Genetic support for the evolutionary theory of reproductive transactions in social wasps

Recent evolutionary models of reproductive partitioning within animal societies (known as 'optimal skew', 'concessions' or 'transactional' models) predict that a dominant individual will often yield some fraction of the group's reproduction to a subordinate as an incentive to stay in the group and help rear the dominant's offspring. These models quantitatively predict how the magnitude of the subordinate's 'staying incentive' will vary with the genetic relatedness between dominant and subordinate, the overall expected group output and the subordinate's expected output if it breeds solitarily. We report that these predictions accord remarkably well with the observed reproductive partitioning between conesting dominant and subordinate queens in the social paper wasp Polistes fuscatus. In particular, the theory correctly predicts that (i) the dominant's share of reproduction, i.e. the skew, increases as the colony cycle progresses and (ii) the skew is positively associated both with the colony's productivity and with the relatedness between dominant and subordinate. Moreover, aggression between foundresses positively correlated with the skew, as predicted by transactional but not alternative tug-of-war models of societal evolution. Thus, our results provide the strongest (quantitative support yet for a unifying model of social evolution.     

Female control of the distribution of paternity in cooperative breeders

Models of reproductive skew have shed light on why animal societies vary in the partitioning of reproduction among group members. However, their application to cooperative vertebrate societies remains controversial. A particular problem is that previous models assume that skew in paternity is determined by interactions among males and males only. This conflicts with observations from many species that indicate that females exert control over the distribution of paternity. Here we address this shortfall in the current theory by developing two models to explore the expected patterns of skew in three member groups in which a female controls the allocation of paternity among two males. The first "staying incentive" model extends previous "transactional" (or "concession") models to examine the conditions where females will be willing to share reproduction among a dominant and a subordinate male to retain the subordinate in the group. The second "work incentive" model explores patterns of skew where females allocate paternity in order to maximize the amount of care their offspring receive. The models make contrasting predictions about the nature of male-female conflict over reproduction and also about the relationships between skew and relatedness, ecological constraints, the relative quality of the subordinate male, and the relative cost of care for the two males. These divergent predictions provide a schema by which the evolutionary causes of variation in skew among males can be evaluated.     

Reproductive skew in birds: models, problems and prospects

In recent years there has been a resurgence of interest in models to explain the partitioning of direct reproduction ('reproductive skew') among members of one sex within social groups. We review models of skew, identify problems of testing models, and consider how to make progress. One series of models assumes that dominants have complete control of subordinate reproduction, but may allow subordinates some reproduction as a way of enticing them to help or getting them to share the cost of reproduction. Another series of models assume that dominants have limited control of subordinate reproduction. Reproductive skew may also be affected by incest avoidance or control by the opposite sex. Models are largely untested because no study of birds has quantified all relevant parameters, and we see no prospect of this happening soon. A common simplifying approach is to test qualitative predictions about the effect on skew of relatedness among group members. However, these data alone cannot distinguish among models because models do not make unique predictions, partly because skew is also affected by other factors. A major problem in cooperatively-breeding birds is that any effect of relatedness will often be confounded by covariation with relatedness asymmetry and subordinate competitiveness. Progress can be made with the development of theory, controlling confounding variables through the choice of study species or types of social group, and, most importantly, testing assumptions underlying hypotheses.     

Patterns of reproductive skew in the polygynandrous acorn woodpecker

We compared observed levels of reproductive skew in the cooperatively breeding acorn woodpecker (Melanerpes formicivorus) with those predicted by two alternative transactional models. "Concession" models predict the degree to which parentage is shared assuming that a single dominant is in complete control of reproduction. Alternatively, "restraint" models predict reproductive sharing assuming that the dominant controls only whether subordinates remain in the group but does not control its share of reproduction. Reproductive skew is high among males: on average, the most successful male sires more than three times as many offspring as the next most successful male. Females share parentage equally and have lower constraints on dispersal and lower survival rates compared with males, which is consistent with predictions from the concessions model. Also as predicted by the concessions model, yearly variation in opportunities for dispersal before the breeding season correlates positively with skew. However, in contrast to concessions but consistent with the restraint model, skew decreases with relatedness. Thus, neither model consistently predicts patterns of reproductive skew in this species. We suggest that models of reproductive skew will need to include competitive interactions among potential breeders and mate choice before they will adequately predict patterns of reproductive partitioning in most vertebrate societies.     

Costly young and reproductive skew in animal societies

Many recent models of reproductive skew explain subordinatereproduction as a staying incentive offered by dominants, whocan produce more young with a helper present than without. Here,we present a new, alternative explanation for subordinate reproduction,which applies whenever the fitness cost to a parent of producingyoung is an accelerating function of the number produced (ascommonly assumed in optimal clutch size theory). Under these circumstances,a dominant individual may be selected to offer a share of reproductionto a related subordinate, not as an incentive to stay, but becauseadditional offspring that would be expensive for the dominantto produce are cheap for the subordinate. "Beneficial sharing"of this kind is more likely the more closely related the subordinateis to the dominant, so that the model predicts a negative relationshipbetween skew and relatedness. This result runs directly counterto the positive relationship predicted by previous incentive-basedmodels. We explore the interaction of these contrasting effectsby developing an integrated model that allows for both beneficialsharing and staying incentives. When offspring are cheap to produce,this integrated model predicts that the incentive effect will dominate,and skew will increase with relatedness. When young are costly,in contrast, beneficial sharing will be of greater importance,and skew will decrease with relatedness.     

A Transactional Theory of Within-Group Conflict

Transactional models of social evolution emphasize that dominant members of the society can be favored to donate parcels of reproduction to subordinate members in return for cooperation. I construct a formal theory of intragroup conflict within the framework of transactional models by determining the maximum extent to which colony members can be selfish without destabilizing the group. The difference between the maximum value of the subordinate's fraction of group reproduction that the dominant can tolerate before ejecting the subordinate and the minimum value required by the subordinate to stay and cooperate peacefully in the group defines the "window of selfishness," which in turn predicts the frequency of within-group conflict. The window of selfishness tends to increase with increasing group reproductive output, increasingly harsh ecological constraints on solitary breeding, and, counterintuitively, increasing relatedness between subordinate and dominant. Increasing fighting ability of the subordinate can either widen or narrow the window of selfishness, the latter being most likely when ecological constraints on group living are strong. Although increasing relatedness is predicted to increase the rate of within-group aggression, the mean intensity of an aggressive act should decline, as predicted by the general theory of honest signaling between relatives and the tug-of-war models of within-group selfishness. In the bidding game, in which multiple dominants bid for the services of a subordinate, the window of selfishness is predicted to have zero width. A zero-width window of selfishness and low conflict also are predicted for saturated N-person groups, that is, groups whose total output is a concave function of group size and in which the dominant is not favored to admit additional subordinates. The model's predictions are compared to empirical evidence and to predictions of alternative models of intragroup aggression, including the value-aggression model and the pure tug-of-war model.     

Reproductive skew and the threat of eviction: a new perspective

Most recent models of the partitioning of reproduction attempt to explain patterns of skew on the assumption that dominant individuals have complete control over breeding opportunities within the group, but may nevertheless concede a share of direct reproduction to subordinates as an incentive to remain peacefully in the association. Although these models may be applicable to some animal societies, we argue that they fail to provide a comprehensive theory of skew. Instead, we suggest that subordinates may often be able to claim unsanctioned reproduction for themselves, but will be forced to exercise a degree of reproductive restraint lest they incite ejection by the dominant. Reproductive skew, in other words, may reflect the threat of ejection (inducing subordinate restraint) rather than the threat of subordinate departure (inducing reproductive concessions by dominants). We present a simple ESS model of reproductive skew under these circumstances, which demonstrates that a shift in emphasis from reproductive concessions by dominants to reproductive restraint on the part of subordinates, radically alters the predictions of skew models. High group productivity, high relatedness and (when group members are related) strong ecological constraints are all expected to lead to reduced skew (the opposite conclusions to those of previous, concession-based analyses). The reason is that these factors reduce the benefits (or increase the costs) of ejection to the dominant, who therefore does best to tolerate more subordinate reproduction.     

Clutch-size adjustments and skew models: effects on reproductive partitioning and group stability

Reproductive skew theory seeks to integrate social and ecologicalfactors thought to influence the division of reproduction amonggroup-living animals. However, most reproductive skew modelsonly examine interactions between individuals of the same sex.Here, we suggest that females can influence group stabilityand conflict among males by modifying their clutch size andmay do so if they benefit from the presence of subordinate malehelpers or from reduced conflict. We develop 3 models, basedon concessions-based, restraint, and tug-of-war models, in whichfemale clutch size is variable and ask when females will increasetheir clutch size above that which would be optimal in the absenceof male–male conflict. In concessions-based and restraintmodels, females should increase clutch size above their optimaif the benefits of staying for subordinate males are relativelylow. Relatedness between males has no effect on clutch size.When females do increase clutch size, the division of reproductionbetween males is not influenced by relatedness and does notdiffer between restraint and concessions-based models. Bothof these predictions are in sharp contrast to previous models.In tug-of-war models, clutch size is strongly influenced byrelatedness between males, with the largest clutches, but thefewest surviving offspring, produced when males are unrelated.These 3 models demonstrate the importance of considering third-partyinterests in the decisions of group-living organisms.     

Power Struggles, Dominance Testing, and Reproductive Skew

Models of reproductive skew are concerned with the partitioning of reproduction between dominant and subordinate members of a group. In an interesting extension of these models, Reeve and Ratnieks briefly considered whether it might benefit subordinates to engage in aggressive behavior to test the fighting ability of a dominant. Their analysis suggested that such testing should be more probable in groups that feature high skew and, hence, perhaps among closer relatives (because high relatedness favors high skew). Here we explore in more detail the possibility of dominance testing. Three models that differ in the outcome of fights over dominance are presented: in the first model, the loser of the challenge is killed; in the second model, the loser is evicted from the nest; and, in the third model, the loser becomes (or remains) subordinate. In each case we consider the independent effects of the parameters that determine skew (namely, relatedness, group productivity, and ecological constraints) on the predicted level of dominance testing. We then construct an amalgamated model to examine situations where fights may lead to any one of the three outcomes. Our analysis reveals that, in the majority of cases, higher relatedness will in fact lead to lower levels of aggression. Moreover, dominance testing need not be associated with high skew. Rather, the relationship between skew and dominance testing will depend on which factor (relatedness, group productivity, or level of ecological constraints) is principally responsible for variation in the distribution of reproduction.     

Tug-of-war has no borders: it is the missing model in reproductive skew theory

Cooperative breeding often results in unequal reproduction between dominant and subordinate group members. Transactional skew models attempt to predict how unequal reproduction can be before the groups themselves become unstable. A number of variants of transactional models have been developed, with a key difference being whether reproduction is controlled by one party or contested by all. It is shown here that ESS solutions for all situations of contested control over reproduction are given by the original tug-of-war model (TOW). Several interesting results follow. First, TOW can escalate enough to destabilize some types of groups. Particularly vulnerable are those that have low relatedness and gain little from cooperative breeding relative to solitary reproduction. Second, TOW can drastically reduce group productivity and especially the inclusive fitness of dominant individuals. Third, these results contrast strongly with those from variants of TOW models that include concessions to maintain group stability. Such models are shown to be special cases of the general and simpler TOW framework, and to have assumptions that may be biologically suspect. Finally, the overall analysis suggests that there is no mechanism within existing TOW framework that will prevent a costly struggle for reproductive control. Because social species rarely exhibit the high levels of aggression predicted by TOW models, alternative evolutionary mechanisms are considered that can limit conflict and produce more mutually beneficial outcomes. The further development of alternative models to predict patterns of reproductive skew are highly recommended.     

From individual control to majority rule: extending transactional models of reproductive skew in animal societies

Transactional concession models of social evolution explain the reproductive skew within groups by assuming that a dominant individual completely controls the allocation of reproduction to other group members. The models predict when the dominant will benefit from donating parcels of reproduction to other members in return for peaceful cooperation. Using linear programming methods, we present a 'majority-rules' model in which the summed actions of all society members, each with equal power, completely determine the reproductive share of any single member. The majority-rules model predicts that, despite the diffusion of power, a 'virtual dominant' (a dominant lacking special behavioural power) will emerge and that the reproductive skew will be exactly that predicted if the virtual dominant were to control completely the group's reproductive partitioning. The virtual dominant is the individual to which group members have the maximum average genetic relatedness. This result greatly broadens the applicability of transactional models of reproductive skew to social groups of any size, such as large-colony eusocial insects, and explains why queens in such colonies can achieve reproductive domination without any behavioural enforcement. Moreover, the majority-rules model unifies transactional-skew theory with models of worker policing and even generates a new theory for the cooperation among somatic cells in a multicellular organism.     

Reproductive skew, costs, and benefits of cooperative breeding in female wood mice (Apodemus sylvaticus)

Two current models seek to explain reproduction of subordinatesin social groups: incentives given by dominants for peacefullyremaining in the group (reproductive skew model) or incompletecontrol by dominants. These models make different predictionsconcerning genetic relatedness between individuals for thedistribution of reproduction and the stability of cooperativebreeding associations. To test these models and to furtherexplore the relationships between reproductive skew, geneticrelatedness, and investment of each participant, we performedbehavioral observations of female wood mice (Apodemus sylvaticus)raising pups communally. Our results do not support previousmodels. Differences in lifetime reproductive success were significantlygreater within mother—daughter pairs than within pairsof sisters or unrelated females. Subordinate females of eitherbreeding unit did not differ in their direct reproduction.Calculations of inclusive fitness based on our results leadto the following predictions: (1) Communal nests should occuronly when ecological circumstances prevent solitary breeding.(2) Subordinate females gain the highest inclusive fitnessjoining their mothers; they also show the highest nursing investment.(3) Mothers should accept daughters, who have no opportunityfor solitary breeding. (4) Dominant sisters and unrelated femalesshould reject subordinate females because cooperative breedingreduces their reproductive success. However, breeding unitsof dominant sisters and unrelated females nevertheless occurand can be explained by our finding that such females significantlyreduce nursing time, which may help them save energy for futurebreeding cycles. Our data demonstrate that both genetic relatednessand investment skew are important in the complex evolutionof reproductive skew in cooperative breeding.     

Reproductive skew and the origin of sterile castes

Reproductive skew theory has not heretofore formally addressed one of the most important questions in evolutionary biology: How can whole-life sterile castes evolve? We construct a transactional skew model investigating under what conditions a subordinate in a multimember group is favored to develop into a morphologically specialized worker caste. Our model demonstrates that, contrary to former expectations, the ecological and genetic conditions favoring caste differentiation are far more restrictive than those favoring high skew. Caste differentiation cannot be selected in saturated, symmetrical relatedness groups unless the genetic relatedness among group members is extremely high. In contrast, it can be selected in the saturated, asymmetrical relatedness (parent-offspring) groups with complete skew. If we also consider the future reproduction of subordinates, caste differentiation is possible only after the group size reaches a certain critical point. Most importantly, caste differentiation in a parent-offspring group increases its saturated group size. The positive feedback between group size and the degree of caste differentiation can continue in principle until completely sterile worker castes emerge. Thus, at least in the case of parent-offspring groups, group size but not the degree of reproductive skew may be a better index of the level of social complexity. A scheme for the evolution of sterile worker castes that integrates the role of group size into the framework of reproductive skew theory is proposed.     

Reproduction in foundress associations of the social wasp, Polistes carolina: conventions, competition, and skew

Who reproduces in colonies of social insects is determined bysome combination of direct competition and more peaceful convention.We studied these two alternatives in foundresses of the paperwasp, Polistes carolina, by examining two different contexts:what determines who becomes the dominant reproductive and whatdetermines the amount of reproduction obtained by subordinates.The dominant queen on most nests was the foundress to arrivefirst, rather than the largest foundress, expected to be bestat fighting. This suggests that dominance is initially determinedby convention, although the persistence of some aggressiveconflict throughout the foundress period suggests that thisconvention is not absolute. Attempts to explain the divisionof reproduction using several skew theories were generallyunsuccessful. Skew was not correlated with relatedness, size differences, colony productivity, and challenges by the subordinate.P. carolina showed high constraints against solitary nesting,with a minority of females attempting to nest alone, and nonesucceeding. In this situation, most skew theories predict thatgroup stability will be independent of relatedness, yet nearlyall collected subordinates were full sisters to the queen.Reproductive partitioning in early P. carolina colonies may have more to do with enhancing worker production than with conflictover direct fitness.     

Transactional skew and assured fitness return models fail to predict patterns of cooperation in wasps

Cooperative breeders often exhibit reproductive skew, where dominant individuals reproduce more than subordinates. Two approaches derived from Hamilton's inclusive fitness model predict when subordinate behavior is favored over living solitarily. The assured fitness return (AFR) model predicts that subordinates help when they are highly likely to gain immediate indirect fitness. Transactional skew models predict dominants and subordinates "agree" on a level of reproductive skew that induces subordinates to join groups. We show the AFR model to be a special case of transactional skew models that assumes no direct reproduction by subordinates. We use data from 11 populations of four wasp species (Polistes, Liostenogaster) as a test of whether transactional frameworks suffice to predict when subordinate behavior should be observed in general and the specific level of skew observed in cooperative groups. The general prediction is supported; in 10 of 11 cases, transactional models correctly predict presence or absence of cooperation. In contrast, the specific prediction is not consistent with the data. Where cooperation occurs, the model accurately predicts highly biased reproductive skew between full sisters. However, the model also predicts that distantly related or unrelated females should cooperate with low skew. This prediction fails: cooperation with high skew is the observed norm. Neither the generalized transactional model nor the special-case AFR model can explain this significant feature of wasp sociobiology. Alternative, nontransactional hypotheses such as parental manipulation and kin recognition errors are discussed.     

The past,present and future of reproductive skew theory and experiments

A major evolutionary question is how reproductive sharing arises in cooperatively breeding species despite the inherent reproductive conflicts in social groups. Reproductive skew theory offers one potential solution: each group member gains or is allotted inclusive fitness equal to or exceeding their expectation from reproducing on their own. Unfortunately, a multitude of skew models with conflicting predictions has led to confusion in both testing and evaluating skew theory. The confusion arises partly because one set of models (the ‘transactional’ type) answer the ultimate evolutionary question of what ranges of reproductive skew can yield fitness‐enhancing solutions for all group members. The second set of models (‘compromise’) give an evolutionarily proximate, game‐theoretic evolutionarily stable state (ESS) solution that determines reproductive shares based on relative competitive abilities. However, several predictions arising from compromise models require a linear payoff to increased competition and do not hold with non‐linear payoffs. Given that for most species it may be very difficult or impossible to determine the true relationship between effort devoted to competition and reproductive share gained, compromise models are much less predictive than previously appreciated. Almost all skew models make one quantitative prediction (e.g. realized skew must fall within ranges predicted by transactional models), and two qualitative predictions (e.g. variation in relatedness or competitive ability across groups affects skew). A thorough review of the data finds that these three predictions are relatively rarely supported. As a general rule, therefore, the evolution of cooperative breeding appears not to be dependent on the ability of group members to monitor relatedness or competitive ability in order to adjust their behaviour dynamically to gain reproductive share. Although reproductive skew theory fails to predict within‐group dynamics consistently, it does better at predicting quantitative differences in skew across populations or species. This suggests that kin selection can play a significant role in the evolution of sociality. To advance our understanding of reproductive skew will require focusing on a broader array of factors, such as the frequency of mistaken identity, delayed fitness payoffs, and selection pressures arising from across‐group competition. We furthermore suggest a novel approach to investigate the sharing of reproduction that focuses on the underlying genetics of skew. A quantitative genetics approach allows the partitioning of variance in reproductive share itself or that of traits closely associated with skew into genetic and non‐genetic sources. Thus, we can determine the heritability of reproductive share and infer whether it actually is the focus of natural selection. We view the ‘animal model’ as the most promising empirical method where the genetics of reproductive share can be directly analyzed in wild populations. In the quest to assess whether skew theory can provide a framework for understanding the evolution of sociality, quantitative genetics will be a central tool in future research.     

Within‐group relatedness is correlated with colony‐level social structure and reproductive sharing in a social fish

In group‐living species, the degree of relatedness among group members often governs the extent of reproductive sharing, cooperation and conflict within a group. Kinship among group members can be shaped by the presence and location of neighbouring groups, as these provide dispersal or mating opportunities that can dilute kinship among current group members. Here, we assessed how within‐group relatedness varies with the density and position of neighbouring social groups in Neolamprologus pulcher, a colonial and group‐living cichlid fish. We used restriction site‐associated DNA sequencing (RADseq) methods to generate thousands of polymorphic SNPs. Relative to microsatellite data, RADseq data provided much tighter confidence intervals around our relatedness estimates. These data allowed us to document novel patterns of relatedness in relation to colony‐level social structure. First, the density of neighbouring groups was negatively correlated with relatedness between subordinates and dominant females within a group, but no such patterns were observed between subordinates and dominant males. Second, subordinates at the colony edge were less related to dominant males in their group than subordinates in the colony centre, suggesting a shorter breeding tenure for dominant males at the colony edge. Finally, subordinates who were closely related to their same‐sex dominant were more likely to reproduce, supporting some restraint models of reproductive skew. Collectively, these results demonstrate that within‐group relatedness is influenced by the broader social context, and variation between groups in the degree of relatedness between dominants and subordinates can be explained by both patterns of reproductive sharing and the nature of the social landscape.     

Reproductive suppression in female Alpine marmots, Marmota marmota

We studied mechanisms responsible for reproductive skew in female Alpine marmots. In each social group, only dominant females produced weaned young although subordinate females had similar body condition, were fertile as indicated by high levels of oestradiol during the mating season, and occasionally started pregnancies. During the period of gestation, dominant females initiated significantly more agonistic interactions against subordinate females, resulting in significantly increased levels of glucocorticoids and decreased levels of progesterone in subordinates. Results suggested that reproductive suppression in female Alpine marmots is mediated by the negative effects of stress (glucocorticoids) on the activity of the hypothalamic-pituitary-gonadal axis. The strength of competition between subordinate and dominant females was affected by their relatedness. Dominant females attacked unrelated subordinate females more, whereas amicable behaviour was observed mainly between dominant females and their daughters. These differences could be explained by differences in indirect fitness: related subordinate females benefited from warming the offspring of the dominant female during hibernation. Reproductive suppression was apparently costly for dominant females, because their reproductive success decreased as the number of adult subordinate females in a group increased. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

High rates of pregnancy loss by subordinates leads to high reproductive skew in wild golden lion tamarins (Leontopithecus rosalia)

Across taxa, cooperative breeding has been associated with high reproductive skew. Cooperatively breeding golden lion tamarins (Leontopithecus rosalia) were long thought to have a monogynous mating system in which reproduction was limited to a single dominant female. Subordinates with few reproductive opportunities delayed dispersal and remained in the natal group to provide alloparental care to siblings, thus allowing dominant reproductive females to meet the energetic needs associated with high rates of reproduction and successful infant rearing. The goal of this study was to re-assess monogyny in wild golden lion tamarin groups based upon pregnancy diagnoses that used non-invasive enzyme immunoassay for progesterone and cortisol, combined with weekly data on individual weight gain, bi-annual physical examinations noting pregnancy and lactation status and daily behavioral observations. We established quantitative and qualitative criteria to detect and determine the timing of pregnancies that did not result in the birth of infants. Pregnancy polygyny occurred in 83% of golden lion tamarin groups studied. The loss of 64% of subordinate pregnancies compared to only 15% by dominant females limited reproductive success mainly to dominant females, thus maintaining high reproductive skew in female golden lion tamarins. Pregnancy loss by subordinate adults did not appear to result from dominant interference in subordinate hormonal mechanisms, but more likely resulted from subordinate abandonment of newborn infants to mitigate dominant aggression.     

Using social parasitism to test reproductive skew models in a primitively eusocial wasp

Remarkable variation exists in the distribution of reproduction (skew) among members of cooperatively breeding groups, both within and between species. Reproductive skew theory has provided an important framework for understanding this variation. In the primitively eusocial Hymenoptera, two models have been routinely tested: concessions models, which assume complete control of reproduction by a dominant individual, and tug-of-war models, which assume on-going competition among group members over reproduction. Current data provide little support for either model, but uncertainty about the ability of individuals to detect genetic relatedness and difficulties in identifying traits conferring competitive ability mean that the relative importance of concessions versus tug-of-war remains unresolved. Here, we suggest that the use of social parasitism to generate meaningful variation in key social variables represents a valuable opportunity to explore the mechanisms underpinning reproductive skew within the social Hymenoptera. We present a direct test of concessions and tug-of-war models in the paper wasp Polistes dominulus by exploiting pronounced changes in relatedness and power structures that occur following replacement of the dominant by a congeneric social parasite. Comparisons of skew in parasitized and unparasitized colonies are consistent with a tug-of-war over reproduction within P. dominulus groups, but provide no evidence for reproductive concessions.