Introduction

• 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.
• The token exchange paradigm has shown that several species of monkeys and great apes can use tokens to request specific food rewards
• This line of research provides insights into the cognitive underpinnings of economic behaviour in non-human primates [3].
• 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]

Objectives

• 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

Findings

• 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

Discussion

• 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.
• The ability to engage in more negotiated successful token/reward-bartering sequences—during which the monkey only returned the token after being proposed more food rewards, or after rejecting more food rewards, or after accepting a type of food reward different from the one(s) previously rejected—was not fully acquired before the subadult stage
• These results lend some support to the ‘experiential learning’ hypothesis, whereby token-robbing and token/ reward-bartering interactions are multi-stepped and complex behavioural sequences requiring perceptual learning, sensorimotor coordination and cognitive skills to be successfully performed; they are gradually acquired through extended individual practice during the juvenile period, in part via experiential trial-anderror learning.
• It is noteworthy to mention that the development ofadult-level proficiency at robbing/bartering is dependent on skill learning, but may be constrained by physical maturation
• 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.
• 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

Conclusion

• 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.
• This spontaneous, population-specific, prevalent, cross-generational, learned and socially influenced practice may be the first example of a culturally maintained 8 token economy in free-ranging animals.
• Further experimental research on the Uluwatu macaques should make future cross-species comparisons of economic decision-making and symbolic tool use more relevant from an evolutionary perspective and may lead to a better understanding of the origins of autonomous monetary systems in humans [29].
• Our study was conducted with research permission from the Indonesian Ministry of Research and Technology (#328/SIP/FRP/E5/Dit.KI/IX/2015; #410/SIP/FRP/E5/Dit.KI/X/ 2015), related Indonesian government and provincial agencies and the local authorities of the Uluwatu Temple