Parochial cooperation in wild chimpanzees: a model to explain the evolution of parochial altruism

Abstract

Parochial altruism, taking individual costs to benefit the in-group and harm the out-group, has been proposed as one of the mechanisms underlying the human ability of large-scale cooperation. How parochial altruism has evolved remains unclear. In this review paper, we formulate a parochial cooperation model in small-scale groups and examine the model in wild chimpanzees. As suggested for human parochial altruism, we review evidence that the oxytocinergic system and in-group cooperation and cohesion during out-group threat are integral parts of chimpanzee collective action during intergroup competition. We expand this model by suggesting that chimpanzee parochial cooperation is supported by the social structure of chimpanzee groups which enables repeated interaction history and established social ties between co-operators. We discuss in detail the role of the oxytocinergic system in supporting parochial cooperation, a pathway that appears integral already in chimpanzees. The reviewed evidence suggests that prerequisites of human parochial altruism were probably present in the last common ancestor between Pan and Homo. This article is part of the theme issue 'Intergroup conflict across taxa'.

Keywords: human evolution; in-group out-group; oxytocin; parochial cooperation model; primates; social ties.

Figure 1.

The parochial cooperation model. Variables and pathways involved in small-scale group cooperation of non-human animals in an in-group out-group context. Blue arrows depict pathways with empirical support, while red arrows depict pathways empirically untested. Solid arrows depict increasing effects while patterned arrows correspond to decreasing effects. Black arrows suggest potential feedback loops in the model.

Figure 2.

Distribution of chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) across Africa. Chimpanzees divided in four subspecies (verus, eliotti, troglodytes and schweinfruthii) marked in different colours. Long-term study sites are marked for better understanding of the geographical distribution.