Abstract

Reciprocation is thought to favour altruism among nonrelatives. Three types of reciprocity have been
proposed: direct, indirect and generalized. All three are theoretically possible, but their role in real
biological systems is unclear. We concurrently examined the occurrence of direct, indirect and generalized reciprocity during grooming exchanges in longtailed macaques, Macaca fascicularis. The occurrence of the grooming monkey A gave B predicted the latency and occurrence of the grooming B gave back to A (direct reciprocity), the latency of the grooming A received from C (indirect reciprocity), but not
the grooming B gave C (generalized reciprocity). The duration of the grooming monkey A gave B predicted the latency and occurrence of the grooming B gave back to A (direct reciprocity) but not the grooming A received from C (indirect reciprocity) or the grooming B gave C generalized reciprocity).
Finally, monkeys directed overall more of their grooming to those individuals that overall groomed them more (direct reciprocity), but not to those that groomed other individuals more (indirect reciprocity); nor did monkeys that received overall more grooming groom others more (generalized reciprocity). Overall, we found strong evidence for direct reciprocity, limited support for indirect reciprocity and no evidence for generalized reciprocity. Our results support the view that direct reciprocity plays a crucial role in the life of primates and suggest indirect and generalized reciprocity are rare or absent in nonhuman animals.
We argue that direct reciprocity may be driven by a system of partner-specific emotional bookkeeping of past social interactions that does not require complex cognitive capacities.