Acquisition of object-robbing and object/food-bartering behaviours: a culturally maintained token economy in free-ranging long-tailed macaques
To assess the relative local availability in the six different types of tokens, we examined a sample of 84 video-recorded token-robbing events, randomly selected from our observational data, that featured 500 potential human targets
The token exchange paradigm is an appealing and heuristically powerful system used to investigate the existence of economic behaviour in non-human primates and to explore the evolutionary origins and developmental pathways of human monetary systems.
Most of these experimental procedures involve human-induced exchanges with relatively small samples of individually trained, laboratory-bred subjects
During the experiments, these subjects (i) are typically placed in isolation from their conspecifics and their other daily activities, (ii) exchanged in constrained environments characterized by a lack of alternative response options, and (iii) received small rewards for the correct actions ([1,2,4,5], but see [6–8]).
These conditions markedly contrast with real-world human economic behaviours that offer many different formats and variants, often occur over extended periods of time, are spontaneously engaged in by a very heterogeneous population, use a range of symbolic currencies and are influenced by a rich social context [3,9]
This study aimed to fill this knowledge gap by testing three hypotheses pertaining to the acquisition and skillful performance of robbing and bartering interactions in the Uluwatu population of long-tailed macaques.
During successful token/reward-bartering sequences, we found statistically significant age differences in the average number of food rewards being proposed to the monkey (H2 = 15.1, p = 0.001), the average number of food rewards being rejected by the monkey (H2 = 13.9, p = 0.001) and the average number of food rewards being rejected by the monkey before accepting a different type of food reward to end the token/reward-bartering sequence (H2 = 8.7, p = 0.013)
In each of these three variables, subadults scored significantly higher than juveniles (Nsubadult = 14, Njuvenile = 9, U = 11.0, p < 0.001; U = 12.5, p = 0.001; U = 19.0, p = 0.004, respectively) and adults scored higher than juveniles (Nadult = 46, U = 41.5, p < 0.001; U = 49.0, p < 0.001; U = 87.5, p = 0.006, respectively).
We found a significant increase in tokenrobbing success from juveniles to subadults to adults, whereas the main behaviour patterns required for the successful performance of token/reward-bartering interactions were already in place from around 4 years
This field observational and experimental study of tokenrobbing and token/reward-bartering interactions in the free-ranging population of Balinese long-tailed macaques produced three main findings: (i) these behaviours need to be learned throughout juvenescence to be successfully performed; (ii) older monkeys preferentially selected tokens that were more valued by humans; and (iii) these more skilful and selective individuals appeared to make economic decisions, as evidenced by clear behavioural associations between value-based token possession and quantity or quality of food rewards rejected and accepted.
(a) Experiential learning
As predicted, we found a significant increase in tokenrobbing success from juveniles to subadults to adults, whereas the main behaviour patterns required for the successful performance of token/reward-bartering interactions were already in place from around 4 years.
This is true during the token-robbing phase that often involves monkey–human body contact and/or requires muscular strength when a monkey has to yank on a flip-flop still worn by an adult human
In these cases, the limited physical capabilities of juveniles, and the maturing bodies of subadults, may partly explain the significant increase in token-robbing success from juveniles to subadults to adults.Token-robbing and token/reward-bartering are cognitively challenging tasks for the Uluwatu macaques that revealed unprecedented economic decision-making processes in a large monkey population living in an anthropogenically impacted habitat.
The research protocol of this study was approved by the