Evolution of warfare by resource raiding favours polymorphism in belligerence and bravery

Abstract

From protists to primates, intergroup aggression and warfare over resources have been observed in several taxa whose populations typically consist of groups connected by limited genetic mixing. Here, we model the coevolution between four traits relevant to this setting: (i) investment into common-pool resource production within groups (helping); (ii) proclivity to raid other groups to appropriate their resources (belligerence); and investments into (iii) defense and (iv) offense of group contests (defensive and offensive bravery). We show that when traits coevolve, the population often experiences disruptive selection favouring two morphs: ‘Hawks’, who express high levels of both belligerence and offensive bravery; and ‘Doves’, who express neither. This social polymorphism involves further among-traits associations when the fitness costs of helping and bravery interact. In particular, if helping is antagonistic with both forms of bravery, coevolution leads to the coexistence of individuals that either: (i) do not participate into common-pool resource production but only in its defense and appropriation (Scrounger Hawks) or (ii) only invest into common pool resource production (Producer Doves). Provided groups are not randomly mixed, these findings are robust to several modelling assumptions. This suggests that inter-group aggression is a potent mechanism in favouring within-group social diversity and behavioural syndromes. This article is part of the theme issue ‘Intergroup conflict across taxa’.

Intergroup conflict: origins, dynamics and consequences across taxa

“…Groups can differ in size and in internal cohesiveness, and group boundaries can be permeable such that individual members are more or less able to migrate into adjacent groups or form new groups. Regardless, groups within such structured populations may collectively compete over scarce resources such as territory and shelter, access to food and water and mating opportunities [48][49][50]52,63]. In humans, intergroup competition and conflict sometimes also involve immaterial resources like political influence, the truth and the validity of worldviews [64,65].…”

“…Sometimes, however, task specializations are flexible and change over time. Human groups deliberately select some community members to join raids on neighbouring tribes, with partner selection depending on both individual abilities and reputations [ 46 , 63 , 114 , 138 ]. Such skill-based partner choice to coordinate collective action is seen in other species too, including chimpanzees [ 139 ], gorillas ( Gorilla g. gorilla ) [ 140 ], marmosets ( Callithrix jacchus ) [ 141 ], harbour porpoises ( Phocoena phocoena ) [ 142 ] and groupers ( Plectropomus pessuliferus marisrubri ) [ 143 ].…”

Correlates of individual participation in boundary patrols by male chimpanzees

“…Individuals may also differ in motivation due to differences in propensity for boldness, exploration or aggression [54]. Inter-group aggression may be a driving force behind such behavioural variation [56]. We considered two additional factors associated with individual fighting ability and motivation: hunting and personality ratings.…”

Evolutionary Game Theory and the Adaptive Dynamics Approach: Adaptation where Individuals Interact

“…More seldom seen is disruptive selection in terms of relatedness coefficient [45,47,108,[114][115][116][117][118]. Under the assumption that individual fitness can be written as !…”