Response Facilitation in the Robbing and Bartering Activities of Balinese Long-Tailed Macaques

Because they live in highly anthropogenic habitats such as tourist temple sites, primates are required to be flexible and innovative in their foraging strategies. To do so, they can use social information and directly learn by observing group members. We investigated the role of response facilitation as a direct social influence mechanism underlying the expression of robbing and bartering (RB) by free-ranging Balinese long-tailed macaques at Uluwatu Temple (Indonesia). RB consists in stealing inedible objects from visitors (e.g., glasses) and exchanging them for food with humans. We video-recorded all occurrences of spontaneous RB events between September 2015 and August 2016, and collected 15-min focal samples on neighbor individuals (NF) immediately after they watched a RB event. We also collected matched-control focal samples (MCF) on the same witnesses located at similar distance to former demonstrators, in the absence of any RB events. The RB practice was customary in the Uluwatu population: 55.4% of all able-bodied individuals robbed and 40.9% could also barter. Our results support the role of response facilitation: watching RB performed by conspecifics temporarily increased the probability for the witness to engage in RB on its own. Macaques robbed/bartered more often and within a shorter time window (N = 115 NF/MCF dyads, p < 0.001) after watching group members engaging in RB than in control conditions (RB by focal occurred in 70% of NFs versus 21% in MCFs). The social influence occurred mainly during the first minutes after watching RB, after which RB frequency returned to baseline. By demonstrating social influence in the expression of RB, our results support the cultural nature of the RB practice at Uluwatu. Supported by the “Discovery Grant from the Natural Sciences and Engineering Research Council of Canada”.