Benefiting friends or dominants: prosocial choices mainly depend on rank position in long-tailed macaques (Macaca fascicularis).

Long-term observational studies in a number of animal species suggest that exchange patterns of social acts depend on long-term emotional bonds. Therefore, it is expected that the frequency of prosocial behavior will depend on the strength of such a bond. In this study we tested whether variation in relationship quality among unrelated individuals, i.e., "friends" and "nonfriends," is predictive of the prosocial behavior of long-tailed macaques in two experiments. First, we related relationship quality to prosociality in a dyadic prosociality test, and second, we gave subjects the choice to give to either a friend or a nonfriend in a triadic choice test. We show that prosocial behavior of long-tailed macaques in the dyadic test is not related to relationship quality. When given the choice to give to either a friend or a nonfriend in the triadic test, there is a minor indication that long-tailed macaques show a preference to give to their friends, yet this indication is neither significant nor consistent. In contrast, subordinate long-tailed macaques make a more "competitive" choice and avoid giving to the individual closest in rank. Therefore, in the short-term situation of experimental tests, prosocial behavior of long-tailed macaques seems unaffected by the relationship quality of the dyad/triad tested, and the relative dominance position of these dyads/triads seems to have a much stronger effect on their prosocial behavior.