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Eusocial insect reproductive castes (in Hymenoptera, female reproductive queens and sterile workers) differ dramatically in behavior. Castes may differ in the cognitive demands that affect patterns of brain tissue investment. Queens and workers diverge most strongly in the advanced eusocial, or swarm-founding species, where queens do not forage and rarely leave their nests. We asked whether reproductive castes of swarm-founding paper wasps in the tribe Epiponini differed in the relative sizes of their mushroom bodies (MB), a key brain region involved in sensory integration, and in learning and memory. We measured brain-size corrected volumes of the MB dendritic-field neuropils (calyces) and the MB axonal bundles (peducles and lobes) for queens and workers from 16 species of 10 genera of the tribe Epiponini. The subject species spanned much of the epiponine phylogeny, differing in colony size and degree of caste differentiation. Queens had significantly higher relative MB investment than workers, both for the MB in toto and for the MB calyces. The magnitude of queen-worker MB size differences did not covary significantly with body size, but species with larger colonies had stronger caste differences in MB size. A review of caste differences in MB volume across a wide range of social Hymenoptera taxa suggested a positive association of MB investment with social dominance is widespread. Significance statement: Social insect castes (reproducing queens and sterile workers) differ strongly in behavior, particularly in swarm-founding species where queens are largely nest-bound. Caste comparisons are a powerful model for understanding brain/behavior relationships. We measured the relative size of a key insect brain region, the mushroom bodies (MB), in 16 swarm-founding wasp species. MB are involved in sensory integration, and in learning and memory. Queens had relatively larger MB than workers, and the magnitude of the queen-worker differences increased with species average colony size. We suggest the reproductive dominance and social contact of nest-bound queens promotes greater mushroom body investment. [ABSTRACT FROM AUTHOR]

Paper wasps of the family Vespidae exhibit a wide range of social lifestyles, from facultative eusocial groups to highly ritualistic swarm-founding societies. Even so, adult caste flexibility is widespread throughout the eusocial tribes. Thus, a common endocrine mechanism for caste determination and maintenance in paper wasps is expected, with Polistes dominula serving as a model for the study of mechanisms controlling phenotypic plasticity. In P. dominula, juvenile hormone (JH) and ecdysteroids have been shown to have important caste-determining functions, are important for reproductive growth, and correlate with hydrocarbon signals on the cuticle. Yet research on swarm-founding vespids has shown that JH functions are surprisingly labile, begging the question as to how conserved JH functions are within Polistes, a non-swarming genus. Here, we compared the JH and ecdysteroid titers, cuticular hydrocarbon (CHC) profiles, and prospective visual signals of dominance of free-living foundresses from two sympatric and closely related species of Polistes in South Africa, the indigenous Polistes smithii and the invasive P. dominula. In contrast to P. dominula, neither the JH titer nor the CHC profile was linked to dominance or reproduction in P. smithii, and in both species, hemolymph ecdysteroids were essentially absent. Moreover, many of the relationships between hormones, reproduction, dominance, and social signals in P. dominula are in contrast to studies performed on northern hemisphere populations. The divergence of endocrine and chemical profiles within Polistes offers an unforeseen opportunity to study the evolution of proximate mechanisms underlying phenotypic plasticity. [ABSTRACT FROM AUTHOR]

Insect social parasites, like other parasites, may benefit from inhibiting their host from reproducing (complete or partial parasitic castration) because they can then exploit more of the host's resources for their own reproduction. In particular, social parasites that kill or expel the host queen need to prevent host workers from reproducing; this is a common worker response to the absence of their queen. Indeed, host workers would benefit from detecting the presence of the parasite and investing in direct and indirect fitness. Studying whether and how social parasites control host worker reproduction can provide information about the degree of integration of the parasite in the host colony and help identify factors regulating workers' reproductive decisions in social insects. We investigated whether the paper wasp social parasite, Polistes sulcifer, suppresses Polistes dominula (host) worker reproduction as efficiently as the dominant host female does in queen-right colonies by comparing worker reproductive efforts in parasitized and non-parasitized (control) colonies. Our results show that 6 weeks after usurpation of their colony by the social parasite, parasitized workers (1) had more developed ovaries than control workers and (2) laid more eggs as soon as the opportunity arose. This reproductive readiness of parasitized workers was not apparent 2 weeks after colony usurpation. This suggests that P. dominula workers have evolved means to react to social parasitism, as occurs in some ants, and that the parasite has only limited control over host reproduction. [ABSTRACT FROM AUTHOR]

Various animals build nests with defensive structures to deter predation on offspring. Construction of nest defensive structures can reduce the probability of predation but will involve various costs. Here, we examined both the costs and benefits of the construction of a nest defensive structure in a paper wasp, Polistes chinensis antennalis, and clarify whether the paper wasp changes the level of defensive structures of nests depending on predation risk. A foundress (queen) of the paper wasp starts a colony in spring and maintains her nest alone until the emergence of workers. At this stage, pupae in the nests are sometimes preyed on by conspecifics of other nests. The intruder needs to break the cocoon, which seals the entrance of the cell, to extract the pupa from the cell. Foundresses often apply nest material (pulp) to the surface of cocoons in their nests. We found that pulp on a cocoon increased the time an intruder required to break the cocoon. This result shows that the pulp structure on cocoons helps to prevent predation on pupae. On the other hand, pulp on cocoons involved costs, including time required to collect pulp and being a potential obstacle to the emergence of workers from the cocoon. Additionally, we found that the amount of pulp on cocoons was greater in nests under higher predation risk than nests under lower predation risk. These results suggest that pulp on cocoons is a nest defensive structure, and foundresses adjusted the construction of the defensive structure depending on predation risk. [ABSTRACT FROM AUTHOR]

Cuticular hydrocarbons (CHCs) are information-rich signals in social insects that coordinate behaviors within nests. However, in some taxa, the precise information conveyed by CHCs is poorly understood. In particular, there is a debate over whether CHCs convey information about their bearer's dominance or fertility. Distinguishing between dominance and fertility signaling is difficult because fertility and rank are frequently correlated within social insect colonies. This study disentangles those relationships by examining CHCs of Polistes dominulus paper wasps during the early nest-founding stage before dominance and fertility become correlated. First, we confirm that dominance and fertility are not associated in early spring foundresses. Then we show that CHCs are more strongly associated with fertility than dominance. There was no relationship between cuticular hydrocarbon profiles and a wasp’s ability to dominate rivals through aggression, suggesting that CHCs are unlikely to provide information about dominance. However, there was a significant correlation between ovarian development and the CHC profile, suggesting that CHCs could convey valuable information about their bearer's fertility. Furthermore, our data provide a potential mechanism for chemical signaling of fecundity, as there is a relationship between endogenous juvenile hormone titer (a gonadotropin), degree of ovarian development, and the CHC profile. Hormonal regulation of CHC profile expression offers a physiological mechanism to coordinate behavior, physical state, and signal expression. [ABSTRACT FROM AUTHOR]

Most research on animal contests has focused on the factors that influence the intensity and outcome of aggressive contests within nonsocial species, while relatively little is known about contests in social taxa. Here, we examine contests among queens of the social paper wasp, Polistes dominulus. Queens use multiple reproductive strategies, including nesting alone, usurping established colonies, and cooperatively joining other queens. We stage contests between a nesting queen and a challenger to test how resource value (RV) and resource holding potential (RHP) influence (a) who occupies the nest at the end of the contest and (b) the extent of conflict between the queen and challenger. We found that RHP, as measured by individuals’ facial patterns and body size, influenced the outcome of the contest. Challengers with high RHP were more likely to successfully usurp the nest than challengers with low RHP. Interestingly, queens with relatively high RHP were more likely to form a cooperative association with the challenger than queens with lower RHP, suggesting that queens may evict individuals that are an aggressive threat. RV influenced the intensity of conflict. There was more aggressive conflict over large nests than over small nests. Overall, social taxa have complex contest dynamics with important parallels to contests in nonsocial taxa. Studying contests in social taxa provides an important perspective on the factors that influence individual decisions about conflict versus cooperation. [ABSTRACT FROM AUTHOR]

A challenge for parasites is how to evade the sophisticated detection and rejection abilities of potential hosts. Many studies have shown how insect social parasites overcome host recognition systems and successfully enter host colonies. However, once a social parasite has successfully usurped an alien nest, its brood still face the challenge of avoiding host recognition. How immature stages of parasites fool the hosts has been little studied in social insects, though this has been deeply investigated in birds. We look at how larvae of the paper wasp obligate social parasite Polistes sulcifer fool their hosts. We focus on cuticular hydrocarbons (CHCs), which are keys for adult recognition, and use behavioral recognition assays. Parasite larvae might camouflage themselves either by underproducing CHCs (odorless hypothesis) or by acquiring a chemical profile that matches that of their hosts. GC/MS analyses show that parasite larvae do not have lower levels of CHCs and that their CHCs profile is similar to the host larval profile but shows a reduced colony specificity. Behavioral tests show that the hosts discriminate against alien conspecific larvae from different colonies but are more tolerant towards parasite larvae. Our results demonstrate that parasite larvae have evolved a host larval profile, which overcomes the host colony recognition system probably because of the lower proportion of branched compounds compared to host larvae. In some ways, this is a similar hypothesis to the odorless hypothesis, but it assumes that the parasite larvae are covered by a chemical blend that is not meaningful to the host. [ABSTRACT FROM AUTHOR]

Social insects are popular models for studying the evolution of cooperation. Casteless taxa where individuals have the flexibility to either nest alone or cooperate are particularly valuable for understanding the causes and consequences of cooperative behavior. For example, some ‘workers’ from Polistes paper wasp nests disappear from their natal colony soon after pupal emergence and nest independently. However, little is known about dispersal behavior. In this paper, I compare predispersal behavior of wasps who leave their natal colony soon after emergence with behavior of individuals who remain on the natal colony as true workers. I found that P. dominulus females with short nest tenure behave much like gynes (reproductive-destined offspring produced at the end of the season), as wasps with short nest tenure are behaviorally selfish while on the natal colony. They spend a smaller proportion of their time foraging and a larger proportion of their time resting than workers with long nest tenure. In addition, I assessed the factors that may favor early dispersal. Nest environment strongly influenced dispersal; large colonies had a smaller proportion of females with short nest tenure. Queen turnover also increased dispersal behavior perhaps because queen turnover reduces relatedness between a colony’s current and future offspring, thereby reducing the kin-selected benefits of cooperation. Therefore, casteless social insects exhibit a surprising degree of reproductive flexibility. Individuals may use information about their internal state and nest environment to optimize their behavioral strategies. [ABSTRACT FROM AUTHOR]

Tug-of-war models of within-group conflict predict that the frequency of conflict will be positively related to the degree of reproductive sharing within the group; in contrast, a negative relationship supports transactional models, in which reproductive payments among group members limit the degree of within-group selfishness. We tested predictions of the tug-of-war and transactional models by examining cofoundress interactions during the founding (preworker) phase of colony development in 30 naturally nesting colonies of the paper wasp Polistes dominulus. We found that the mean rate of foundress aggression and the mean probability of food sharing were significantly negatively associated, which supports the prediction of the transactional, not the tug-of-war model. Further, cofoundress aggression significantly increased over the founding phase (independently of temperature), while the fraction of aggression initiated by the dominant (alpha) foundress significantly decreased over this period. We show that both of these results are predicted by the transactional model of within-group conflict. Interestingly, the alpha’s rate of aggression was significantly positively temperature dependent, while the beta’s was not. This indicates that the alpha’s aggression level may often be near her physiological maximum, while the beta’s aggression is limi- ted by other factors, contradicting the prediction of the tug-of-war model. Moreover, the alpha’s aggression was significantly positively temperature dependent only in the second half of the founding period, as predicted by the transactional model since this is when there is least reproductive sharing. Finally, our results indicate that the alpha’s level of aggression depends on the resources controlled by the beta. [ABSTRACT FROM AUTHOR]

Dominance interactions affected patterns of non-reproductive division of labor (polyethism) in the eusocial wasp Mischocyttarus mastigophorus. Socially dominant individuals foraged for food (nectar and insect prey) at lower rates than subordinate individuals. In contrast, dominant wasps performed most of the foraging for the wood pulp used in nest construction. Social dominance also affected partitioning of materials collected by foragers when they returned to the nest. Wood pulp loads were never shared with nest mates, while food loads, especially insect prey, were often partitioned with other wasps. Dominant individuals on the nest were more likely to take food from arriving foragers than subordinate individuals. The role of dominance interactions in regulating polyethism has evolved in the eusocial paper wasps (Polistinae). Both specialization by foragers and task partitioning have increased from basal genera (independent-founding wasps, including Mischo-cyttarus spp.) to more derived genera (swarm-founding Epiponini). Dominance interactions do not regulate forager specialization or task partitioning in epiponines. I hypothesize that these changes in polyethism were enabled by the evolution of increased colony size in the Epiponini. [ABSTRACT FROM AUTHOR]

1. Overwintering females of the paper wasp, Polistes fuscatus, were marked on their home colonies in 1977 and identified in the spring of 1978. Most females which joined foundress associations in 1978 did so with former nestmates, which are close relatives. The probability that any two associating females were nestmates ranges between 0.77 and 0.93. 2. Philopatric females - those which nested at the site where they emerged enjoyed a higher probability that an associate would be a nestmate (0.90-1.00) than did nonphilopatric females (0.50-0.79). 3. Philopatric females formed associations of 2-7 wasps. Nonphilopatric females which did not found singly almost always formed associations of only two foundresses. The bias to two-foundress colonies in nonphilopatric females might be explained by a hypothesis of 'selfish joining'. [ABSTRACT FROM AUTHOR]

A recent study (Izzo et al., Behav Ecol Sociobiol 64: 857–864, ) reported that cuticular hydrocarbons (CHCs) correlate with fertility, not dominance, in the paper wasp Polistes dominulus thus contradicting the results of recent investigations which concluded that social dominance is the main determinant for CHC signatures in this species. We suggest here that different forms of dominance in the pre-nesting and post-nesting phases caused the apparently contradictory results. Thus the assumption that dominance behaviour in the pre-nesting stage is synonymous with dominance after colony foundation is incorrect. We provide standardised definitions for forms of “dominance” observed in the P. dominulus life cycle to avoid apparent discrepancies in the future among studies dealing with the same topics in different annual stages. [ABSTRACT FROM AUTHOR]

In the laboratory, gynes (potential queens) of the paper wasp, Polistes fuscatus, exposed to both their natal nest and female nestmates (presumably their sisters) discriminate between female nestmates and unrelated females. However, gynes not exposed to their natal nest or conspecifics and gynes exposed only to female nestmates do not discriminate between female nestmates and unrelated females. Thus, the presence of the nest appears to be a requisite for the development of nestmate discrimination. [ABSTRACT FROM AUTHOR]

Summary. Laboratory-overwintered paper wasp (Polistes metricus) females preferentially associate on new nests with former nestmates (presumably their sisters) rather than with non-nestmates in the absence of cues associated with their natal nests or nest sites. Females isolated from conspecifics and nests for 74-99 days retain the ability to discriminate nestmates from non-nestmates. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: Division of labor in social groups can be influenced by differential nutrition. Consumption of more food or higher-quality food often affects individuals’ capacities for reproduction. In social insects, nutrients consumed during immature (e.g., larval) stages often affect adult reproductive capacity, but adult nutrition may also impact reproductive status. This study tested whether ovary development, an indicator of reproductive status, corresponded to higher trophic-level feeding for adults in the primitively eusocial paper wasp Mischocyttarus mastigophorus. Our main prediction was that females’ ovary development would correlate positively with evidence of feeding at higher trophic levels, as indicated by stable isotope ratios of nitrogen (δ15N) and carbon (δ13C). We first asked whether isotope ratios of mature females co-varied with ovary development. δ15N values were higher for mature females with better-developed ovaries, as expected if they fed on a diet richer in animal tissue. There was a negative relationship of δ13C with ovary development in mature females, as would be expected if females with developed ovaries had higher body lipid stores. To test for evidence of nutritional biasing of caste during immature development, we measured changes in isotope ratios across pupal development leading up to early adulthood (i.e., immediately before and after adult eclosion). The δ15N and δ13C values for mature pupae were similar to those of newly emerged adults and to those of mature adults lacking developed ovaries. In contrast, mature females with developed ovaries showed N-isotope signatures of a more prey-based diet and C-isotope signatures of elevated lipid content. We conclude the N and C isotopic signatures of ovary-developed mature females diverged from their levels at the end of immature development. The findings suggest reproductive caste status was associated with differences in nutrient acquisition and reflects differences in consumption of animal-derived versus plant-based foods during adulthood.Significance statement: Pre-adult nutrition is known to affect caste development, and thereby influence division of labor, in diverse insect societies. In temperate independent-founding (IF) paper wasps, the effects of larval nutrition on adult caste may reflect adaptations for overwintering and diapause by reproductive females. We asked whether adult nutritional content showed caste-related trophic differences in the Neotropical IF paper wasp Mischocyttarus mastigophorus. Previous studies suggested interactions among adult nest-mates affected food sharing and caused biased nutrient flow within colonies. Our data suggest adult nutrition affected reproductive physiology. We propose this pattern may be widespread in tropical IF paper wasps, where opportunities for adult reproductive plasticity are favored by long colony cycles and relatively aseasonal nest founding. [ABSTRACT FROM AUTHOR]