Escalated conflict in a social hierarchy

Animals that live in cooperative societies form hierarchies in which dominant individuals reap disproportionate benefits from group cooperation. The stability of these societies requires subordinates to accept their inferior status rather than engage in escalated conflict with dominants over rank. Applying the logic of animal contests to these cases predicts that escalated conflict is more likely where subordinates are reproductively suppressed, where group productivity is high, relatedness is low, and where subordinates are relatively strong. We tested these four predictions in the field on co-foundress associations of the paper wasp Polistes dominulus by inducing contests over dominance rank experimentally. Subordinates with lower levels of ovarian development, and those in larger, more productive groups, were more likely to escalate in conflict with their dominant, as predicted. Neither genetic relatedness nor relative body size had significant effects on the probability of escalation. The original dominant emerged as the winner in all except one escalated contest. The results provide the first evidence that reproductive suppression of subordinates increases the threat of escalated conflict, and hence that reproductive sharing can promote stability of the dominant-subordinate relationship.     

Worker-queen conflict in annual social hymenoptera

There is a conflict of interest between the queen and her worker-daughters in social hymenoptera over which party shall lay the haploid eggs which become males and over the ratio of investment in male and female reproductives. A model of this conflict in annual colonies is developed in this paper which can be analysed by mathematical optimisation methods. In the absence of worker-laying the queen prefers a 1:1 investment ratio while the workers prefer a 3: 1 female-biased ratio; it is concluded that the queen is in a strong position to impose her preference because she can lay only haploid eggs in the last generation. If the workers can lay and rear haploid eggs in the last generation (but not in the previous generation because of the danger that physical conflict will destroy the queen together with her contribution to future generations), the queen can gain control of male production by forcing the workers to rear her haploid eggs; but in so doing she must relinquish some control over the investment ratio.     

Theory of genomic imprinting conflict in social insects

Background  

Genomic imprinting refers to the differential expression of genes inherited from the mother and father (matrigenes and patrigenes). The kinship theory of genomic imprinting treats parent-specific gene expression as products of within-genome conflict. Specifically, matrigenes and patrigenes will be in conflict over treatment of relatives to which they are differently related. Haplodiploid females have many such relatives, and social insects have many contexts in which they affect relatives, so haplodiploid social insects are prime candidates for tests of the kinship theory of imprinting.     

Colony size,social complexity and reproductive conflict in social insects

The broad limits of mature colony size in social insect species are likely to be set by ecological factors. However, any change in colony size has a number of important social consequences. The most fundamental is a change in the expected reproductive potential of workers. If colony size rises, workers experience a fall in their chances of becoming replacement reproductives and, it is shown, increasing selection for mutual inhibition of one another's reproduction (worker policing). As workers’ reproductive potential falls, the degree of dimorphism between reproductive and worker castes (morphological skew) can rise. This helps explain why small societies have low morphological skew and tend to be simple in organization, whereas large societies have high morphological skew and tend to be complex. The social consequences of change in colony size may also alter colony size itself in a process of positive feedback. For these reasons, small societies should be characterized by intense, direct conflict over reproduction and caste determination. By contrast, conflict in large societies should predominantly be over brood composition, and members of these societies should be relatively compliant to manipulation of their caste. Colony size therefore deserves fuller recognition as a key determinant, along with kin structure, of social complexity, the reproductive potential of helpers, the degree of caste differentiation, and the nature of within-group conflict.     

The role of conformity in foraging when personal and social information conflict

There is currently considerable interest in the interplay betweenpersonal and social information in decision-making processes.Two experiments are presented exploring the relative use ofprior personal information and subsequent social informationin foraging decisions of guppies. Experiment 1 tested the assumptionthat when the use of information acquired through personal experienceis not costly, conflicting social information will be ignored.The assumption was confirmed because, when given a choice betweenfeeding at two food patches, at one of which they had previouslyseen conspecifics feed, individual fish with prior experienceof feeding at the alternative site chose the alternative, whereasfish with no prior experience chose the site at which theirconspecifics had fed. Experiment 2 tested theoretical predictionsthat when the use of information acquired through personal experienceis potentially costly, conflicting social information will beweighed more heavily than will personal information. The predictionwas confirmed because, when given a choice between feeding attwo food patches, one at which they had previously seen conspecificsfeed and one behind a visual barrier, individual fish with priorexperience of feeding behind the barrier chose the site at whichtheir conspecifics had fed. These findings suggest that conformitycan promote social learning in naïve individuals, but priorexperience can insulate individuals from conformity providedthe costs of relying on that experience are small. In addition,the experiments highlight the fact that personal and socialinformation are not always weighed equally.     

Caste totipotency and conflict in a large-colony social insect

In most social insects with large, complex colonies workers and queens are morphologically quite distinct. This means that caste determination must occur prior to adulthood. However workers and queens in the swarm-founding epiponine wasps are often morphologically indistinguishable, or nearly so, suggesting that caste determination in these wasps could be quite different. To determine the extent of caste lability in the epiponine, Parachartergus colobopterus, we removed all the queens from one colony and all but one from another colony. Worker aggression diminished after queen removal. A week later the colony with no queens had a new, young cohort of mated queens. These must have been either adults or pupae at the time of queen removal, and so could not have been fed any differently from workers. Relatedness patterns confirmed that these new queens would normally have been workers and not queens. A model of inclusive fitness interests shows that workers ought to suppress new queen production, except at low queen numbers, a prediction supported by our empirical results. The patterns of social conflict over queenship resulting from swarm founding in a many-queen society may help to explain the unusually weak caste differentiation in the epiponines.     

Resolving social conflict among females without overt aggression

Members of animal societies compete over resources and reproduction, but the extent to which such conflicts of interest are resolved peacefully (without recourse to costly or wasteful acts of aggression) varies widely. Here, we describe two theoretical mechanisms that can help to understand variation in the incidence of overt behavioural conflict: (i) destruction competition and (ii) the use of threats. The two mechanisms make different assumptions about the degree to which competitors are socially sensitive (responsive to real-time changes in the behaviour of their social partners). In each case, we discuss how the model assumptions relate to biological reality and highlight the genetic, ecological and informational factors that are likely to promote peaceful conflict resolution, drawing on empirical examples. We suggest that, relative to males, reproductive conflict among females may be more frequently resolved peacefully through threats of punishment, rather than overt acts of punishment, because (i) offspring are more costly to produce for females and (ii) reproduction is more difficult to conceal. The main need now is for empirical work to test whether the mechanisms described here can indeed explain how social conflict can be resolved without overt aggression.     

Suppression of social conflict and evolutionary transitions to cooperation

Evolutionary conflict arises at all levels of biological organization and presents a barrier to the evolution of cooperation. This barrier can be overcome by mechanisms that reduce the disparity between the fitness optima of subunits, sometimes called the "battleground" of conflict. An alternative, unstudied possibility is that effort invested in conflict is unprofitable. This possibility has received little attention because most existing models of social conflict assume that fitness depends on the ratio of players' conflict efforts, so that "peaceful" outcomes featuring zero conflict effort are evolutionarily unstable. Here I show that peaceful outcomes are stable where success depends on the difference rather than the ratio of efforts invested in conflict. These difference form models are particularly appropriate to model strategies of suppression or policing. The model suggests that incomplete information and asymmetries in strength can act to eliminate costly conflict within groups, even among unrelated individuals, and thereby facilitate the evolution of cooperation.     

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.     

The evolution of social communication systems in a subdivided population

Evolution of social communication systems is modeled with a quantitative genetic model. The mathematical model describes the coevolutionary process of a social signal (a social character) and responsiveness (a social preference) to the signal. The responsiveness is postulated to influence fitness of senders of the signal. Considerations are extended to subdivided population structure by combining the social selection model with a group selection model. The numerical results derived from the models indicate that the evolutionary rate of social communication systems depends largely on genetic correlation between the signal and the responsiveness. Group selection can reinforce the evolutionary rate and relax its dependence on genetic correlation. The origin of genetic correlation is discussed in relation to group selection.     

Reification of social systems

In his 1991 book, Intraspecific Variation in the Social Systems of Wild Vertebrates, Dale Lott¹ reviews the evidence that a single social system does not characterize a species. Rather, he says, the longer a species is studied and the wider the range of habitats in which it is studied, the more variation is discovered. I strongly recommend this book as a concise review of recent studies on nonprimate vertebrates for anyone needing a comparative perspective on a question that deeply concerns students of primates. The clarity of Lott's exposition led me to ask several questions. If differences in the behavior of populations can be described in terms of differences in the behavior of individuals, what is gained by speaking in terms of a social system? What is a social system anyway? Is it possible that the social system is a concept that might actually get in the way of understanding social behavior?     

Family feuds: social competition and sexual conflict in complex societies

Darwin was initially puzzled by the processes that led to ornamentation in males-what he termed sexual selection-and those that led to extreme cooperation and altruism in complex animal societies-what was later termed kin selection. Here, I explore the relationships between sexual and kin selection theory by examining how social competition for reproductive opportunities-particularly in females-and sexual conflict over mating partners are inherent and critical parts of complex altruistic societies. I argue that (i) patterns of reproductive sharing within complex societies can drive levels of social competition and reproductive conflict not only in males but also in females living in social groups, and ultimately the evolution of female traits such as ornaments and armaments; (ii) mating conflict over female choice of sexual partners can influence kin structure within groups and drive the evolution of complex societies; and (iii) patterns of reproductive sharing and conflict among females may also drive the evolution of complex societies by influencing kin structure within groups. Ultimately, complex societies exhibiting altruistic behaviour appear to have only arisen in taxa where social competition over reproductive opportunities and sexual conflict over mating partners were low. Once such societies evolved, there were important selective feedbacks on traits used to regulate and mediate intra-sexual competition over reproductive opportunities, particularly in females.     

Reproductive conflict delays the recovery of an endangered social species

1. Evolutionary theory predicts that individuals, in order to increase their relative fitness, can evolve behaviours that are detrimental for the group or population. This mismatch is particularly visible in social organisms. Despite its potential to affect the population dynamics of social animals, this principle has not yet been applied to real-life conservation. 2. Social group structure has been argued to stabilize population dynamics due to the buffering effects of nonreproducing subordinates. However, competition for breeding positions in such species can also interfere with the reproduction of breeding pairs. 3. Seychelles magpie robins, Copsychus sechellarum, live in social groups where subordinate individuals do not breed. Analysis of long-term individual-based data and short-term behavioural observations show that subordinates increase the territorial takeover frequency of established breeders. Such takeovers delay offspring production and decrease territory productivity. 4. Individual-based simulations of the Seychelles magpie robin population parameterized with the long-term data show that this process has significantly postponed the recovery of the species from the Critically Endangered status. 5. Social conflict thus can extend the period of high extinction risk, which we show to have population consequences that should be taken into account in management programmes. This is the first quantitative assessment of the effects of social conflict on conservation.     

Lack of conflict during queen production in the social wasp Vespula maculifrons

Social insects display extreme cooperative and helping behaviours. However, social insect colonies are also arenas of intense competitive interactions. One particularly important matter over which colony members may compete centres on the development of sexual offspring. Specifically, colony members may engage in selfish behaviours leading to reproductive competition, whereby individuals either strive to develop as sexuals or assist kin so that close relatives emerge as new reproductives. We investigated whether reproductive competition occurred in the polyandrous social wasp Vespula maculifrons. We genotyped V. maculifrons workers and new queens at eight polymorphic microsatellite loci to determine if larvae of particular genotypes were reared as gynes more frequently than expected by chance. However, we found no significant evidence of reproductive competition in this species. The proportional contributions of males to workers and new queens did not vary within colonies. Moreover, male reproductive skew did not differ between workers and new queens. Finally, novel statistical techniques uncovered no evidence of patriline reversal, the phenomenon whereby males that contribute little to worker production contribute substantially to new queen production. Consequently, we conclude that individual level selection operating to increase the frequency of selfish behaviours that would lead to reproductive competition has been nullified by colony-level selection acting to maintain colony efficiency and cooperation.     

Kin selection and the evolution of social information use in animal conflict

Animals often use social information about conspecifics in making decisions about cooperation and conflict. While the importance of kin selection in the evolution of intraspecific cooperation and conflict is widely acknowledged, few studies have examined how relatedness influences the evolution of social information use. Here we specifically examine how relatedness affects the evolution of a stylised form of social information use known as eavesdropping. Eavesdropping involves individuals escalating conflicts with rivals observed to have lost their last encounter and avoiding fights with those seen to have won. We use a game theoretical model to examine how relatedness affects the evolution of eavesdropping, both when strategies are discrete and when they are continuous or mixed. We show that relatedness influences the evolution of eavesdropping, such that information use peaks at intermediate relatedness. Our study highlights the importance of considering kin selection when exploring the evolution of complex forms of information use.     

Caste fate conflict in swarm-founding social hymenoptera: an inclusive fitness analysis

A caste system in which females develop into morphologically distinct queens or workers has evolved independently in ants, wasps and bees. Although such reproductive division of labour may benefit the colony it is also a source of conflict because individual immature females can benefit from developing into a queen in order to gain greater direct reproduction. Here we present a formal inclusive fitness analysis of caste fate conflict appropriate for swarm-founding social Hymenoptera. Three major conclusions are reached: (1) when caste is self-determined, many females should selfishly choose to become queens and the resulting depletion of the workforce can substantially reduce colony productivity; (2) greater relatedness among colony members reduces this excess queen production; (3) if workers can prevent excess queen production at low cost by controlled feeding, a transition to nutritional caste determination should occur. These predictions generalize results derived earlier using an allele-frequency model [Behav. Ecol. Sociobiol. (2001) 50: 467] and are supported by observed levels of queen production in various taxa, especially stingless bees, where caste can be either individually or nutritionally controlled.     

Sex differences in hormonal responses to social conflict in the monogamous California mouse

Monogamous species are usually considered to be less likely to exhibit sex differences in behavior or brain structure. Most previous studies examining sex differences in stress hormone responses have used relatively sexually dimorphic species such as rats. We examined the stress hormone responses of monogamous California mice (Peromyscus californicus) to resident-intruder tests. We also tested males and females under different photoperiods, because photoperiod has been shown to affect both aggression and stress hormone responses. Females, but not males showed a significant increase in corticosterone levels immediately following a resident-intruder test. Males but not females showed elevated corticosterone levels under short days. Females tested in aggression tests also showed a significant increase in plasma oxytocin levels, but only when housed in long days. This was consistent with our observation that females but not males had more oxytocin positive cells in the paraventricular nucleus (PVN) when housed under long days. Our data show that sex differences in glucocorticoid responses identified in other rodents are present in a monogamous species.     

The evolution of unusual chromosomal systems in sciarid flies: intragenomic conflict and the sex ratio

A model is presented for the evolution of the sciarid chromosomal system. In this model, a driving X chromosome caused female-biased sex ratios. The drive was exploited by maternal autosomes that segregated with the X at spermatogenesis. Genes in mothers converted some of their XX daughters into sons by eliminating a paternal X from the embryonic soma. L chromosomes were derived from X chromosomes and favored male-biased sex ratios. An X' chromosome arose that suppressed the effects of L chromosomes. The 1:1 sex ratio is a stalemate between the X' and X'X mothers causing all-female broods and the L chromosomes in XX mothers causing all-male broods. Any element (such as an L chromosome) that is preferentially transmitted through one sex will be selected to bias the sex ratio towards this sex.