From inter-group conflict to inter-group cooperation: insights from social insects

Abstract

The conflict between social groups is widespread, often imposing significant costs across multiple groups. The social insects make an ideal system for investigating inter-group relationships, because their interaction types span the full harming-helping continuum, from aggressive conflict, to mutual tolerance, to cooperation between spatially separate groups. Here we review inter-group conflict in the social insects and the various means by which they reduce the costs of conflict, including individual or colony-level avoidance, ritualistic behaviours and even group fusion. At the opposite extreme of the harming-helping continuum, social insect groups may peacefully exchange resources and thus cooperate between groups in a manner rare outside human societies. We discuss the role of population viscosity in favouring inter-group cooperation. We present a model encompassing intra- and inter-group interactions, and local and long-distance dispersal. We show that in this multi-level population structure, the increased likelihood of cooperative partners being kin is balanced by increased kin competition, such that neither cooperation (helping) nor conflict (harming) is favoured. This model provides a baseline context in which other intra- and inter-group processes act, tipping the balance toward or away from conflict. We discuss future directions for research into the ecological factors shaping the evolution of inter-group interactions. This article is part of the theme issue 'Intergroup conflict across taxa'.

Keywords: class-structure; inclusive fitness; intergroup conflict; intergroup cooperation; population viscosity; social insects.

Figure 1.

Conceptual schematic of model of intra- and inter-group relationships. We assume a large population composed of patches, each of which contains a fixed and equal number N of groups (here depicted as three). A group comprises two identical individuals (n = 2). (a) Each individual can have an impact on the members of its own group and on other groups within the patch. Impacts can be positive (cooperation = helping), neutral (tolerance) or negative (conflict = harming); as an example, here the individual has a positive impact on the other member of its own group and on Group B, but a negative impact on Group C. (b) Offspring can stay in their own group, move (m) to another group in the same patch, or disperse (d) with long-distance dispersal-related mortality risk k, to a random group in a new patch (see box 2 for more details). (Online version in colour.)

Figure 2.

Relatedness, kin competition, and the potential for helping (i.e. cooperation) as a function of long-distance dispersal, d. (a–c) Taylor's cancellation result: limited dispersal increases relatedness among group members, but it also increases the intensity of competition among related individuals; these two forces cancel each other out, such that population viscosity has no net effect on the evolution of helping or harming (negative interactions, i.e. conflict). (d–i) Taylor's cancellation result extends to higher levels of biological organization when individuals form groups within patches, for both intra- and inter-group helping and harming, irrespective of the amount of movement, m, between groups within the focal patch, where m is the fraction of offspring that move to a different group among those offspring that remain in the local patch, i.e. 1 – d. This cancellation result holds irrespective of the number of individuals within each group, n, the number of groups within a patch, N, and the long-distance dispersal mortality risk, k (see electronic supplementary material for details). Parameter values: (a–c) k = 0.5, n_T = 6; (d–i) k = 0.5, n = 2, N = 3; (d–f) m = 0.4; (g–i) m = 0.1. (See figure 1, box 1 and box 2 for definitions of parameters.)