Behavioral and physiological response to inequity in bonobos (Pan paniscus)

Inequity aversion (IA), the affective, cognitive, and behavioral response to inequitable outcomes, allows individuals to avoid exploitation and therefore stabilizes cooperation. The presence of IA varies across animal species, which has stimulated research to investigate factors that might explain this variation, and to investigate underlying affective responses. Among great apes, IA is most often studied in chimpanzees. Here, we investigate IA in bonobos, a reputedly tolerant and cooperative species for which few IA studies are available. We describe how bonobos respond to receiving less preferred rewards than a partner in a token exchange task. We show that bonobos respond to receiving less preferred rewards by refusing tokens and rewards, and by leaving the experimental area. Bonobos never refused a trial when receiving preferred rewards, and thus showed no advantageous IA. We also investigate the variability in the disadvantageous IA response on a dyadic level, because the level of IA is expected to vary, depending on characteristics of the dyad. Like in humans and chimpanzees, we show that the tolerance towards inequity was higher in bonobo dyads with more valuable relationships. To study the affective component of IA, we included behavioral and physiological measures of arousal: a displacement behavior (rough self-scratching) and changes in salivary cortisol levels. Both measures of arousal showed large variability, and while analyses on rough self-scratching showed no significant effects, salivary cortisol levels seemed to be lower in subjects that received less than their partner, but higher in subjects that received more than their partner, albeit that both were not significantly different from the equity condition. This suggests that although overcompensated bonobos showed no behavioral response, they might be more aroused. Our data support the cooperation hypothesis on an interspecific and intraspecific level. They show inequity aversion in bonobos, a reputedly cooperative species, and suggest that the variability in IA in bonobos can be explained by their socioecology. Most successful cooperative interactions happen between mothers and their sons and among closely bonded females. The limited need to monitor the partners' investment within these dyads can result in a higher tolerance towards inequity. We therefore suggest future studies to consider relevant socioecological characteristics of the species when designing and analyzing IA studies.