A social learning model of conflict and cooperation in human societies

Patterns of conflict and cooperation both within and between societies may be related to the degree of cultural similarity within and between the same societies. A simple model of social learning is used to predict patterns of conflict and cooperation in hypothetical societies that differ in the roles of relatives and nonrelatives in the enculturation of children. The model is illustrated by comparing its predictions to known differences in the patterns of conflict between males inpatrilocal and matrilocal societies.     

Longevity suppresses conflict in animal societies

Models of social conflict in animal societies generally assume that within-group conflict reduces the value of a communal resource. For many animals, however, the primary cost of conflict is increased mortality. We develop a simple inclusive fitness model of social conflict that takes this cost into account. We show that longevity substantially reduces the level of within-group conflict, which can lead to the evolution of peaceful animal societies if relatedness among group members is high. By contrast, peaceful outcomes are never possible in models where the primary cost of social conflict is resource depletion. Incorporating mortality costs into models of social conflict can explain why many animal societies are so remarkably peaceful despite great potential for conflict.     

The cost of conflict in aphid societies

Most social aphids are found within plant galls, inside of which clonally‐derived family groups feed, and specialized larval castes forego reproduction and perform various cooperative tasks, including group defence. When unrelated aphids move between clones, conditions are ripe for conflict because galls and cooperative defence are shared resources that are vulnerable to exploitation. A key unknown is whether conflict is costly in aphid social groups. We show that diversity within groups is negatively correlated with performance in the North American social aphid, Pemphigus obesinymphae. A substantial fraction of productivity is invested into drifting. However, drifting aphids tend to mature and depart non‐natal galls prior to the seasonal peak in fecundity. These results suggest that when unrelated individuals move between groups, social aphids may experience conditions consistent with a tragedy of the commons. These results also emphasize the strongly convergent properties associated with conflict across the spectrum of animal and microbial sociality.     

Sex allocation conflict in insect societies: who wins?

Sex allocation in colonies of eusocial Hymenoptera is one of the best studied social conflicts. We outline a framework for analysing conflict outcome through power and the costs of manipulation and suggest that the conflict will often be unresolved because both major parties of interest, the queen and the workers, should manipulate allocation even at considerable costs to the colony. We suggest future work for analysing power in the conflict between queen and workers over sex allocation and discuss the extent of male power.     

Kin conflict in insect societies: a new epigenetic perspective

The social hymenopterans (ants, wasps and bees) have all the enzymatic and genetic mechanisms necessary for the functional modification of DNA by methylation. Methylation appears to play a central role in shaping the developmental processes that give rise to the different castes. However, could DNA methylation have other roles in social insects? Theoretical arguments predict that male and female hymenopterans can be in conflict over the reproductive potential of their female offspring. An exciting prospect for future research is to examine the possibility that queens and males imprint the genomes of their gametes using DNA methylation to manipulate the reproductive potential of their progeny in ways that favour the inclusive fitness of the parent.     

Perspective: sexual conflict and sexual selection: chasing away paradigm shifts

Abstract.— Traditional models of sexual selection propose that partner choice increases both average male and average female fitness in a population. Recent theoretical and empirical work, however, has stressed that sexual conflict may be a potent broker of sexual selection. When the fitness interests of males and females diverge, a reproductive strategy that increases the fitness of one sex may decrease the fitness of the other sex. The chase-away hypothesis proposes that sexual conflict promotes sexually antagonistic, rather than mutualistic, coevolution, whereby manipulative reproductive strategies in one sex are counteracted by the evolution of resistance to such strategies in the other sex. In this paper, we consider the criteria necessary to demonstrate the chase-away hypothesis. Specifically, we review sexual conflict with particular emphasis on the chase-away hypothesis; discuss the problems associated with testing the predictions of the chase-away hypothesis and the extent to which these predictions and the predictions of traditional models of sexual selection are mutually exclusive; discuss misconceptions and mismeasures of sexual conflict; and suggest an alternative approach to demonstrate sexual conflict, measure the intensity of sexually antagonistic selection in a population, and elucidate the coevolutionary trajectories of the sexes.     

The evolution of female ornaments and weaponry: social selection, sexual selection and ecological competition

Ornaments, weapons and aggressive behaviours may evolve in female animals by mate choice and intrasexual competition for mating opportunities-the standard forms of sexual selection in males. However, a growing body of evidence suggests that selection tends to operate in different ways in males and females, with female traits more often mediating competition for ecological resources, rather than mate acquisition. Two main solutions have been proposed to accommodate this disparity. One is to expand the concept of sexual selection to include all mechanisms related to fecundity; another is to adopt an alternative conceptual framework-the theory of social selection-in which sexual selection is one component of a more general form of selection resulting from all social interactions. In this study, we summarize the history of the debate about female ornaments and weapons, and discuss potential resolutions. We review the components of fitness driving ornamentation in a wide range of systems, and show that selection often falls outside the limits of traditional sexual selection theory, particularly in females. We conclude that the evolution of these traits in both sexes is best understood within the unifying framework of social selection.     

Sensory exploitation and sexual conflict

Much of the literature on male-female coevolution concerns the processes by which male traits and female preferences for these can coevolve and be maintained by selection. There has been less explicit focus on the origin of male traits and female preferences. Here, I argue that it is important to distinguish origin from subsequent coevolution and that insights into the origin can help us appreciate the relative roles of various coevolutionary processes for the evolution of diversity in sexual dimorphism. I delineate four distinct scenarios for the origin of male traits and female preferences that build on past contributions, two of which are based on pre-existing variation in quality indicators among males and two on exploitation of pre-existing sensory biases among females. Recent empirical research, and theoretical models, suggest that origin by sensory exploitation has been widespread. I argue that this points to a key, but perhaps transient, role for sexually antagonistic coevolution (SAC) in the subsequent evolutionary elaboration of sexual traits, because (i) sensory exploitation is often likely to be initially costly for individuals of the exploited sex and (ii) the subsequent evolution of resistance to sensory exploitation should often be associated with costs due to selective constraints. A review of a few case studies is used to illustrate these points. Empirical data directly relevant to the costs of being sensory exploited and the costs of evolving resistance is largely lacking, and I stress that such data would help determining the general importance of sexual conflict and SAC for the evolution of sexual dimorphism.     

Interactive effects of competition and social environment on the expression of sexual dimorphism

The expression of sexual dimorphism is expected to be influenced by the acquisition of resources available to allocate to trait growth, combined with sex‐specific patterns of resource allocation. Resource acquisition in the wild may be mediated by a variety of ecological factors, such as the density of interspecific competitors. Allocation may in turn depend on social contexts, such as sex ratio, that alter the pay‐off for investment in sexual traits. How these factors interact to promote or constrain the expression and evolution of sexual dimorphism is poorly understood. We manipulated sex ratio and interspecific resource competition over the growing season of red‐spotted newts (Notophthalmus viridescens) in artificial ponds. Fish competitors had a stronger effect on female than male growth, which effectively eliminated the expression of sexual size dimorphism. In addition, newt sex ratio influenced fish growth, leading to reduction in fish mass with an increase in female newt frequency. Fish also reduced the expression of male tail height, a sexually selected trait, but only in tanks with a female‐biased sex ratio. This suggests males alter their resource allocation pattern in response to the strength of sexual selection. Our results demonstrate that ecologically and socially mediated interactions between sex‐specific resource acquisition and allocation can contribute to variation in the expression of sexual dimorphism.     

Sexual conflict and sexual selection: lost in the chase

The emergent field of evolutionary biology that studies disparities between the evolutionary interests of alleles expressed in the two sexes, or sexual conflict, promises to offer novel insights into male-female coevolution and speciation. Our theoretical understanding of basic concepts is, however, still incomplete. In a recent perspective paper, Pizzari and Snook provided a framework for understanding sexually antagonistic coevolution and for distinguishing this process from other models of male-female coevolution and suggested an experimental protocol to test for sexually antagonistic coevolution. Here, I show that the framework is flawed, primarily because it is built upon the mistaken assumption that male and female fitness can evolve independently. Further, while the empirical strategy advocated has indeed offered important insights in the past, it does not allow unambiguous discrimination between competing hypotheses.     

Tales of two snails: sexual selection and sexual conflict in Lymnaea stagnalis and Helix aspersa

Sexual selection and sexual conflict have been shown to playkey roles in the evolution of species with separate sexes. Experimentalevidence is accumulating that this is also true for simultaneoushermaphrodites. For example, many species of land snails forcefullystab their mating partners with love darts. In the brown gardensnail (Helix aspersa, now called Cantareus asperses), this dartincreases sperm storage and paternity, probably via the transferof an allohormone that inhibits sperm digestion. A recent interspeciescomparison of dart-possessing land snails revealed coevolutionbetween darts and spermatophore-receiving organs that is consistentwith counteradaptation against an allohormonal manipulation.The great pond snail (Lymnaea stagnalis) seems to use a seminalproduct to manipulate its partner and mates in the male rolewhen enough seminal fluid is available in the prostate gland.Receipt of semen not only initiates egg laying in virgin animals,but also feminizes the mating partner later in life. These increasesin the female function have been shown to go at the expenseof growth and seminal fluid production of the sperm recipient.Although in Helix, and probably also Lymnaea, the sperm donorbenefits from the induced changes through increased fertilizationsuccess, the sperm recipient may experience injury, imposedreallocation of resources, and altered sperm storage. Thesefindings support the existence of sexual conflict in simultaneouslyhermaphroditic snails, and its importance for the evolutionof mating behaviors and reproductive morphologies is discussed.     

Wake up and smell the conflict: odour signals in female competition

Odour signals used in competitive and aggressive interactions between males are well studied in the context of sexual selection. By contrast, relatively little is known about comparable signals used by females, despite current interest in the evolution of female ornaments and weaponry. Available evidence suggests that odour signals are important in competitive interactions between female mammals, with reductions or reversals of male-biased sexual dimorphism in signalling where female competition is intense. Scent marking is often associated with conflict between females over access to resources or reproductive opportunities. Female scent marks may therefore provide reliable signals of competitive ability that could be used both by competitors and potential mates. Consistent with this hypothesis, we report that aggressive behaviour of female house mice is correlated with the amount of major urinary protein (MUP) excreted in their urine, a polymorphic set of proteins that are used in scent mark signalling. Under semi-natural conditions, females with high MUP output are more likely to produce offspring sired by males that have high reproductive success, and less likely to produce offspring by multiple different sires, suggesting that females with strong MUP signals are monopolized by males of particularly high quality. We conclude that odour signals are worthy of more detailed investigation as mediators of female competition.     

Spatial environmental complexity mediates sexual conflict and sexual selection in Drosophila melanogaster

Sexual selection is an important agent of evolutionary change, but the strength and direction of selection often vary over space and time. One potential source of heterogeneity may lie in the opportunity for male–male and/or male–female interactions imposed by the spatial environment. It has been suggested that increased spatial complexity permits sexual selection to act in a complementary fashion with natural selection (hastening the loss of deleterious alleles and/or promoting the spread of beneficial alleles) via two (not mutually exclusive) pathways. In the first scenario, sexual selection potentially acts more strongly on males in complex environments, allowing males of greater genetic “quality” a greater chance of outcompeting rivals, with benefits manifested indirectly in offspring. In the second scenario, increased spatial complexity reduces opportunities for males to antagonistically harm females, allowing females (especially those of greater potential fecundities) to achieve greater reproductive success (direct fitness benefits). Here, using Drosophila melanogaster, we explore the importance of these mechanisms by measuring direct and indirect fitness of females housed in simple vial environments or in vials in which spatial complexity has been increased. We find strong evidence in favor of the female conflict‐mediated pathway as individuals in complex environments remated less frequently and produced more offspring than those housed in a simpler spatial environment, but no difference in the fitness of sons or daughters. We discuss these results in the context of other recent studies and what they mean for our understanding of how sexual selection operates.     

Crossing the taxonomic divide: conflict and its resolution in societies of reproductively totipotent individuals

Reproduction in groups may be unequal, with one or a few individuals monopolizing direct reproduction assisted by nonbreeding helpers. In social insects this has frequently led to a pronounced queen-worker dichotomy and a loss of reproductive totipotency among workers. However, in some invertebrate and all vertebrate societies, all or most individuals remain reproductively totipotent. In these groups, conflicts of interest over reproduction are potentially greatest. Here, we synthesize previous analyses of reproductive conflict, aggression and breeder replacement in haplodiploid societies of totipotent individuals and extend them to cover diploid (vertebrate) examples. We test predictions arising from this approach using the best-studied invertebrate (Dinoponera queenless ants) and vertebrate (naked mole-rat, Heterocephalus glaber) examples, although in principle our analysis applies to all similar groups. We find that premature replacement of a parent breeder by nonbreeders (overthrow) is rare. Dominant coercive control of nonbreeders by the breeder is often unnecessary and honest signalling of breeder vitality can maintain group stability and resolve conflicts over reproduction. We hope that by providing an explicit transfer of social theory between ants and naked mole-rats we will stimulate further cross-taxonomic studies that will greatly broaden our understanding of sociality.     

Adaptations to sexual selection and sexual conflict: insights from experimental evolution and artificial selection

Artificial selection and experimental evolution document natural selection under controlled conditions. Collectively, these techniques are continuing to provide fresh and important insights into the genetic basis of evolutionary change, and are now being employed to investigate mating behaviour. Here, we focus on how selection techniques can reveal the genetic basis of post-mating adaptations to sexual selection and sexual conflict. Alteration of the operational sex ratio of adult Drosophila over just a few tens of generations can lead to altered ejaculate allocation patterns and the evolution of resistance in females to the costly effects of elevated mating rates. We provide new data to show how male responses to the presence of rivals can evolve. For several traits, the way in which males responded to rivals was opposite in lines selected for male-biased, as opposed to female-biased, adult sex ratio. This shows that the manipulation of the relative intensity of intra- and inter-sexual selection can lead to replicable and repeatable effects on mating systems, and reveals the potential for significant contemporary evolutionary change. Such studies, with important safeguards, have potential utility for understanding sexual selection and sexual conflict across many taxa. We discuss how artificial selection studies combined with genomics will continue to deepen our knowledge of the evolutionary principles first laid down by Darwin 150 years ago.