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.

Summary

In this chapter, we compare the predictions of reproductive-skew models with data from primitively eusocial wasps, the insect taxon in which skew has been best studied. These wasps share some key biological features with cooperatively breeding vertebrates, but represent a more experimentally tractable system. We describe a useful classification of skew models based on concepts of battleground and resolution models, and suggest how the basic biology of a taxon can help to identify which models and predictions in our classification are relevant. In primitively eusocial wasps, dominants have been assumed to control the allocation of reproductive shares at low cost. A priori, we therefore expect dominants to offer the minimum share required to retain a subordinate in the group (the staying incentive), or deter it from fighting (the peace-incentive). Optimization constraints are unlikely to apply because the cost of producing eggs is relatively low and non-accelerating.

Among eight detailed genetic studies of primitively eusocial wasps, only one has found strong support for the concession model of skew. None of the other studies found clear relationships between skew and relatedness, productivity, or relative body size. Skew was typically high, often uniformly high across groups. There are several possible explanations for this apparent lack of fit between empirical studies and the concession model. First, there are shortcomings of the data, such as small sample sizes and uncertainty concerning the chance of inheritance by subordinates. Second, strong ecological constraints and a good chance of inheritance reduce the need for staying incentives, in which case other factors such as the threat of fighting must be invoked to explain reproductive sharing.

Summary

The two main types of skew models, transactional and compromise models, make different assumptions about the division of reproduction. Transactional models assume that one individual has full control over reproduction within the group, but may have to refrain from claiming all reproduction in order to prevent others leaving or evicting it from the group. Compromise models, by contrast, ignore outside options such as departing to breed elsewhere, but allow for incomplete control over reproduction within the group. Attempts to synthesize these two approaches have proved controversial. Here, we show that this controversy can be resolved using a simple principle from the economic literature on bargaining – the “outside option principle.” Even if outside options are available, they will influence the outcome of reproductive conflict within a group only if they yield greater payoffs than are available within the group. We present a novel synthetic model based on this principle, in which individuals engage in a tug-of-war over reproduction within a group, but may “ease off ” in their competitive effort in response to the threat of departure or eviction. We show that over a large range of parameter space, particularly when group productivity and relatedness among group members are high, these threats are not credible, so that opportunities outside the group do not influence the stable level of skew. However, when group productivity and relatedness are low, one or other of the players will typically ease off in competition in order to maintain group stability. Under these circumstances, outside options do influence skew. Tests which examine the relationship between skew and factors such as group productivity or ecological constraints are thus expected to yield variable results.

Summary

Human females (Homo sapiens) exhibit a dramatic form of reproductive skew in which half the age classes of adults contain only nonbreeders. Among other mammals, only pilot (Globicephala spp.) and killer whales (Orcinus orca) exhibit a similar pattern. The “grandmother” hypothesis suggests that selection can favor post-reproductive survival because older females help their offspring to reproduce. But the indirect fitness gains of helping appear insufficient to outweigh the potential benefits of continued direct reproduction, so this hypothesis cannot explain why women cease reproducing in the first place. Here we present some background on menopause and describe new research which helps to understand both the strange taxonomic distribution of menopause and the timing of reproductive cessation in humans. Specifically, recent models have explored the potential reproductive conflicts that may have arisen in ancestral human families, and the influence of demography on the resolution of such conflicts. These studies suggest that an integrated model which takes into account the potential costs of reproductive competition, as well as the benefits of helping, offers a fuller understanding of the evolution of menopause.

Reproductive skew in human societies

Human societies are characterized by a dramatic and puzzling pattern of reproductive skew. In populations exposed to natural schedules of mortality and fertility (i.e. without access to modern medicine and technology), almost half the age classes of adult human females contain only nonbreeders (Figure 2.1).