Monkey business: Inter-group differences in the object/food bartering practice in Balinese macaques (Macaca fascicularis) at the Uluwatu Temple, Indonesia

While there is increasing evidence for the social transmission of behavioural innovations and intergroup behavioural variation in a wide range of nonhuman primate taxa, some behavioural domains (e.g., tool use) are far more represented than others (e.g., arbitrary social conventions) in the literature. Our study explores the ‘object/food bartering’ activity in the free-ranging long-tailed macaques, Macaca fascicularis, living around the Uluwatu temple, southern Bali (Indonesia). This practice occurs in two steps: after robbing temple visitors of non-edible objects, the monkeys use these objects as tokens, by returning them in exchange for specific food rewards. This spontaneous population-level activity is customary and enduring at Uluwatu, whereas it is very rare or absent at other macaque-tourism sites across the island. During a fourmonth study in 2010 at Uluwatu, we used the all-occurrence sampling technique to record 186 successful events of object-robbing, where 95 (51%) were followed by object/food bartering attempts. In line with the ‘needing-to-learn’ hypothesis, we found that older individuals were significantly more efficient at robbing valuable objects and more successful at exchanging them for food than younger individuals. We also found substantial differences in the frequency and form of the bartering practice (n = 95 events) among the four social groups constituting the Uluwatu population, with two groups (‘Temple’: 60% of the bartering events, ‘Tear’: 34%) being responsible for more frequent bartering events than the two other groups (‘Scarface’: 4%, ‘Nez’: 2%). We investigated the role of group-specific environmental and anthropic influences (food provisioning and degree of human presence) in such intergroup differences. Taken together, these preliminary results suggest that the bartering practice could be a local behavioural tradition in
Balinese macaques.