Grooming reciprocity in Geoffroy’s spider monkeys, and the influence of the opportunity of interaction

Reciprocity allows animals to balance the costs and benefits of cooperative interactions by switching roles over time. Reciprocity can be based on two different processes: a within-dyad process based on temporal relations between cooperative events (partner control) and an across-dyad process based on preferences for the most cooperative partners (partner choice). The relative roles of the two processes remain debated. The aim of our study was to assess how the opportunity of interaction modulates grooming reciprocity in wild Geoffroy’s spider monkeys ( Ateles geoffroyi ), a species with low grooming rates and variable opportunities for group members to interact due to a high degree of fission-fusion dynamics. We investigated the roles of the aforementioned processes in grooming reciprocity, and the factors affecting them. Receiving grooming increased the probability of immediately returning grooming to the same partner (within-dyad temporal relations between cooperative events). In contrast, we found no evidence that spider monkeys directed most of their grooming to those individuals from which they received most grooming during the entire study period (across-dyad preferences for the most cooperative partners). The probability of returning grooming to the same partner was higher for dyads that spent less time together in the same subgroup, but was unaffected by maternal kinship. Our results suggest the opportunity to interact is a potent modulator of reciprocity. In species characterized by a high degree of fission-fusion dynamics and low grooming rates, short-term reciprocation based on partner control can be favored at the expense of a longer-term process based on partner choice. Significance statement Reciprocity is considered one of the key mechanisms allowing the evolution of cooperation. However, the processes guiding reciprocity are seldom explored, as the relative prevalence of within-dyad temporal relations between cooperative events (partner control) and of across-dyad preferences for the most cooperative partners (partner choice) is rarely investigated. Our study is the first to quantify the roles of these two separate processes in supporting reciprocity in spider monkeys. Specifically, our study is the first to assess how the opportunity two group members have to interact with each other modulates reciprocity. We found that the within-dyad process based on temporal relations between grooming events is the prevalent process, and that a low pro