no one pulled, they would both lose the reward. There were two experimental manipulations: the tray's weight (low or high weight condition) and the time to solve the dilemma before the rewards became inaccessible (40 seconds or 10 seconds). When the costs were high (i.e high weight condition), chimpanzees waited longer to act. Cooperation tended to increase in frequency across sessions. The pulling effort invested in the task also became more skewed between subjects. The subjects also adjusted their behaviour by changing their pulling effort for different partners. These results demonstrate that chimpanzees can coordinate their actions in situations where there is a conflict of interest.

We investigated how apes allocated their choices between 2 food options that varied in terms of their quantity and quality. Experiment 1 tested whether subjects preferred an AB option over an A option, where the A item is preferred to the B item (e.g., apple + carrot vs. apple). Additionally, we tested whether the length of the intertrial interval (ITI) affected subjects' choices. Five orangutans, 4 gorillas, 7 bonobos, and 10 chimpanzees received 3 types of trials: preference (A vs. B), quantity (AA vs. A), and mixed (AB vs. A where A is the preferred food). We used 3 food items that substantially differed in terms of preference (carrots, apples, and pellets). Subjects showed no overall preference for the mixed option (AB) compared with the single option (A), even though they showed clear preferences during both the preference and quantity trials. The intertrial length had no effect on choice behavior. Experiment 2 further explored apes' choices by using 3 highly preferred food items (bananas, grapes, and pellets) in 6 orangutans, 4 gorillas, 8 bonobos, and 18 chimpanzees. Unlike the results of Experiment 1, apes generally chose the mixed option. Our results indicated that apes did not show a general "selective-value" effect but chose depending on the relative value of the food items involved. Subjects were more likely to select the mixed over the single option when (a) the mixed option was composed of items that were closer in value and (b) they were compared against the less valuable item forming the mixed option.

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Cooperation often comes with the temptation to defect and benefit at the cost of others. This tension between cooperation and defection is best captured in social dilemmas like the Prisoner's Dilemma. Adult humans have specific strategies to maintain cooperation during Prisoner's Dilemma interactions. Yet, little is known about the ontogenetic and phylogenetic origins of human decision-making strategies in conflict scenarios. To shed light on this question, we compared the strategies used by chimpanzees and 5-year old children to overcome a social dilemma. In our task, waiting for the partner to act first produced the best results for the subject. Alternatively, they could mutually cooperate and divide the rewards. Our findings indicate that the two species differed substantially in their strategies to solve the task. Chimpanzees became more strategic across the study period by waiting longer to act in the social dilemma. Children developed a more efficient strategy of taking turns to reciprocate their rewards. Moreover, children used specific types of communication to coordinate with their partners. These results suggest that while both species behaved strategically to overcome a conflict situation, only children engaged in active cooperation to solve a social dilemma.

Social primates can influence others through the control of resources. For instance, dominant male chimpanzees might allow subordinates access to mate with females in exchange for social support. However, little is known about how chimpanzees strategically use a position of leverage to maximize their own benefits. We address this question by presenting dyads of captive chimpanzee (N = 6) with a task resulting in an unequal reward distribution. To gain the higher reward each individual should wait for their partner to act. In addition, one participant had leverage: access to an alternative secure reward. By varying the presence and value of the leverage we tested whether individuals used it strategically (e.g. by waiting longer for partners to act when they had leverage in the form of alternatives). Additionally, non-social controls served to show if chimpanzees understood the social dilemma. We measured the likelihood to choose the leverage and their latencies to act. The final decision made by the chimpanzees did not differ as a function of condition (test versus non-social control) or the value of the leverage, but they did wait longer to act when the leverage was smaller—particularly in test (versus non-social control) trials suggesting that they understood the conflict of interest involved. The chimpanzees thus recognized the existence of social leverage, but did not use it strategically to maximize their rewards.

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