Queen-worker conflicts over male production and sex allocation in a primitively eusocial wasp

Abstract In a colony headed by a single monandrous foundress, theories predict that conflicts between a queen and her workers over both sex ratio and male production should be intense. If production of males by workers is a function of colony size, this should affect sex ratios, but few studies have examined how queens and workers resolve both conflicts simultaneously. We conducted field and laboratory studies to test whether sex-ratio variation can be explained by conflict over male production between queen and workers in the primitively eusocial wasp Polistes chinensis antennalis.
Worker oviposition rate increased more rapidly with colony size than did queen oviposition. Allozyme and micro-satellite markers revealed that the mean frequency of workers' sons among male adults in queen-right colonies was 0.39 ± 0.08 SE (n = 22). Genetic relatedness among female nestmates was high (0.654–0.796), showing that colonies usually had a single, monandrous queen. The mean sex allocation ratio (male investment/male and gyne investments) of 46 queen-right colonies was 0.47 ± 0.02, and for 25 orphaned colonies was 0.86 ± 0.04. The observed sex allocation ratio was likely to be under queen control. For queen-right colonies, the larger colonies invested more in males and produced reproductives protandrously and/or simultaneously, whereas the smaller colonies invested more in females and produced reproductives protogynously. Instead of positive relationships between colony size and worker oviposition rate, the frequency of workers' sons within queen-right colonies did not increase with colony size. These results suggest that queens control colony investment, even though they allow worker oviposition in queen-right colonies. Eggs laid by workers may be policed by the queen and/or fellow workers. Worker oviposition did not influence the outcome of sex allocation ratio as a straightforward function of colony size.     

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.     

Sexual dimorphism and intraspecific aggression,and their relationship to sex ratios in Caprella gorgonia Laubitz & Lewbel (Crustacea: Amphipoda: Caprellidae)

The amphipod Caprella gorgonia Laubitz & Lewbel is an obligate commensal on gorgonian octocorals. Its primary host is Lophogorgia chilensis (Verrill), found below 20 m.C. gorgonia breeds throughout the year, with wide fluctuations in abundance. Mating and oviposition follow molting. Sex reversal does not occur; two distinct sexes are present from the first instar after emergence from the brood pouch.Young males and females grow at approximately the same rate, but males are larger by a relatively constant increment. Males continue to grow at their original rate to a maximum size (about twice that of females). The growth rate of females is not limited by the onset of reproduction and brooding, but rather by an approach to maximum size when the rate is greatly reduced. Fecundity of females is not affected by size.The population sex ratio is about 1:3 (males:females), and about 1:4 among adults. The secondary sex ratio is 1:1. The post-emergence sex ratio bias is a result of heavier mortality among males. Sex ratios drop from 50% at emergence to 25% as females approach maximum size, then rise to 100% in larger size classes.Differential predation on males did not appear to be a source of any sex ratio bias. Adult males possess a “poison spine”, a puncturing weapon on the large second gnathopod, which functions in mating-related intraspecific combat with other males. Intraspecific male aggression during mating is a major cause of sex ratio bias. In the laboratory, increased density in breeding groups may affect mortality due to male aggression. In nature, adult sex ratios are negatively correlated with population density. The reproductive capacity of the population is not limited by a shortage of adult males, despite the low adult sex ratio.     

Relationship between prey consumption and colony size in an orb spider

Summary I studied the relationship between prey consumption and colony size in the orb spiderPhiloponella semiplumosa. Observations of unmanipulated colonies showed that prey biomass per juvenile spider was positively correlated with colony size, indicating that prey consumption was highest in the largest colonies observed. In contrast, the relationship between prey biomass per adult female and colony size was curvilinear; prey consumption tended to be highest in intermediatesized colonies. Adult female cephalothorax width was positively correlated with colony size. Number of egg sacs per adult female tended to be highest in intermediate-sized colonies. Prey biomass per juvenile was lower in experimentally reduced colonies than in large control colonies. Aerial-arthropod abundance was not correlated with colony size, and experimental prey supplementation did not affect colony size. Thus, the relationship between prey consumption and colony size was influenced by coloniality directly, rather than by a correlation between prey abundance at a site and colony size.     

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.     

Limited male dispersal in a social spider with extreme inbreeding

Cooperatively breeding animals commonly avoid incestuous mating through pre-mating dispersal. However, a few group-living organisms, including the social spiders, have low pre-mating dispersal, intra-colony mating, and inbreeding. This results in limited gene flow among colonies and sub-structured populations. The social spiders also exhibit female-biased sex ratios because survival benefits to large colonies favour high group productivity, which selects against 1 : 1 sex ratios. Although propagule dispersal of mated females may occasionally bring about limited gene flow, little is known about the role of male dispersal. We assessed the extent of male movement between colonies in natural populations both experimentally and by studying colony sex ratios over the mating season. We show that males frequently move to neighbouring colonies, whereas only 4% of incipient nests were visited by dispersing males. Neighbouring colonies are genetically similar and movement within colony clusters does not contribute to gene flow. Post-mating sex ratio bias was high early in the mating season due to protandry, and also in colonies at the end of the season, suggesting that males remain in the colony when mated females have dispersed. Thus, male dispersal is unlikely to facilitate gene flow between different matrilineages. This is consistent with models of non-Fisherian group-level selection for the maintenance of female biased sex ratios, which predict the elimination of male dispersal.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society 2009, 97 , 227–234.     

Sex investment ratios in monogyne and polygyne populations of the fire ant Solenopsis invicta

Both monogyne (single queen per colony) and polygyne (multiple queens per colony) populations of the fire ant Solenopsis invicta are good subjects for tests of kin selection theory because their genetic and reproductive attributes are well-characterized, permitting quantitative predictions about the degree to which sex investment ratios should be female-biased if workers and not queens control reproductive allocation. In the study populations, an investment ratio of 3 females: 1 male is predicted (a proportional investment in females of 0.75) in the monogyne form, whereas a proportional investment in females between 0.637 and 0.740 is expected in the polygyne form. To test these predictions, colonies from a single population of each social form were collected and censused during three different seasons. Consistent with their alternative modes of colony founding, monogyne colonies invested more in reproduction (sexual production) and less in growth/maintenance (worker production) than did the polygyne colonies. Overall, the sex investment ratios were female-biased in both forms, although there was considerable seasonal variation. After adjusting for sex-specific energetic costs, the proportional investment in females was 0.607 in the monogyne population, a value in between those expected under complete control by either the queen or the workers. However, when combined with data from four other previously studied monogyne populations in the U.S.A., the mean investment ratio did not differ significantly from the value predicted if workers have exclusive control. In the polygyne population, the proportional investment in females of 0.616 was consistent with the level of female bias expected under partial to complete worker control, although the potential influence of two confounding factors — possible contact with monogyne colonies and the preponderance of sterile diploid males — weakens this conclusion somewhat. Taken as a whole, the sex investment ratios of monogyne and polygyne populations of S. invicta are consistent with at least partial worker control. Of several ultimate and proximate explanations that have been proposed to explain inter-colonial variation in the sex investment ratio, only the effect of the primary sex ratio (female-determined eggs: male-determined eggs) laid by the queen appears to account for the observed variation among monogyne colonies. In the polygyne population, there is limited support for the hypothesis that greater resource abundance favors investment in females.     

Colony sex ratios vary with queen number but not relatedness asymmetry in the ant Formica exsecta

Split-sex-ratio theory assumes that conflict over whether to produce predominately males or female reproductives (gynes) is won by the workers in haplodiploid insect societies and the outcome is determined by colony kin structure. Tests of the theory have the potential to provide support for kin-selection theory and evidence of social conflict. We use natural variation in kinship among polygynous (multiple-queen) colonies of the ant Formica exsecta to study the associations between sex ratios and the relatedness of workers to female versus male brood (relatedness asymmetry). The population showed split sex ratios with about 89% of the colonies producing only males, resulting in an extremely male-biased investment ratio in the population. We make two important points with our data. First, we show that queen number may affect sex ratio independently of relatedness asymmetry. Colonies producing only males had greater genetic effective queen number but did not have greater relatedness asymmetry from the perspective of the adult workers that rear the brood. This lack of a difference in relatedness asymmetry between colonies producing females and those producing only males was associated with a generally low relatedness between workers and brood. Second, studies that suggest support for the relatedness-asymmetry hypothesis based on indirect measures of relatedness asymmetry (e.g. queen number estimated from relatedness data taken from the brood only) should be considered with caution. We propose a new hypothesis that explains split sex ratios in polygynous social insects based on the value of producing replacement queens.     

Sex-ratio conflict between queens and workers in eusocial Hymenoptera: mechanisms, costs, and the evolution of split colony sex ratios

Because workers in the eusocial Hymenoptera are more closely related to sisters than to brothers, theory predicts that natural selection should act on them to bias (change) sex allocation to favor reproductive females over males. However, selection should also act on queens to prevent worker bias. We use a simulation approach to analyze the coevolution of this conflict in colonies with single, once-mated queens. We assume that queens bias the primary (egg) sex ratio and workers bias the secondary (adult) sex ratio, both at some cost to colony productivity. Workers can bias either by eliminating males or by directly increasing female caste determination. Although variation among colonies in kin structure is absent, simulations often result in bimodal (split) colony sex ratios. This occurs because of the evolution of two alternative queen or two alternative worker biasing strategies, one that biases strongly and another that does not bias at all. Alternative strategies evolve because the mechanisms of biasing result in accelerating benefits per unit cost with increasing bias, resulting in greater fitness for strategies that bias more and bias less than the population equilibrium. Strategies biasing more gain from increased biasing efficiency whereas strategies biasing less gain from decreased biasing cost. Our study predicts that whether queens or workers evolve alternative strategies depends upon the mechanisms that workers use to bias the sex ratio, the relative cost of queen and worker biasing, and the rates at which queen and worker strategies evolve. Our study also predicts that population and colony level sex allocation, as well as colony productivity, will differ diagnostically according to whether queens or workers evolve alternative biasing strategies and according to what mechanism workers use to bias sex allocation.     

Sex ratios and the distribution of elaiosomes in colonies of the ant, Aphaenogaster rudis

Summary: Genetic theory predicts that workers in monogynous ant colonies with singly-mated queens should capitalize on higher relatedness with sisters than with brothers by altering the sex investment ratio of a colony in favor of females. Sex investment ratios, however, may also be influenced by the amount of resources available to colonies, in part because more mating opportunities might be obtained by investing scarce resources in males, which are much smaller than queens. Female larvae that reach a critical size by a particular point in development become queens while underfed larvae develop into workers, so workers could potentially influence the sex investment ratio of a colony by selectively feeding female larvae. In a previous experiment on the ant, Aphaenogaster rudis, colonies increased female sex investment after their diet was supplemented with elaiosomes, a lipid-rich food gained from a seed dispersal mutualism. In order to investigate the mechanisms producing this shift, we radio-labeled Sanguinaria canadensis elaiosomes with fatty acids and compared uptake among castes within a colony. The experiment was performed in both the laboratory and field. Lab colonies produced female-biased sex investment ratios, while field colonies mainly invested in males. We hypothesize that this discrepancy is related to differing levels of background food availability in the lab and field. The results of the elaiosome distribution experiment do not support a hypothesis that elaiosomes play a qualitative role in queen determination, because all individuals in a colony receive this nutrient. There is, however, support for the hypothesis that elaiosomes have a quantitative effect on larval development because larvae that accumulated more radio-label from elaiosomes tended to develop into gynes (virgin queens), while other female larvae developed into workers.     

Mating frequency, genetic structure, and sex ratio in the intermorphic female producing ant species Myrmecina nipponica

1. Myrmecina nipponica has two types of colonies: a queen colony type, in which the reproductive females are queens and new colonies are made by independent founding, and an intermorphic female colony type, in which reproductive females belong to a wingless intermediate morphology between queen and worker, and where colonies multiply through colonial budding. 2. The mating frequencies of reproductive females in both types indicate monoandry. The relatedness among nestmates in both types was almost 0.75, however relatedness between mother and daughter in intermorphic female colonies was slightly higher than that of queen colonies. 3. The sex ratio (corrected investment female ratio) was 0.70 at the population level, suggesting that the sex ratio is controlled by workers in this species, however the ratio differed greatly between the two types of colonies. Queen colonies (n = 37) had a female‐biased sex ratio of 0.77 while intermorphic female colonies (n = 33) had a ratio of 0.56. 4. Each reproductive intermorphic female was accompanied by an average of 2.9 workers (including virgin intermorphic females) in the colonial budding, and when the investment to those workers was added to the female investment, the sex ratio reached 0.81. 5. The frequency distribution of sex ratio was bimodal, with many colonies producing exclusively males or females, however mean estimated relatedness within colonies was almost 0.75. These data are inconsistent with the genetic variation hypothesis, which is one of the predominant hypotheses accounting for the between‐colony variation in sex ratio.     

Progeny sex ratios in a short-lived lizard: seasonal invariance despite sex-specific effects of hatching date on fitness

When fitness returns are sex-specific, selection should favor the facultative adjustment of offspring sex ratios. Seasonal shifts in offspring sex ratios are predicted to be particularly beneficial in short-lived, sexually dimorphic species in which hatching date is linked to adult size, which is related to fitness in a sex-specific fashion. We used four time series of hatching dates and progeny sex ratios in the brown anole (Anolis sagrei), a short-lived lizard with male-biased sexual size dimorphism, to test for such a seasonal shift in progeny sex ratio. In 2 of the 4 years, we also released hatchlings to their natural environment to test for sex-specific effects of hatching date on juvenile survival and adult size. We found that the relationship between hatching date and size the following year was significantly steeper in males than in females, and previous work has shown that adult size is more strongly tied to fitness in males than in females. Based on those results and on further evidence linking hatching date and body size to sex-specific survival and reproductive success, we predicted that sex ratios should shift from male- to female-biased as the breeding season progressed. Contrary to our prediction, we detected no clear seasonal shift in progeny sex ratio. Furthermore, although juvenile survival was correlated with hatching date, this relationship did not consistently differ between the sexes. The observation that progeny sex ratios are seasonally invariant despite several apparent links to adult fitness suggests that the evolution of a seasonal sex-ratio bias is either inherently constrained or requires a stronger selective advantage with respect to juvenile survival.     

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.     

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.     

Agonistic Interactions of Juvenile Savanna Baboons

19 juvenile members of known genealogies in two wild baboon groups were studied over a 16-month period to compare the ontogeny of agonistic experience and dominance relations for males and females. Juveniles of all age-sex classes were disproportionately likely to receive aggression from and submit to adult males per unit of time spent in proximity. This pattern intensified with increasing juvenile age. With age, juvenile females more often submitted to unrelated adult females from higher-ranking families, whereas this was not true for juvenile males. All juveniles received aggression from older group members more often during feeding than was expected by chance. High rates of agonistic interaction with unrelated adult females accounted for old juvenile females (3–5.5 years-old) interacting agonistically more frequently than male age peers and young juveniles of either sex (1–2.5 years-old). Adult females were also more aggressive toward females among young juveniles, suggesting that adult females target females among juveniles for aggression and resistance to rank reversal. Within juvenile age groups, males dominated all females and all younger males, irrespective of maternal dominance status. Dominance relations among female age-peers were generally isomorphic with relations among their mothers. No juvenile targeted any older male for rank reversal. Males targeted all older females, whereas females typically targeted only older females from families lower-ranking than their own. The strong sexual dimorphism in adult body size in baboons may explain why juvenile males' dominance relations with peers and adult females are not structured along lines of family membership as is true for the less dimorphic macaques. Acquisition of higher agonistic status probably allows juveniles of both sexes to increase their success in within-group feeding competition during late stages of juvenility, which, in turn, could affect important life-history traits such as age at menarche and adult body size.     

Seasonal variation in the sex and age composition of the woodcock bag in Denmark

The Eurasian woodcock is a highly valued game bird in Western Europe from which c. 2.7 million individuals are harvested annually from an estimated population of 20–26 million birds. The population size and status remains uncertain due to the cryptic behaviour and widespread and solitary occurrence of woodcock, on breeding and wintering areas, making reliable population surveys difficult. Hunting bag records provide age ratios amongst bagged birds, but sex ratios remain poorly known because of the sexually monomorphic nature of this species. We used DNA analysis to determine sex ratios amongst 327 shot woodcocks from two hunting seasons in Denmark (1 October–31 January, 2012/13 and 2013/14). Based on bag totals, age ratios and sex ratios, juvenile females constituted 37%, juvenile males 27%, adult females 16% and adult males 20% of the annual woodcock bag. The female bias was related to a significant deviation from parity in the sex ratio amongst juvenile birds in October, although no such deviation was found at other times or amongst adults. Compared to limited data from other European countries, our data suggest that autumn migration of woodcock involves an initial wave of juvenile females followed by juvenile males and adults, and perhaps that males stay further north in Europe than females during autumn and winter. This migratory pattern would suggest that postponing the opening of the hunting season could reduce the hunting bag on reproductively valuable females in this polygamous species.     

Dioecy and the evolution of sex ratios in ants

Split sex ratios, when some colonies produce only male and others only female reproductives, is a common feature of social insects, especially ants. The most widely accepted explanation for split sex ratios was proposed by Boomsma and Grafen, and is driven by conflicts of interest among colonies that vary in relatedness. The predictions of the Boomsma–Grafen model have been confirmed in many cases, but contradicted in several others. We adapt a model for the evolution of dioecy in plants to make predictions about the evolution of split sex ratios in social insects. Reproductive specialization results from the instability of the evolutionarily stable strategy (ESS) sex ratio, and is independent of variation in relatedness. We test predictions of the model with data from a long-term study of harvester ants, and show that it correctly predicts the intermediate sex ratios we observe in our study species. The dioecy model provides a comprehensive framework for sex allocation that is based on the pay-offs to the colony via production of males and females, and is independent of the genetic variation among colonies. However, in populations where the conditions for the Boomsma–Grafen model hold, kin selection will still lead to an association between sex ratio and relatedness.     

Bumblebee sex ratios: why do bumblebees produce so many males?

Sex investment ratios in populations of bumblebees are male biased, which contradicts theoretical predictions. Male-biased investment ratios in eusocial Hymenoptera are assumed to be non-stable for both the queen and her workers. In this paper, we show that male-biased sex allocation does not necessarily decrease fitness in the bumblebee Bombus terrestris. A male-biased investment ratio can be the result of an optimal allocation of resources when resources are scarce if (i) there is a large cost difference between male and female production, (ii) there is uncertainty about the amount of resources a colony can invest, and (iii) only a proportion of the investment made in an individual can be reused. This resource allocation then leads to split sex ratios depending on the amount of resources available to a bumblebee colony: colonies under low resource conditions will show a male-biased investment ratio, whereas colonies under high resource conditions allocate more resources towards females. However, the extent to which bumblebee populations show a male-biased sex allocation cannot be explained by cost differences between male and female production alone. In a recent paper, A. F. G. Bourke argued that male-biased investment ratios in bumblebee populations are a by-product of the occurrence of protandry (males emerge before females). Here we will extend Bourke''s argument and show that within a protandrous population, both protandrous and protogynous (females emerge before males) colonies exist. The existence of protandrous and protogynous colonies results in split sex ratios in time, because protogynous colonies rely on males produced by protandrous colonies (partial protandry).     

Stochastic effects in LMC models.

LMC (local mate competition) was first introduced by W. D. Hamilton to explain extraordinary female-biased sex ratios observed in a variety of insects and mites. In the original model, the population is subdivided into an infinite number of colonies founded by a fixed number of inseminated females producing the same very large number of offspring. The male offspring compete within the colonies to inseminate the female offspring and then these disperse at random to found new colonies. An unbeatable sex ratio strategy is found to be female-biased. In this paper, the effects of having colonies of random size and foundresses producing a random finite number of offspring are considered. The exact evolutionarily stable strategy (ESS) sex ratio is deduced and comparisons with previous approximate or numerical results are made. As the mean or the variance of brood size increases, the ESS sex ratio becomes more female-biased. An increase in the variance of colony size increases the ESS proportion of males when the mean brood size and colony size are both small, but decreases this proportion when the mean brood size or the mean colony size is large.     

Male-biased sex ratios,female promiscuity,and copulatory mate guarding in an aggregating tropical bug,Dysdercus bimaculatus

The ecological and social bases of the mating system of the seed-feeding bug, Dysdercus bimaculatus(Hemiptera: Pyrrhocoridae), were studied in the lab and in aggregations at the host tree, Sterculia apetala(Malvales: Malvaceae), in Panama. On theoretical grounds, two factors are predicted to be of importance in determining the evolution of male mating tactics in Ms species: the operational sex ratio and the probability that undefended females will mate with other males, subjecting the gametes of deserters to sperm competition. Results of a study of a related species suggested that sperm displacement is probably substantial. Adult sex ratios at numerous sites were significantly male biased, and females whose mates were removed remated before oviposition (i. e., sperm utilization). These results predict that a mate defense tactic is likely to be superior to a nondefense tactic. The biological significance of the parameters is supported by observations that captive pairs often remained in copulafor several days, until just before oviposition. However, substantial variation in copulation duration was also observed, and possible causes of this variation are considered. Causes of male biased adult sex ratios were investigated by monitoring demographic changes within a single aggregation over 2 months. Both female juvenile and adult mortality rates were greater than male. In addition, dissections of reproductive adults showed that the flight muscles of females, but not males, had histolyzed, so that female reproduction is physiologically limited to a single site. Greater rates of immigration among both mature and young males suggests that an excess of males may also be found in the populations of bugs that subsequently colonize other host plants, so that female scarcity is typical of aggregations in all stages of development. The evolution of sex-limtied flight muscle histolysis may be explained by greater patchiness of females than males as mating resources, plus a lower energetic benefit/cost ratio of histolysis for males.