Making friends in an asymmetric game: the establishment of male-female grooming exchanges in vervet monkeys

An important challenge for individuals of group-living species is to build cooperative relationships with new partners. One famous strategy for doing so is "raise-the-stakes", proposing low initial investments that increase if the partner matches these investments in a series of exchanges. Contrarily, the "all-in" strategy predicts high initial investment that may be maintained, but downregulated if not fully matched. We tested these predictions on grooming exchanges between wild male-female vervet monkeys, a species with regular male dispersal. Contrary to model predictions, we found uneven initial investment between novel partners that reached near reciprocity after approximately six months. To identify which sex altered investment, we examined grooming durations and frequencies of novel and established partners. Females showed consistently high initial investment and gradually reduced grooming over the first six months. Males did not alter grooming duration but had higher grooming frequency after a year of residency. Eventually, grooming exchanges nearly evened out. Female behaviour aligns more closely with the "all-in" strategy, which suggests competition among female groups over dispersing males. Male behaviour partly fits the "raise-the-stakes" hypothesis. The study highlights the complexity of real-life cooperation, emphasizing the need to explicitly incorporate life history parameters to understand cooperative strategies.