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The origin of eusociality in the Hymenoptera is a question of major interest. Theory has tended to focus on genetic relatedness, but ecology can be just as important a determinant of whether eusociality evolves. Using the model of Fu et al. (2015), we show how ecological assumptions critically affect the conclusions drawn. Fu et al. inferred that eusociality rarely evolves because it faces a fundamental ‘risk‐return tradeoff’. The intuitive logic was that worker production represents an opportunity cost because it delays realising a reproductive payoff. However, making empirically justified assumptions that (1) workers take over egg‐laying following queen death and (2) productivity increases gradually with each additional worker, we find that the risk‐return tradeoff disappears. We then survey Hymenoptera with more specialised morphological castes, and show how the interaction between two common features of eusociality – saturating birth rates and group size‐dependent helping decisions – can determine whether eusociality outperforms other strategies., Theories concerning the origin of eusociality in Hymenoptera have tended to focus on genetic relatedness, but ecology can be just as important. With realistic ecological assumptions, we show that eusociality does not face a previously postulated ‘risk‐return tradeoff’. We find that the interaction between two common features of eusociality – birth rates that saturate at large group sizes, and group‐size dependent helping decisions – can instead determine whether eusociality outperforms other strategies.

The productivity of social groups depends critically on effective regulation of work effort among group members. In social insect colonies, regulation of work may be decentralised or alternatively may be controlled by one or a few individuals (‘pacemakers’) within the colony. Social parasites, which usurp host colonies and replace the dominant as the principal reproductive, similarly depend on efficient regulation of work by hosts to rear parasite offspring, but few studies have explored the strategies used by parasites to achieve this. We compared the role of the social parasite Polistes semenowi in regulating host activity with that of the dominant individual on unparasitized nests of the host species, P. dominula. Dominant foundresses acted as pacemakers within unparasitized colonies, interacting frequently with colony members to initiate activity bursts and foraging trips, whereas parasites did not initiate more activity than the average colony member. Nonetheless, overall activity levels were similar in parasitized and unparasitized colonies, indicating that parasites may use other, indirect means to control the host activity. Colony activity did not change significantly following the removal of parasites or dominant host foundresses, perhaps because other individuals rapidly assumed the dominant position, or because of persistent indirect effects on colony activity. The role of P. semenowi in regulating the host activity differs strikingly from that reported for a second Polistes social parasite, P. atrimandibularis, suggesting that different Polistes social parasites may have fundamentally different social roles within host colonies, despite being closely phylogenetically related to one another.

Recent theory and empirical studies of avian biparental systems suggest that animals resolve conflict over parental care via a process of behavioral negotiation or “rules for responding.” Less is known, however, about whether negotiation over helping effort occurs in cooperatively breeding animal societies or whether behavioral negotiation requires a relatively large brain. In this study, we tested whether negotiation over help occurs in a social insect, the paper wasp Polistes dominulus, by recording individual responses to both observed and experimentally induced foraging returns by other group members. In our experiments, we manipulated food delivery to the nest in 2 ways: 1) by catching departing foragers and giving them larval food to take back to the nest and 2) by giving larval food directly to wasps on the nest, which they then fed to larvae, so increasing food delivery independently of helper effort. We found no evidence from Experiment 1 that helpers adjusted their own foraging effort according to the foraging effort of other group members. However, when food was provided directly to the nest, wasps did respond by reducing their own foraging effort. One interpretation of this result is that paper wasp helpers adjust their helping effort according to the level of offspring need rather than the work rate of other helpers. Negotiation based on indicators of demand rather than work rate is a likely mechanism to resolve conflict over investment in teams where helpers cannot observe each other’s work rate directly, as is commonly the case in insect and vertebrate societies.

Relatedness is predicted to be a key determinant of cooperative behavior, but kin discrimination within social insect colonies is surprisingly rare. A lack of reliable cuticular hydrocarbon (CHC) cues is thought to be responsible, but here we show that in a high-profile paper wasp model, kin recognition cues are available for some individuals that found nests with nonrelatives. Thus, unrelated Polistes dominulus helpers could potentially recognize themselves as such. On this basis, we reanalyzed a behavioral data set to investigate whether foraging effort, defense contributions and aggression toward nest mates might thus reflect CHC profiles. Both foraging behavior and aggression varied with genetic relatedness, but genetic relatedness itself was a better predictor of this variation than differences in CHC profiles. We propose that wasps use specific components of the CHC profile, the identity of which is as yet unknown, to identify relatives among nest mates. Our data provide the first evidence of within-nest kin discrimination in primitively eusocial wasps but leave open the question of which cues are responsible.

Eusocial animal societies are typified by the presence of a helper (worker) caste which predominantly cares for young offspring in a social group while investing little in their own direct reproduction. A key question is what determines whether an individual becomes a worker or leaves to initiate her own reproduction. In some insects, caste is determined nutritionally during development. In others, and in vertebrate societies, adults are totipotent and the cues that determine caste are less well known. The mate limitation hypothesis (MLH) states that a female's mating status acts as a cue for caste determination: females that mate become reproductives, while those that fail to mate become workers. The MLH is consistent with empirical observations in sweat bees showing that over the course of the nesting season, there are increases in both the proportion of females that become reproductives and the frequency of males in the mating pool. We modelled a foundress's offspring sex-ratio strategy to investigate whether an increasingly male-biased operational sex-ratio over time is evolutionarily stable under the MLH. Our results indicate that such a pattern could occur if early workers were more valuable than late workers. This pattern was then more likely if male mortality was high, if worker mortality was low, if the value of a worker was high and if the period over which workers can help was short. Our results suggest that the MLH can be evolutionarily stable, but only under restrictive conditions. Manipulative experiments are now required to investigate whether mating determines caste in nature.

In cooperatively breeding vertebrates, the existence of individuals that help to raise the offspring of non-relatives is well established, but unrelated helpers are less well known in the social insects. Eusocial insect groups overwhelmingly consist of close relatives, so populations where unrelated helpers are common are intriguing. Here, we focus on Polistes dominula— the best-studied primitively eusocial wasp, and a species in which nesting with non-relatives is not only present but frequent. We address two major questions: why individuals should choose to nest with non-relatives, and why such individuals participate in the costly rearing of unrelated offspring. Polistes dominula foundresses produce more offspring of their own as subordinates than when they nest independently, providing a potential explanation for co-founding by non-relatives. There is some evidence that unrelated subordinates tailor their behaviour towards direct fitness, while the role of recognition errors in generating unrelated co-foundresses is less clear. Remarkably, the remote but potentially highly rewarding chance of inheriting the dominant position appears to strongly influence behaviour, suggesting that primitively eusocial insects may have much more in common with their social vertebrate counterparts than has commonly been thought.

Within a species, variation in the use of sexual signals is observed between different populations. For intersexual traits, differences in the environmental conditions experienced by populations can play an important role in driving variation in male ornaments and female preferences. However, little is known about the factors maintaining variation in intrasexual traits used in competition. In this study, we investigate the role of climatic conditions in maintaining variation in the expression of an intrasexual signal in the paper wasp Polistes dominulus. The results of an experiment in which pupae were housed under different temperature and humidity conditions revealed a strong effect of temperature during pupal development on the expression of the signal. Furthermore, a comparison of survival and body weight between wasps reared at different temperatures indicates that signal expression exhibits phenotypic plasticity in response to developmental temperature. The effect of temperature on signal expression is consistent with patterns of signal expression observed across populations in the wild and suggests that climatic conditions may act to constrain signal expression in some populations but not in others, driving variation in signal use within P. dominulus. Environmental conditions may therefore be important in defining the scope for intrasexual signalling in animal populations and, in doing so, may play a role in maintaining variation in intrasexual traits in the face of sexual selection.

Phenotypic plasticity may evolve when conditions vary temporally or spatially on a small enough scale. Plasticity is thought to play a central role in the early stages of evolutionary transitions, including major transitions such as those between non-sociality and sociality. The sweat bee Halictus rubicundus is of special interest in this respect, because it is socially plastic in the British Isles: Nests are social or non-social depending on the environment. However, sociality comprises a complex suite of inter-related traits. To further investigate social plasticity in H. rubicundus, we measured traits that are potentially integral to social phenotype at a northern site, where nests are non-social, and a southern site where nests can be social. We found that foundresses at non-social sites were smaller, produced offspring of a size more similar to themselves, initiated nesting later, and took longer to produce their first female offspring. They began provisioning earlier in the day, finished earlier, and collected more pollen loads. Common garden experiments suggested that these differences represent mainly plasticity, as expected for traits involved in the overall plastic social phenotype, with only limited evidence for fixed genetic differences in foraging. Conditions during overwintering did not have major effects on a foundress' subsequent behaviour.

Within-group power asymmetries and the resulting reproductive skew, common in most social groups, may effectively be set at the very early stages of group formation, that is, when group membership is determined. Hence, groupmate choices can define an individual’s future reproductive success. We examined how groups of Polistes dominulus formed under natural, unconstrained conditions, using data on the nesting history, kinship and morphology of individually marked foundresses obtained during two consecutive seasons in southern Spain. Foundresses that hibernated in the same aggregation were more likely to start a nest together, but all of the foundresses at a nest were seldom from a single aggregation. Changes in group composition were frequent throughout the preworker period, mainly because some foundresses disappeared and other wasps joined established groups. Within-group relatedness, however, was not affected by the late arrival of wasps. Our results suggest that waiting to join an established group is a common nesting strategy in P. dominulu s. Only 16% of marked wasps used more than one nest. Foundresses that moved between groups tended to move to groups in which genetic relatedness among the resident foundresses was higher, but not necessarily relatedness to the moving wasp herself. Overall, nestmate choices were not associated with a single factor. High failure rates, particularly of single-foundress nests, however, suggest that ecological constraints (e.g. risk of predation, lack of resources) may have a stronger effect on individual nesting choices than previously considered.

Altruism in the social Hymenoptera is generally considered to be a feature of females rather than males. A popular explanation for this is that in the solitary ancestors of today's social species, males provided little brood care. Males might therefore lack the preadaptations necessary to evolve altruism in social contexts. While anecdotal observations of male contributions to colony life have been reported, there are few reports of male participation in nest defence. In apoid wasps, there have been several reports of male nest-guarding behaviour in solitary species, potentially setting the evolutionary stage for similar behaviours in social lineages. Here, we present evidence of active and effective nest defence in males of the social apoid wasp Microstigmus nigrophthalmus. Males were observed chasing intruders away from the nest, and the presence of males had a significant effect on nest survival when females were removed. Males potentially obtained direct benefits through defence, so that defence may not represent male altruism. However, our results do show that males can perform acts that benefit their colony.

Our knowledge of primitively eusocial societies is focused particularly on cooperatively breeding vertebrates and vespid wasps, whereas numerous taxa representing independent origins of social behavior have been largely overlooked. The lineage of apoid wasps including the genus Microstigmus represents a relatively neglected independent origin of eusociality. We present the first use of modern hypervariable genetic markers, in combination with behavioral observations, to investigate reproductive division of labor and cooperative brood care in an apoid wasp, the Brazilian M. nigrophthalmus. Microstigmus nigrophthalmus is unusual because, although there is cooperative brood care, reproductively dominant females carry out at least as much risky foraging as their subordinate nest mates. Empirical studies of reproductive skew are often hampered by a lack of variation in skew. We find that reproductive skew is highly variable between nests in M. nigrophthalmus. There was no correlation between skew and either body size or group size. The absence of an effect of body size is typical of studies of skew in insects and may indicate that body size is a poor measure of an individual's ability to control a group. However, skew was positively correlated with genetic relatedness. This provides rare support for concession models based on "social contracts" between dominant individuals and their subordinates. Copyright 2011, Oxford University Press.

Contests between individuals over resources may be costly in terms of both time and energy expended and the risk of injury. Signals of status, or ‘status badges’, are thought to have evolved to minimize these costs by providing information about an individual’s fighting ability or resource-holding potential (RHP) at the start of a contest. Studies on recently established North American populations of the paper wasp Polistes dominulus have demonstrated the existence of a status badge, in the form of black clypeal patterns, and have shown that rivals attend to these patterns during competitive interactions. However, observational data from studies in this wasp’s native European range have failed to demonstrate a strong link between clypeal patterning and RHP. We undertook the first direct test of status signalling in a European population of P. dominulus, by testing receiver responses to clypeal pattern manipulations in a competitive foraging context. We found no evidence that individuals assessed rivals using the clypeal ‘badge’. We discuss possible reasons for variation in signal use between the American and European populations, including genetic drift and environmental effects of the development and transmission of the signal.

The origin of eusociality is often regarded as a change of macroevolutionary proportions [[1] and [2]]. Its hallmark is a reproductive division of labor between the members of a society: some individuals (?helpers? or ?workers?) forfeit their own reproduction to rear offspring of others (?queens?). In the Hymenoptera (ants, bees, wasps), there have been many transitions in both directions between solitary nesting and sociality [[2], [3], [4] and [5]]. How have such transitions occurred? One possibility is that multiple transitions represent repeated evolutionary gains and losses of the traits underpinning sociality. A second possibility, however, is that once sociality has evolved, subsequent transitions represent selection at just one or a small number of loci controlling developmental switches between preexisting alternative phenotypes [[2] and [6]]. We might then expect transitional populations that can express either sociality or solitary nesting, depending on environmental conditions. Here, we use field transplants to directly induce transitions in British and Irish populations of the sweat bee Halictus rubicundus. Individual variation in social phenotype was linked to time available for offspring production, and to the genetic benefits of sociality, suggesting that helping was not simply misplaced parental care [7]. We thereby demonstrate that sociality itself can be truly plastic in a hymenopteran.

Environmental factors, and particularly climate, play an important role in influencing behaviour in many insects. In social species, climate is known to have a strong influence on social traits, but how this manifests itself in tropical ecosystems is poorly understood. In Peninsular Malaysia, the climate is characterised by relatively consistent annual temperatures with wet/dry cycles, and the tropical hover-wasp Liostenogaster flavolineata Cameron is active year-round. Newly emerged females can choose to remain at the natal nest and help, or disperse and found their own nest depending on a balance of ecological and demographic factors. We collated long-term adult and brood census data for populations of L. flavolineata in Peninsular Malaysia in three different years to investigate temporal variation in demographics (brood and adult numbers) and how this might be related to climatic factors. Our data indicate that there are multiple, temporally distinct peaks of brood production in this population. The number of newly eclosing females and number of mature brood were positively associated with temperature and negatively associated with the number of rain-days during the observation period. Furthermore, larger females were produced during the peaks of brood production. We speculate how these patterns may influence the staying or leaving decisions of newly emerged females in a primitively eusocial species such as L. flavolineata.

In primitively eusocial societies, all individuals can potentially reproduce independently. The key fact that we focus on in this paper is that individuals in such societies instead often queue to inherit breeding positions. Queuing leads to systematic differences in expected future fitness. We first discuss the implications this has for variation in behaviour. For example, because helpers nearer to the front of the queue have more to lose, they should work less hard to rear the dominant's offspring. However, higher rankers may be more aggressive than low rankers, even if they risk injury in the process, if aggression functions to maintain or enhance queue position. Second, we discuss how queuing rules may be enforced through hidden threats that rarely have to be carried out. In fishes, rule breakers face the threat of eviction from the group. In contrast, subordinate paper wasps are not injured or evicted during escalated challenges against the dominant, perhaps because they are more valuable to the dominant. We discuss evidence that paper-wasp dominants avoid escalated conflicts by ceding reproduction to subordinates. Queuing rules appear usually to be enforced by individuals adjacent in the queue rather than by dominants. Further manipulative studies are required to reveal mechanisms underlying queue stability and to elucidate what determines queue position in the first place.

Hierarchies constitute the base of many social groups. Hence, understanding how they are established is critical. Here we examine how hierarchies are formed in foundresses associations of the common paper wasp Polistes dominulus. By comparing field data with computer simulations, we evaluate order of arrival at the nest, body size, facial color patterns, and within-group kinship structure as determinants of inheritance rank. Hierarchies (ranks 1--5) were experimentally inferred for 53 nests. Overall, the order in which foundresses arrived at the nest and their body size were not significantly correlated with rank. A foundress's rank was negatively correlated with the number of full sisters it had in its group. Highly ranked wasps (ranks 1 and 2) were less likely to share a nest with their full sisters than wasps of lower rank. A wasp's rank was not determined by the relative rank of its nest-mate sisters. A foundress's rank was significantly correlated with the size of its black clypeal marks, but the number of foundresses with clypeal marks in each nest was small. On 15 of 20 nests where wasps with marks were present, only 1 wasp had such marks. Overall, our results suggest that within-group relatedness structure is important in the establishment of dominance hierarchies in P. dominulus foundress associations. Copyright 2009, Oxford University Press.

The sweat bees (Family Halictidae) are a socially diverse taxon in which eusociality has arisen independently numerous times. The obligate, primitively eusocial Lasioglossum malachurum, distributed widely throughout Europe, has been considered the zenith of sociality within halictids. A single queen heads a colony of smaller daughter workers which, by mid-summer, produce new sexuals (males and gynes), of which only the mated gynes overwinter to found new colonies the following spring. We excavated successfully 18 nests during the worker- and gyne-producing phases of the colony cycle and analysed each nest's queen and either all workers or all gynes using highly variable microsatellite loci developed specifically for this species. Three important points arise from our analyses. First, queens are facultatively polyandrous (queen effective mating frequency: range 1?3, harmonic mean 1.13). Second, queens may head colonies containing unrelated individuals (n= 6 of 18 nests), most probably a consequence of colony usurpation during the early phase of the colony cycle before worker emergence. Third, nonqueen's workers may, but the queen's own workers do not, lay fertilized eggs in the presence of the queen that successfully develop into gynes, in agreement with so-called `concession? models of reproductive skew.

Indirect benefits obtained through the reproduction of relatives are fundamental in the formation and maintenance of groups. Here, we examine the hypothesis that females of the temperate paper wasp Polistes dominulus preferentially form groups with close relatives. Genetic relatedness data were obtained for 180 groups of females collected at the early stages of the nesting cycle of a large population of P. dominulus in two sites in southwestern Spain. Average within-group relatedness values ranged from 0.189 to 0.491. Foundresses on early nests were significantly more closely related than females in winter aggregations or in stable groups (just before workers emerged). Within-group relatedness values were independent of group size. The vast majority of worker-producing nests (c. 85%) had one or more females that were unrelated (or distantly related) to the remaining members of the group. These results provide further support to the hypothesis that indirect fitness benefits alone are unlikely to explain why P. dominulus foundresses form cooperative associations.

The mechanisms through which dominance is inherited within social groups vary from direct interactions such as fighting to non-confrontational conventions. Liostenogaster flavolineata is a primitively eusocial hover wasp in which one female, the ‘dominant’, is the only reproductive upon the nest. The remaining females, although capable of reproduction, behave as helpers. In this study, we investigate the rules by which helpers inherit dominance. We removed successive dominants from 56 nests and recorded accession on un-manipulated nests. The results showed that L. flavolineata has a strict age-based inheritance queue: new dominants are the oldest female in their groups 87% of the time. Thirteen cases of queue-jumping were found in which young individuals were able to supplant older nestmates and inherit dominance precociously. Queue jumpers did not differ from other wasps in terms of relatedness to other group members or body size. Individuals that had previously worked less hard than other females of equivalent rank were significantly more likely to later jump the queue. Queue-jumping may represent a cheating strategy or could indicate that the rule for inheriting dominance is not based purely on relative age. We also discuss possible reasons why age-based queuing has evolved and its potential to promote the evolution of helping behaviour.

An important benefit of social living is increased capacity for defense. Highly eusocial species have often evolved a fighting caste for this purpose, but many facultatively eusocial insects and cooperatively breeding vertebrates lack morphological castes and the decision to defend or not can depend on costs and benefits to each individual. Defense by subordinates in a social group can be regarded as a form of helping, and helping input often varies among subordinates of different age or size. Several hypotheses attempt to explain variation in helping effort, including the effects of relatedness and differences in the costs of helping. Evidence for these hypotheses is mixed and often lacks data on the rank of subordinates, an important determinant of expected future fitness. We examined individual variation in propensity to defend the nest against conspecifics in the tropical hairy-faced hover wasp Liostenogaster flavolineata. Prior to experimentation, we determined the positions of all wasps in the age-based queue to inherit the single egg-laying position in each L. flavolineata group. Two approaches were then used: observations of defense against natural intrusions by conspecifics and experimental trials where wasps were presented attached to a wire. Higher ranks were more likely to defend the nest than lower ranks, opposite to the pattern previously documented for another form of helping: foraging effort. Possible explanations for this result are that higher ranked females are better defenders and that they suffer a larger decrease in expected future fitness when an intruder usurps their position in the inheritance queue. Copyright 2007, Oxford University Press.

Queues, in which individuals inherit resources in a predictable, temporally stable order, are widespread in animal social groups. We develop an analytic model to explore the effect of differential survivorship on the stability of a reproductive queue. We show that unless fighting for dominance is potentially fatal, future direct benefits are not alone sufficient to stabilize a queue of non-relatives under constant (age-independent) mortality rates, regardless of whether a dominant becomes an isolate or remains a dominant on the death of the first subordinate. In the absence of fatal fighting, stabilization of such a queue by future direct benefits alone requires either the dominant or the subordinate to have age-dependent mortality rates. Even when the queue is stabilized by present direct reproduction, however, the shape of the lifespan distribution can make a significant difference to the size of the required incentive. In contrast to non-relatives, queues of relatives can be stable without age-dependent mortality, so long as relatedness exceeds a critical value; however, age-dependent mortality can lower this critical value.

Recent theory suggests that much of the wide variation in individual behavior that exists within cooperative animal societies can be explained by variation in the future direct component of fitness, or the probability of inheritance. Here we develop two models to explore the effect of variation in future fitness on social aggression. The models predict that rates of aggression will be highest toward the front of the queue to inherit and will be higher in larger, more productive groups. A third prediction is that, in seasonal animals, aggression will increase as the time available to inherit the breeding position runs out. We tested these predictions using a model social species, the paper wasp Polistes dominulus. We found that rates of both aggressive "displays" (aimed at individuals of lower rank) and aggressive "tests" (aimed at individuals of higher rank) decreased down the hierarchy, as predicted by our models. The only other significant factor affecting aggression rates was date, with more aggression observed later in the season, also as predicted. Variation in future fitness due to inheritance rank is the hidden factor accounting for much of the variation in aggressiveness among apparently equivalent individuals in this species.

Most nonsocial wasps and bees are mass provisioners (MP), sealing each egg into its own cell containing all of the food required to reach maturity. Other species instead provision progressively, feeding their offspring only gradually as they develop and usually provisioning more than one offspring simultaneously (SPP). The evolution of progressive provisioning is interesting because it has obvious drawbacks. In particular, it could prolong the period of offspring dependency, and hence increase the chance that a mother will die before her offspring reach independence. Prolonged dependency could in turn facilitate the evolution of helping through insurance-based mechanisms. In this paper, I outline two ecologically extreme models of how reproductive success is accrued during provisioning. In Model 1, immature offspring become independent as soon as they are fully provisioned. SPP is then disadvantageous because it prolongs the provisioning period compared with MP. If SPP does evolve, Model 1 predicts that brood size, the number of offspring provisioned simultaneously in a batch, should be minimized. Model 2 differs from Model 1 in that offspring become independent only at adulthood. SPP can then be advantageous because investment is converted into independent offspring more quickly than under MP. Model 2 predicts an intermediate brood size, positively correlated with larval development time and the abundance of provisions. Ammophiline digger wasps and eusocial vespids may correspond to Models 1 and 2, respectively, whereas ground-nesting wasps and bees with multicellular nests may be intermediate. Copyright 2005.

In many cooperatively breeding species, group members form a dominance hierarchy or queue to inherit the position of breeder. Models aimed at understanding individual variation in helping behavior, however, rarely take into account the effect of dominance rank on expected future reproductive success and thus the potential direct fitness costs of helping. Here we develop a kin-selection model of helping behavior in multimember groups in which only the highest ranking individual breeds. Each group member can invest in the dominant's offspring at a cost to its own survivorship. The model predicts that lower ranked subordinates, who have a smaller probability of inheriting the group, should work harder than higher ranked subordinates. This prediction holds regardless of whether the intrinsic mortality rate of subordinates increases or decreases with rank. The prediction does not necessarily hold, however, where the costs of helping are higher for lower ranked individuals: a situation that may be common in vertebrates. The model makes two further testable predictions: that the helping effort of an individual of given rank should be lower in larger groups, and the reproductive success of dominants should be greater where group members are more closely related. Empirical evidence for these predictions is discussed. We argue that the effects of rank on stable helping effort may explain why attempts to correlate individual helping effort with relatedness in cooperatively breeding species have met with limited success. Copyright 2005.

We explore the effects of group size on the direct reproductive success of subordinate helpers in eusocial animals where only a single, dominant individual reproduces at one time. Helpers can reproduce directly if they inherit dominance, but when dominance is age based, an individual born into a larger group has a longer wait to inherit. We show that this disincentive to help can potentially be offset by increased productivity, increased life span, and insurance-based benefits for helpers if they survive to inherit dominance in larger groups. We analyze a field experiment in which group size was manipulated in the hover wasp Liostenogaster flavolineata. Productivity increased linearly with group size, larger groups were less likely to fail, and dominants in larger groups may have lived longer. Combined with the probability of inheriting dominance, these effects led overall to a negative correlation between group size and expected direct fitness, mainly because group size decreased during our study period, so that helpers could not expect to inherit as large a group as they started queuing in. Our analysis suggests that the relationship between group size and productivity plays a central role in determining the fitness consequences of helping.

Low-level social aggression is a conspicuous feature of cooperative animal societies, but its precise function is usually unclear. One long-standing hypothesis is that aggressive displays by dominant individuals serve to reduce uncertainty about relative strength and deter subordinates from starting fights that they are unlikely to win. However, most formal theoretical models of this idea do not consider how the credibility of deterrent signals might change over time in social groups. We developed a simple model of dominant aggression as a deterrent signal, which takes into account how credibility changes over time and how selection should act on receiver memory. We then carried out an experimental test of the predictions of our model on a field population of the paper wasp, Polistes dominulus. The match between our theoretical and empirical results suggests that low-level social aggression can help to maintain the stability and productivity of cooperative associations in this species. Moreover, our work suggests that rates of aggression in animal societies and the robustness of social memories are likely to be intimately related.

In facultatively eusocial hover wasps, some females leave their natal nests while others choose to stay and become helpers. We tested whether the decisions of 126 newly emerged females to stay or leave depended on group size: the number of females already resident on their natal nests. We predicted that females would be less likely to stay in larger groups, where the benefits of helping are probably smaller and there is a smaller chance of inheriting the dominant position. We also predicted that unrelated females would be less likely to join larger groups. We manipulated group size by removing residents from nests. Newly emerged females disappeared from their natal nests at a rate of 2.5% per day, but did not disappear from manipulated nests at higher rates than controls. Experimentally reducing group size also did not increase the frequency of joiners. Newly emerged females disappeared at twice the rate of older subordinate females, suggesting the existence of a `leaving window' early in life. One problem is the difficulty of distinguishing between leaving and death.

Using simple stochastic models, we discuss how cooperative breeders, especially wasps and bees, can improve their productivity by reducing foraging work. In a harsh environment, where foraging is the main cause of mortality, such breeders achieve greater productivity by reducing their foraging effort below full capacity, and they may thrive by adopting cooperative breeding. This could prevent the population extinction of cooperative breeders under conditions where a population of lone breeders cannot be maintained.

Status signals function in a number of species to communicate competitive ability to conspecific rivals during competition for resources. In the paper wasp Polistes dominulus, variable black clypeal patterns are thought to be important in mediating competition among females. Results of previous behavioral experiments in the lab indicate that P. dominulus clypeal patterns provide information about an individual’s competitive ability to rivals during agonistic interactions. To date, however, there has been no detailed examination of the adaptive value of clypeal patterns in the wild. To address this, we looked for correlations between clypeal patterning and various fitness measures, including reproductive success, hierarchical rank, and survival, in a large, free-living population of P. dominulus in southern Spain. Reproductive success over the nesting season was not correlated with clypeal patterning. Furthermore, there was no relationship between a female’s clypeal patterning and the rank she achieved within the hierarchy or her survival during nest founding. Overall, we found no evidence that P. dominulus clypeal patterns are related to competitive ability or other aspects of quality in our population. This result is consistent with geographical variation in the adaptive value of clypeal patterns between P. dominulus populations; however, data on the relationship between patterning and fitness from other populations are required to test this hypothesis. Key wor ds: clypeal pattern, fitness, Polistes dominulus, quality, status signalling, wild. [Behav Ecol]

The evolution of helping, in which some individuals forfeit their own reproduction and help others to reproduce, is a central problem in evolutionary biology. Recently proposed insurance-based mechanisms rely on a pre-existing life history with a long period of offspring dependency relative to the short life expectancies of adult carers1, 2, 3, 4: a lone mother's offspring are doomed if she dies young, whereas after a helper dies, other group members can finish rearing the offspring5, 6. A critical question, however, is how this life history could evolve in ancestral non-social populations, as offspring survival would then depend on a single, short-lived carer. Here, we resolve this paradox by focusing on the extended parental care inherent in prolonged dependency. We show experimentally that in non-social wasps, extended care can significantly reduce the impact of interspecific parasites. Under extended care, offspring are less vulnerable by the time they are exposed to parasites, and costs of parasitism are reduced because mothers have the option to terminate investment in failing offspring. By experimentally simulating aspects of extended care in a species where it is lacking, we demonstrate that neither benefit requires specialized behaviour. Such benefits could therefore offset the disadvantage of prolonged dependency in non-social species, thereby facilitating the evolution of helping.

The functional significance of nest co-occupation inCerceris arenaria(Hymenoptera: Sphecidae) was investigated. Willmer (1985a,Behav. Ecol. Sociobiol.,17,151-160, 1987,Proc. 3rd Eur. Congr. Entomol,2,229-240) suggested that in this species co-occupation is a mutualistic response to ecophysiological pressures, and that these pressures might have played a general role in the evolution of eusociality. In this study nest-less ‘Searcher’ females almost always (99%,N=80) accepted (started using) pre-existing nests rather than digging new burrows. Burrows were occupied by sequences of females whose periods of occupation overlapped briefly. Co-occupations began when Searchers accepted nests that already had owners. The behaviour of Searchers after nest acceptance depended on time of acceptance. Late in the day, Searchers remained inside nests and usually closed them. Early in the day, Searchers started provisioning accepted nests. If these nests were already occupied, the result was joint-provisioning. When a nest was joint-provisioned, it received significantly more prey than the owner could provision alone, but less than twice what she could provision alone. Co-occupations rarely (4%,N=47) lasted more than 1 day, and ended when one female left the nest for good. Searchers remained in possession of nests after 49% (N=47) of co-occupations. Searchers appear functionally to be usurpers: they are known to prefer nests with owners and on average, owners lose out if Searchers accept their nests. Willmer's hypothesis that co-occupations are mutualistic is rejected. An analysis of time costs suggests that accepting a pre-existing nest has larger payoffs than digging a new burrow. The high rate of replacement of nest owners (11-16% per day) will reduce the relatedness of offspring emerging from the same nest, presenting a barrier against the kin-selected evolution of nest-sharing. Variation in social interactions in the genusCercerismay be consistent with the predictions of a recent model of cooperation based on reciprocity.

Sweat bees (family Halictidae) comprise a numerous and diverse group that are arguably among the most socially labile of all insect taxa. Given the lack of highly variable markers for eusocial species of the family, we developed a suite of dinucleotide and trinucleotide markers for one of its members, the Eurasian Lasioglossum malachurum, and used them to amplify DNA from other halictids. Loci were highly variable in L. malachurum and amplified DNA from many other halictids.

Climate has long been suggested to affect population genetic structures of eusocial insect societies. For instance, Hamilton [Journal of Theoretical Biology 7 (1964) 17] discusses whether temperate and tropical eusocial insects may show differences in population-level genetic structure and viscosity, and how this might relate to differences in the degree of synchrony in their life cycles or modes of nest founding. Despite the importance of Hamilton's 1964 papers, this specific idea has not been tested in actual populations of wasps, probably due to the paucity of studies on tropical species. Here, we compare colony and population genetic structures in two species of primitively eusocial paper wasps with contrasting ecologies: the tropical species Polistes canadensis and the temperate species P. dominulus. Our results provide important clarifications of Hamilton's discussion. Specifically, we show that the genetic structures of the temperate and tropical species were very similar, indicating that seasonality does not greatly affect population viscosity or inbreeding. For both species, the high genetic differentiation between nests suggests strong selection at the nest level to live with relatives, whereas low population viscosity and low genetic differentiation between nest aggregations might reflect balancing selection to disperse, avoiding competition with relatives. Overall, our study suggests no prevalence of seasonal constraints of the life cycle in affecting the population genetic structure of eusocial paper wasps. These conclusions are likely to apply also to other primitively eusocial insects, such as halictine bees. They also highlight how selection for a kin structure that promotes altruism can override potential effects of ecology in eusocial insects. © 2012 The Authors.Ecology and Evolution published by Blackwell Publishing Ltd.

Abstract. This paper demonstrates that intraspecific nest parasites are selective in their choice of host nests. In a large nest aggregation, instead of digging new burrows, nest-less females of the digger wasp Cerceris arenaria entered pre-existing burrows and eventually used one of them. Data on host nest selection showed that the likelihood that a nest-less female would use a burrow increased if the burrow contained more prey, if the burrow had a conspecific owner, and if the owner was absent from the nest. This indicates that during their brief visits to burrows, nest-less females are capable of rapidly assessing the value of the burrow and its contents. They appear to be true 'usurpers', unlike nest-less females of other species which use burrows at random ('entering'). The evolution of an ability to assess prey may not have involved large costs since a similar ability may have already been present, associated with provisioning and sex allocation strategies. A significant cost of intraspecific parasitism could arise if the same nests tend to be repeatedly parasitized, but this was not the case in C. arenaria.

Animals that cooperate with nonrelatives represent a challenge to inclusive fitness theory, unless cooperative behavior is shown to provide direct fitness benefits. Inheritance of breeding resources could provide such benefits, but this route to cooperation has been little investigated in the social insects. We show that nest inheritance can explain the presence of unrelated helpers in a classic social insect model, the primitively eusocial wasp Polistes dominulus. We found that subordinate helpers produced more direct offspring than lone breeders, some while still subordinate but most after inheriting the dominant position. Thus, while indirect fitness obtained through helping relatives has been the dominant paradigm for understanding eusociality in insects, direct fitness is vital to explain cooperation in P. dominulus.

The theory of assured fitness returns proposes that individuals nesting in groups gain fitness benefits from effort expended in brood-rearing, even if they die before the young that they have raised reach independence. These benefits, however, require that surviving nest-mates take up the task of rearing these young. It has been suggested that assured fitness returns could have favoured group nesting even at the origin of sociality (that is, in species without a dedicated worker caste). We show that experimentally orphaned brood of the apoid wasp Microstigmus nigrophthalmus continue to be provisioned by surviving adults for at least two weeks after the orphaning. This was the case for brood of both sexes. There was no evidence that naturally orphaned offspring received less food than those that still had mothers in the nest. Assured fitness returns can therefore represent a real benefit to nesting in groups, even in species without a dedicated worker caste.

We describe eight polymorphic microsatellite loci for facultatively eusocial hover wasps (Hymenoptera: Stenogastrinae). Loci were isolated from a partial genomic library of Liostenogaster flavolineata DNA, enriched for GA repeats and an unenriched library of Liostenogaster vechti DNA. We present our results of cross-species amplification in three species: L. flavolineata, L. vechti and Parischnogaster alternata.

The paper wasp Polistes dominulus is unique among the social insects in that nearly one-third of co-foundresses are completely unrelated to the dominant individual whose offspring they help to rear and yet reproductive skew is high. These unrelated subordinates stand to gain direct fitness through nest inheritance, raising the question of whether their behaviour is adaptively tailored towards maximizing inheritance prospects. Unusually, in this species, a wealth of theory and empirical data allows us to predict how unrelated subordinates should behave. Based on these predictions, here we compare helping in subordinates that are unrelated or related to the dominant wasp across an extensive range of field-based behavioural contexts. We find no differences in foraging effort, defense behaviour, aggression or inheritance rank between unrelated helpers and their related counterparts. Our study provides no evidence, across a number of behavioural scenarios, that the behaviour of unrelated subordinates is adaptively modified to promote direct fitness interests.\ud \ud

The nesting behaviour of individually marked female pompilid wasps, Anoplius viaticus, was observed at a Breckland heath site with particular emphasis on intraspecific parasitism and nest defence. Prey was stolen from conspecifics while it was being carried to the nest site, and while it was left unattended during nest construction. Females also appeared to brood parasitize each other's completed nests. Parasitism appeared to be opportunistic. Brood parasitism may be a tactic by which time-limited females can increase their fecundity. By placing prey in vegetation tufts during nest construction, females may reduce the risk of prey theft. An individual female's successive unicellular nests were clustered and therefore easier to defend, in many ways resembling a multicellular nest. Females defended their clusters vigorously, visiting them every few minutes during foraging and expelling conspecifics from the vicinity. This type of nest defence may be costly, and has rarely been observed in solitary wasps.

1 Three aspects of prey utilization are documented in a guild of spider-hunting pompilid wasps at a Breckland heath site: female phenology, size, and microhabitat utilization. 2 Twenty-four species were present at the site, 59% of the British fauna. Ten species individually represented more than 1% of the guild. 3 Pompilid abundance peaked in early July and mid-late August. Anoplius viaticus had a different life-history from other common guild members, making its inclusion in the guild questionable. 4 Most species represented by large samples occurred in all microhabitats and time intervals, and all species overlapped in size with all other species except A. viaticus. Arachnospila anceps was numerically dominant in all microhabitats and most time intervals. 5 Mean pair-wise overlaps in phenology and microhabitat utilization were significantly lower than predicted by null models, consistent with the idea that interspecific competition has been important in determining guild structure. 6 Female size is highly correlated with prey size, but the distribution of mean female sizes did not generally differ from null expectations. 7 Interpretation of comparisons with null models is problematic, particularly because it is difficult to quantify evolutionary ‘favourability’ of different resource states. Null models are currently of limited use because the patterns expected to result from key processes such as competition are uncertain in multi-dimensional systems.

1 In nest-building wasps there is a continuum from species that provision each offspring cell with one large prey item (single provisioning) to species that provision each cell with many small items (multiple provisioning). The significance of number of prey per cell was examined during two seasons in Ammophila sabulosa (L.), a species that provisions between one and five caterpillars per cell. 2 There was no difference between the total weight of prey in singly-provisioned and multiply-provisioned cells: individual caterpillars placed in multiply-provisioned cells were smaller. 3 Small caterpillars were captured and transported to the nest faster than large ones. 4 If the first prey provisioned was large, the cell was usually then permanently closed. If the first prey was small, additional prey were captured, and females appeared to become more selective: additional prey were rarely large. 5 It is suggested that the flexible provisioning strategy used by A.sabulosa allows a wide range of prey sizes to be utilized without affecting maternal control of offspring size and sex. This could be important when available prey sizes vary temporally and spatially. 6 There was a male-biased first generation investment ratio. Female-producing cells were provisioned with a higher total prey weight than male-producing cells, and conversion of prey weight to adult weight was more efficient for females. 7 Total prey weight provisioned is probably a good indirect measure of parental time investment in A.sabulosa. Other measures, particularly number of prey provisioned, will be less accurate.

Recent models postulate that the members of a social group assess their ecological and social environments and agree a "social contract" of reproductive partitioning (skew). We tested social contracts theory by using DNA microsatellites to measure skew in 24 cofoundress associations of paper wasps, Polistes bellicosus. In contrast to theoretical predictions, there was little variation in cofoundress relatedness, and relatedness either did not predict skew or was negatively correlated with it; the dominant/subordinate size ratio, assumed to reflect relative fighting ability, did not predict skew; and high skew was associated with decreased aggression by the rank 2 subordinate toward the dominant. High skew was associated with increased group size. A difficulty with measuring skew in real systems is the frequent changes in group composition that commonly occur in social animals. In P. bellicosus, 61% of egg layers and an unknown number of non-egg layers were absent by the time nests were collected. The social contracts models provide an attractive general framework linking genetics, ecology, and behavior, but there have been few direct tests of their predictions. We question assumptions underlying the models and suggest directions for future research.

Nest-building Hymenoptera have been a major testing ground for theories of parental investment and sex allocation. Investment has usually been estimated by the likely costs of offspring provisioning, ignoring other aspects of parental care. Using three experimental treatments, we estimated the costs of egg-laying and provisioning separately under field conditions in a digger wasp Ammophila pubescens . In one treatment, we increased the provisioning effort required per offspring by removing alternate prey items as they were brought to the nest. In two other treatments, we reduced parental effort by either preventing females from provisioning alternate nests or preventing them from both ovipositing and provisioning. Our results indicate that both egg-laying and provisioning represent significant costs of reproduction, expressed as differences in productivity but not survival. A trade-off-based model suggests that other components of parental care such as nest initiation may also represent significant costs. Costs of egg production and nest initiation are probably similar for male and female offspring, so that taking them into account leads to a less male-biased expected sex ratio. Mothers compensated only partially for prey removal in terms of the total provisions they gave to individual offspring.

A plethora of recent models examines how genetic and environmental factors might influence partitioning of reproduction ('skew') in animal societies, but empirical data are sparse. We used three microsatellite loci to estimate skew on 13 nests of the Malaysian hover wasp, Liostenogaster flavolineata. Groups are small in L. flavolineata (1-10 females) and all females are capable of mating and laying eggs. Despite considerable variation between nests in parameters expected to influence skew, skew was uniformly high. On 11 of the 13 nests, all female eggs had been laid by a single dominant female. A second female had laid one to two out of 5-10 eggs respectively on the two remaining nests. A likelihood analysis suggested that on average, 90% of the male eggs had also been laid by the dominant. The slightly lower skew among male eggs might reflect the lower average relatedness of subordinates to male versus female offspring of the dominant. We suggest that high skew in L. flavolineata may result from strong ecological constraints and a relatively high probability that a subordinate will eventually inherit the dominant, egg-laying position.

The origin and maintenance of eusociality is a central problem in evolutionary biology1, 2. Eusocial groups contain individuals that forfeit their own reproduction in order to help others reproduce. In facultatively eusocial taxa, offspring can choose whether to found new nests or become helpers in their natal groups. In many facultatively eusocial insects, offspring need continuous care during development, but adult carers have life expectancies shorter than the developmental period3, 4, 5, 6, 7. When a lone foundress dies, her partly reared brood are usually doomed. Here, we show that helpers in a tropical hover wasp (Liostenogaster flavolineata) have an insurance-based advantage over lone foundresses because after a helper dies, most of the brood that she has partly reared will be brought to maturity by surviving nest-mates. After some of the helpers are experimentally removed from a multi-female nest, the reduced group is left with more brood than it would normally rear. We found that larger, more valuable extra brood were reared through to maturity, but not smaller, less valuable brood. Smaller brood may be sacrificed to feed larger brood, and reduced groups probably benefited from increased short-term helper recruitment. Rearing extra brood did not increase adult mortality or brood development time.

A candidate explanation for the evolution of eusociality is that helpers are physiologically constrained such that helping is their only realistic option. We tested this subfertility hypothesis in a species of facultatively eusocial hover wasp (Hymenoptera, Stenogastrinae: Liostenogaster flavolineata) by seeing whether helpers that were forced to nest on their own were able to mature their own eggs. One focal helper was left alone on each of 22 nests, from which all other adult wasps (including the dominant) were permanently removed. After 18 days, all but one of the 19 focal helpers that remained on their nests had ovarian development and insemination status characteristic of dominants, and the majority had probably laid eggs. This was in striking contrast to the reproductive status of other helpers removed from the same nests at the start of the experiment. These results provide convincing experimental evidence that females do not become helpers because of some unconditional physiological constraint. There is currently no unequivocal support for the subfertility hypothesis in facultatively eusocial Hymenoptera lacking morphological castes. Copyright 1999 The Association for the Study of Animal Behaviour.

Recent field experiments suggest that cooperative breeding in vertebrates can be driven by a shortage of breeding territories. We did analogous experiments on facultatively eusocial hover wasps (Stenogastrinae: Liostenogaster flavolineata). We provided nesting opportunities by removing residents from 39 nests within a large aggregation (1995), and by glueing 20 nests obtained from a distant site into a second aggregation (1996). We prevented nest-less floaters from competing for these opportunities in 1995 but not in 1996. In both years, helpers in unmanipulated groups were given opportunities to nest independently without having to incur nest-building costs and with a reduced wait before potential helpers emerged. Helpers visited the nests we provided, but adopted only a small proportion (5% of 111 vacancies created in 1995). Others were adopted by floaters, but a significant proportion of nests were never adopted (nine out of 20 in 1995, seven out of 20 in 1996). Helpers that visited nests did not originate from particular kinds of social group. Nests containing older brood were more likely to be adopted, and adopting females rarely destroyed older brood. A general feature of social insect but not vertebrate life-histories, the long period of offspring dependency relative to the short life expectancy of adult carers, may be a key factor constraining independent nesting.

Nesting tactics of individually marked female solitary wasps, Ammophila sabulosa (L.) (Sphecidae), were studied over two seasons at a Norfolk heathland site. Females provision unicellular nests with 1-5 paralysed caterpillars. The entire nesting cycle requires approximately 8-10 hours of activity and there is no evidence that nesting success depends on female size. Prey are obtained by hunting or are stolen from other females' nests, which are found by searching in the nesting area. Females search on the nesting area and open other females' nests at significantly later times of day and lower temperatures than those at which they are absent from the nesting area (presumably hunting) and provision hunted prey. This may reflect thermal constraints or changes in prey availability or availability of detectable nests. Hunting and searching are mutually exclusive alternatives, but because females frequently switch between them it is difficult to estimate the time costs of either. Brood parasitism is an alternative way in which an A. sabulosa female can provide her larva with food and shelter. After completing her previous nest she must find a new digging site. While searching in the nesting area she may come across another female's nest, which she opens and usually brood parasitizes if it contains prey. It is again difficult to compare the time costs of non-parasitic nesting and parasitism, this time because alternative behaviours are not mutually exclusive - finding digging sites and finding other females' nests both involve searching on the nesting area. Females might find other females' nests opportunistically, or spend time searching for them. My methods and interpretation are compared with those of BROCKMANN et al., who analysed alternative nesting strategies in Sphex ichneumoneus. The alternative behaviours in Sphex are 'digging' and 'entering', two ways of acquiring a burrow. BROCKMANN et al. included nights and periods of poor weather in their time cost measurements, decreasing the likelihood of detecting differences between the net benefits of digging and entering. They also assumed that eggs laid after digging and entering had equal chances of survival, and that a female decides unconditionally to either dig or enter immediately after closing her previous nest. A major problem in both studies is that of pin-pointing when decisions are made and hence defining the strategy set. This is particularly difficult when alternative behaviours are not mutually exclusive.

The nesting behaviour of marked female solitary wasps, Ammophila sabulosa (L.) (Sphecidae), was studied over two seasons at a Norfolk heathland site. Females mass provisioned unicellular nests with 1-5 paralysed caterpillars. Intraspecific parasitism was the major mortality factor before the egg hatched - approximately 28% of eggs were destroyed by parasites. Parasites stole caterpillars to provision their own nests, replaced host eggs with their own eggs (brood parasitism), or simply discarded prey from host nests. Hosts were ineffective at preventing parasitism. Individual females both provisioned their own nests and parasitized those of other females. Parasitism usually occurred soon after the previous activity at the host nest. Parasitism events were clumped at certain nests, which were parasitized up to five times each. Once a nest had been brood parasitized, the prey were significantly more likely to be stolen or discarded than were prey in unparasitized nests, and only 17 % of eggs laid by brood parasites survived. Circumstantial evidence suggests that chemical stimuli are important in detection of host nests by parasites. High levels of intraspecific parasitism have rarely been reported in solitary wasps and bees, and parasitic behaviour is rare and unspecialized in some other Ammophila species. Parasitism by miltogrammine flies was of relatively minor importance in A. sabulosa as in other Ammophila species, probably because provisioning events, during which flies larviposit, are few and well separated in time.