Molecular signatures of alternative reproductive strategies in a facultatively social hover wasp

Social insect reproductives and non-reproductives represent ideal models with which to understand the expression and regulation of alternative phenotypes. Most research in this area has focused on the developmental regulation of reproductive phenotypes in obligately social taxa such as honey bees, while relatively few studies have addressed the molecular correlates of reproductive differentiation in species in which the division of reproductive labour is established only in plastic dominance hierarchies. To address this knowledge gap, we generate the first genome for any stenogastrine wasp and analyse brain transcriptomic data for non-reproductives and reproductives of the facultatively social species Liostenogaster flavolineata, a representative of one of the simplest forms of social living. By experimentally manipulating the reproductive 'queues' exhibited by social colonies of this species, we show that reproductive division of labour in this species is associated with transcriptomic signatures that are more subtle and variable than those observed in social taxa in which colony living has become obligate; that variation in gene expression among non-reproductives reflects their investment into foraging effort more than their social rank; and that genes associated with reproductive division of labour overlap to some extent with those underlying division of labour in the separate polistine origin of wasp sociality but only explain a small portion of overall variation in this trait. These results indicate that broad patterns of within-colony transcriptomic differentiation in this species are similar to those in Polistinae but offer little support for the existence of a strongly conserved 'toolkit' for sociality.

Conflict and conflict resolution in the major transitions

Conflict and conflict resolution have been argued to be fundamental to the major transitions in evolution. These were key events in life's history in which previously independently living individuals cooperatively formed a higher-level individual, such as a multicellular organism or eusocial colony. Conflict has its central role because, to proceed stably, the evolution of individuality in each major transition required within-individual conflict to be held in check. This review revisits the role of conflict and conflict resolution in the major transitions, addressing recent work arguing for a minor role. Inclusive fitness logic suggests that differences between the kin structures of clones and sexual families support the absence of conflict at the origin of multicellularity but, by contrast, suggest that key conflicts existed at the origin of eusociality. A principal example is conflict over replacing the founding queen (queen replacement). Following the origin of each transition, conflict remained important, because within-individual conflict potentially disrupts the attainment of maximal individuality (organismality) in the system. The conclusion is that conflict remains central to understanding the major transitions, essentially because conflict arises from differences in inclusive fitness optima while conflict resolution can help the system attain a high degree of coincidence of inclusive fitness interests.

Why do Hymenopteran workers drift to non-natal groups? Generalized reciprocity and the maximization of group and parental success

Eusocial Hymenoptera are often characterized by having facultatively or obligately sterile worker castes. However, findings across an increasing number of species are that some workers are non-natal-they have 'drifted' away from where they were born and raised. Moreover, drifters are often indistinguishable from natal workers in the work and benefits provided to joined groups. This seems an evolutionary paradox of providing benefits to potentially unrelated individuals over close kin. Rather than being mistakes, drifting is proposed to be adaptive if joiners either gain inclusive fitness by preferentially moving to other kin groups or through generalized reciprocity in which exchanging workers across groups raises group-level genetic diversity and creates social heterosis. It is unclear, however, if reciprocity is unlikely because of a susceptibility to cheating. In resolving this question, a series of evolutionary simulations show: (1) Reciprocity can persist under a range of genetic assumptions and scenarios of cheating, (2) cheating almost always evolves, but can be expressed in a variety of ways that are not always predictable, (3) the inclusive fitness hypothesis is equally or more susceptible to cheating. Moreover, existing data in Hymenoptera (although not extensive) are more consistent with generalized reciprocity. This supports a hypothesis that drifting, as a phenomenon, may more often reflect maximization of group and parental fitness rather than fitness gains for the individual drifters.

Philopatry yields higher fitness than dispersal in a cooperative breeder with sex-specific life history trajectories

Social evolution is tightly linked to dispersal decisions, but the ecological and social factors selecting for philopatry or dispersal often remain obscure. Elucidating selection mechanisms underlying alternative life histories requires measurement of fitness effects in the wild. We report on a long-term field study of 496 individually marked cooperatively breeding fish, showing that philopatry is beneficial as it increases breeding tenure and lifetime reproductive success in both sexes. Dispersers predominantly join established groups and end up in smaller groups when they ascend to dominance. Life history trajectories are sex specific, with males growing faster, dying earlier, and dispersing more, whereas females more likely inherit a breeding position. Increased male dispersal does not seem to reflect an adaptive preference but rather sex-specific differences in intrasexual competition. Cooperative groups may thus be maintained because of inherent benefits of philopatry, of which females seem to get the greater share in social cichlids.

The suppressive potential of a gene drive in populations of invasive social wasps is currently limited

Social insects are very successful invasive species, and the continued increase of global trade and transportation has exacerbated this problem. The yellow-legged hornet, Vespa velutina nigrithorax (henceforth Asian hornet), is drastically expanding its range in Western Europe. As an apex insect predator, this hornet poses a serious threat to the honey bee industry and endemic pollinators. Current suppression methods have proven too inefficient and expensive to limit its spread. Gene drives might be an effective tool to control this species, but their use has not yet been thoroughly investigated in social insects. Here, we built a model that matches the hornet's life history and modelled the effect of different gene drive scenarios on an established invasive population. To test the broader applicability and sensitivity of the model, we also incorporated the invasive European paper wasp Polistes dominula. We find that, due to the haplodiploidy of social hymenopterans, only a gene drive targeting female fertility is promising for population control. Our results show that although a gene drive can suppress a social wasp population, it can only do so under fairly stringent gene drive-specific conditions. This is due to a combination of two factors: first, the large number of surviving offspring that social wasp colonies produce make it possible that, even with very limited formation of resistance alleles, such alleles can quickly spread and rescue the population. Second, due to social wasp life history, infertile individuals do not compete with fertile ones, allowing fertile individuals to maintain a large population size even when drive alleles are widespread. Nevertheless, continued improvements in gene drive technology may make it a promising method for the control of invasive social insects in the future.

The evolution of ageing in cooperative breeders

Cooperatively breeding animals live longer than their solitary counterparts. This has been suggested for birds, mole rats, and social insects. A common explanation for these long lifespans is that cooperative breeding evolves more readily in long-lived species because lower mortality reduces the rate of territory turnover and thus leads to a limitation of breeding territories. Here, we reverse this argument and show that-rather than being a cause for its evolution-long lifespans are an evolutionary consequence of cooperative breeding. In evolutionary individual-based simulations, we show that natural selection favors a delayed onset of senescence in cooperative breeders, relative to solitary breeders, because cooperative breeders have a delayed age of first reproduction as helpers wait in a reproductive queue to obtain breeder status. Especially long lifespans evolve in cooperative breeders in which queue positions depend on the helpers' age rank among the helpers within the breeding territory. Furthermore, we show that lower genetic relatedness among group members leads to the evolution of longer lifespans. This is because selection against higher mortality is weaker when mortality reduces competition for breeding between relatives. Our results link the evolutionary theory of ageing with kin selection theory, demonstrating that the evolution of ageing in cooperative breeders is driven by the timing of reproduction and kin structure within breeding territories.

Nepotistic colony fission in dense colony aggregations of an Australian paper wasp

Social insects are highly diverse in their social structures, aside from the consistent presence of reproductive castes. Among social insects, the Australian paper wasp Ropalidia plebeiana constructs extremely dense colony aggregations consisting of hundreds of colonies within a few square meters; however, little is known about the aggregation structures. We genetically analyzed the colony and population structure of R. plebeiana, and concomitant variations in colony sex ratios. In spring, the foundress (candidate queen) group started their colonies on a single old comb from the previous season, subsequently dividing these old combs via relatedness-based comb-cutting. Female philopatry, a prerequisite condition of Local Resource Competition (LRC), was confirmed. The colony sex ratio of reproductive individuals (male and female offspring for the next generation) became slightly male-biased in larger colonies, as predicted under LRC. However, the number of foundresses was positively associated with the number of reproductive individuals, suggesting that Local Resource Enhancement (LRE) also operates. Although the population structure appears to meet the prerequisites of LRC, the sex ratio appears to be modulated by factors other than LRC. Rather, through LRE, the availability of female helpers at the founding stage is likely to mitigate the sex ratios predicted under LRC.

Sisters doing it for themselves: extensive reproductive plasticity in workers of a primitively eusocial bee

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.

Who to help? Helping decisions in a cooperatively breeding bird with redirected care

Abstract 

Cooperative breeding sometimes occurs when adult breeders form groups following natal dispersal and mating. In such cases, individuals typically face a choice of social partner with whom to cooperate. Selecting appropriate social partners is crucial to maximising the fitness payoffs from cooperation, but our understanding of the criteria guiding partner choice is limited. Here, we analyse helping decisions by long-tailed tits (Aegithalos caudatus), which may redirect their care to assist breeders in raising offspring following the failure of their own nests. In this species, helpers prefer to help relatives at nearby nests, but it is unclear whether other criteria that may affect helper fitness also influence helping decisions. When choosing among broods of equivalent kinship, we found that helpers did not prefer those broods that offered the greatest indirect fitness returns. Further analyses revealed that helpers did not choose nests on the basis of brood size or age, but were more likely to help broods that were closer to their own failed nests and that were already being cared for by other helpers. Both effects likely reflect the limited choice available to helpers: although individuals breed close to relatives within kin neighbourhoods, a high rate of nest predation constrains helpers’ choice of broods. In other species where cooperatively breeding groups form after natal dispersal, a greater range of options may be available and here detailed analysis of group formation will be helpful for determining the decision rules that underpin partner choice and permit stable cooperation in the face of alternative options.

Significance statement

Cooperative breeding occurs most frequently when offspring delay dispersal from their natal site and help to care for their younger siblings. In some species, however, individuals first disperse and then come together as adults to cooperate in rearing young. In the latter case, multiple social partners may be available — what then determines which partner is helped? We studied partner choice in long-tailed tits, which may help to feed other broods if their own brood fails. When multiple related broods were available, individuals were more likely to help those close by but showed no preference for broods offering the greatest indirect fitness returns. One explanation for this result is that helping options for most individuals are limited by high levels of nest predation, favouring a simpler decision-making process based on identifying close relatives breeding in close proximity.

Extreme reproductive skew at the dawn of sociality is consistent with inclusive fitness theory but problematic for routes to eusociality

To understand the earliest stages of social evolution, we need to identify species that are undergoing the initial steps into sociality. Amphylaeus morosus is the only unambiguously known social species in the bee family Colletidae and represents an independent origin of sociality within the Apoidea. This allows us to investigate the selective factors promoting the transition from solitary to social nesting. Using genome-wide SNP genotyping, we infer robust pedigree relationships to identify maternity of brood and intracolony relatedness for colonies at the end of the reproductive season. We show that A. morosus forms both matrifilial and full-sibling colonies, both involving complete or almost complete monopolization over reproduction. In social colonies, the reproductive primary was also the primary forager with the secondary female remaining in the nest, presumably as a guard. Social nesting provided significant protection against parasitism and increased brood survivorship in general. We show that secondary females gain large indirect fitness benefits from defensive outcomes, enough to satisfy the conditions of inclusive fitness theory, despite an over-production of males in social colonies. These results suggest an avenue to sociality that involves high relatedness and, very surprisingly, extreme reproductive skew in its earliest stages and raises important questions about the evolutionary steps in pathways to eusociality.

Complementary interactions between indirect and direct fitness in a cooperatively breeding bird

Altruism is difficult to explain evolutionarily and to understand it, there is a need to quantify the benefits and costs to altruists. Hamilton's theory of kin selection argues that altruism can persist if the costs to altruists are offset by indirect fitness payoffs from helping related recipients. Nevertheless, helping non-kin is also common and in such situations the costs must be compensated for by direct benefits. While previous researchers tended to evaluate the indirect and direct fitness in isolation, we expect that they have a complementary interaction where altruists are associated with recipients of different relatedness within a population. The prediction is tested with 12 years of data on lifetime reproductive success for a cooperatively breeding bird, Tibetan ground tits Pseudopodoces humilis. Helpers who helped distantly related recipients gained significantly lower indirect benefits than those who helped closely related recipients, but the opposite was true for direct fitness, thereby making these helpers have an equal inclusive fitness. Helping efforts were independent of helpers' relatedness to recipients, but those helping distantly related recipients were more likely to inherit the resident territory, which could be responsible for their high direct reproductive success. Our findings provide an explanatory model for the widespread coexistence of altruists and recipients with varying relatedness within a single population.

Five decades of misunderstanding in the social Hymenoptera: a review and meta-analysis of Michener's paradox

In a much-cited 1964 paper entitled "Reproductive efficiency in relation to colony size in hymenopterous societies," Charles Michener investigated the correlation between a colony's size and its reproductive efficiency - the ability of its adult females to produce reproductives, measured as per-capita output. Based on his analysis of published data from destructively sampled colonies in 18 species, he reported that in most of these species efficiency decreased with increasing colony size. His conclusion that efficiency is higher in smaller groups has since gained widespread acceptance. But it created a seeming paradox: how can natural selection maintain social behaviour when a female apparently enjoys her highest per-capita output by working alone? Here we treat Michener's pattern as a hypothesis and perform the first large-scale test of its prediction across the eusocial Hymenoptera. Because data on actual output of reproductives were not available for most species, Michener used various proxies, such as nest size, numbers of brood, or amounts of stored food. We show that for each of Michener's data sets the reported decline in per-capita productivity can be explained by factors other than decreasing efficiency, calling into question his conclusion that declining efficiency is the cause of the pattern. The most prominent cause of bias is the failure of the proxy to capture all forms of output in which the colony invests during the course of its ontogeny. Other biasing factors include seasonal effects and a variety of methodological flaws in the data sets he used. We then summarize the results of 215 data sets drawn from post-1964 studies of 80 species in 33 genera that better control for these factors. Of these, 163 data sets are included in two meta-analyses that statistically synthesize the available data on the relationship between colony size and efficiency, accounting for variable sample sizes and non-independence among the data sets. The overall effect, and those for most taxonomic subgroups, indicates no loss of efficiency with increasing colony size. Two exceptional taxa, the halictid bees and independent-founding paper wasps, show negative trends consistent with the Michener hypothesis in some species. We conclude that in most species, particularly those with large colony sizes, the hypothesis of decreasing efficiency with increasing colony size is not supported. Finally, we explore potential mechanisms through which the level of efficiency can decrease, be maintained, or even increase, as colonies increase in size.

The build-up of dominance hierarchies in eusocial insects

Reproductive division of labour is a hallmark of eusocial insects. However, its stability can often be hampered by the potential for reproduction by otherwise sterile nest-mates. Dominance hierarchy has a crucial role in some species in regulating which individuals reproduce. Compared with those in vertebrates, the dominance hierarchies in eusocial insects tend to involve many more individuals, and should require additional selective forces unique to them. Here, we provide an overview of a series of studies on dominance hierarchies in eusocial insects. Although reported from diverse eusocial taxa, dominance hierarchies have been extensively studied in paper wasps and ponerine ants. Starting from molecular physiological attributes of individuals, we describe how the emergence of dominance hierarchies can be understood as a kind of self-organizing process through individual memory and local behavioural interactions. The resulting global structures can be captured by using network analyses. Lastly, we argue the adaptive significance of dominance hierarchies from the standpoint of sterile subordinates. Kin selection, underpinned by relatedness between nest-mates, is key to the subordinates' acceptance of their positions in the hierarchies. This article is part of the theme issue 'The centennial of the pecking order: current state and future prospects for the study of dominance hierarchies'.

Ecological Drivers of Non-kin Cooperation in the Hymenoptera

Despite the prominence of kin selection as a framework for understanding the evolution of sociality, many animal groups are comprised of unrelated individuals. These non-kin systems provide valuable models that can illuminate drivers of social evolution beyond indirect fitness benefits. Within the Hymenoptera, whose highly related eusocial groups have long been cornerstones of kin selection theory, groups may form even when indirect fitness benefits for helpers are low or absent. These non-kin groups are widespread and abundant, yet have received relatively little attention. We review the diversity and organization of non-kin sociality across the Hymenoptera, particularly among the communal bees and polygynous ants and wasps. Further, we discuss common drivers of sociality across these groups, with a particular focus on ecological factors. Ecological contexts that favor non-kin sociality include those dominated by resource scarcity or competition, climatic stressors, predation and parasitism, and/or physiological constraints associated with reproduction and resource exploitation. Finally, we situate Hymenopteran non-kin sociality within a broader biological context by extending insights from these systems across diverse taxa, especially the social vertebrates. Non-kin social groups thus provide unique demonstrations of the importance of ecological factors in mediating the evolutionary transition from solitary to group living.

Life History and the Transitions to Eusociality in the Hymenoptera

Although indirect selection through relatives (kin selection) can explain the evolution of effectively sterile offspring that act as helpers at the nest (eusociality) in the ants, bees, and stinging wasps (aculeate Hymenoptera), the genetic, ecological, and life history conditions that favor transitions to eusociality are poorly understood. In this study, ancestral state reconstruction on recently published phylogenies was used to identify the independent transitions to eusociality in each of the taxonomic families that exhibit eusociality. Semisociality, in which a single nest co-foundress monopolizes reproduction, often precedes eusociality outside the vespid wasps. Such a route to eusociality, which is consistent with groups consisting of a mother and her daughters (subsocial) at some stage and ancestral monogamy, is favored by the haplodiploid genetic sex determination of the Hymenoptera (diploid females and haploid males) and thus may explain why eusociality is common in the Hymenoptera. Ancestral states were also reconstructed for life history characters that have been implicated in the origins of eusociality. A loss of larval diapause during unfavorable seasons or conditions precedes, or coincides with, all but one transition to eusociality. This pattern is confirmed using phylogenetic tests of associations between state transition rates for sweat bees and apid bees. A loss of larval diapause may simply reflect the subsocial route to eusociality since subsociality is defined as females interacting with their adult daughters. A loss of larval diapause and a gain of subsociality may be associated with an extended breeding season that permits the production of at least two broods, which is necessary for helpers to evolve. Adult diapause may also lower the selective barrier to a first-brood daughter becoming a helper. Obligate eusociality meets the definition of a major evolutionary transition, and such transitions have occurred five times in the Hymenoptera.

Selection Forces Driving Herding of Herbivorous Insect Larvae

Herding behavior is widespread among herbivorous insect larvae across several orders. These larval societies represent one of several different forms of insect sociality that have historically received less attention than the well-known eusocial model but are showing us that social diversity in insects is broader than originally imagined. These alternative forms of sociality often focus attention on the ecology, rather than the genetics, of sociality. Indeed, mutually beneficial cooperation among individuals is increasingly recognized as important relative to relatedness in the evolution of sociality, and I will explore its role in larval insect herds. Larval herds vary in in the complexity of their social behavior but what they have in common includes exhibiting specialized social behaviors that are ineffective in isolated individuals but mutually beneficial in groups. They hence constitute cooperation with direct advantages that doesn’t require kinship between cooperators to be adaptive. Examples include: trail following, head-to-tail processions and other behaviors that keep groups together, huddling tightly to bask, synchronized biting and edge-feeding to overwhelm plant defenses, silk production for shelter building or covering plant trichomes and collective defensive behaviors like head-swaying. Various selective advantages to group living have been suggested and I propose that different benefits are at play in different taxa where herding has evolved independently. Proposed benefits include those relative to selection pressure from abiotic factors (e.g., thermoregulation), to bottom-up pressures from plants or to top-down pressures from natural enemies. The adaptive value of herding cooperation must be understood in the context of the organism’s niche and suite of traits. I propose several such suites in herbivorous larvae that occupy different niches. First, some herds aggregate to thermoregulate collectively, particularly in early spring feeders of the temperate zone. Second, other species aggregate to overwhelm host plant defenses, frequently observed in tropical species. Third, species that feed on toxic plants can aggregate to enhance the warning signal produced by aposematic coloration or stereotyped defensive behaviors. Finally, the combination of traits including gregariousness, conspicuous behavior and warning signals can be favored by a synergy between bottom-up and top-down selective forces. When larvae on toxic plants aggregate to overcome plant defenses, this grouping makes them conspicuous to predators and favors warning signals. I thus conclude that a single explanation is not sufficient for the broad range of herding behaviors that occurs in phylogenetically diverse insect larvae in different environments.

The nature of privilege: intergenerational wealth in animal societies

Wealth inequality is widespread across human societies, from pastoral and small-scale agricultural groups to large modern social structures. The intergenerational transfer of wealth privileges some individuals over others through the transmission of resources external to an individual organism. Privileged access to household wealth (e.g., land, shelter, silver) positively influences the destinies of some (and their descendants) over others in human societies. Strikingly parallel phenomena exist in animal societies. Inheritance of nongenetic commodities (e.g., a nest, territory, tool) external to an individual also contributes greatly to direct fitness in animals. Here, we illustrate the evolutionary diversity of privilege and its disparity-generating effects on the evolutionary trajectories of lineages across the Tree of Life. We propose that integration of approaches used to study these patterns in humans may offer new insights into a core principle from behavioral ecology—differential access to inherited resources—and help to establish a broad, comparative framework for studying inequality in animals.

Competition Drives Group Formation and Reduces Within Nest Relatedness in a Facultatively Social Carpenter Bee

Animals respond to competition among kin for critical breeding resources in two ways: avoidance of direct fitness costs via dispersal of siblings to breed separately, and formation of kin-based societies in which subordinates offset direct fitness costs of breeding competition via altruism and increased indirect fitness. In the facultatively social eastern carpenter bee, nests are a critical breeding resource in perpetually short supply, leading to strong competition among females. Observations of individually marked and genotyped females in conditions of high and low resource competition demonstrate that competition leads to resource sharing and group nesting. However, in contrast to almost all known animal societies, females avoid nesting with relatives, and disperse from their natal nests to join social groups of non-relatives. This is the first example of a structured insect society with cooperation nestmates, the majority of which are unrelated; thus cooperation is more likely based on selection for direct, rather than indirect fitness. By forming social groups of non-kin, females avoid the indirect fitness costs of kin competition among sisters, yet increase their chances of successful reproduction, and thus direct fitness, when forming colonies of non-relatives.

Sex-Specific Routes to Independent Breeding in a Polygynous Cooperative Breeder

How can individuals obtain a breeding position and what are the benefits associated with philopatry compared to dispersal? These questions are particularly intriguing in polygamous cooperative breeders, where dispersal strategies reflect major life history decisions, and routes to independent breeding may utterly differ between the sexes. We scrutinized sex-dependent life-history routes by investigating dispersal patterns, growth rates and mortality in a wild colony of the cooperatively breeding cichlid Neolamprologus savoryi. Our data reveal that female helpers typically obtain dominant breeding positions immediately after reaching sexual maturity, which is associated with strongly reduced growth. In contrast, males obtain breeder status only at twice the age of females. After reaching sexual maturity, males follow one of two strategies: (i) they may retain their subordinate status within the harem of a dominant male, which may provide protection against predators but involves costs by helping in territory maintenance, defence and brood care; or (ii) they may disperse and adopt a solitary status, which diminishes survival chances and apparently reflects a best-of-a-bad-job strategy, as there are no obvious compensating future fitness benefits associated with this pathway. Our study illustrates that sex-dependent life history strategies strongly relate to specific social structures and mating patterns, with important implications for growth rates, the age at which breeding status is obtained, and survival.

Age- and sex-dependent variation in relatedness corresponds to reproductive skew, territory inheritance, and workload in cooperatively breeding cichlids

Kin selection plays a major role in the evolution of cooperative systems. However, many social species exhibit complex within-group relatedness structures, where kin selection alone cannot explain the occurrence of cooperative behavior. Understanding such social structures is crucial to elucidate the evolution and maintenance of multi-layered cooperative societies. In lamprologine cichlids, intragroup relatedness seems to correlate positively with reproductive skew, suggesting that in this clade dominants tend to provide reproductive concessions to unrelated subordinates to secure their participation in brood care. We investigate how patterns of within-group relatedness covary with direct and indirect fitness benefits of cooperation in a highly social vertebrate, the cooperatively breeding, polygynous lamprologine cichlid Neolamprologus savoryi. Behavioral and genetic data from 43 groups containing 578 individuals show that groups are socially and genetically structured into subgroups. About 17% of group members were unrelated immigrants, and average relatedness between breeders and brood care helpers declined with helper age due to group membership dynamics. Hence the relative importance of direct and indirect fitness benefits of cooperation depends on helper age. Our findings highlight how both direct and indirect fitness benefits of cooperation and group membership can select for cooperative behavior in societies comprising complex social and relatedness structures.

Social consequences of energetically costly nest construction in a facultatively social bee

Social groups form when the costs of breeding independently exceed fitness costs imposed by group living. The costs of independent breeding can often be energetic, especially for animals performing expensive behaviours, such as nest construction. To test the hypothesis that nesting costs can drive sociality by disincentivizing independent nest founding, we measured the energetics of nest construction and inheritance in a facultatively social carpenter bee (Xylocopa sonorina Smith), which bores tunnel nests in wood. We measured metabolic rates of bees excavating wood and used computerized tomography images of nesting logs to measure excavation volumes. From these data, we demonstrate costly energetic investments in nest excavation of a minimum 4.3 kJ per offspring provisioned, an expense equivalent to nearly 7 h of flight. This high, potentially prohibitive cost of nest founding may explain why females compete for existing nests rather than constructing new ones, often leading to the formation of social groups. Further, we found that nest inheritors varied considerably in their investment in nest renovation, with costs ranging more than 12-fold (from 7.08 to 89.1 kJ energy), probably reflecting differences in inherited nest quality. On average, renovation costs were lower than estimated new nest construction costs, with some nests providing major savings. These results suggest that females may join social groups to avoid steep energetic costs, but that the benefits of this strategy are not experienced equally.

Ten recent insights for our understanding of cooperation

Since Hamilton published his seminal papers in 1964, our understanding of the importance of cooperation for life on Earth has evolved beyond recognition. Early research was focused on altruism in the social insects, where the problem of cooperation was easy to see. In more recent years, research into cooperation has expanded across the entire tree of life, and has been revolutionized by advances in genetic, microbiological and analytical techniques. We highlight ten insights that have arisen from these advances, which have illuminated generalizations across different taxa, making the world simpler to explain. Furthermore, progress in these areas has opened up numerous new problems to solve, suggesting exciting directions for future research.

Connecting social polymorphism to single nucleotide polymorphism: population genomics of the small carpenter bee, Ceratina australensis

How do social insects expand and adapt to new ranges and how does sociality per se contribute to their success (or failure)? These questions can become tractable with the use of population genomics. We explored the population genomics of the socially polymorphic small carpenter bee, Ceratina australensis, across its range in eastern and southern Australia to search for evidence of selection and identify loci associated with social nesting. We sampled and sequenced fully the genomes of 54 socially and solitarily nesting C. australensis within Queensland, Victoria and South Australia, yielding 2 061 234 single nucleotide polymorphisms across the genome. We found strong evidence of population-specific selection and evidence of genetic variants associated with social nesting behaviour. Both the sets of associated loci and differentially expressed ‘social’ genes had evidence of positive selection, suggesting that alleles at genes associated with social nesting might provide different fitness benefits.

Split sex ratios and genetic relatedness in a primitively eusocial sweat bee

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.

Substantial direct fitness gains of workers in a highly eusocial ant

Hamilton's theory of inclusive fitness suggests that helpers in animal societies gain fitness indirectly by increasing the reproductive performance of a related beneficiary. Helpers in cooperatively breeding birds, mammals and primitively eusocial wasps may additionally obtain direct fitness through inheriting the nest or mating partner of the former reproductive. Here, we show that also workers of a highly eusocial ant may achieve considerable direct fitness by producing males in both queenless and queenright colonies. We investigated the reproductive success of workers of the ant Temnothorax crassispinus in nature and the laboratory by dissecting workers and determining the origin of males by microsatellite analysis. We show that workers are capable of activating their ovaries and successfully producing their sons independently of the presence of a queen. Genotypes revealed that at least one fifth of the males in natural queenright colonies were not offspring of the queen. Most worker-produced males could be assigned to workers that were unrelated to the queen, suggesting egg-laying by drifting workers.

Queen succession conflict in the paper wasp Polistes dominula is mitigated by age-based convention

Reproduction in cooperative animal groups is often dominated by one or a few individuals, with the remaining group members relegated to nonreproductive helping roles. This reproductive skew can evolve if helpers receive fitness benefits such as potential future inheritance of the breeding position, but the mechanisms by which inheritance is determined are not well resolved. Polistes paper wasps form highly reproductively skewed groups and inheritance of the breeding position is likely to play a key role in the maintenance of this social structure, making them excellent models for the processes by which simple societies are maintained. Reproductive succession is thought to be determined via an age-based convention in some Polistes species, but there is also evidence for contest-based succession systems in which the replacement queen uses physical aggression to overpower and thereby subordinate her nestmates. Here, we provide evidence that queen succession in colonies of the European paper wasp Polistes dominula is determined via convention rather than contest, with little disruption to the colony’s social functioning. We use queen removal experiments and fine-scale behavioral analyses to confirm that age is a strong predictor of succession, and that behavioral responses to queen removal are restricted to the oldest individuals rather than being experienced equally across the group. We provide the most comprehensive and detailed experimental analysis on the dynamics of breeder succession in a cooperatively breeding invertebrate to date, thereby shedding light on the mechanisms by which animal societies are able to maintain cohesion in the face of within-group conflict.

Rethinking recognition: social context in adult life rather than early experience shapes recognition in a social wasp

Social recognition represents the foundation of social living. To what extent social recognition is hard-wired by early-life experience or flexible and influenced by social context of later life stages is a crucial question in animal behaviour studies. Social insects have represented classic models to investigate the subject, and the acknowledged idea is that relevant information to create the referent template for nest-mate recognition (NMR) is usually acquired during an early sensitive period in adult life. Experimental evidence, however, highlighted that other processes may also be at work in creating the template and that such a template may be updated during adult life according to social requirements. However, currently, we lack an ad hoc experiment testing the alternative hypotheses at the basis of NMR ontogeny in social insects. Thus, to investigate the mechanisms underlying the ontogeny of NMR in Polistes wasps, a model genus in recognition studies, and their different role in determining recognition abilities, we subjected Polistes dominula workers to different olfactory experiences in different phases of their life before inserting them into the social environment of a novel colony and testing them in recognition bioassays. Our results show that workers develop their NMR abilities based on their social context rather than through pre-imaginal and early learning or self-referencing. Our study demonstrates that the social context represents the major component shaping recognition abilities in a social wasp, therefore shedding new light on the ontogeny of recognition in paper wasps and prompting the reader to rethink about the traditional knowledge at the basis of the recognition in social insects. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

Body size and sperm quality in queen- And worker-produced ant males

Workers of many species of social Hymenoptera have functional ovaries and are capable of laying haploid, unfertilized eggs, at least in the absence of a queen. Except for honeybees, it remains largely unknown whether worker-produced males have the same quality as queen-produced males and whether workers benefit in direct fitness by producing their sons. Previous studies in the monogynous ant Temnothorax crassispinus revealed that a high proportion of males in natural and laboratory colonies are worker offspring. Here, we compare longevity, body size, sperm length and sperm viability between queen- and worker-produced males. We either split queenright colonies into queenright and queenless halves or removed the queen from a fraction of the queenright colonies and then examined the newly produced males. Male quality traits varied considerably among colonies but differed only slightly between queen- and worker-produced males. Worker-produced males outnumbered queen-produced males and also had a longer lifespan, but under certain rearing conditions sperm from queen-produced males had a higher viability.

The evolution of eusociality: no risk-return tradeoff but the ecology matters

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.

Egg discrimination is mediated by individual differences in queen olfactory responsiveness and boldness

Nest parasites attempt to shift the cost of rearing young from themselves to others. Despite strong selection to avoid this exploitation, there is considerable variation among-individuals in susceptibility to nest parasites. We evaluated the effects of individual variation in boldness, aggressiveness, and olfactory responsiveness on egg discrimination in wasps: Polistes metricus, which founds nests as singleton individuals, and P. dominula, which founds nests in small coalitions. Aggressiveness and boldness were evaluated using individuals’ response to mechanical disturbance, and olfactory responsiveness was evaluated using individuals’ tendency to respond to a novel rewarding stimulus. Egg discrimination was evaluated by presenting each queen with a variety of foreign eggs: 1) unaltered eggs laid by the resident (negative control group), 2) eggs produced by the resident female that were removed and replaced (procedural control), 3) eggs of foreign conspecifics (conspecific egg), and 4) eggs of a heterospecific congener (heterospecific egg). Females of both species never rejected untampered eggs and rejected procedural controls in only 35% of cases. Both species were twice as likely (70% rejection rate) to reject eggs of foreign conspecifics or heterospecifics. In P. dominula, bolder individuals and those with low olfactory responsiveness were more likely to reject foreign eggs. In P. metricus, boldness was not associated with egg rejection, but individuals with heightened olfactory responsiveness were more likely to reject foreign eggs. Thus, there are contrasting associations between behavioral phenotypes and egg rejection across species. These results are discussed in light of differences in the colony founding behavior of these species.

Medial Cortical Structures Mediate Implicit Trustworthiness Judgments about Kin Faces, but Not Familiar Faces: A Brief Report

Human kin recognition activates substrates of the extended facial processing network, notably the right-hemisphere structures involved in self-face recognition and posterior medial cortical substrates. To understand the mechanisms underlying prosociality toward kin faces in comparison to other familiar faces, we investigated the neural correlates of implicit trustworthiness ratings to faces of actual kin and personal friends, controlling for activation to distracter faces. When controlling for activation associated with unknown faces, trustworthiness ratings of faces of kin, compared to friends, were associated with increased activation in the dorsal anterior cingulate cortex, posterior cingulate, and precuneous. On the other hand, trustworthiness ratings of friend faces, relative to kin faces, were associated with the lateral occipital gyrus and insular cortex. Trustworthiness ratings for unknown faces were only associated with activation in the fusiform gyrus. These findings suggest that we should employ medial cortical substrates known to be part of the self-other network when making implicit social judgements about kin, but not other classes of facial stimuli.

Reproductive skew in cooperative breeding: Environmental variability, antagonistic selection, choice, and control

A multitude of factors may determine reproductive skew among cooperative breeders. One explanation, derived from inclusive fitness theory, is that groups can partition reproduction such that subordinates do at least as well as noncooperative solitary individuals. The majority of recent data, however, fails to support this prediction; possibly because inclusive fitness models cannot easily incorporate multiple factors simultaneously to predict skew. Notable omissions are antagonistic selection (across generations, genes will be in both dominant and subordinate bodies), constraints on the number of sites suitable for successful reproduction, choice in which group an individual might join, and within-group control or suppression of competition. All of these factors and more are explored through agent-based evolutionary simulations. The results suggest the primary drivers for the initial evolution of cooperative breeding may be a combination of limited suitable sites, choice across those sites, and parental manipulation of offspring into helping roles. Antagonistic selection may be important when subordinates are more frequent than dominants. Kinship matters, but its main effect may be in offspring being available for manipulation while unrelated individuals are not. The greater flexibility of evolutionary simulations allows the incorporation of species-specific life histories and ecological constraints to better predict sociobiology.

Partner choice correlates with fine scale kin structuring in the paper wasp Polistes dominula

Cooperation among kin is common in animal societies. Kin groups may form by individuals directly discriminating relatives based on kin recognition cues, or form passively through natal philopatry and limited dispersal. We describe the genetic landscape for a primitively eusocial wasp, Polistes dominula, and ask whether individuals choose cooperative partners that are nearby and/or that are genetic relatives. Firstly, we genotyped an entire sub-population of 1361 wasps and found genetic structuring on an extremely fine scale: the probability of finding genetic relatives decreases exponentially within just a few meters of an individual's nest. At the same time, however, we found a lack of genetic structuring between natural nest aggregations within the population. Secondly, in a separate dataset where ~2000 wasps were genotyped, we show that wasps forced experimentally to make a new nest choice tended to choose new nests near to their original nests, and that these nests tended to contain some full sisters. However, a significant fraction of wasps chose nests that did not contain sisters, despite sisters being present in nearby nests. Although we cannot rule out a role for direct kin recognition or natal nest-mate recognition, our data suggest that kin groups may form via a philopatric rule-of-thumb, whereby wasps simply select groups and nesting sites that are nearby. The result is that most subordinate helpers obtain indirect fitness benefits by breeding cooperatively.

Evolutionary drivers of group foraging: A new framework for investigating variance in food intake and reproduction

A proposed fundamental driver of group living is more reliable, predictable foraging and reproduction, i.e., reduced variance in food intake and reproductive output. However, existing theories on variance reduction in group foraging are simplistic, refer to variance at the level of individuals and groups without linking the two, and do not spell out crucial underlying assumptions. We provide a new, widely applicable framework for identifying when variance reduction conveys fitness benefits of group foraging in a wide range of organisms. We discuss critical limitations of established theories, the Central Limit Theorem and Risk-Sensitive Foraging Theory applied to group foraging, and incorporate them into our framework while addressing the confusion over the levels of variance and identifying previously unaddressed assumptions. Through a field study on colonial spiders, Cyrtophora citricola, we demonstrate the importance of evaluating the level of food sharing as a critical first step, previously overlooked in the literature. We conclude that variance reduction provides selective advantages only under narrow conditions and does not provide a universal benefit to group foraging as previously proposed. Our framework provides an important tool for identifying evolutionary drivers of group foraging and understanding the role of fitness variance in the evolution of group living.

Heritabilities, social environment effects and genetic correlations of social behaviours in a cooperatively breeding vertebrate

Social animals interact frequently with conspecifics, and their behaviour is influenced by social context, environmental cues and the behaviours of interaction partners, allowing for adaptive, flexible adjustments to social encounters. This flexibility can be limited by part of the behavioural variation being genetically determined. Furthermore, behaviours can be genetically correlated, potentially constraining independent evolution. Understanding social behaviour thus requires carefully disentangling genetic, environmental, maternal and social sources of variations as well as the correlation structure between behaviours. Here, we assessed heritability, maternal, common environment and social effects of eight social behaviours in Neolamprologus pulcher, a cooperatively breeding cichlid. We bred wild-caught fish in a paternal half-sibling design and scored ability to defend a resource against conspecifics, to integrate into a group and the propensity to help defending the group territory ("helping behaviour"). We assessed genetic, social and phenotypic correlations within clusters of behaviours predicted to be functionally related, namely "competition," "aggression," "aggression-sociability," "integration" and "integration-help." Helping behaviour and two affiliative behaviours were heritable, whereas there was little evidence for a genetic basis in all other traits. Phenotypic social effects explained part of the variation in a sociable and a submissive behaviour, but there were no maternal or common environment effects. Genetic and phenotypic correlation within clusters was mostly positive. A group's social environment influenced covariances of social behaviours. Genetic correlations were similar in magnitude but usually exceeding the phenotypic ones, indicating that conclusions about the evolution of social behaviours in this species could be provisionally drawn from phenotypic data in cases where data for genetic analyses are unobtainable.

Parental manipulation of offspring size in social groups: a test using paper wasps

Abstract

Maternal effects should be especially likely when mothers actively provision offspring with resources that influence offspring phenotype. In cooperatively breeding and eusocial taxa, there is potential for parents to strategically manipulate offspring phenotype in their own interests. Social insect queens are nearly always larger than their worker offspring, and queens could benefit by producing small daughter workers in several ways. If queens use aggression to dominate or coerce workers, a queen producing small workers might minimize potential conflict or competition from her offspring. In addition, because of the trade-off between the number of workers she is able to produce and their individual size, a queen may produce small workers to optimize colony work effort. In this study, we investigate why queens of the primitively eusocial paper wasp Polistes gallicus limit the size of their workers. We created queen–worker size mismatches by cross-fostering queens between nests. We then tested whether the queen–worker size difference affects worker foraging and reproductive effort, or the amount of aggression in the group. Some of our results were consistent with the idea that queens limit worker size strategically: small workers were no less successful foragers, so that producing a larger number of smaller workers may overall increase queen fitness. We found that queens were less likely to attack large workers, perhaps because attempting to coerce large workers is riskier. However, larger workers did not forage less, did not invest more in ovarian development, and were not more aggressive themselves. There was therefore little evidence overall that queens limit conflict by producing smaller workers.

Significance statement

In social animals, parents might manipulate phenotypic traits of their offspring in their own interests. In paper wasps (Polistes), the first offspring produced are smaller than the queen and become workers: instead of founding their own nests, they stay and help their mother to rear new queens and males. We investigated whether P. gallicus queens could benefit by producing small daughter workers by using cross-fostering to create size mismatches between queens and their offspring. We then recorded foraging activity, reproductive effort, and aggression on nests. Queens were less likely to attack larger workers, but overall, there was limited evidence of size-based queen–worker conflict. However, because small workers were no less successful foragers, producing a larger number of smaller workers may optimize colony work effort.

Scent of death: Evolution from sea to land of an extreme collective attraction to conspecific death

All living organisms must eventually die, though in some cases their death can bring life-giving opportunities. Few studies, however, have experimentally tested how animals capitalize on conspecific death and why this specialization would evolve. Here, we conducted experiments on the phylogenetically most closely-related marine and terrestrial hermit crabs to investigate the evolution of responses to death during the sea-to-land transition. In the sea, death of both conspecifics and heterospecifics generates unremodeled shells needed by marine hermit crabs. In contrast, on land, terrestrial hermit crabs are specialized to live in architecturally remodeled shells, and the sole opportunity to acquire these essential resources is conspecific death. We experimentally tested these different species' responsiveness to the scent of conspecific versus heterospecific death, predicting that conspecific death would have special attractive value for the terrestrial species. We found the terrestrial species was overwhelmingly attracted to conspecific death, rapidly approaching and forming social groupings around conspecific death sites that dwarfed those around heterospecific death sites. This differential responsiveness to conspecific versus heterospecific death was absent in marine species. Our results thus reveal that on land a reliance on resources associated exclusively with conspecifics has favored the evolution of an extreme collective attraction to conspecific death.

Shortening self-report mental health symptom measures through optimal test assembly methods: Development and validation of the Patient Health Questionnaire-Depression-4

BACKGROUND

The objective of this study was to develop and validate a short form of the Patient Health Questionnaire-9 (PHQ-9), a self-report questionnaire for assessing depressive symptomatology, using objective criteria.

METHODS

Responses on the PHQ-9 were obtained from 7,850 English-speaking participants enrolled in 20 primary diagnostic test accuracy studies. PHQ unidimensionality was verified using confirmatory factor analysis, and an item response theory model was fit. Optimal test assembly (OTA) methods identified a maximally precise short form for each possible length between one and eight items, including and excluding the ninth item. The final short form was selected based on prespecified validity, reliability, and diagnostic accuracy criteria.

RESULTS

A four-item short form of the PHQ (PHQ-Dep-4) was selected. The PHQ-Dep-4 had a Cronbach's alpha of 0.805. Sensitivity and specificity of the PHQ-Dep-4 were 0.788 and 0.837, respectively, and were statistically equivalent to the PHQ-9 (sensitivity = 0.761, specificity = 0.866). The correlation of total scores with the full PHQ-9 was high (r = 0.919).

CONCLUSION

The PHQ-Dep-4 is a valid short form with minimal loss of information of scores when compared to the full-length PHQ-9. Although OTA methods have been used to shorten patient-reported outcome measures based on objective, prespecified criteria, further studies are required to validate this general procedure for broader use in health research. Furthermore, due to unexamined heterogeneity, there is a need to replicate the results of this study in different patient populations.

Queen Longevity and Fecundity Affect Conflict with Workers over Resource Inheritance in a Social Insect

Resource inheritance is a major source of conflict in animal societies. However, the assumptions and predictions of models of conflict over resource inheritance have not been systematically tested within a single system. We developed an inclusive fitness model for annual eusocial Hymenoptera that predicts a zone of conflict in which future reproductive workers are selected to enforce nest inheritance before the queen is selected to cede the nest. We experimentally tested key elements of this model in the bumblebee Bombus terrestris. In colonies from which queens were sequentially removed, queen tenure was significantly negatively associated with worker male production, confirming that workers gain direct fitness by usurping the queen. In unmanipulated colonies, queen fecundity decreased significantly over the latter part of the colony cycle, confirming that workers' indirect fitness from maintaining queens declines over time. Finally, in an experiment simulating loss of queen fecundity by removal of queens' eggs, worker-to-queen aggression increased significantly and aggressive workers were significantly more likely to become egg layers, consistent with workers monitoring queen fecundity to assess the net benefit of future reproduction. Overall, by upholding key assumptions and predictions of the model, our results provide novel empirical support for kin-selected conflict over resource inheritance.

Current indirect fitness and future direct fitness are not incompatible

In primitively eusocial insects, many individuals function as workers despite being capable of independent reproduction. Such altruistic behaviour is usually explained by the argument that workers gain indirect fitness by helping close genetic relatives. The focus on indirect fitness has left open the question of whether workers are also capable of getting direct fitness in the future in spite of working towards indirect fitness in the present. To investigate this question, we recorded behavioural profiles of all wasps on six naturally occurring nests of Ropalidia marginata, and then isolated all wasps in individual plastic boxes, giving them an opportunity to initiate nests and lay eggs. We found that 41% of the wasps successfully did so. Compared to those that failed to initiate nests, those that did were significantly younger, had significantly higher frequency of self-feeding behaviour on their parent nests but were not different in the levels of work performed in the parent nests. Thus ageing and poor feeding, rather than working for their colonies, constrain individuals for future independent reproduction. Hence, future direct fitness and present work towards gaining indirect fitness are not incompatible, making it easier for worker behaviour to be selected by kin selection or multilevel selection.

Adult-larval vibrational communication in paper wasps: the role of abdominal wagging in Polistes dominula

Communication through vibrational signals is widespread among social insects and regulates crucial social activities. Females of the social wasp Polistes dominula produce substrate-borne vibrations on the combs by performing a conspicuous abdominal oscillatory behavior, known as abdominal wagging. Several studies have reported correlative evidence in support of its signaling role, but direct evidence is still lacking. Because abdominal wagging is strictly associated with the presence of larvae in the nest and with cell inspection, it has been suggested that it could be involved in adult-larvae communication. According to this hypothesis, abdominal wagging vibrations would have short-term effects related to food and trophallactic exchanges between adults and larvae by modulating salivary secretion (decreasing its amount, to prepare larvae to receive food, or stimulating the release of larval saliva to adults). Here, by using an electro-magnetic shaker, we assessed, for the first time, the short-term effects of abdominal wagging on larval behavior by recording larval responses and by measuring the amount of saliva released immediately after abdominal wagging playback. Our results show that larvae are able to perceive the substrate-borne vibrations produced by abdominal wagging and react by increasing the movement of their body, possibly in order to attract the attention of adult females during feeding nest inspection. Yet, we found that vibrations neither increase nor decrease the release of larval saliva. Our results support the hypothesis of the alleged role of vibrations in adult-larvae communications; however, they do not support the long-lasting hypothesis of salivary release modulation.

Predictors of nest growth: diminishing returns for subordinates in the paper wasp Polistes dominula

ABSTRACT

In cooperative breeders, subordinates that have alternative reproductive options are expected to stay and help dominant breeders only as long as they contribute to group productivity, if their fitness is linked with colony success. Female Polistes dominula paper wasps live as cooperative breeders in small groups of typically fewer than 10 females. Subordinates tend to have high-quality outside options, and so could choose alternative breeding tactics if their work efforts increased productivity negligibly. In the founding stage before workers emerge, we tested the effect of various predictors on nest growth, as a proxy for group productivity, and explored the shape of the relationship between group size and nest growth. We found group size to be the only significant predictor of nest growth: variation among body sizes within the group showed no effect, suggesting a lack of size-dependent task specialization in this species. Average body size and average genetic relatedness between group members similarly showed no effects on nest growth. Group size had a non-linear effect so that per-capita benefits to nest growth decreased in larger groups, and groups of 10 or more would benefit negligibly from additional group members. Hence, females might be better off pursuing other options than joining a large group. This finding helps to explain why P. dominula groups are usually relatively small in our study population. Further studies may illuminate the mechanisms behind the smaller per-capita nest growth that we found in larger groups.

SIGNIFICANCE STATEMENT

Identifying which factors influence the productivity of animal groups is key to understanding why different species breed cooperatively in groups of varying sizes. In the paper wasp Polistes dominula, we investigated the growth rate of nests as a measure of group productivity. We found that average body size, the variation in body sizes within the group, and average genetic relatedness between group members did not affect nest growth, while group size had a strong, positive effect: nests grew faster with more group members, but the per-capita benefit decreased in larger groups. The addition of extra group members in groups of 10 or more had negligible effects on nest growth. Hence, wasps may be better off pursuing other options than joining large groups. This finding helps to explain why groups normally consist of fewer than 10 wasps in this population.