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The neotropical apoid wasp genus Microstigmus is of particular interest biologically because it represents an origin of eusociality independent of vespid wasps and bees, and is part of the only eusocial lineage among the approximately 10,000 solitary species of apoid wasps. Females construct nests made of silk and the species exhibit an unusual diversity of nesting strategies. However, research is hampered because many species remain undescribed and the basic nesting biology of only a few species is known. I describe three new species from north-west Ecuador related to M. bicolor Richards, including diagnostic morphological characters, altitudinal ranges and molecular data as well as descriptions of their nests and nesting biologies. M. rosae sp. nov. is a mass provisioner that preys on nymphal Thysanoptera, while M. lydiae sp. nov. and M. mirandae sp. nov. are progressive provisioners that prey on nymphal leafhoppers (Cicadellidae). Nests of all three species can contain multiple adult females but more than half of nests contain only a single female. http://www.zoobank.org/urn:lsid:zoobank.org:pub:E46768B9-FD13-4370-8E31-8D1819B724F4

The evolution of eusociality is regarded as a major evolutionary transition, where units that previously reproduced independently function as one complex entity. Advanced eusocial societies are characterised by morphologically differentiated castes and reduced conflict. We explore conditions under which morphological castes may arise and the factors constraining their evolution. Control over offspring morphology and behaviour may often be decoupled. Queens and provisioners can influence morphology directly, through the nutrition they provide, while offspring control their own behaviour as adults. Queens and provisioners may, however, influence worker behaviour indirectly, if offspring modify their behaviour in response to their morphology. Our results suggest that the evolution of a morphologically differentiated worker caste depends on the prior presence of a behavioural caste: specialist worker morphology will be mismatched with behaviour unless some offspring already choose to work. A mother’s certainty about her offspring’s behaviour should also be critical – less certainty results in greater mismatch. Decoupled control is important in maintaining a worker caste, and may result in reduced or no conflict between offspring and provisioners. We also show how worker productivity in the absence of a morphological trait can affect the likelihood of that trait being favoured by natural selection.

Massive transfusions guided by massive transfusion protocols are commonly used to manage critical bleeding, when the patient is at significant risk of morbidity and mortality, and multiple timely decisions must be made by clinicians. Clinical decision support systems are increasingly used to provide patient-specific recommendations by comparing patient information to a knowledge base, and have been shown to improve patient outcomes. To investigate current massive transfusion practice and the experiences and attitudes of anaesthetists towards massive transfusion and clinical decision support systems, we anonymously surveyed 1000 anaesthetists and anaesthesia trainees across Australia and New Zealand. A total of 228 surveys (23.6%) were successfully completed and 227 were analysed for a 23.3% response rate. Most respondents were involved in massive transfusions infrequently (88.1% managed five or fewer massive transfusion protocols per year) and worked at hospitals which have massive transfusion protocols (89.4%). Massive transfusion management was predominantly limited by timely access to point-of-care coagulation assessment and by competition with other tasks, with trainees reporting more significant limitations compared to specialists. The majority of respondents reported that they were likely, or very likely, both to use (73.1%) and to trust (85%) a clinical decision support system for massive transfusions, with no significant difference between anaesthesia trainees and specialists ( P = 0.375 and P = 0.73, respectively). While the response rate to our survey was poor, there was still a wide range of massive transfusion experience among respondents, with multiple subjective factors identified limiting massive transfusion practice. We identified several potential design features and barriers to implementation to assist with the future development of a clinical decision support system for massive transfusion, and overall wide support for a clinical decision support system for massive transfusion among respondents.

Abstract Plasticity is a key trait when an individual’s role in the social environment, and hence its optimum phenotype, fluctuates unpredictably. Plasticity is especially important in primitively eusocial insects where small colony sizes and little morphological caste differentiation mean that individuals may find themselves switching from non-reproductive to reproductive roles. To understand the scope of this plasticity, workers of the primitively eusocial sweat bee Lasioglossum malachurum were experimentally promoted to the reproductive role (worker-queens) and their performance compared with foundress-queens. We focussed on how their developmental trajectory as workers influenced three key traits: group productivity, monopolisation of reproduction, and social control of foraging nest-mates. No significant difference was found between the number of offspring produced by worker-queens and foundress-queens. Genotyping of larvae showed that worker-queens monopolised reproduction in their nests to the same extent as foundress queens. However, non-reproductives foraged less and produced a smaller total offspring biomass when the reproductive was a promoted worker: offspring of worker-queens were all males, which are the cheaper sex to produce. Greater investment in each offspring as the number of foragers increased suggests a limit to both worker-queen and foundress-queen offspring production when a greater quantity of pollen arrives at the nest. The data presented here suggest a remarkable level of plasticity and represent one of the first quantitative studies of worker reproductive plasticity in a non-model primitively eusocial species. Significance statement The ability of workers to take on a reproductive role and produce offspring is expected to relate strongly to the size of their colony. Workers in species with smaller colony sizes should have greater reproductive potential to insure against the death of the queen. We quantified the reproductive plasticity of workers in small colonies of sweat bees by removing the queen and allowing the workers to control the reproductive output of the nest. A single worker then took on the reproductive role and hence prevented her fellow workers from producing offspring of their own. These worker-queens produced as many offspring as control queens, demonstrating remarkable worker plasticity in a primitively eusocial species.

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.

Eusociality, where workers typically forfeit their own reproduction to assist their mothers in raising siblings, is a fundamental paradox in evolutionary biology. By sacrificing personal reproduction, helpers pay a significant cost, which must be outweighed by indirect fitness benefits of helping to raise siblings. In 1983, Jon Seger developed a model showing how in the haplodiploid Hymenoptera (ants, wasps and bees), a partially bivoltine life cycle with alternating sex ratios may have promoted the evolution of eusociality. Seger predicted that eusociality would be more likely to evolve in hymenopterans where a foundress produces a male-biased first brood sex ratio and a female-biased second brood. This allows first brood females to capitalize on super-sister relatedness through helping to produce the female-biased second brood. In Seger’s model, the key factor driving alternating sex ratios was that first brood males survive to mate with females of both the second and the first brood, reducing the reproductive value of second brood males. Despite being potentially critical in the evolution of eusociality, however, male survivorship has received little empirical attention. Here, we tested whether first brood males survive across broods in the facultatively eusocial sweat bee Halictus rubicundus. We obtained high estimates of survival and, while recapture rates were low, at least 10% of first brood males survived until the second brood. We provide empirical evidence supporting Seger’s model. Further work, measuring brood sex ratios and comparing abilities of first and second brood males to compete for fertilizations, is required to fully parameterize the model.

Abstract In eusocial Hymenoptera, queens and their helper offspring should favour different sex investment ratios. Queens should prefer a 1:1 investment ratio, as they are equally related to offspring of both sexes (r = 0.5). In contrast, helpers should favour an investment ratio of 3:1 towards the production of female brood. This conflict arises because helpers are more closely related to full sisters (r = 0.75) than brothers (r = 0.25). However, helpers should invest relatively more in male brood if relatedness asymmetry within their colony is reduced. This can occur due to queen replacement after colony orphaning, multiple paternity and the presence of unrelated alien helpers. We analysed an unprecedentedly large number of colonies (n = 109) from a UK population of Lasioglossum malachurum, an obligate eusocial sweat bee, to tease apart the effects of these factors on colony-level investment ratios. We found that multiple paternity, unrelated alien helpers and colony orphaning were all common. Queen-right colonies invested relatively more in females than did orphaned colonies, producing a split sex ratio. However, investment ratios did not change due to multiple paternity or the presence of alien helpers, reducing inclusive fitness pay-offs for helpers. Queen control may also have been important: helpers rarely laid male eggs, and investment in female brood was lower when queens were large relative to their helpers. Genetic relatedness between helpers and the brood that they rear was 0.43 in one year and 0.37 in another year, suggesting that ecological benefits, as well as relatedness benefits, are necessary for the maintenance of helping behaviour. Significance statement How helping behaviour is maintained in eusocial species is a key topic in evolutionary biology. Colony-level sex investment ratio changes in response to relatedness asymmetries can dramatically influence inclusive fitness benefits for helpers in eusocial Hymenoptera. The extent to which helpers in primitively eusocial colonies can respond adaptively to different sources of variation in relatedness asymmetry is unclear. Using data from 109 colonies of the sweat bee Lasioglossum malachurum, we found that queen loss, but not multiple paternity or the presence of alien helpers, was correlated with colony sex investment ratios. Moreover, we quantified average helper-brood genetic relatedness to test whether it is higher than that predicted under solitary reproduction (r = 0.5). Values equal to and below r = 0.5 suggest that relatedness benefits alone cannot explain the maintenance of helping behaviour. Ecological benefits of group living and/or coercion must also contribute.

Abstract Insect parental care strategies are particularly diverse, and prolonged association between parents and offspring may be a key precursor to the evolution of complex social traits. Macroevolutionary patterns remain obscure, however, due to the few rigorous phylogenetic analyses. The subsocial sphecid wasps are a useful group in which to study parental care because of the diverse range of strategies they exhibit. These strategies range from placing a single prey item in a pre-existing cavity to mass provisioning a pre-built nest, through to complex progressive provisioning where a female feeds larvae in different nests simultaneously as they grow. We show that this diversity stems from multiple independent transitions between states. The strategies we focus on were previously thought of in terms of a stepping-stone model in which complexity increases during evolution, ending with progressive provisioning which is a likely precursor to eusociality. We find that evolution has not always followed this model: reverse transitions are common, and the ancestral state is the most flexible rather than the simplest strategy. Progressive provisioning has evolved several times independently, but transitions away from it appear rare. We discuss the possibility that ancestral plasticity has played a role in the evolution of extended parental care. Significance statement Parental care behaviour leads to prolonged associations between parents and offspring, which is thought to drive the evolution of social living. Despite the importance of insect parental care for shaping the evolution of sociality, relatively few studies have attempted to reconstruct how different strategies evolve in the insects. In this study, we use phylogenetic methods to reconstruct the evolution of the diverse parental care strategies exhibited by the subsocial digger wasps (Sphecidae). Contrary to expectations, we show that parental care in this group has not increased in complexity over evolutionary time. We find that the ancestral state is not the simplest, but may be the most flexible strategy. We suggest that this flexible ancestral strategy may have allowed rapid response to changing environmental conditions which might explain the diversity in parental care strategies that we see in the digger wasps today.

The breeding and non-breeding 'castes' of eusocial insects provide a striking example of role-specific selection, where each caste maximises fitness through different morphological, behavioural and physiological trait values. Typically, queens are long-lived egg-layers, while workers are short-lived, largely sterile foragers. Remarkably, the two castes are nevertheless produced by the same genome. The existence of inter-caste genetic correlations is a neglected consequence of this shared genome, potentially hindering the evolution of caste dimorphism: alleles that increase the productivity of queens may decrease the productivity of workers and vice versa, such that each caste is prevented from reaching optimal trait values. A likely consequence of this 'intralocus caste antagonism' should be the maintenance of genetic variation for fitness and maladaptation within castes (termed 'caste load'), analogous to the result of intralocus sexual antagonism. The aim of this review is to create a research framework for understanding caste antagonism, drawing in part upon conceptual similarities with sexual antagonism. By reviewing both the social insect and sexual antagonism literature, we highlight the current empirical evidence for caste antagonism, discuss social systems of interest, how antagonism might be resolved, and challenges for future research. We also introduce the idea that sexual and caste antagonism could interact, creating a three-way antagonism over gene expression. This includes unpacking the implications of haplodiploidy for the outcome of this complex interaction.

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.

Eusociality is characterised by a reproductive division of labour, where some individuals forgo direct reproduction to instead help raise kin. Socially polymorphic sweat bees are ideal models for addressing the mechanisms underlying the transition from solitary living to eusociality, because different individuals in the same species can express either eusocial or solitary behaviour. A key question is whether alternative social phenotypes represent environmentally induced plasticity or predominantly genetic differentiation between populations. In this paper, we focus on the sweat bee Lasioglossum calceatum, in which northern or high-altitude populations are solitary, whereas more southern or low-altitude populations are typically eusocial. To test whether social phenotype responds to local environmental cues, we transplanted adult females from a solitary, northern population, to a southern site where native bees are typically eusocial. Nearly all native nests were eusocial, with foundresses producing small first brood (B1) females that became workers. In contrast, nine out of ten nests initiated by transplanted bees were solitary, producing female offspring that were the same size as the foundress and entered directly into hibernation. Only one of these ten nests became eusocial. Social phenotype was unlikely to be related to temperature experienced by nest foundresses when provisioning B1 offspring, or by B1 emergence time, both previously implicated in social plasticity seen in two other socially polymorphic sweat bees. Our results suggest that social polymorphism in L. calceatum predominantly reflects genetic differentiation between populations, and that plasticity is in the process of being lost by bees in northern populations. Phenotypic plasticity is thought to play a key role in the early stages of the transition from solitary to eusocial behaviour, but may then be lost if environmental conditions become less variable. Socially polymorphic sweat bees exhibit either solitary or eusocial behaviour in different geographic populations, depending on the length of the nesting season. We tested for plasticity in the socially polymorphic sweat bee Lasioglossum calceatum by transplanting nest foundresses from a northern, non-eusocial population to a southern, eusocial population. Plasticity would be detected if transplanted bees exhibited eusocial behaviour. We found that while native bees were eusocial, 90% of transplanted bees and their offspring did not exhibit traits associated with eusociality. Environmental variables such as time of offspring emergence or temperatures experienced by foundresses during provisioning could not explain these differences. Our results suggest that the ability of transplanted bees to express eusociality is being lost, and that social polymorphism predominantly reflects genetic differences between populations.

One of the most difficult aspects of persuading the public to support climate change policy is the lack of recognition that climate change will likely have a direct impact on an individual’s life. Anecdotal evidence and arguments within the media suggest that those who are skeptical of climate change are more likely to believe that the negative externalities associated with climate change will be experienced by others, and, therefore, are not a concern to that individual. This project examines public perceptions of the health risk posed by climate change. Using a large national public opinion survey of adults in the United States, respondents were asked to evaluate the health risk for themselves, their community, the United States, and the world. The results suggest that individuals evaluate the risk for each of these contexts differently. Statistical analyses are estimated to identify the determinants of each risk perception to identify their respective differences. The implications of these findings on support for climate change policy are discussed.

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.

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.

Objective: Electronic communication mediums provide an opportunity for clinicians to enhance communication, collaboration, and sharing of clinical experience, especially via mobile devices. In 2016, the authors implemented a private online discussion forum in a tertiary-level anaesthesia department to improve communication and collaboration amongst members. The objective of this survey was to assess if these aims were met, to determine the degree of communication medium duplication incurred by its introduction, and to assess departmental communication practices more generally.Methods: A qualitative anonymous online survey was conducted 18 months following the introduction of the online discussion forum, over a two-month period. All 120 medical staff were invited via forum message and hard-copy invitation and responses were stratified by training status.Results: Forty-seven responses were collected (39% response rate), comprising 31 anaesthesia specialists and 16 anaesthesia trainees. Sixty-one percent of respondents reported that discussion had improved following the introduction of the online discussion forum, with no significant difference between specialists and trainees. Despite this, 57% overall maintained a preference for email discussion.Conclusions: This study demonstrated that the introduction of an online discussion forum resulted in a perceived improvement in overall departmental communication. However, it is important that workplaces considering implementation of a similar communication medium determine their employees’ cultural perspectives on technology, established communication preferences and aims of implementation to ensure success.

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.

The evolution of parental care strategies in aculeate (stinging) wasps and bees has been much studied from a functional perspective, but relatively little phylogenetic information is available to place this in a rigorous historical context, especially at the species level. We used mitochondrial cytochrome oxidase I and two nuclear genes, the elongation factor-1α and LW rhodopsin, to investigate the phylogeny of Sphecidae digger wasps. We focus particularly on the tribe Ammophilini, a clade of nonsocial apoid wasps that exhibit unusually diverse parental care strategies. We analysed a 2232 bp dataset for 40 ammophilines plus nine other taxa from within the remaining Sphecidae. Our Bayesian phylogeny provides strong support for the monophyly of Ammophilini and for the monophyly of all six individual ammophiline genera, except that the position of P. affinis within the genus Podalonia is only weakly supported. The monophyly of some, but not all, previously designated species groups within the genus Ammophila is supported. We discuss the implications of our results for the evolution of morphological traits used previously in ammophiline systematics.

In his famous haplodiploidy hypothesis, W. D. Hamilton proposed that high sister–sister relatedness facilitates the evolution of kin-selected reproductive altruism among Hymenopteran females. Subsequent analyses, however, suggested that haplodiploidy cannot promote altruism unless altruists capitalize on relatedness asymmetries by helping to raise offspring whose sex ratio is more female-biased than the population at large. Here, we show that haplodiploidy is in fact more favourable than is diploidy to the evolution of reproductive altruism on the part of females, provided only that dispersal is male-biased (no sex-ratio bias or active kin discrimination is required). The effect is strong, and applies to the evolution both of sterile female helpers and of helping among breeding females. Moreover, a review of existing data suggests that female philopatry and non-local mating are widespread among nest-building Hymenoptera. We thus conclude that Hamilton was correct in his claim that ‘family relationships in the Hymenoptera are potentially very favourable to the evolution of reproductive altruism’.

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.

Microstigmus (Hymenoptera: Crabronidae) is a genus of social apoid wasps which represents an origin of sociality independent from vespoids, but which has so far received little attention. Though group-nesting is widespread in Microstigmus, genetic relatedness has so far been studied in only one species, M. comes. We report on the social biology of M. nigrophthalmus, drawing from behavioural observations and molecular genetic analyses of relatedness and kinship. There was no evidence of distinctive behavioural suites that distinguished reproductive and non-reproductive individuals. Females could mate more than once, but mating frequency was low. Mean relatedness within nests was high, particularly between females (close to 0.5), but pairwise relatedness values were very variable, as nestmates displayed a wide range of relationships. Such high levels of relatedness should be a factor promoting social nesting and cooperative brood care in this species, as females gain only a slight genetic advantage through rearing their own offspring rather than those of nestmates. This study provides the finest analysis of genetic structure so far in an apoid wasp, and indicates that the form of sociality varies greatly between species of Microstigmus.

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.

Eusociality is widely considered a major evolutionary transition. The socially polymorphic sweat bee Halictus rubicundus, solitary in cooler regions of its Holarctic range and eusocial in warmer parts, is an excellent model organism to address this transition, and specifically the question of whether sociality is associated with a strong barrier to gene flow between phenotypically divergent populations. Mitochondrial DNA (COI) from specimens collected across the British Isles, where both solitary and social phenotypes are represented, displayed limited variation, but placed all specimens in the same European lineage; haplotype network analysis failed to differentiate solitary and social lineages. Microsatellite genetic variability was high and enabled us to quantify genetic differentiation among populations and social phenotypes across Great Britain and Ireland. Results from conceptually different analyses consistently showed greater genetic differentiation between geographically distant populations, independently of their social phenotype, suggesting that the two social forms are not reproductively isolated. A landscape genetic approach revealed significant isolation by distance (Mantel test r = 0.622, P < 0.001). The Irish Sea acts as physical barrier to gene flow (partial Mantel test r = 0.453, P < 0.01), indicating that geography, rather than expression of solitary or social behaviour (partial Mantel test r = -0.238, P = 0.053), had a significant effect on the genetic structure of H. rubicundus across the British Isles. Although we cannot reject the hypothesis of a genetic underpinning to differences in solitary and eusocial phenotypes, our data clearly demonstrate a lack of reproductive isolation between the two social forms.

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.

Social aggression arises from conflicts of interest over reproduction in animal societies. Aggression is often highly variable between individuals in a group, may be correlated with dominance, and is often integral to the establishment of dominance hierarchies that in turn determine reproductive opportunities. However, reproductive dominance is not always linked with social dominance: 'queens' are not always the most aggressive individuals in a group. Furthermore, in some animals social rank is determined without aggression, and derived through another means, such as gerontocracy. In such instances, what is the role of aggression, and what underlies the variation between individuals? Here, we investigate the relationship between inheritance rank and aggression in the hover wasp Liostenogaster flavolineata, which has an age-based inheritance queue. All females in this study were of known age and, thus, rank could be determined independently of behaviour. Observations of intra-colony aggression indicated that aggression increased with inheritance rank and occurred among non-breeding subordinates. This cannot be explained by models that do not account for aggression between non-breeders. It is likely that contests over inheritance rank and the higher future fitness anticipated by high-ranking individuals account for this pattern.

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.

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

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.

Helpers in primitively eusocial and cooperatively breeding animal societies forfeit their own reproduction to rear the offspring of a queen or breeding pair, but may eventually attain breeding status themselves. Kin selection1 provides a widely accepted theoretical framework for understanding these societies, but differences in genetic relatedness do not explain a universal societal feature: the huge variation between individuals in helping effort2,3,4,5,6,7,8,9,10. An alternative explanation for this variation lies in a fundamental trade-off faced by helpers: by working harder, they increase the indirect component of their fitness, but simultaneously decrease their own future survival and fecundity2,4,8. Here, we show that individuals work less hard when they stand to lose more future fitness through working. We experimentally manipulated two components of future fitness in social queues of hover wasps (Stenogastrinae): a helper's chance of inheriting an egg-laying position, and the workforce available to rear her offspring should she inherit. After each manipulation, helpers increased or decreased their effort as appropriate to the change in expected future fitness that they experienced. Although helping provides significant indirect fitness benefits for hover wasps11, our study shows that variation in the costs associated with helping is the major determinant of helping effort.

The degree of genetic heterogeneity among the individuals in an animal society depends on the society's genetic structure. Genetic heterogeneity, in turn, means that group members will differ in their reproductive objectives and conflicts over reproduction may arise. The resolution of these conflicts may be reflected in the way that reproduction is partitioned between potential reproductives. We used 5 microsatellite loci to investigate genetic structure and reproductive skew in 17 nests of the Malaysian hover wasp, Parischnogaster alternata. Parischnogaster alternata colonies are small (1--10 females), and all adult colony members are capable of mating and producing offspring. We found that colonies tended to consist of closely related individuals and that at any one time the production of both female and male offspring was nearly always monopolized by a single dominant female, despite considerable variation between nests in parameters predicted to affect skew. Subordinate females that remained in their natal colonies obtained indirect fitness benefits by helping to raise offspring to which they were related. Subordinate females also appeared to be positioned within an age-based queue for inheritance of the dominant egg-laying position. We suggest that the high skew in P. alternata may result from strong ecological constraints on solitary nesting, high relatedness, and a relatively high probability that subordinates will eventually inherit the position of dominance. Copyright 2006.

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.

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.