Variation between species, populations, groups and individuals in the fitness consequences of out-group conflict

Abstract

Out-group conflict is rife in the natural world, occurring from primates to ants. Traditionally, research on this aspect of sociality has focused on the interactions between groups and their conspecific rivals, investigating contest function and characteristics, which group members participate and what determines who wins. In recent years, however, there has been increasing interest in the consequences of out-group conflict. In this review, we first set the scene by outlining the fitness consequences that can arise immediately to contest participants, as well as a broader range of delayed, cumulative and third-party effects of out-group conflict on survival and reproductive success. For the majority of the review, we then focus on variation in these fitness consequences of out-group conflict, describing known examples both between species and between populations, groups and individuals of the same species. Throughout, we suggest possible reasons for the variation, provide examples from a diverse array of taxa, and suggest what is needed to advance this burgeoning area of social evolution. This article is part of the theme issue ‘Intergroup conflict across taxa’.

Intergroup conflict: origins, dynamics and consequences across taxa

“…In some species, males are more likely to be directly involved in group conflict than females, and the impact of conflict differs significantly for male and female group members [17]. Moreover, Morris-Drake et al [26] discuss a range of individual-level pay-offs, some being more 'hidden' than others. For example, non-participating individuals (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]. Social insects raid neighbouring colonies and kill enemy conspecifics [29][30][31][32][33].…”

“…Chimpanzees kill and die from battlerelated injuries [16], and inter-colony warfare among social insects such as ants and bees can kill tens of thousands [31,36]. Moreover, and in addition to battle-related trauma, intergroup conflict has, across species, been linked to environmental degradation and famine, migration and forced relocation and the spreading of infectious diseases [26,[37][38][39].…”

“…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].…”

Leaders of war: modelling the evolution of conflict among heterogeneous groups

“…serious injury or death [9,11,13], are outweighed by the potential benefits of victory, e.g. the expansion of territory [9,14], access to limited resources [8,11,13,15,16] or increases to prestige, status and reputation ( [17]; for a review see [18]). However, this simple understanding-that war should evolve when the benefits are greater than the costs-becomes more complicated when considering heterogeneous groups [19].…”