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Intergroup variation in robbing and bartering by long-tailed macaques at Uluwatu Temple (Bali, Indonesia)

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Robbing and bartering (RB) is a behavioral practice anecdotally reported in free-ranging commensal macaques. It usually occurs in two steps: after taking inedible objects (e.g., glasses) from humans, the macaques appear to use them as tokens, returning them to humans in exchange for food. While extensively studied in captivity, our research is the first to investigate the object/food exchange between humans and primates in a natural setting. During a 4-month study in 2010, we used both focal and event sampling to record 201 RB events in a population of long-tailed macaques (Macaca fascicularis), including four neighboring groups ranging freely around Uluwatu Temple, Bali (Indonesia). In each group, we documented the RB frequency, prevalence and outcome, and tested the underpinning anthropogenic and demographic determinants. In line with the environmental opportunity hypothesis, we found a positive qualitative relation at the group level between time spent in tourist zones and RB frequency or prevalence. For two of the four groups, RB events were significantly more frequent when humans were more present in the environment. We also found qualitative partial support for the male-biased sex ratio hypothesis [i.e., RB was more frequent and prevalent in groups with higher ratios of (sub)adult males], whereas the group density hypothesis was not supported. This preliminary study showed that RB is a spontaneous, customary (in some groups), and enduring population-specific practice characterized by intergroup variation in Balinese macaques. As such, RB is a candidate for a new behavioral tradition in this species.

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This study was carried out with the financial support of the funding agencies mentioned hereinafter. F. B. has received research grants from the Belgian National Fund for Scientific Research and la Fondation Belge de la Vocation. J.-B. L. has received the following grants: Natural Sciences and Engineering Research Council of Canada, Research Start-Up Allowance from the Dean’s Office of the Faculty of Arts & Science at the University of Lethbridge, and conservation grants from the International Primatological Society and the American Society of Primatologists. N. G. has received the Rufford Small Grant for Nature Conservation. We thank the Indonesian Ministry of Research and Technology, Sri Wahyono, and the Uluwatu Temple management committee for permission to conduct this research in Indonesia. Finally, we are very grateful to the Associate Editor of Primates, Brian Hare, and the two anonymous reviewers for their very constructive comments on earlier versions of this manuscript.

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Correspondence to Fany Brotcorne.

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The authors declare that they have no conflict of interest.

Human/animal rights

This research was exclusively observational and non-invasive, with no animal experiments. Non-human animals observed in the study were already used to human presence.

Ethical approval

Our study followed all Indonesian laws and was conducted under research permission from the Indonesian Ministry of Research and Technology (no. 0109/SIP/FRP/SM/VI/2010 (F. B.), no. 03B/TKPIPA/FRP/SM/III/2011 (F. B.), no. 355/SIP/FRP/SM/IX/2012 (F. B.), no. 0011/EXT/FRP/SM/I/2010 (J-B. L. and N. G.), the Provincial Government of Bali and the local authorities.

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Supplementary material 1. This video clip illustrates a typical RB sequence displayed by an adult male macaque at Uluwatu Temple (Bali, Indonesia). The sequence occurs in three steps. First the macaque approaches a female temple visitor from behind, jumps on her shoulder and takes her eyeglasses. Then, the macaque stays put while handling the eyeglasses and looking around. Second, a male tourist tries, unsuccessfully, to exchange the eyeglasses for a non-food item (i.e., an eyeglasses case). Third, the macaque moves toward a male member of temple staff who offers a food item (i.e., a cracker) to the macaque holding the eyeglasses in his mouth. Then, the macaque drops almost instantaneously the eyeglasses and steps aside to consume the food reward (i.e., successful bartering). (MPG 37140 kb)


Supplementary material 2. Home ranges (Kernel 95%) of the four groups of long-tailed macaques (Riting, Celagi, Melum, Gading) at Uluwatu Temple in 2010. (TIFF 17510 kb)

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Brotcorne, F., Giraud, G., Gunst, N. et al. Intergroup variation in robbing and bartering by long-tailed macaques at Uluwatu Temple (Bali, Indonesia). Primates 58, 505–516 (2017).

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