In the previous chapters we have discussed how to succeed in obtaining resources by attempting to be quicker (race) or stronger (fight) than competitors. The third principle tactic to cope with competition for resources is to share them, which involves conceding a quota to competitors. This may prove to be a better choice than either racing or fighting, particularly if these tactics are too costly or unprofitable because of limited competitiveness, or due to concordance of fitness interests (Frank 2003; Taborsky et al. 2016). Sharing can also be favoured if coordinating or cooperating with other individuals increases the value of a resource, or helps to produce resources (Clark & Mangel 1986; Garfinkel & Skaperdas 2007; e.g. in cooperative hunting: Packer & Ruttan 1988; Dumke et al. 2018). Cooperation often guarantees the most efficient use of resources, because of synergistic effects (Maynard Smith 1982b; Queller 1985; Hauert et al. 2006; Gore et al. 2009; Cornforth et al. 2012; Van Cleve & Akcay 2014; Corning & Szathmary 2015). For instance, if several individuals coordinate to capture a prey that is normally difficult or risky to hunt on their own, each individual predator may gain more food at lower cost. Similarly, raising offspring in a group may allow individuals to specialize in different tasks, such as tending young, warding off predators and supplying resources, which enhances the efficiency of offspring care.

Humans exhibit a rare life history in which females stop reproducing midway through life. Only a handful of other wild mammals exhibit a similar menopausal life history. The theory of kin selection suggests that post-reproductive survival can be favoured where older females confer fitness benefits on their offspring (the grandmother hypothesis). Numerous studies have shown that older females do help to boost the fitness of their offspring, but such helping benefits are too small to favour reproductive cessation in humans at the observed age of 40–50 years old. Recent models of ‘kinship dynamics’ suggest that reproductive conflict between generations is a missing factor in the evolution of post-reproductive lifespan. The models further suggest that humans and some cetaceans are predisposed to the evolution of menopause as a consequence of their unusual, and distinct, patterns of mating and dispersal, providing an explanation for the strange taxonomic distribution of this trait. Tests of these models in humans support the predicted effect of demography on patterns of kinship and provide evidence of intergenerational reproductive conflict in some societies. Recent tests of the models in a wild population of another menopausal mammal, the killer whale, have found strong support for the model predictions. Studies of the family conflict in other long-lived mammals can shed new light on how the unusual human life history evolved.

The previous chapter examined the factors that shape the evolution of competitive behaviour, focusing primarily on forms of scramble competition or racing to consume or exploit resources. Competition also occurs between individuals that interact repeatedly – what we term social conflict – which may lead to the evolution of more complex strategies of competition. In some circumstances, individuals may be selected to put their differences aside and work together as a team to outcompete other teams. Such transitions from outright conflict to cooperation have been called ‘major transitions in evolution’. The theory of major transitions tries to explain how and why many forms of life have become more complex over time, from self-replicating molecules to animal societies. Understanding how major transitions occur requires an explanation of how individual conflicts of interest can be suppressed for the good of the group. Many of the major transitions in evolution happened billions or hundreds of millions of years ago, and are difficult to study. However, a recent major transition occurred with the evolution of cooperative animal societies from solitary ancestors, and hence these societies are tractable systems to investigate how strategies of conflict and cooperation coevolve. This chapter explores the forms of conflict that arise in cooperative societies, and the social behaviours that individuals use to shift the resolution of conflict in their own favour, from aggression and escalated fighting to more subtle forms of negotiation. We show how selection can lead to the suppression of competition and peaceful resolution of conflict among social partners, uniting their fitness interests and paving the way for the final stage of a major transition, the evolution of a new, higher level of biological complexity.

Competition for resources is the fundamental process generating selection on social behaviour. Individuals compete with family members, with other conspecifics, and with the members of other species for food, shelter and other resources that are essential for survival and reproduction. Conspecifics also compete for access to social partners and mates, and hence selection acting on strategies of social competition is particularly intense (West-Eberhard 1979). However, competition is not simply a repellent force in the lives of organisms, driving them apart; it is often a social attractor, bringing individuals together and setting the stage for social evolution. In particular, where resources such as food and mates are clumped in the environment or predictable in time, competition has the effect of drawing individuals together into aggregation and possible social interaction, selecting for strategies that maximize fitness by exploiting, parasitizing, following, or even cooperating with other individuals of the same or different species.

In nature, conflict and cooperation are prevalent not only between individuals of the same species, as discussed in Chapters 2–4, but also between individuals belonging to different species. If thinking of interspecific relations, what perhaps first crosses one’s mind are the salient relations between predators and prey, or between parasites and hosts. However, different organisms also cooperate with each other, which in fact may lead to entirely new organisms (Kiers & West 2015). Here we focus on the behavioural responses of individuals to competition with individuals of other species, based on the concepts we outlined in the introduction to this book: non-interference rivalry (race), conflict (fight), or cooperation (share).

Why is the evolution of social behaviour interesting? For one thing, if we wish to comprehend the origin, maintenance and functionality of any biological trait, we need to understand its evolution. At the same time, each behaviour is social in essence; it affects the survival, production and reproduction of others in some way or another. ‘Others’ encompasses social partners including mates, offspring, competitors, friends and foes regardless.

The behaviour that evolves in response to competition depends on the ecological distribution of resources, and the physical and social attributes of competing organisms. In Chapter 2 we explored the ecological and social factors that influence the form and intensity of competition, and the consequences of competition for animal distributions and social behaviour. We have focused on scramble competition – a race for resources – which is the most basic of social interactions and generally involves the least-complicated behaviour. By contrast, contest competition – a fight for resources – which we addressed in Chapter 3, leads to more varied, durable and complex social relationships.

Animals behave according to their previous social experience. This has been demonstrated in a wide range of species all across the animal kingdom (Rutte et al. 2006). Remarkably, this response to previous social interactions is not confined to experience with known individuals. It appears to be a much simpler trait than we might assume from our own intuition. If an animal fights with any conspecific, it will behave differently in future encounters, depending on whether it won or lost. These renowned winner and loser effects are among the most predictable traits in animal interactions (see Chapter 14). In humans, we term the psychological mechanism involved ‘self-confidence’. But, most interestingly, animals respond contingently upon social experience also in a sociopositive context. If they received help, they are more likely to help others as well, even if donors and receivers are completely unknown to each other. This generalised form of reciprocity has been demonstrated in humans and Norway rats so far (Bartlett & DeSteno 2006, Rutte & Taborsky 2007), but we assume it to be a general phenomenon, just like the ubiquitous winner and loser effects.

In the early 1970s, after Hamilton's (1964) and Trivers' (1971) revelations on kin selection and reciprocity as key mechanisms of altruistic behaviour and advanced sociality, we were tempted to believe that the major riddles in this field had been solved. The vast literature that emerged on the evolutionary mechanisms of altruism and (eu)sociality since then, at both theoretical and empirical levels, proved us dead wrong.

Summary

Four types of cooperation between conspecific competitors can be differentiated in fish reproduction: joint defense of a spawning site or territory, joint preparations for spawning, cooperative spawning, and cooperative brood care. Long-lasting associations allowing for different reproductive shares of partners are mainly found in species that cooperate in territory defense and brood care. Here I use skew theory to scrutinize patterns of reproductive participation among related and unrelated group members in cooperatively reproducing cichlids and wrasses. A comparison of five species from which sufficient data are available does not reveal an obvious relationship between average relatedness, or group size, and reproductive skew levels, as predicted from respective skew models. It is remarkable that superficially similar cooperative systems in fish may be based on distinctly different parameter combinations, even in closely related species. I discuss five alternative schemes to understand the patterns of reproductive participation in cooperatively reproducing fish, including kin-selection theory, reciprocity models, manipulation or coercive strategies, models of alternative reproductive tactics, and a dynamic modeling approach. A comparison of approaches suggests that conventional skew models do not account for the complexity of evolutionary mechanisms involved in reproductive skew among members of fish groups. Alternative approaches, such as reciprocity theory or models to explain the coexistence of alternative reproductive tactics, may have greater explanatory potential, at least in some cases. However, they have generally not been developed sufficiently to derive predictions allowing for a conclusive test. To understand how decisions of distinctly different types of group members evolve, an approach is needed that takes account of the state dynamics involved, as in fish, conditions constantly change due to their continued growth after maturation.