Social bonds facilitate cooperative resource sharing in wild chimpanzees

Abstract

Why share when access to benefits is uncertain is crucial to our understanding of the evolution of humans' extensive cooperation. Here, we investigated some of the different human sharing hypotheses and potential neuroendocrine mechanisms, in one of our closest living relatives, chimpanzees. The strongest predictor of sharing across food types was the presence of enduring and mutually preferred grooming partners, more than harassment, direct signalling, or trade. Moreover, urinary oxytocin levels were higher after the sharing of both individually and jointly acquired resources compared with controls. We conclude that the emotional connection inherent in social bonds was a key factor determining sharing patterns, with the oxytocinergic system potentially facilitating long-term cooperative exchanges. Testing for the role of social bonds in increasing predictability of sharing behaviour, a feature frequently overlooked, may help us to identify the evolutionary drivers of resource sharing and mechanisms that sustain delayed reciprocity between non-kin.

Keywords: Pan troglodytes; cooperation; friendship; harassment; oxytocin.

Figure 1.

Effect of the interaction between the rank of the possessor, rank of beggar and the sex combination from the perspective of the food possessor on the likelihood to share. Sex symbols on the top and left side represent beggars and possessors, respectively. Shown are the observed probabilities to share food (larger point volumes denote a larger number of observations), as well as the model results (surface).

Figure 2.

Effects of hunting and sharing on urinary oxytocin levels in wild chimpanzees in the East and South groups (n = 246 samples; 20 subjects; 186 events). Shown are medians (thin horizontal lines), quartiles (boxes), percentiles (2.5 and 97.5%; vertical lines), minimum and maximum (laying crosses) as well as the fitted model and its 95% confidence intervals (thick grey lines and error bars). ***p < 0.001.