Bonobos voluntarily hand food to others but not toys or tools

Abstract

A key feature of human prosociality is direct transfers, the most active form of sharing in which donors voluntarily hand over resources in their possession Direct transfers buffer hunter-gatherers against foraging shortfalls. The emergence and elaboration of this behaviour thus likely played a key role in human evolution by promoting cooperative interdependence and ensuring that humans' growing energetic needs (e.g. for increasing brain size) were more reliably met. According to the strong prosociality hypothesis, among great apes only humans exhibit sufficiently strong prosocial motivations to directly transfer food. The versatile prosociality hypothesis suggests instead that while other apes may make transfers in constrained settings, only humans share flexibly across food and non-food contexts. In controlled experiments, chimpanzees typically transfer objects but not food, supporting both hypotheses. In this paper, we show in two experiments that bonobos directly transfer food but not non-food items. These findings show that, in some contexts, bonobos exhibit a human-like motivation for direct food transfer. However, humans share across a far wider range of contexts, lending support to the versatile prosociality hypothesis. Our species' unusual prosocial flexibility is likely built on a prosocial foundation we share through common descent with the other apes.

Keywords: bonobo; chimpanzee; cooperation; human evolution; prosociality; sharing.

Figure 1.

(a) Testing set-up and (b) results of Experiment 1.

Figure 2.

Testing set-up and results of Experiment 2. (a) Experimental condition. (b) Control condition. Located in adjacent rooms, subjects could interact through a single window (dashed line). Round dots represent nuts provided to subjects in each trial while rectangles attached to rounded lines depict rocks and their tethers. Nuts and rocks were provisioned far beyond the reach of the bonobo in the adjacent room. E1 and E2 served as experimenters and camera-people. (c) Per cent of trials in which subjects exhibited tolerated theft of nuts and rocks in the experimental and control conditions. (d) Per cent of trials in which subjects exhibited direct transfer of nuts and rocks in the experimental and control conditions. Error bars denote standard error.