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«Non-reciprocal but peaceful fruit sharing in wild bonobos in Wamba Shinya Yamamoto a,b,∗ a Graduate School of Intercultural Studies, Kobe ...»

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Behaviour 152 (2015) 335–357 brill.com/beh

Non-reciprocal but peaceful fruit sharing in

wild bonobos in Wamba

Shinya Yamamoto a,b,∗


Graduate School of Intercultural Studies, Kobe University, 1-2-1 Tsurukabuto, Nada-ku,

657-8501 Kobe, Japan


Wildlife Research Center, Kyoto University, Yoshida-honmachi, Sakyo-ku,

606-8501 Kyoto, Japan

* Author’s e-mail address: shinyayamamoto1981@gmail.com

Accepted 30 December 2014; published online 29 January 2015

Abstract Food sharing is considered to be a driving force in the evolution of cooperation in human societies.

Previously postulated hypotheses for the mechanism and evolution of food sharing, e.g., reciprocity and sharing-under-pressure, were primarily proposed on the basis of meat sharing in chimpanzees.

However, food sharing in bonobos has some remarkably different characteristics. Here I report details pertaining to fruit sharing in wild bonobos in Wamba based on 150 events of junglesop fruit sharing between independent individuals. The bonobos, primarily adult females, shared fruit that could be obtained individually without any cooperation or specialized skills. There was no evidence for reciprocal exchange, and their peaceful sharing seems to contradict the sharing-under-pressure explanation. Subordinate females begged for abundant fruit from dominants; this might indicate that they tested the dominants’ tolerance based on social bonds rather than simply begging for the food itself, suggesting existence of courtesy food sharing in bonobos.

Keywords bonobo, Pan paniscus, plant-food sharing, courtesy food sharing, begging for social bond, prosocial behaviour, reciprocity, sharing-under-pressure.

1. Introduction Food sharing is one of the most prevalent forms of prosocial behaviour in the animal kingdom, and it has attracted much attention in investigations of the evolution of cooperation. Why does the owner relinquish food rather than keep it? This is the most basic and common question when discussing the evolution of apparently non-self-rewarding acts. Sharing with dependent re- lated offspring is relatively common in various animal taxa (Clutton-Brock, © Koninklijke Brill NV, Leiden, 2015 DOI 10.1163/1568539X-00003257 336 Fruit sharing in wild bonobos 1991; Ydenberg, 1994; Brown et al., 2004), and its evolution can be ex- plained by kin selection (Hamilton, 1964). In contrast, sharing among inde- pendent individuals is far less common and the benefit of sharing is often questioned or is controversial (Stevens & Gilby, 2004; Jaeggi & van Schaik, 2011). Considerable efforts have been made to find answers for these ques- tions regarding the mechanism and evolution of food sharing by investigating non-human primates, particularly chimpanzees (Pan troglodytes).

To explain non-kin sharing, two main hypotheses have been proposed on the basis of previous studies involving chimpanzees: reciprocity and harassment. The reciprocity hypothesis explains that animals share in exchange for a past or future benefit (e.g., receiving the same food items or a different currency such as coalitions or mating). This hypothesis is supported by previous studies on chimpanzees, which detected statistical associations between sharing food with socially established partners (de Waal, 1989a, 1997; Mitani & Watts, 2001; Hockings et al., 2007; Gomes & Boesch, 2009). In contrast, the harassment hypothesis explains that there is an immediate benefit for the owner. Thus, when the beggar negatively affects the owner’s feeding

rate, the owner may give up some food but retain the majority, i.e., sharing to avoid further harassment from the beggar (‘sharing-under-pressure’:

Wrangham, 1975; ‘tolerated theft’ for human sharing: Blurton Jones, 1984).

This hypothesis is supported by some previous observational and experimental studies involving chimpanzees (Stevens, 2004; Gilby, 2006; Gilby et al., 2010), and it provides a more simple and parsimonious explanation for sharing behaviour (Stevens & Gilby, 2004; Stevens & Hauser, 2004). However, these two hypotheses are not exclusive and each may partially explain the sharing behaviour of chimpanzees (Jaeggi & van Schaik, 2011).

Previous studies have been mainly based on meat sharing among chimpanzees (e.g., in Gombe: Teleki, 1973; Goodall, 1986; in Mahale: Kawanaka, 1982; Nishida et al., 1992; in Kibale: Mitani & Watts, 1999, 2001; and in Tai: Boesch & Boesch, 1989; Boesch, 1994). Meat is a rare commodity and at some sites might be difficult to obtain without cooperation between individuals (Boesch, 1994). Some researchers have claimed that the contribution of an individual during cooperative hunting can predict the amount of meat they obtain during the following sharing session (Boesch, 1994). This is why sharing is often discussed in relation to hunting cooperation. In a comparative context, adult male chimpanzees at Bossou often enter ‘risky’ parts of their habitat to acquire large cultivated foods. In these cases, only a few 337 S. Yamamoto / Behaviour 152 (2015) 335–357 ‘brave’ adult males could be fruit owners, and they consequently share these difficult-to-obtain foods with other community members (Hockings et al., 2007; Ohashi, 2007). The evolutionary story of human sharing cooperation has been constructed on the basis of these studies involving chimpanzees, frequently using comparisons with human hunter–gatherers.

The bonobo (Pan paniscus) is the sister species of the chimpanzee; however, it has been much less studied in this context, although previous studies have reported considerable differences in food sharing between these two species (Kano, 1980; Kuroda, 1980, 1984; White, 1994; Fruth & Hohmann, 2002). The unique features of sharing in bonobos are particularly characterized by their frequent fruit sharing and female participation in this social interaction. These sharing traits of bonobos are clearly different from the hunting–sharing observed in chimpanzees. A previous study involving wild bonobos in Lomako forest suggested that sharing under pressure, or mutualism, is a more plausible explanation than reciprocity (Fruth & Hohmann,

2002) because sharing increased with the number of beggars. Lomako bonobos might allow a few recipients to take pieces of food in order to avoid other begging hands; thus, both the owner and recipients seem to be mutually benefited. Meanwhile, detailed video analysis of meat and fruit sharing by a specific dyad of non-kin adult female bonobos in Wamba forest suggested that neither reciprocity nor harassment appeared to completely explain the observations (Hirata et al., 2010). More specifically, reciprocity cannot explain why the owner only tolerated the transfer of non-valuable parts, because it would be more reasonable from a psychological viewpoint to assume that the owner would share the valuable parts if he/she expected a future return benefit. Likewise, harassment cannot fully explain the differential rate of food transfer (more transfers of fruit than of meat), when the degree of pressure given by the recipient toward the two types of food remained more or less constant (Hirata et al., 2010). At present, considering that sufficient quantitative data are not available, it seems too early to make any conclusions regarding the mechanisms underlying fruit sharing in bonobos. We should also consider that the environments where animals live may influence the nature of food sharing. Unfortunately, to my knowledge, no specific study has previously compared food sharing in different wild bonobo sites. In captivity, experiments with chimpanzees and bonobos in African sanctuaries suggested that bonobos are more tolerant of each other than of chimpanzees, where they outperformed chimpanzees in a cooperaFruit sharing in wild bonobos tive task and subsequent co-feeding (Hare et al., 2007). The bonobos even exhibited a preference for voluntary food sharing rather than eating alone with monopolisable food sources (Hare & Kwetuenda, 2010), even with unfamiliar individuals (Tan & Hare, 2013). However, an experimental study with chimpanzees and bonobos in several European zoos reported the opposite; chimpanzees shared food more tolerantly, actively and reciprocally than bonobos who demonstrated a more despotic nature (Jaeggi et al., 2010a).

The environments in the zoos and sanctuaries may have affected the results, although we currently do not know what exactly might determine this difference. Therefore, bonobo–chimpanzee comparisons as well as comparisons between same-species communities are required from both captive and wild contexts to understand the complexities of ape sharing behaviours. In this sense, comparisons between bonobo field sites where food sharing can be frequently observed are important to obtain ecological insights into the nature of food sharing.

So far, however, insufficient data are available on food sharing among fully identified wild bonobos in their natural environment. Therefore, the main aim of the present study was to obtain details of fruit sharing in wild bonobos in Wamba forest, where long-term research (since 1973) has established a tracking record that began 1976 and allows for individual identification. Thus, I could investigate the detailed social relationships among sharing individuals. The excellent observational conditions and rich environment with abundant junglesop (Anonidium mannii) fruit also allowed me to focus on a single fruit species in the quantitative data analysis, thereby avoiding possible confusion due to different food types, which may affect the characteristics of sharing (Kano, 1980; Kuroda, 1984; White, 1994; Fruth & Hohmann, 2002; Hirata et al., 2010).

The shared item, i.e., abundant fruit and not rare meat, also raises the following question: why do independent recipients beg for fruit from other individuals when it can be obtained without any cooperation or specialized skills? The previous hypotheses, i.e., reciprocity and harassment, assume nutritional benefits to the recipients and discusses the benefits accrued to individuals that share (Stevens & Gilby, 2004). However, if the begging individual could acquire more of the same food by implementing an alternate strategy, another perspective may be required to understand this sharing behaviour (Slocombe & Newton-Fisher, 2005). The target in the present study, i.e., junglesop fruit, is often available in small quantities simultaneously, but it can still be found much more frequently and in greater quantities in Wamba 339 S. Yamamoto / Behaviour 152 (2015) 335–357 than meat in the chimpanzee studies and Treculia fruit in the bonobo studies in Lomako. In the high season for junglesop fruits in Wamba, I counted a maximum of nine individuals who ate junglesop fruits simultaneously at a feeding site, and up to 45 individuals in total during a single observation day (Yamamoto, unpublished data), although on average 0.49 Treculia fruits were eaten per day in Lomako (Fruth & Hohmann, 2002). On the basis of these quantitative data, I examined some previously postulated hypotheses and propose another hypothesis from the recipient’s perspective to explain this sharing behaviour and its evolution.

2. Materials and methods

2.1. Study conditions I studied bonobos in the E1 group at Wamba (0°11 08 N, 22°37 58 E) in the northern sector of the Luo Scientific Reserve, Democratic Republic of Congo. The E1 group size varied between 27–31 individuals during the study periods, including 9–10 adult/adolescent females, 8–10 adult/adolescent males, 1–2 juvenile females, 2–3 juvenile males and 6–9 dependent infants, during the study periods. These numbers did not include some individuals from other groups who made short-term visits (1 month at the point of study). The age–sex class was based on Hashimoto’s (1997) categorization for Wamba bonobos, i.e., age 4: infant, 4–7: juvenile, 8–14: adolescent, 15: adult; and I adjusted it for the purpose of the present study. In the present study, the distinction between a dependent infant and a juvenile was based on whether an individual was carried by his/her mother or walked independently for 20 m during travel on the ground. In this sense, a juvenile can be considered as an ‘independent individual’ who could obtain his/her food by him/herself. The age when the transition occurred was approximately 4 years. I categorized a young female as juvenile if she remained in her natal group. Corresponding to this categorization of females, I used females’ average emigration age, i.e., 8 years (Furuichi, 1989; Hashimoto, 1997; Furuichi et al., 1998, 2012; Hashimoto et al., 2008), for the categorization of males into juveniles and adults. The distinction between adolescent and adult is not that meaningful for the present study; hence, hereafter both classes have been taken together as adult. In this study, I focused on sharing among independent individuals, i.e., within and between adults and juveniles. Not all of the individuals lived in the community simultaneously (some vanished or emigrated, whereas others immigrated or matured), but 315 dyads of inFruit sharing in wild bonobos dependent individuals were considered. For each age–sex class ratio and relevant analyses, I used the median number of individuals in each age–sex class during the study period, with a total of 22 independent individuals, i.e., 10 adult females (45.5%), nine adult males (40.9%), one juvenile female (4.5%) and two juvenile males (9.1%). All individuals in the group were well identified and habituated. Artificial provisioning was abolished in 1996.

The history of the E1 group and the details of the study site have been previously described by Kano (1992), Furuichi et al. (1998, 2012), Hashimoto et al. (2008) and Idani et al. (2008).

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