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’.

Intergroup conflict: origins, dynamics and consequences across taxa

“…Individuals may be averse to such negative impacts and have prosocial concerns for out-group conspecifics (∂p i /∂α O < 0). This should reduce conflict participation and afford cooperative intergroup interactions as seen in, for instance, ant species [33], bonobos [18,100] and humans [101][102][103]. However, in the case of antisocial preferences for out-groups, individuals may initiate and escalate conflict out of 'spite' [104][105][106].…”

“…In humans, leader rhetoric sometimes aims at creating hatred for rivalling out-groups (α O ) [46,62], and social bonding rituals increase parochial in-group concern (α I ) [122][123][124]. Lemoine et al [18] review evidence that such a mechanism generalizes beyond humans-collective grooming and food sharing in chimpanzees prior to intergroup encounters can increase social ties among group members.…”

“…R. Soc. B 377: 20210134 and chimpanzees engage in more cooperative grooming after aggressive intergroup encounters [18,164]. These social behaviours can reduce the presence of ectoparasites, alleviate stress from conflict interactions [165,166] and relax withingroup competition and aggression.…”

“…For example, nonhuman primates, including chimpanzees (Pan troglodytes) and baboons (e.g. Papio cynocephalus), engage in violent conflict with neighbouring groups [13][14][15][16][17][18]. Intergroup conflict has also been observed in spider monkeys (Ateles paniscus) [19], hyaenas (Crocuta crocuta) [20], wolves (Canis lupus) [21], meerkats (Suricata suricatta) [22], banded mongooses (Mungos mungo) [23,24], in various group-living birds [25][26][27] and social fishes [28].…”

“…It provides, for the first time, to our knowledge, a side-by-side treatment of intergroup conflict across taxa, detailing the origins and consequences of intergroup conflict. Each in their own way, the contributions provide insight on intergroup conflict for social insects [32,33], social fishes [40], group-living birds [26,27], non-primate mammals such as banded mongoose [24], lions and wolves [17], several monkey species [41][42][43], chimpanzees [18,44] and humans [45][46][47].…”