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

From protists to primates, intergroup aggression and warfare over resources has been observed in several taxa whose populations typically consist of groups connected by limited genetic mixing. Here, we model the co-evolution 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 co-evolve, 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, co-evolution 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 favoring within-group social diversity and behavioural syndromes.