What drives cooperation? Proximate factors explaining the variability in inequity aversion and prosociality in captive Bonobos.

Cooperation is a key component of social life but seems an evolutionary puzzle as it involves behaviours that benefit others. Because it often involves a cost to the actor, natural selection must have produced mechanisms to regulate cooperation to overcome any adverse effects of these costs. The main proximate mechanisms that regulate cooperation are prosociality and inequity aversion, respectively the promotor and stabiliser of cooperation. In this thesis, I study a group of bonobos (Pan paniscus) in Zoo Planckendael, combining behavioural and physiological measures in different experimental paradigms to explain the variability in these proximate mechanisms of cooperation in bonobos. Bonobos are an ideal species to study prosociality and IA. First, because they are one of our closest living relatives and studying prosociality and IA in bonobos increases our knowledge on how unique the level of prosociality and IA in humans is. Second, because bonobos have been described as 'hippies of the primate world', who are highly tolerant, prosocial, empathic and cooperative, but prosociality and inequity aversion as drivers of cooperation have not been extensively studied in this species.To study prosociality, I conducted three group experiments that differed in the payoff distribution between the actor and receiver. I used a juice provisioning experiment that had previously been used to measure prosociality in chimpanzees and I implemented two food provisioning paradigms, the prosocial choice task (PCT) and the group service paradigm (GSP). To study inequity aversion, I used the standard token exchange task. To complement the standard behavioural measures with the emotional component of inequity aversion, I also investigated a behavioural and a physiological measure of arousal. The results of all prosociality experiments showed that the Zoo Planckendael bonobos mainly behaved out of self-interest: in more than half of the juice-provisioning acts, the subject also benefitted; bonobos did not prefer the prosocial above the selfish option in the PCT and adult bonobos did not provision group members in the GSP. Thus, bonobos, like chimpanzees, behaved indifferently to the welfare of others, which contrasts with the popular image of the prosocial and food sharing bonobo, who is often portrayed as a "hippie of the primate world". I concluded that this popular image is mainly the result of an age bias in previous experimental studies. I also demonstrated that bonobos reacted to receiving less than a partner by refusing trials and moving away from the experimenter while they never refused trials when receiving more than a partner. The level of inequity aversion was influenced by the relationship quality between individuals. I showed that stronger bonded individuals were more tolerant towards inequity. Further, subjects were more aroused when receiving a better reward than a partner, suggesting that bonobos do notice when being favoured but do not respond to it behaviourally.This thesis highlights the importance of validated methodologies and provides supporting evidence for the nuanced view of the prosocial, food-sharing and tolerant hippie ape. I show that adult bonobos do not behave prosocially in food-related paradigms, which can be explained by the competitive nature around the preferred food items, and which corresponds to the food-related behaviour of bonobos in the wild. I also showed that in bonobos, like chimpanzees, the tolerance to inequity is limited to a certain level and linked to specific partners.

Keywords:BONOBO (PAN PANISCUS)

Disciplines:Animal biology