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.