Acquisition of object-robbing and object/food-bartering behaviours: a culturally maintained token economy in free-ranging long-tailed macaques

Jean-Baptiste Leca

2021

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
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Summary

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 [6,7,8]).
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

Results

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

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