Food sharing is linked to urinary oxytocin levels and bonding in related and unrelated wild chimpanzees

Abstract

Humans excel in cooperative exchanges between unrelated individuals. Although this trait is fundamental to the success of our species, its evolution and mechanisms are poorly understood. Other social mammals also build long-term cooperative relationships between non-kin, and recent evidence shows that oxytocin, a hormone involved in parent-offspring bonding, is likely to facilitate non-kin as well as kin bonds. In a population of wild chimpanzees, we measured urinary oxytocin levels following a rare cooperative event--food sharing. Subjects showed higher urinary oxytocin levels after single food-sharing events compared with other types of social feeding, irrespective of previous social bond levels. Also, urinary oxytocin levels following food sharing were higher than following grooming, another cooperative behaviour. Therefore, food sharing in chimpanzees may play a key role in social bonding under the influence of oxytocin. We propose that food-sharing events co-opt neurobiological mechanisms evolved to support mother-infant bonding during lactation bouts, and may act as facilitators of bonding and cooperation between unrelated individuals via the oxytocinergic system across social mammals.

Keywords: bonding mechanism; chimpanzee; cooperation; food-sharing; non-invasive hormone sampling; oxytocin.

Figure 1.

Effect of food sharing versus social feeding without sharing on the urinary oxytocin concentrations in wild chimpanzees. Residuals are shown ± standard error following model 1 (total of n = 79 samples; 26 subjects; **p < 0.01).

Figure 2.

Effect of (a) food sharing compared with grooming and (b) five behavioural and social contexts on the urinary oxytocin concentration in wild chimpanzees. Residuals are shown ± standard error following model 3. Number of samples included in each context are shown beneath each plot (total n = 182 samples, 34 subjects; *p < 0.05, **p < 0.01, ***p < 0.001).