Female vervet monkeys fine-tune decisions on tolerance versus conflict in a communication network

Abstract

Group living promotes opportunities for both cooperation and competition. Selection on the ability to cope with such opposing social opportunities has been proposed as a driving force in the evolution of large brains in primates and other social species. However, we still know little about the degree of complexity involved in such social strategies. Here, we report advanced social strategies in wild vervet monkeys. Building on recent experimental evidence that subordinate females trade grooming for tolerance from higher-ranking individuals during foraging activities, we show that the audience composition strongly affects this trade. First, tolerance was lower if the audience contained individuals that outranked the subordinate partner, independently of audience size and kinship relationships. Second, we found a significant interaction between previous grooming and relative rank of bystanders: dominant subjects valued recent grooming by subordinates while intermediate ranked subjects valued the option to aggress subordinate partners in the presence of a dominant audience. Aggressors were also more likely to emit coalition recruitment calls if the audience contained individuals that outranked the subordinate partner. In conclusion, vervet monkeys include both recent grooming and knowledge about third-party relationships to make complex decisions when trading grooming for tolerance, leading to a finely balanced trade-off between reciprocation and opportunities to reinforce rank relationships.

Keywords: Machiavellian intelligence; audience effect; cognition; strategic behaviour; vervet monkeys.

Figure 1.

Proportion of conflicts within all trials with a dominant, intermediate, and subordinate audience in the two situations: partners did not groom (no grooming), partners groomed each other (post-grooming). The average proportion is based on the proportion of conflict for each pair. Bars represent +/− the standard error.

Figure 2.

Proportion of conflicts within all trials with a single individual as an audience and according to its rank (dominant, intermediate, and subordinate). The average proportion is based on the proportion of conflict for each pair. Bars represent +/− the standard error.

Figure 3.

Proportion of conflicts according to the relatedness between the subject and the highest ranking individual within the audience. Lines represent the trend line and the confidence intervals.

Figure 4.

Proportion of conflicts where vocalizations were produced within all trials with a dominant, intermediate, and subordinate audience in the two situations: partners did not groom (no grooming), partners groomed each other (post-grooming). The average proportion is based on the proportion of conflict for each pair. Bars represent +/− the standard error.