Reproductive Dominance Research Articles

Reproductive behaviour and fertilized spawns in cultured Solea senegalensis broodstock co-housed with wild breeders during their juvenile stages

The reproductive failure of Senegalese sole (Solea senegalensis) cultured males (reared entirely in captivity from egg through to adult) that do not participate in reproductive behaviours to fertilise spawns, results in a problem to achieve reproductive control in captivity. However, cohabitation with wild males has led to an increase in the involvement of cultured males in reproductive behaviour, although their contribution to fertilised spawning is still lower than that of wild breeders. This study aimed to examine the effect of different social conditions, on the reproductive behaviour and spawning success of cultured breeders over three reproductive seasons. Before starting this study, different social learning opportunities were provided to the breeders from the juvenile to the pubertal stages of the individuals. Behaviour and spawning were evaluated in four experimental groups of cultured breeders: two groups (W1 and W2) that prior to this study were reared during the juvenile stage with wild breeders that fertilized spawns, a Culture breeder group (CB) that was previously reared with cultured breeders that spawned unfertile eggs, and a negative control group (CN) that was reared in isolation from adult fish.During the three reproductive seasons, spawning was obtained from all groups. Generally, the first year had the highest egg production and the third year the lowest. However, fertilised eggs were only obtained from W1 in the first year. A total of eight fertilised spawns were collected with a fertilisation rate of 28.02 ± 13.80 % and a hatching rate of 15.04 ± 10.40 %. The mean number of larvae obtained per spawn was 7,683 ± 5,947 and the total number of larvae from all eight spawns was 61,468. The paternity analysis assigned 64.3 % of larvae to a single couple of breeders, while 34.3 % of larvae were not assigned to any single family, but inconclusively to more than three parents. The highest locomotor activity was observed in W1, while no significant differences were observed in the number of movements within W2, CB and CN. In all groups, during the peak of locomotor activity (19h00-20h00), the main reproductive behaviours observed were Rest the Head and Follow, while the Guardian behaviour was low and Coupled behaviour was only observed in W1. Over time, the reproductive behaviours decreased, except for Follow. The social learning opportunities provided by cohabitation with wild fish during juvenile stages prior to spawning in W1, increased activity and fertilised spawning. However, the number of successful spawns was low and over time stopped in association with a decrease in reproductive behaviour. This suggests that other mechanisms of behavioural learning could be involved in reproductive success, such as reproductive dominance, environmental conditions or hormonal interactions that could affect physiological processes in the reproduction of captive breeders.

Individual- and group-level sex ratios under local mate competition: consequences of infanticide and reproductive dominance.

Extremely female-biased sex ratios of parasitoid wasps in multiple-foundress groups challenges evolutionary theory which predicts diminishing bias as foundress numbers increase. Recent theory based on foundress cooperation has achieved qualitative rather than quantitative success in explaining bias among parasitoids in the genus Sclerodermus. Here, we develop an explanation, expanding the theory of local mate competition, based on the observation that male production seems dominated by some foundresses within groups. Two sex ratio effects arise from such reproductive dominance: an immediate effect via suppression of male production, and a long-term evolutionary response to reproductive skew. We analyze the outcome of these effects at the individual and group level, the latter being more readily observable. Three model scenarios are analyzed: (1) random killing of developing sons in a group by all foundresses, without reproductive skew, (2) the development of reproductive dominance by some foundresses after sex allocation decisions by all foundresses have been implemented, and (3) reproductive dominance within foundress groups before sex allocation decisions are implemented. The 3 scenarios have subtly different implications for sex ratio evolution, with Models 2 and 3 being novel additions to theory, showing how reproductive dominance can alter the outcome of sex ratio evolution. All models match observations in their outcomes better than other recently proposed theory, but Models 2 and 3 are closest to observations in their underlying assumptions. Further, Model 2 shows that differential offspring mortality after parental investment can influence the primary sex ratio even when random with respect to parental and offspring characters, but targeted at entire clutches. The novel models are solved for both diploid and haplodiploid genetic systems, and confirmed with simulations. Overall, these models provide a feasible explanation for the extremely female-biased sex ratios produced by multi-foundress groups and expand the scope of local mate competition theory to consider reproductive dominance.

Honeybee queen mandibular pheromone induces a starvation response in Drosophila melanogaster

Eusocial insect societies are defined by the reproductive division of labour, a social structure that is generally enforced by the reproductive dominant(s) or ‘queen(s)’. Reproductive dominance is maintained through behavioural dominance or production of queen pheromones, or a mixture of both.Queen mandibular pheromone (QMP) is a queen pheromone produced by queen honeybees (Apis mellifera) which represses reproduction in worker honeybees. How QMP acts to repress worker reproduction, the mechanisms by which this repression is induced, and how it has evolved this activity, remain poorly understood. Surprisingly, QMP is capable of repressing reproduction in non-target arthropods.Here we show that in Drosophila melanogaster QMP treatment mimics the starvation response, disrupting reproduction. QMP exposure induces an increase in food consumption and activation of checkpoints in the ovary that reduce fecundity and depresses insulin signalling. The magnitude of these effects is indistinguishable between QMP-treated and starved individuals. As QMP triggers a starvation response in an insect diverged from honeybees, we propose that QMP originally evolved by co-opting nutrition signalling pathways to regulate reproduction.

Identification of a queen primer pheromone in higher termites

It is long established that queens of social insects, including termites, maintain their reproductive dominance with queen primer pheromones (QPPs). Yet, the QPP chemistry has only been elucidated in a single species of lower termites. By contrast, the most diversified termite family Termitidae (higher termites), comprising over 70% of termite species, has so far resisted all attempts at QPP identification. Here, we show that the queen- and egg-specific sesquiterpene (3R,6E)-nerolidol acts as the QPP in the higher termite Embiratermes neotenicus. This species has a polygynous breeding system, in which the primary queen is replaced by multiple neotenic queens of parthenogenetic origin. We demonstrate that (3R,6E)-nerolidol suppresses the development of these parthenogenetic queens and thus mimics the presence of mature queen(s). It acts as an airborne signal and may be used to optimize the number of queens, thus being the key regulatory element in the special breeding system of E. neotenicus.

How many cooperators are too many? Foundress number, reproduction and sex ratio in a quasi‐social parasitoid

AbstractIn the parasitoid genusSclerodermus, multiple foundresses produce and care for communal broods on large hosts, which can lead to greater reproductive success for group members than attempting to reproduce alone.We explore the consequences of foundress group size on the benefits of cooperative brooding and on brood sex ratios by providing groups of 10–55 foundresses with a single host and no alternative reproductive options.Within this range, increasing foundress group size leads to increasingly common failure in brood production and diminishedper capitasuccess. Group production of adult offspring declines once foundress number reaches around 25.Brood failure is usually at the early developmental stages, and current evidence suggests that there may be competition among foundresses for oviposition sites, possibly involving reproductive dominance and ovicide, which also delays initial brood production. Once broods become established, their rate of development is enhanced by large foundress numbers.The sex ratios of broods are very strongly female biased, irrespective of the foundress number. As this bias is not easily explained by standard models of local mate competition or by a recent model of local resource enhancement, we suggest an explanation based on control of sex allocation by a minority of dominant foundresses, which monopolise the production of adult males.

Built to change: dominance strategy changes with life stage in a primitively eusocial bee

AbstractAccess to reproduction is determined by an individual’s dominance rank in many species and is achieved through aggression and/or dominance signaling. In eusocial insects, one or several dominant females (queens) monopolize reproduction but to what extent queens rely on aggression and signaling remains obscure. Aggression is costly and its efficiency depends on the group size, whereas signaling may reduce the risks and costs of aggression. Both strategies are used to regulate reproduction in social taxa, with aggression being more common in small social groups, compared to signaling in larger societies. Here, we examine the use of aggression and chemical signaling in a social species (Bombus impatiens) where the dominant queen interacts with increasing numbers of workers as she ages. We found that the queen’s strategy to monopolize reproduction changes with life stage, shifting from overt aggression to chemical signaling as the queen gets older. Particularly, old queens exhibited a higher ratio of short to long cuticular hydrocarbons compared to young queens, an endogenous shift that was attributed to age, as all egg-laying queens were fecund and kept with the same number of workers. Our findings contribute to the understanding of reproductive dominance in the context of an individual’s life history.

Identification of a queen pheromone mediating the rearing of adult sexuals in the pharaoh ant Monomorium pharaonis.

The division of labour between reproductive queens and mostly sterile workers is among the defining characteristics of social insects. Queen-produced chemical signals advertising her presence and fertility status, i.e. queen pheromones, are normally used to assert the queen's reproductive dominance in the colony. Most queen pheromones identified to date are chemicals that stop the daughter workers from reproducing. Nevertheless, it has long been suggested that queen pheromones could also regulate reproduction in different ways. In some multiple-queen ants with obligately sterile workers, for example-such as fire ants and pharaoh ants-queen pheromones are thought to regulate reproduction by inhibiting the rearing of new sexuals. Here, we identify the first such queen pheromone in the pharaoh ant Monomorium pharaonis and demonstrate its mode of action via bioassays with the pure biosynthesized compound. In particular, we show that the monocyclic diterpene neocembrene, which in different Monomorium species is produced solely by fertile, egg-laying queens, strongly inhibits the rearing of new sexuals (queens and males) and also exerts a weakly attractive 'queen retinue' effect on the workers. This is the first time that a queen pheromone with such a dual function has been identified in a social insect species with obligately sterile workers.

Cues of dominance hierarchy, fertility and nestmate recognition in the primitively eusocial wasp Mischocyttarus parallelogrammus (Vespidae: Polistinae: Mischocyttarini)

Chemical communication is pivotal for social insects to ensure proper functioning of their colonies. Cuticular hydrocarbons (CHCs) are the most well-known class of compounds used to regulate different types of behavioural interaction within a social context. Queens of highly eusocial insects rely on the use of chemical communication to keep their reproductive monopoly, whereas queens of primitively eusocial insects often exert physical control to maintain reproductive dominance. However, in the past years, ample evidence has demonstrated that primitively eusocial insects also use chemical compounds to communicate. Based on this evidence, we aimed to elucidate whether CHCs carry some information regarding female hierarchical position, ovary activation and nestmate recognition in the primitively eusocial wasp Mischocyttarus parallelogrammus. Additionally, females were classified by their ovary activation. Finally, the cuticular profiles of females originating from different nests were compared to check whether CHCs convey information about their nest of origin. Our results confirmed that the chemical composition of alpha and subordinate females differed significantly in post-worker emergence nests, but that alpha and beta females surprisingly were not chemically different from each other in either of the colony phases. Furthermore, females with activated ovaries expressed a chemical profile distinct from that of females with non-activated ovaries. Lastly, we showed that CHCs might convey information about nest origin, since females hailing from different nests showed distinct chemical profiles. Based on our results, we conclude that CHCs might play a critical role in the nest-functioning of M. parallelogrammus, since they mirror social status.

Hydroxylation patterns associated with pheromone synthesis and composition in two honey bee subspecies Apis mellifera scutellata and A. m. capensis laying workers

Colony losses due to social parasitism in the form of reproductive workers of the Apis mellifera capensis clones results from the production of queen-like pheromonal signals coupled with ovarian activation in these socially parasitic honey bees. While the behavioral attributes of these social parasites have been described, their genetic attributes require more detailed exploration. Here, we investigate the production of mandibular gland pheromones in queenless workers of two sub-species of African honey bees; A. m. scutellata (low reproductive potential) and A. m. capensis clones (high reproductive potential). We used standard techniques in gas chromatography to assess the amounts of various pheromone components present, and qPCR to assess the expression of cytochrome P450 genes cyp6bd1 and cyp6as8, thought to be involved in the caste-dependent hydroxylation of acylated stearic acid in queens and workers, respectively. We found that, for both subspecies, the quality and quantity of the individual pheromone components vary with age, and that from the onset, A. m. capensis parasites make use of gene pathways typically upregulated in queens in achieving reproductive dominance. Due to the high production of 9-hydroxy-decenoic acid (9-HDA) the precursor to the queen substance 9-oxo-decenoic acid (9-ODA) in newly emerged capensis clones, we argue that clones are primed for parasitism upon emergence and develop into fully fledged parasites depending on the colony's social environment.

Genetic signatures of dominance hierarchies reveal conserved cis-regulatory and brain gene expression underlying aggression in a facultatively social bee.

Agonistic interactions among individuals can result in the formation of dominance hierarches that can reinforce individual behavior and social status. Such dominance hierarches precede the establishment of reproductive dominance, division of labor and caste formation in highly social insect taxa. As such, deciphering the molecular basis of aggression is fundamental in understanding the mechanisms of social evolution. Assessing the proximate mechanisms of aggression in incipiently social bees can provide insights into the foundations of genomic mechanisms of social behavior. Here, we measured the effects of aggression on brain gene expression in the incipiently social bee, Ceratina australensis. We examine the brain transcriptomic differences between individuals who have experienced recurrent winning, losing, or a change in rank during repeated encounters. Using comparative analyses across taxa, we identify deeply conserved candidate genes, pathways, and regulatory networks for the formation of social hierarchies.

Combining molecular and incomplete observational data to inform management of southern white rhinoceros (Ceratotherium simum simum)

Conservation efforts have preserved the southern white rhinoceros (SWR) in protected areas and have resulted in substantial overall growth in population size, but in small, fragmented populations in which inbreeding is an important risk. However, field observation of breeding often lacks sufficient accuracy to inform translocation strategies that are intended to increase genetic variation. The purpose of this study was to integrate microsatellite genotypes with an incomplete, field-observed pedigree to make inferences about mean kinship and basic demographic data that could be used to inform translocation programmes for SWR in a confined population in Botswana. Using this approach, we identified parents for 29 out of 45 offspring born in the reserve between 1993 and 2013 and detected eight non-breeding bulls with high mean kinship as candidates for translocation. The method also allowed inferences about demographic parameters that could influence the effectiveness of intervention strategies, such as age and timing of reproduction, and natal sex ratios. Importantly, the reproductive dominance of the bulls was not as skewed as expected after the original dominant bull was removed from the population, suggesting that closed populations can maintain multiple, simultaneously breeding males. The genetic data also confirmed that the accuracy of field-based parentage assignment was increased after implementation of an ear-notching programme. This study demonstrates the value of combining genetic information with ongoing surveillance to inform management of threatened populations, and of using mean kinship to inform metapopulation management by identifying candidates for translocation.

The role of juvenile hormone in regulating reproductive physiology and dominance in Dinoponera quadriceps ants.

Unequal reproductive output among members of the same sex (reproductive skew) is a common phenomenon in a wide range of communally breeding animals. In such species, reproductive dominance is often acquired during antagonistic interactions between group members that establish a reproductive hierarchy in which only a few individuals reproduce. Rank-specific syndromes of behavioural and physiological traits characterize such hierarchies, but how antagonistic behavioural interactions translate into stable rank-specific syndromes remains poorly understood. The pleiotropic nature of hormones makes them prime candidates for generating such syndromes as they physiologically integrate environmental (social) information, and often affect reproduction and behaviour simultaneously. Juvenile hormone (JH) is one of several hormones that occupy such a central regulatory role in insects and has been suggested to regulate reproductive hierarchies in a wide range of social insects including ants. Here we use experimental manipulation to investigate the effect of JH levels on reproductive physiology and social dominance in high-ranked workers of the eusocial ant Dinoponera quadriceps, a species that has secondarily reverted to queenless, simple societies. We show that JH regulated reproductive physiology, with ants in which JH levels were experimentally elevated having more regressed ovaries. In contrast, we found no evidence of JH levels affecting dominance in social interactions. This could indicate that JH and ovary development are decoupled from dominance in this species, however only high-ranked workers were investigated. The results therefore confirm that the regulatory role of JH in reproductive physiology in this ant species is in keeping with its highly eusocial ancestors rather than its secondary reversion to simple societies, but more investigation is needed to disentangle the relationships between hormones, behaviour and hierarchies.

Antecedents of behavioural and reproductive dominance in pairs of the primitively eusocial wasp Ropalidia marginata

What factors predispose some individuals to become reproductively dominant in a group where every member can reproduce? Antecedents of reproductive dominance have often been investigated in primitively eusocial species where reproductive skew exists despite adult reproductive potential displayed by every group-member, but such studies have rarely focused on small, incipient colonies. Here, I investigated antecedents of behavioural and reproductive dominance in pairs of the Indian paper wasp Ropalidia marginata. Common antecedents of behavioural dominance such as body size and age were inoperative in pairs of R. marginata. Moreover, age and behavioural dominance, but not body size, influenced reproductive dominance in pairs. These findings are not only different from other primitively eusocial insects, but also different from the colonies of R. marginata. It is likely that antecedents of reproductive dominance are different not only in different species, but also change with group size within a species, such that the role of behavioural dominance to achieve reproductive monopoly remains more effective in small groups such as pairs, and becomes less effective as the group size increases. These results require further investigations into the effect of group size on individual behaviour in group-living animals.

Identification and Enantiodivergent Synthesis of (5 Z,9 S)-Tetradec-5-en-9-olide, a Queen-Specific Volatile of the Termite Silvestritermes minutus.

The queens of social insects differ from sterile colony members in many aspects of their physiology. Besides adaptations linked with their specialization for reproduction and extended lifespan, the queens also invest in the maintenance of their reproductive dominance by producing exocrine chemicals signaling their presence to the nestmates. The knowledge of the chemistry of queen-specific cues in termites is scarce. In addition to the contact recognition based on cuticular hydrocarbons, long-range signals mediated by volatiles are expected to participate in queen signaling, especially in populous colonies of higher termites (Termitidae). In queens of the higher termite Silvestritermes minutus (Syntermitinae), we have detected a previously undescribed volatile. It is present in important quantities on the body surface and in the headspace, ovaries, and body cavity. MS and GC-FTIR data analyses led us to propose the structure of the compound to be a macrolide 10-pentyl-3,4,5,8,9,10-hexahydro-2 H-oxecin-2-one. We performed enantiodivergent syntheses of two possible enantiomers starting from enantiopure ( S)-glycidyl tosylate. The synthetic sequence involved macrolide-closing metathesis quenched with a ruthenium scavenging agent. The absolute and relative configuration of the compound was assigned to be (5 Z,9 S)-tetradec-5-en-9-olide. Identification and preparation of the compound allow for investigation of its biological significance.

Reproductive parasitism by worker honey bees suppressed by queens through regulation of worker mandibular secretions

Social cohesion in social insect colonies can be achieved through the use of chemical signals whose production is caste-specific and regulated by social contexts. In honey bees, queen mandibular gland pheromones (QMP) maintain reproductive dominance by inhibiting ovary activation and production of queen-like mandibular gland signals in workers. We investigated whether honey bee queens can control reproductively active workers of the intraspecific social parasite Apis mellifera capensis, parasitising A. m. scutellata host colonies. Our results show that the queen’s QMP suppresses ovarian activation and inhibits the production of QMP pheromone signals by the parasitic workers, achieved through differential expression of enzymes involved in the biosynthesis of these pheromones at two points in the biosynthetic pathway. This is the first report showing that honey bee queens can regulate reproduction in intraspecific social parasites and deepens our understanding of the molecular mechanisms involved in the regulation of worker reproduction in social insects.

Reproductive Dominance Strategies in Insect Social Parasites.

In eusocial insects, the high cost of altruistic cooperation between colony members has favoured the evolution of cheaters that exploit social services of other species. In the most extreme forms of insect social parasitism, which has evolved multiple times across most social lineages, obligately parasitic species invade the nests of social species and manipulate the workforce of their hosts to rear their own reproductive offspring. As alien species that have lost their own sociality, these social parasites still face social challenges to infiltrate and control their hosts, thus providing independent replicates for understanding the mechanisms essential to social dominance. This review compares socially parasitic insect lineages to find general trends and build a hypothetical framework for the means by which social parasites achieve reproductive dominance. It highlights how host social organization and social parasite life history traits may impact the way they achieve reproductive supremacy, including the potential role of chemical cues. The review discusses the coevolutionary dynamics between host and parasite during this process. Altogether, this review emphasizes the value of social parasites for understanding social evolution and the need for future research in this area.

Contribuição parental em progênies de Curimba usando diferentes sistemas reprodutivos

O conhecimento de manejos apropriados para a reprodução é fundamental para a gestão e direcionamento das atividades em centrais de repovoamento. O objetivo deste estudo foi avaliar a influência dos sistemas reprodutivos por extrusão e seminatural e da proporção de sexo na contribuição parental de progênies de P. lineatus. Amostras de nadadeiras caudais de 25 reprodutores submetidos ao sistema reprodutivo por extrusão e 10 reprodutores submetidos ao sistema reprodutivo seminatural foram coletados em diferentes proporções de sexo (1ââ„¢"š:1♀ e 2ââ„¢"š:1♀, e 1ââ„¢"š:1♀, respectivamente). Duzentos e oito alevinos oriundos desses acasalamentos foram avaliados. A mortalidade dos reprodutores foi registrada logo após os procedimentos reprodutivos. Para a análise de paternidade, foram amplificados 10 loci microssatélites. Três reprodutores foram a óbito no sistema extrusão (dois na proporção 1:1 e um na proporção 2:1). No sistema extrusão 1:1 observou-se que um macho (M1.4) obteve maior contribuição na formação progênie (41,8%), enquanto que na proporção 2:1, quatro machos contribuí­ram entre 14,5 a 21,7%. A menor dominí­¢ncia reprodutiva foi identificada no sistema seminatural, com valores variando de 22,86% a 25,71% para quatro machos. Na formação das famí­­lias, foi observada dominí­¢ncia reprodutiva de alguns casais em ambos os sistemas. Conclui-se que o sistema seminatural permitiu maior homogeneidade na contribuição na progênie sem perdas por mortalidade.

Modeling Developmental Class Provides Insights into Individual Contributions to Infant Survival in Callitrichids

Cooperative breeders live in social groups in which individuals in an age–sex class vary in reproductive development due to reproductive dominance by a few individuals in each group. Among callitrichids, adult males have been implicated in driving group reproductive output, but uneven sampling efforts, the underlying effects of group size, and pseudoreplication at the group and species levels are confounding variables in these analyses. We examined the drivers of group reproductive output in callitrichids by 1) conducting a meta-analysis of published studies of callitrichid group composition; 2) assigning developmental class based on reproductive morphology; and 3) using multivariate modeling to test whether the proportion of individuals of each developmental class predicts the presence and the number of surviving offspring among free-ranging Weddell’s saddleback tamarins (Leontocebus weddelli) and emperor tamarins (Saguinus imperator) in Peru. The meta-analysis revealed that the number of adult females and group size, but not the number of adult males, are significantly correlated with group reproductive output. Statistical models of the new dataset revealed that the proportion of primary breeding males, primary breeding females, and group size predicted whether groups had surviving infants, and that only the proportion of primary breeding females and group size predicted the number of surviving infants. Thus, primary breeding males appear to be necessary for groups to raise any infants, but a higher proportion of primary breeding females and a larger group size increase group reproductive output overall.

Glandular sources of pheromones used to control host workers (Apis mellifera scutellata) by socially parasitic workers of Apis mellifera capensis

Pheromonal control by the honey bee queen is achieved through the use of secretions from diverse glandular sources, but the use of pheromones from a variety of glandular sources by reproductively dominant workers, has not previously been explored. Using the social parasite, Apis mellifera capensis clonal worker we studied the diversity of glandular sources used for pheromonal control of reproductively subordinate A. m. scutellata workers. To determine whether pheromones from different glandular sources are used by reproductively active workers to achieve dominance and evaluate the degree of pheromonal competition between workers of the two sub-species, we housed groups of workers of the two sub-species together in cages and analysed mandibular and tergal gland secretions as well as, ovarian activation status of each worker after 21days. The results showed that A. m. capensis invasive clones used both mandibular and tergal gland secretions to achieve reproductive dominance and suppress ovarian activation in their A. m. scutellata host workers. The reproductively dominant workers (false queens) produced more queen-like pheromones and inhibited ovarian activation in subordinate A. m. scutellata workers. These results show that tergal gland pheromones working in synergy with pheromones from other glands allow individual workers (false queens) to establish reproductive dominance within these social groups and to act in a manner similar to that of queens. Thus suggesting that, the evolution of reproductively dominant individuals (queens or false queens) and subordinate individuals (workers) in social insects like the honey bee is the result of a complex interplay of pheromonal signals from different exocrine glands.

Caste differences in the mushroom bodies of swarm-founding paper wasps: implications for brain plasticity and brain evolution (Vespidae, Epiponini)

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. 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.