No evidence that grooming is exchanged for coalitionary support in the short- or long-term via direct or generalized reciprocity in unrelated rhesus macaques

Abstract

Reciprocity is a prominent explanation for cooperation between non-kin. Studies designed to demonstrate reciprocity often focus on direct reciprocity in the timescale of minutes to hours, whereas alternative mechanisms like generalized reciprocity and the possibility of reciprocation over longer timescales of months and years are less often explored. Using a playback experiment, we tested for evidence of direct and generalized reciprocity across short and longer timescales. We examined the exchange of grooming for coalitionary support between unrelated female rhesus macaques in a population with a complete genetic pedigree. Females that received grooming were not more responsive to calls for coalitionary support from unrelated female group mates compared to control females that received agonism or no interaction — even when the call belonged to a females’ most recent grooming partner. Similarly, females were not more responsive to calls for support from their most frequent unrelated grooming partner of the last two years, nor if they received large amounts of grooming from all other females in their group. We interpret these results as an absence of evidence for direct or generalized reciprocity on any timescale in the exchange of grooming for coalitionary support in rhesus macaques. If grooming is exchanged for support in this population, it is with an intensity below our ability to detect it or over a longer timescale than we examined. We propose by-product explanations may also be at play and highlight the importance of investigating multiple mechanisms when testing apparently cooperative behaviors.Significance statementThe receipt of help can make some animals more likely to provide help in return, whether it be a singular act, or many acts accumulated over months. Similarly, the receipt of help, be it one act of aid, or a group’s worth of help over time, can make some animals more likely to pay help forward to others. Studies on Old World monkeys suggest females may give grooming and in return receive aid in future physical conflicts. Using a playback experiment, we found female rhesus macaques were not more responsive to calls for intervention in a simulated conflict after being groomed by unrelated females, even if the calling combatant was her most recent, or a long-time grooming partner. Our results suggest females in our study population may be receiving benefits other than support in conflicts for the grooming they provide.