Bonds smooth conflicts

Humans tend to respond differently between each other to rewards being dished out unfairly in different social groups, yet little is known about the potential for such variation in other species. A new study has examined responses to such inequity in several groups of chimpanzees (Pan troglodytes). The study, by Sarah Brosnan, Hillary Schiff and Frans de Waal, at the Yerkes National Primate Center at Emory University, Atlanta, in the Proceedings of the Royal Society series B (published online), demonstrates that chimpanzees show a response to inequity that depends to considerable extent on the social context that the rewards are given. The researchers used an experimental procedure whereby animals were given a token, in this case a piece of white PVC pipe, which they then had to give back to the researcher to receive a food reward. Food rewards were placed in identical buckets on the floor in front of the chimpanzees so that all animals in the experiment could see what was in the buckets, but none knew what reward they would receive until they had successfully returned the token. The rewards were chosen on the basis of previous preference tests in which grapes were most highly prized by the animals and cucumber and celery, the least sought. The researchers found that the animals were averse to any inequity but only in subjects that lived in pairs or in a relatively newly established social group. In a far older group, with a tightly knit social structure characterised by intense integration and social reciprocity, inequity caused barely a ripple from the animals disappointingly receiving a cucumber or celery reward. This finding may reflect human responses in close relationships, and inequity may be tolerated more as apes develop the mutual dependencies and bonds that serve a wide range of benefits derived from sociality. “If so, tolerance of inequity may increase with social closeness between partners, such as friends and family, in a wide variety of species, a hypothesis that deserves further testing in humans and non-human primates,” the authors say. Such tolerance may not marry with the experience of some humans, but its apparent existence in our closest cousins is intriguing.