India, Sept. 4 -- In his landmark book, Peacemaking Among Primates (1989), primatologist Frans de Waal describes a behaviour he first documented among the chimpanzees of Arnhem Zoo in the Netherlands a decade earlier. After a fight, former opponents would seek each other out and re-establish contact by grooming, embracing, kissing, and holding hands. Reconciliation seemed to occur most reliably between individuals whose relationship is more highly valued - such as kin, allies, high-ranking partners, and frequent grooming partners - and measurably lowered the probability of aggression in the future. But, what makes this more than a social nicety is that this process of reconciliation is likely both external and physiological.

Filippo Aure...