Animals commonly face fluctuations in their environment and resources. To maximize their benefits,they need to integrate the risks attached to potential pay-offs. We do not know, however, to what extent individuals account for irregularity in the latter. We tested the sensitivity of monkeys (Cebusapella, Macaca tonkeana, M. fascicularis) to the irregularity of pay-offs in two different tasks. In a first experiment, the subjects were given an exchange task where the reward probability varied between different conditions, but yielded the same average pay-off. There was no evidence of subjects favoring either condition, meaning that they behaved in accordance with the predictions ofthe classical decision theory (Expected Utility Theory). In a second experiment, we offered to subjects a choice between two options involving different pay-off regularity. In this case, a wide range of inter-individual variation was found in the choices of individuals. Whereas monkeys accepted irregular pay-off in a rational way, there were individual biases in their preferences. These results indicate that the preferences of animals in a risky situation were not unequivocally shaped by the environment in which species have evolved.

Macaques are the most geographically widespread and behaviorally diverse primate genus, and although macaque species share the same, basic social structure, they display broad inter specificvariation in patterns of adult social behavior. Based on these patterns, macaque species have been arranged along a 4-grade scale for social style. At one end of the scale, there are grade 1 species (e.g., Japanese macaques) that have highly hierarchical and despotic social systems, and at the other end, grade 4 species (e.g., Tonkean macaques) that have more relaxed and egalitarian social systems. We predicted that a species from the more despotic end of the spectrum should have more competitive play fights and that a species from the egalitarian end, more cooperative ones. A detailed analysis of videotaped sequences of juvenile play fighting in Tonkean and Japanese macaques was used to characterize the targets and tactics of attack and defense. Even though the two species have a similar behavioral repertoire, there are significant differences in how that repertoire is used and these differences are consistent with one species having more competitive interactions than the other. Contrasting multi-animal play fights versus pairs showed that the more cooperative style of the Tonkean macaques is further exaggerated. The results suggest that differences in styles of attack anddefense in play fighting may be influenced by differences in the species’ social systems.

Network optimality has been described in genes, proteins and human communicative networks. In the latter, optimality leads to the efficient transmission of information with a minimum number of connections. Whilst studies show that differences in centrality exist in animal networks with central individuals having higher fitness, network efficiency has never been studied in animal groups. Here we studied 78 groups of primates (24 species). We found that group size and neocortex ratio were correlated with network efficiency. Centralisation (whether several individuals are central in the group) and modularity (how a group is clustered) had opposing effects on network efficiency, showing that tolerant species have more efficient networks. Such network properties affecting individual fitness could be shaped by natural selection. Our results are in accordance with the social brain and cultural intelligence hypotheses, which suggest that the importance of network efficiency and information flow through social learning relates to cognitive abilities.