REPRODUCTIVE SKEW AND SPLIT SEX RATIOS IN SOCIAL HYMENOPTERA

Abstract I present a model demonstrating that, in social Hymenoptera, split sex allocation can influence the evolution of reproductive partitioning (skew). In a facultatively polygynous population (with one to several queens per colony), workers vary in their relative relatedness to females (relatedness asymmetry). Split sex‐ratio theory predicts that workers in monogynous (single‐queen) colonies should concentrate on female production, as their relatedness asymmetry is relatively high, whereas workers in the polygynous colonies should concentrate on male production, as their relatedness asymmetry is relatively low. By contrast, queens in all colonies value males more highly per capita than they value females, because the worker‐controlled population sex ratio is too female‐biased from the queens' standpoint. Consider a polygynous colony in a facultatively polygynous population of perennial, social Hymenoptera with split sex ratios. A mutant queen achieving reproductive monopoly would gain from increasing her share of offspring but, because the workers would assess her colony as monogynous, would lose from the workers rearing a greater proportion of less‐valuable females from the colony's brood. This sets an upper limit on skew. Therefore, in social Hymenoptera, skew evolution is potentially affected by queen‐worker conflict over sex allocation.     

Ant workers selfishly bias sex ratios by manipulating female development.

Kin selection theory predicts that social insects should perform selfish manipulations as a function of colony genetic structure. We describe a novel mechanism by which this occurs. First, we use microsatellite analyses to show that, in a population of the ant Leptothorax acervorum, workers' relatedness asymmetry (ratio of relatedness to females and relatedness to males) is significantly higher in monogynous (single-queen) colonies than in polygynous (multiple-queen) colonies. Workers rear mainly queens in monogynous colonies and males in polygynous colonies. Therefore, split sex ratios in this population are correlated with workers' relatedness asymmetry. Together with significant female bias in the population numerical and investment sex ratios, this finding strongly supports kin-selection theory. Second, by determining the primary sex ratio using microsatellite markers to sex eggs, we show that the ratio of male to female eggs is the same in both monogynous and polygynous colonies and equals the overall ratio of haploids (males) to diploids (queens and workers) among adults. In contrast to workers of species with selective destruction of male brood, L. acervorum workers therefore rear eggs randomly with respect to sex and must achieve their favoured sex ratios by selectively biasing the final caste (queen or worker) of developing females.     

Resource allocation in the red ant Myrmica ruginodis– an interplay of genetics and ecology

Worker‐queen conflicts over reproductive allocation (colony maintenance vs. reproduction) and sex allocation (females vs. males) were examined in two populations of the facultatively polygynous ant Myrmica ruginodis. Plasticity of social organization in the form of two co‐existing social types (microgyna and macrogyna) has a profound effect on reproductive allocation. Workers control sex allocation by biasing sex ratios towards their own interest, but local resource competition (LRC) because of restricted dispersal of microgyna females resulted in male bias in one study population. Colony sex ratios were split and followed the predictions of the split sex ratio theory: single queen colonies with higher relatedness asymmetry (RA) produced more females than multiple queen colonies with lower RA. Single and multiple queen colonies showed similar patterns in most aspects of their reproduction, and reproductive allocation could not be explained by the hypothesis tested. This suggests that reproductive allocation conflict is of minor importance in M. ruginodis.     

Colony sex ratios vary with breeding system but not relatedness asymmetry in the facultatively polygynous ant Pheidole pallidula

Abstract.— We investigated sex allocation in a Mediterranean population of the facultatively polygynous (multiple queen per colony) ant Pheidole pallidula . This species shows a strong split sex ratio, with most colonies producing almost exclusively a single-sex brood. Our genetic (microsatellite) analyses reveal that P. pallidula has an unusual breeding system, with colonies being headed by a single or a few unrelated queens. As expected in such a breeding system, our results show no variation in relatedness asymmetry between monogynous (single queen per colony) and polygynous colonies. Nevertheless, sex allocation was tightly associated with the breeding structure, with monogynous colonies producing a male-biased brood and polygynous colonies almost only females. In addition, sex allocation was closely correlated with colony total sexual productivity. Overall, our data show that when colonies become more productive (and presumably larger) they shift from monogyny to polygyny and from male production to female production, a pattern that has never been reported in social insects.     

Split sex ratios in the social Hymenoptera: a meta-analysis

The study of sex allocation in social Hymenoptera (ants, bees,and wasps) provides an excellent opportunity for testing kin-selectiontheory and studying conflict resolution. A queen–workerconflict over sex allocation is expected because workers aremore related to sisters than to brothers, whereas queens areequally related to daughters and sons. If workers fully controlsex allocation, split sex ratio theory predicts that colonieswith relatively high or low relatedness asymmetry (the relatednessof workers to females divided by the relatedness of workersto males) should specialize in females or males, respectively.We performed a meta-analysis to assess the magnitude of adaptivesex allocation biasing by workers and degree of support forsplit sex ratio theory in the social Hymenoptera. Overall, variationin relatedness asymmetry (due to mate number or queen replacement)and variation in queen number (which also affects relatednessasymmetry in some conditions) explained 20.9% and 5% of thevariance in sex allocation among colonies, respectively. Theseresults show that workers often bias colony sex allocation intheir favor as predicted by split sex ratio theory, even iftheir control is incomplete and a large part of the variationamong colonies has other causes. The explanatory power of splitsex ratio theory was close to that of local mate competitionand local resource competition in the few species of socialHymenoptera where these factors apply. Hence, three of the mostsuccessful theories explaining quantitative variation in sexallocation are based on kin selection.     

Queen recruitment and split sex ratios in polygynous colonies of the ant Formica exsecta

Sex ratios in social insects have become a general model for tests of inclusive fitness theory, sex ratio theory and parent–offspring conflict. In populations of Formica exsecta with multiple queens per colony , sex ratios vary greatly among colonies and the dry-weight sex ratio is extremely male-biased, with 89% of the colonies producing males but no gynes (reproductive females). Here we test the queen-replenishment hypothesis, which was proposed to explain sex ratio specialization in this and other highly polygynous ants (i.e. those with many queens per nest). This hypothesis proposes that, in such ants, colonies produce gynes to recruit them back into the colony when the number of resident queens falls below a given threshold limiting colony productivity or survival. We tested predictions of the queen-replenishment hypothesis by following F. exsecta colonies across two breeding seasons and relating the change in effective queen number with changes in sex ratio, colony size and brood production. As predicted by the queen-replenishment hypothesis, we found that colonies that specialized in producing females increased their effective queen number and were significantly more likely to specialize in male production the following year. The switch to male production also coincided with a drop in productivity per queen as predicted. However, adoption of new queens did not result in a significant increase in total colony productivity the following year. We suggest that this is because queen production comes at the expense of worker production and thus queen production leads to resource limitation the following year, buffering the effect of greater queen number on total productivity.     

COLONY SEX RATIOS,CONFLICT BETWEEN QUEENS AND WORKERS,AND APPARENT QUEEN CONTROL IN THE ANT PHEIDOLE DESERTORUM

Sex-ratio conflict between queens and workers was explored in a study of colony sex ratios, relatedness, and population investment in the ant Pheidole desertorum. Colony reproductive broods consist of only females, only males, or have a sex ratio that is extremely male biased. Colonies producing females (female specialists) and colonies producing males (male specialists) occur at near equal frequency in the population. Most colonies apparently specialize in producing one reproductive sex throughout their life. Allozyme analyses show that relatedness does not differ within male-specialist and female-specialist colonies and they do not appear to differ in available resources. In the population, workers are nearly three times more closely related to females than males; however, the investment sex ratio is near equal (1.01, female/male), which is consistent with queen control. Selection should be strong on workers to increase investment in reproductive females, so why do workers in male-specialist colonies produce only (or nearly only) males? One hypothesis is that queens in male-specialist colonies prevent the occurrence of reproductive females, perhaps by producing worker-biased female eggs. An earlier simulation study of genetic evolution of sex ratios in social Hymenoptera (Pamilo 1982b) predicts that such mechanisms can result in the evolution of bimodal colony sex ratios and queen control. Results on P. desertorum are generally consistent with that study; however, information is not currently available to test some of the model's predictions and assumptions.     

Sex allocation in a facultatively polygynous ant: between-population and between-colony variation

We investigated sex allocation in three U.K. populations ofthe facultatively polygynous ant Leptothorax acervorum over1-3 years. The first main finding was that, across sites, thepopulation sex-investment ratio changed from significantly femalebiased to significantly male biased with increasing polygyny.This was consistent with workers controlling sex allocationand reacting to changes in their population-level relatedness asymmetry.It was also consistent with local resource competition due to reproductionby colony budding under polygyny. Worker control was supportedby the finding that queen number had no effect on sex allocationamong polygynous colonies. The second main result was that monogynouscolonies consistently produced more female-biased sex-investmentratios than polygynous colonies in one site only (Santon). Theresults from Santon supported both the relative relatednessasymmetry hypothesis and the idea of sex ratio compensationdue to colony budding. The workers' response to their population-levelrelatedness asymmetry reinforced the case for relatedness asymmetrybeing influential at the colony level. The other populationscould have lacked split sex ratios because polygynous colonieswere either comparatively rare or common, making them behaveas almost entirely monogynous (Aberfoyle) or polygynous (Roydon) populations.In Roydon, this was consistent with the inference from allozyme datathat monogynous and polygynous colonies did not differ in theirworker relatedness asymmetries. The final principal findingwas that, of hypotheses linking the colony sex-investment ratiowith sexual productivity, there was support for the constantfemale hypothesis but not for the constant male, cost variation,or multifaceted parental investment hypotheses.     

Sex investment ratios in eusocial Hymenoptera support inclusive fitness theory

Inclusive fitness theory predicts that sex investment ratios in eusocial Hymenoptera are a function of the relatedness asymmetry (relative relatedness to females and males) of the individuals controlling sex allocation. In monogynous ants (with one queen per colony), assuming worker control, the theory therefore predicts female‐biased sex investment ratios, as found in natural populations. Recently, E.O. Wilson and M.A. Nowak criticized this explanation and presented an alternative hypothesis. The Wilson–Nowak sex ratio hypothesis proposes that, in monogynous ants, there is selection for a 1 : 1 numerical sex ratio to avoid males remaining unmated, which, given queens exceed males in size, results in a female‐biased sex investment ratio. The hypothesis also asserts that, contrary to inclusive fitness theory, queens not workers control sex allocation and queen–worker conflict over sex allocation is absent. Here, I argue that the Wilson–Nowak sex ratio hypothesis is flawed because it contradicts Fisher's sex ratio theory, which shows that selection on sex ratio does not maximize the number of mated offspring and that the sex ratio proposed by the hypothesis is not an equilibrium for the queen. In addition, the hypothesis is not supported by empirical evidence, as it fails to explain ‘split’ (bimodal) sex ratios or data showing queen and worker control and ongoing queen–worker conflict. By contrast, these phenomena match predictions of inclusive fitness theory. Hence, the Wilson–Nowak sex ratio hypothesis fails both as an alternative hypothesis for sex investment ratios in eusocial Hymenoptera and as a critique of inclusive fitness theory.     

Internest sex-ratio variation and male brood survival in the ant Pheidole pallidula

Sex allocation in social insects has become a general modelin tests of inclusive fitness theory, sex-ratio theory, andparent-offspring conflict. Several studies have shown that colonysex ratios are often bimodally distributed, with some coloniesproducing mainly females and others mainly males. Sex specializationmay result from workers assessing their relatedness to malebrood versus female brood, relative to the average worker-relatednessasymmetry in other colonies of their population. Workers thenadjust the sex ratio in their own interest This hypothesis assumesthat workers can recognize the sex of the brood in their colonyand selectively eliminate males. We compared the primary sexratio (at the egg stage) and secondary sex ratio (reproductivepupae and adults) of colonies in the ant Pheidole pallidula.There was a strong bimodal distribution of secondary sex ratios,with most colonies producing mainly reproductives of one sex.In contrast, there was no evidence of a bimodal distributionof primary sex ratios. The proportion of haploid eggs producedby queens was 0.35 in early spring and decreased to about 0.1in summer. Male eggs also were present in virtually all fieldcolonies sampled in July, although eggs laid at this time ofyear never give rise to males. All male brood is, therefore,selectively eliminated beginning in July and continue to beeliminated through the rest of the year. Finally, the populationsex-ratio investment was female-biased. Together, these resultsare consistent with the hypothesis that workers control thesecondary sex ratio by selectively eliminating male brood inabout half the colonies, perhaps those with high relatednessasymmetry.[Behav Ecol 7: 292–298 (1996)]     

Reproductive alliances and posthumous fitness enhancement in male ants

Ants provide excellent opportunities for studying the evolutionary aspects of reproductive conflict. Relatedness asymmetries owing to the haplodiploid sex determination of Hymenoptera create substantial fitness incentives for gaining control over sex allocation, often at the expense of the fitness interests of nest-mates. Under worker-controlled split sex ratios either the reproductive interests of the mother queen (when workers male bias the sex ratio) or the father (when workers female bias the sex ratio), but never that of both parents simultaneously, are fulfilled. When workers bias sex ratios according to the frequency of queen mating, males which co-sire a colony have a joint interest in manipulating their daughter workers into rearing a more female-biased sex ratio. Here we show that males of the ant Formica truncorum achieve such manipulation by partial sperm clumping, so that the cohort-specific relatedness asymmetry of the workers in colonies with multiple fathers is higher than the cumulative relatedness asymmetry across worker cohorts. This occurs because a single male fathers the majority of the offspring within a cohort. Colonies with higher average cohort-specific relatedness asymmetry produce more female-biased sex ratios. Posthumously expressed male genes are thus able to oppose the reproductive interests of the genes expressed in queens and the latter apparently lack mechanisms for enforcing full control over sperm mixing and sperm allocation.     

Kin-selected conflict in the bumble-bee Bombus terrestris (Hymenoptera: Apidae)

Kin selection theory predicts conflict in social Hymenoptera between the queen and workers over male parentage because each party is more closely related to its own male offspring. Some aspects of the reproductive biology of the bumble-bee Bombus terrestris support kin selection theory but others arguably do not. We present a novel hypothesis for how conflict over male parentage should unfold in B. terrestris colonies. We propose that workers delay laying eggs until they possess information showing that egg laying suits their kin-selected interests. In colonies where queens start to lay haploid eggs early, we hypothesize that this occurs when workers detect the presence of queen-produced male brood in the brood's larval stage. In colonies where queens start to lay haploid eggs late, we hypothesize that it occurs when workers detect a signal from the queen to female larvae to commence development as queens. Our hypothesis accounts for previously unexplained aspects of the timing of reproductive events in B. terrestris, provides ultimate explanations for the results of a recent study of mechanisms underlying queen-worker conflict and helps explain this species' characteristic bimodal (split) sex ratios. Therefore, kin selection theory potentially provides a good explanation for reproductive patterns in B. terrestris.     

Split sex ratios in perennial social Hymenoptera: a mixed evolutionary stable strategy from the queens' perspective?

In social Hymenoptera, relatedness asymmetries due to haplodiploidy often generate conflicts of genetic interest between queens and workers. Split sex ratios are common in ant populations and may result from such conflicts, with workers favoring the production of males in some colonies and of gynes in others. Such intercolonial differences may result from variations in relatedness asymmetries among colony members, but several examples are now known in which this hypothesis does not hold. We develop here a simple model assuming monogynous, monoandrous, worker-sterile, perennial colonies without dispersal restrictions. Workers may eliminate eggs of either sex and determine the caste of the female brood, but the queen controls the number of eggs of each sex she lays. In such conditions, we demonstrate that split sex ratios can result from queens adopting a mixed evolutionary stable strategy (ESS), with one option being to put a strict limit to the number of diploid eggs available and the alternative one to provide diploid eggs ad lib. In the former situation, workers should raise all diploid eggs as workers and release only male sexuals. In the latter, workers should adjust the caste ratio so as to reach the maximum sexual productivity for the colony, which is entirely invested into gynes. For a particular relative investment in gynes at the population level, between 0.5 (ESS under full queen control) and 0.75 (ESS under full worker control), an equilibrium is reached at which both strategies yield an equal genetic payoff to the queen. Male-specialized colonies are predicted to be equally abundant but less populous and less productive than gyne-specialized ones. Available data on the monogyne form of the fire ant, Solenopsis invicta, suggest that this model may apply in this case, although more specific studies are required to test these predictions.     

INTRASPECIFIC VARIATION IN ANT SEX RATIOS AND THE TRIVERS-HARE HYPOTHESIS

We consider worker-controlled sex investments in eusocial Hymenoptera (ants in particular) and assume that relatedness asymmetry is variable among colonies and that workers are able to assess the relatedness asymmetry in their own colony. We predict that such “assessing” workers should maximize their inclusive fitness by specializing in the production of the sex to which they are relatively most related, i.e., colonies whose workers have a relatedness asymmetry below the population average should specialize in males, whereas colonies whose workers have a higher than average relatedness asymmetry should specialize in making females. Our argument yields the expectation that colony sex ratios will be bimodally distributed in ant populations where relatedness asymmetry is variable owing to multiple mating, worker reproduction, and/or polygyny. No such bimodality is expected, however, in ant species where relatedness asymmetry is known to be constant, or in cases where relatedness asymmetry is supposed to be irrelevant due to allospecific brood rearing under queen control, as in the slave-making ants. Comparative data on colony sex ratios in ants are reviewed to test the predictions. The data partly support our contentions, but are as yet insufficient to be considered as decisive evidence.     

WHY DO SOME SOCIAL INSECT QUEENS MATE WITH SEVERAL MALES? TESTING THE SEX‐RATIO MANIPULATION HYPOTHESIS IN LASIUS NIGER

Although multiple mating most likely increases mortality risk for social insect queens and lowers the kin benefits for nonreproductive workers, a significant proportion of hymenopteran queens mate with several males. It has been suggested that queens may mate multiply as a means to manipulate sex ratios to their advantage. Multiple paternity reduces the extreme relatedness value of females for workers, selecting for workers to invest more in males. In populations with female-biased sex ratios, queens heading such male-producing colonies would achieve a higher fitness. We tested this hypothesis in a Swiss and a Swedish population of the ant Lasius niger. There was substantial and consistent variation in queen mating frequency and colony sex allocation within and among populations, but no evidence that workers regulated sex allocation in response to queen mating frequency; the investment in females did not differ among paternity classes. Moreover, population-mean sex ratios were consistently less female biased than expected under worker control and were close to the queen optimum. Queens therefore had no incentive to manipulate sex ratios because their fitness did not depend on the sex ratio of their colony. Thus, we found no evidence that the sex-ratio manipulation theory can explain the evolution and maintenance of multiple mating in L. niger.     

Evolution of sex ratios in social hymenoptera: kin selection,local mate competition,polyandry and kin recognition

A model is constructed to study the effects of local mate competition and multiple mating on the optimum allocation of resources between the male and female reproductive brood in social hymenopteran colonies from the ‘points of view’ of the queen (parental manipulation theory) as well as the workers (kin selection theory). Competition between pairs of alleles specifying different sex investment ratios is investigated in a game theoretic frame work. All other things being equal, local mate competition shifts the sex allocation ratio in favour of females both under queen and worker control. While multiple mating has no effect on the queen’s optimum investment ratio, it leads to a relatively male biased investment ratio under worker control. Under queen control a true Evolutionarily Stable Strategy(ess) does not exist but the ‘best’ strategy is merely immune from extinction. A trueess exists under worker control in colonies with singly mated queens but there is an asymmetry between the dominant and recessive alleles so that for some values of sex ratio a recessive allele goes to fixation but a dominant allele with the same properties fails to do so. Under multiple mating, again, a trueess does not exist but a frequency dependent region emerges. The best strategy here is one that is guaranteed fixation against any competing allele with a lower relative frequency. Our results emphasize the need to determine levels of local mate competition and multiple mating before drawing any conclusions regarding the outcome of queen-worker conflict in social hymenoptera. Multiple mating followed by sperm mixing, both of which are known to occur in social hymenoptera, lower average genetic relatedness between workers and their reproductive sisters. This not only shifts the optimum sex ratio from the workers’ ‘point of view’ in favour of males but also poses problems for the kin selection theory. We show that kin recognition resulting in the ability to invest in full but not in half sisters reverts the sex ratio back to that in the case of single mating and thus completely overcomes the hurdles for the operation of kin selection.     

Experimental manipulation of queen number affects colony sex ratio investment in the highly polygynous ant Formica exsecta

In polygynous (multiple queens per nest) ants, queen dispersal is often limited with young queens being recruited within the parental colony. This mode of dispersal leads to local resource competition between nestmate queens and is frequently associated with extremely male-biased sex ratios at the population level. The queen-replenishment hypothesis has been recently proposed to explain colony sex ratio investment under such conditions. It predicts that colonies containing many queens (subject to high local resource competition) should only produce males, whereas colonies hosting few queens (reduced or no local resource competition) should produce new queens in addition to males. We experimentally tested this hypothesis in the ant Formica exsecta by manipulating queen number over three consecutive years in 120 colonies of a highly polygynous population. Queens were transferred from 40 colonies into another 40 colonies while queen number was not manipulated in 40 control colonies. Genetic analyses of worker offspring revealed that our treatment significantly changed the number of reproductive queens. The sex ratio of colonies was significantly different between treatments in the third breeding season following the experiment initiation. We found that, as predicted by the queen-replenishment hypothesis, queen removal resulted in a significant increase in the proportion of colonies that produced new queens. These results provide the first experimental evidence for the queen-replenishment hypothesis, which might account for sex ratio specialization in many highly polygynous ant species.     

Male parentage and queen mating frequency in the bumblebee <Emphasis Type="Italic">Bombus ignitus</Emphasis> (Hymenoptera: bombinae)

Kin selection theory predicts conflict between queens and workers in the social insect colony with respect to male production. This conflict arises from the haplodiploid system of sex determination in Hymenoptera that creates relatedness asymmetries in which workers are more closely related to the sons of other workers than to those of the queen. In annual hymenopteran societies that are headed by a single queen, the mating frequency of the queen is the only factor that affects the colony kin structure. Therefore, we examined the mating structure of queens and the parentage of males in a monogynous bumblebee, Bombus ignitus, using DNA microsatellites. In the seven colonies that were studied, B. ignitus queens mated once, thereby leading to the prediction of conflict between the queen and workers regarding male production. In each of the five queen-right colonies, the majority of the males (95%) were produced by the colony’s queen. In contrast, workers produced approximately 47% of all the males in two queenless colonies. These results suggest that male production in B. ignitus is a conflict between queen and workers.     

Local resource competition and sex ratio in the ant Cataglyphis cursor

The local resource competition (LRC) hypothesis predicts thatwherever philopatric offspring compete for resources with theirmothers, offspring sex ratios should be biased in favor of thedispersing sex. In ants, LRC is typically found in polygynous(multiple queen) species where foundation of new nests occursby budding, which results in a strong population structure anda male-biased population-wide sex ratio. However, under polygyny,the effect of LRC on sex allocation is often blurred by theeffect of lowered relatedness asymmetries among colony members.Moreover, environmental factors, such as the availability ofresources, have also been shown to deeply influence sex ratioin ants. We investigated sex allocation in the monogynous (singlequeen) ant Cataglyphis cursor, a species where colonies reproduceby budding and both male and female sexuals are produced throughparthenogenesis, so that between-colony variations in relatednessasymmetries should be reduced. Our results show that sex allocationin C. cursor is highly male biased both at the colony and populationlevels. Genetic analyses indicate a significant isolation-by-distancein the study population, consistent with limited dispersal offemales. As expected from asexual reproduction, only weak variationsin relatedness asymmetry of workers toward sexual offspringoccur across colonies, and they are not associated with colonysex ratio. Inconsistent with the predictions of the resourceavailability hypothesis, the male bias significantly increaseswith colony size, and investment in males, but not in females,is positively correlated with total investment in sexuals. Overall,our results are consistent with the predictions of the LRC hypothesisto account for sex ratio variation in this species.     

Sex ratio determination by queens and workers in the ant Pheidole desertorum

Because workers in colonies of eusocial Hymenoptera are more closely related to sisters than to brothers, theory predicts workers should bias investment in reproductive broods to favour reproductive females over males. However, conflict between queens and workers is predicted. Queens are equally related to daughters and sons, and should act to prevent workers from biasing investment. Previous study of the ant Pheidole desertorum showed that workers are nearly three times more closely related to reproductive females than males; however, the investment sex ratio is very near equal, consistent with substantial queen control of workers. Near-equal investment is produced by an equal frequency of colonies whose reproductive broods consist of only females (female specialists) and colonies whose reproductive broods consist of only males or whose sex ratios are extremely male biased (male specialists). Because natural selection should act on P. desertorum workers to bias investment in favour of reproductive females, why do workers in male-specialist colonies rear only (or mostly) males? We tested the hypothesis that queens prevent workers from rearing reproductive females by experimentally providing workers with immature reproductive broods of both sexes. Workers reared available reproductive females, while failing to rear available males. Worker preference for rearing reproductive females is consistent with queens preventing their occurrence in colonies of male specialists. These results provide evidence that queens and workers will act in opposition to determine the sex ratio, a fundamental prediction of queen-worker conflict theory. Copyright 2000 The Association for the Study of Animal Behaviour.