Impatient traders or contingent reciprocators? Evidence for the extended time-course of grooming exchanges in baboons

The scarcity of evidence for contingent reciprocity has led to a growing interest in how market forces shape the distribution of exchanges in animal groups. In a biological market, supply and demand determines the value of an exchange, and individuals choose to trade with the partner offering the highest value. Partners maximize their immediate benefits without the need to monitor the balance of their exchange over time. Applied to grooming exchanges in primate groups, a market model predicts that females will primarily balance the amount of grooming they trade within single bouts, particularly when all partners offer similar value. If some partners can offer other benefits, like reduced aggression, females may exchange grooming for those benefits. In such cases, grooming will not be evenly balanced within bouts. Here, we examine the patterning of grooming in a group of free-ranging olive baboons (Papio anubis). In contrast to predictions derived from a biological market model, two-thirds of all grooming bouts in this group were completely one-sided and females did not consistently provide more grooming to higher-ranking partners. Grooming was more evenly balanced across multiple bouts than within single bouts, suggesting that females are not constrained to complete exchanges within single transactions.