Factors affecting tolerance persistence after grooming interactions in wild female vervet monkeys, Chlorocebus pygerythrus

Abstract

Grooming is the most common cooperative behaviour in primates. It has helped advance our understanding of how animals achieve reciprocal cooperation without using precise counting strategies. For example, attitudinal reciprocity refers to recipients making short-term adjustments like reciprocated grooming or tolerance at food sources based on a change in emotional states. Here, we used an experimental set-up to investigate in unprecedented detail the building blocks of attitudinal reciprocity that govern wild dominant females' tolerance around food after a grooming interaction with a subordinate female partner in vervet monkeys. We measured the tolerance of dominant females towards the previous grooming partner within 4 h after a grooming interaction. Tolerance levels remained elevated compared to baseline for the entire period, decreasing with time and predicted to fall to baseline levels after approximately 6 h. The decline in tolerance was not affected by the relationship quality between females, further grooming received from other group members or the surplus grooming that the dominant received relative to what she gave. However, the duration of grooming by the subordinate influenced the dominant's tolerance positively. These results suggest that the temporal dynamics of tolerance are best described as grooming triggering the start of a partner-specific ‘hourglass’, where the amount of remaining ‘sand’ equates to the dominant's current tolerance level above baseline. The results challenge the notion that dominants use scorekeeping regarding their grooming debts towards subordinates to make decisions about reciprocating with tolerance.