Biased Sex Ratios Research Articles

Timing matters: corticosterone injections 4 h before ovulation bias sex ratios towards females in chickens.

Birds have the ability to influence offspring sex prior to egg laying and may use hormones to mediate these skews. Corticosterone is of particular interest as a mediator of offspring sex because, as the primary stress hormone in birds, it regulates responses to environmental and social stimuli that trigger sex ratio biases. In previous studies in birds, chronic elevations of corticosterone stimulated female-biased sex ratios while acute pharmacological elevations that were provided 5 h prior to the expected time of ovulation stimulated male-biased sex ratios. Here, we aimed to determine the magnitude of corticosterone necessary to influence offspring sex and to further pinpoint the timing of the hormonal influence. Because high-dose injections of corticosterone stimulated male-biased sex ratios in hens, we hypothesized that females receiving acute pharmacological elevations of corticosterone would produce more male offspring while females with acute physiological elevations would produce an intermediate proportion of males compared to controls. We tested our hypotheses in laying hens by elevating corticosterone in the physiological or pharmacological range through injections of corticosterone administered 4 or 5 h prior to the expected time of ovulation. Contrary to our hypothesis, a physiological dose of corticosterone provided 5 h prior to the expected time of ovulation did not bias offspring sex ratios when compared to controls. Further, when corticosterone injections were given at 4 h prior to the expected time of ovulation, sex ratios were instead biased towards females. These results suggest that the timing and magnitude of the corticosterone elevation are both critical not only to whether a sex ratio bias occurs, but also the direction of the bias.

Effects of social and climatic factors on birth sex ratio in Macaca mulatta in Mount Taihangshan area.

Sex allocation theory predicts the optimal investment to male and female offspring. However, a biased sex ratio requires explanations as to why the deviation occurs. Rhesus macaque (Macaca mulatta) is the most widely distributed nonhuman primate species and the Taihangshan macaque (Macaca mulatta tcheliensis) occupies the northern limit of all rhesus macaque natural populations worldwide. We observed one macaque troop (Wangwu-1 [WW-1]) inhabiting Taihangshan Macaque National Nature Reserve and recorded all birth events and the sex of newborn macaques from 2004 to 2013. Our aim was to apply the Trivers-Willard hypothesis to this free-ranging rhesus macaque troop, and to understand the relationship between climatic parameters (precipitation and temperature) and birth sex ratio. We found that the total newborn macaques showed a female-biased sex ratio at birth in the WW-1 troop, but there were no significant biased birth sex ratios in all matriarchs and in high-ranking and middle-ranking matrilineal units. However, the low-ranking macaque matrilineal unit was significantly female-biased. Moreover, we found that the annual precipitation of the previous year was positively associated with the birth sex ratio, and there was an interactive effect of troop size and current winter temperature on the birth sex ratio. The underlying mechanisms for the effects of social and climatic factors on birth sex ratio could be complex, and we discuss several plausible explanations.

First evidence of biased sex ratio at birth in a calanoid copepod

To test the hypothesis that the sex ratio of the copepod Acartia tonsa is biased at birth, adult sex ratios of the offspring of families (21 families in 2011 and 72 in 2013) produced by field-caught females were determined in the laboratory under controlled conditions. Sex ratios at birth were estimated from the adult sex ratio, and after applying a correction in which all dead/missing individuals were counted as the rarer sex (males). Before correction the overall population sex ratio was female biased during both years, with over 42% of mothers producing clutches that were significantly different from unity. After correction, 25% (2013) to 33% (2011) of mothers still produced significantly biased sex ratios. The ratio of deaths of males to females had to be 1.75 to 1 (2011) and 7.25 to 1 (2013) for sex ratios not to be biased at birth. There was no evidence of sex reversal in any individual from juvenile (C4 stage) to adult, implying that juvenile sex ratios were also significantly biased. Hence, neither sex reversal nor differential mortality of the sexes is a likely explanation for the observed skewed sex ratios. The sex ratios at birth varied widely from the expectation from Mendelian inheritance of sex chromosomes, suggesting that another mechanism is responsible for skewed sex ratios at birth. Since the majority of families with biased sex ratios at birth were female dominated as adults, biased sex ratio at birth may contribute to the skewed sex ratio of adults observed in the field.

Effect of photoperiod on slow and fast developing individuals in aphidophagous ladybirds, Menochilus sexmaculatus and Propylea dissecta (Coleoptera: Coccinellidae).

The effects of environmental parameters on insect development have been studied extensively. But the reasons for 2 differential developmental rates within same cohort under varying environmental factors have not been explored. For the purpose, in this study the existence and stability of slow and fast development under 5 photoperiods (i.e., 8L: 16D, 10L : 14D, 12L : 12D, 14L : 10D and 16L : 8D; light and dark hours per day) and its effect on body mass and reproductive attributes in 2 aphidophagous ladybirds, Menochilus sexmaculatus (Fabricius) and Propylea dissecta (Mulsant) was examined on Aphis craccivora Koch at 27 ± 1 °C temperature. A clear bimodal (2 peaks, where the first peak represented the fast developing and the 2nd peak slow developing individuals) pattern of distribution at each photoperiod was found. The proportion of slow and fast developing individuals in a cohort differed with photoperiods. The slow developing individuals were more in numbers at 8L : 16D, in equal numbers at 14L : 10D and in less numbers at 16L: 8D, indicating that the variation in emergence was owing to exogenous cues influenced differential rates of mortality. Slow developing individuals had female biased sex ratio, higher longevity and lower body mass than fast developing individuals. Fast developing females laid higher numbers of eggs with higher egg viability than slow developing females. Study of such variations in development at different photoperiods is helpful to understand its role in the development of insects particularly ladybirds and permits the selection of fast developing bioagents for their use in biocontrol of pest species.

Can Wolbachia (Rickettsiaceae) explain female-biased sex ratios in mountain pine beetles (Coleoptera: Curculionidae)?

AbstractWolbachia Hertig (Rickettsiales: Rickettsiaceae) is a genus of endosymbionts that are widespread in insects and can have important implications on population dynamics through cytoplasmic incompatibility or sex-ratio distortion. We considered Wolbachia as a possible explanation for female-biased sex ratios often observed in mountain pine beetle populations, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae: Scolytinae), a major pest species in western Canada whose most recent outbreak and range expansion has killed many millions of mature lodgepole pine, Pinus contorta var. latifolia Engelmann (Pinaceae). We used wsp primers to screen for Wolbachia in 10 mountain pine beetle populations with varying degrees of female bias in southeast British Columbia and southwest Alberta, Canada. We found no evidence of Wolbachia in these populations and suggest that other factors may explain the sex-ratio bias.

Retrospective study of the prevalence of Pseudoloma neurophilia shows male sex bias in zebrafish Danio rerio (Hamilton-Buchanan).

Retrospective study of the prevalence of Pseudoloma neurophilia shows male sex bias in zebrafish Danio rerio (Hamilton-Buchanan).

Sex-ratio meiotic drive and Y-linked resistance in Drosophila affinis.

Genetic elements that cheat Mendelian segregation by biasing transmission in their favor gain a significant fitness benefit. Several examples of sex-ratio meiotic drive, where one sex chromosome biases its own transmission at the cost of the opposite sex chromosome, exist in animals and plants. While the distorting sex chromosome gains a significant advantage by biasing sex ratio, the autosomes, and especially the opposite sex chromosome, experience strong selection to resist this transmission bias. In most well-studied sex-ratio meiotic drive systems, autosomal and/or Y-linked resistance has been identified. We specifically surveyed for Y-linked resistance to sex-ratio meiotic drive in Drosophila affinis by scoring the sex ratio of offspring sired by males with a driving X and one of several Y chromosomes. Two distinct types of resistance were identified: a restoration to 50/50 sex ratios and a complete reversal of sex ratio to all sons. We confirmed that fathers siring all sons lacked a Y chromosome, consistent with previously published work. Considerable variation in Y-chromosome morphology exists in D. affinis, but we showed that morphology does not appear to be associated with resistance to sex-ratio meiotic drive. We then used two X chromosomes (driving and standard) and three Y chromosomes (susceptible, resistant, and lacking) to examine fertility effects of all possible combinations. We find that both the driving X and resistant and lacking Y have significant fertility defects manifested in microscopic examination of testes and a 48-hr sperm depletion assay. Maintenance of variation in this sex-ratio meiotic drive system, including both the X-linked distorter and the Y-resistant effects, appear to be mediated by a complex interaction between fertility fitness and transmission dynamics.

Do mothers prefer helpers or smaller litters? Birth sex ratio and litter size adjustment in cotton-top tamarins (Saguinus oedipus).

Sex allocation theory has been a remarkably productive field in behavioral ecology with empirical evidence regularly supporting quantitative theoretical predictions. Across mammals in general and primates in particular, however, support for the various hypotheses has been more equivocal. Population-level sex ratio biases have often been interpreted as supportive, but evidence for small-scale facultative adjustment has rarely been found. The helper repayment (HR) also named the local resource enhancement (LRE) hypothesis predicts that, in cooperatively breeding species, mothers invest more in the sex which assists with rearing future offspring and that this bias will be more pronounced in mothers who require extra assistance (i.e., due to inexperience or a lack of available alloparents). We tested these hypotheses in captive cotton-top tamarins (Saguinus oedipus) utilizing the international studbook and birth records obtained through a questionnaire from ISIS-registered institutions. Infant sex, litter size, mother's age, parity, and group composition (presence of nonreproductive subordinate males and females) were determined from these records. The HR hypothesis was supported over the entire population, which was significantly biased toward males (the “helpful” sex). We found little support for helper repayment at the individual level, as primiparous females and those in groups without alloparents did not exhibit more extreme tendencies to produce male infants. Primiparous females were, however, more likely to produce singleton litters. Singleton births were more likely to be male, which suggests that there may be an interaction between litter size adjustment and sex allocation. This may be interpreted as supportive of the HR hypothesis, but alternative explanations at both the proximate and ultimate levels are possible. These possibilities warrant further consideration when attempting to understand the ambiguous results of primate sex ratio studies so far.

Identifying the selective pressures underlying offspring sex-ratio adjustments: a case study in a wild seabird

Sex allocation theory predicts that parents should bias offspring sex according to the costs and benefits associated with producing either sex in a given context. Accurately interpreting sex-ratio biases, therefore, requires a precise identification of these selective pressures. However, such information is generally lacking. This may partly explain the inconsistency in reported sex allocation patterns, especially in vertebrates. We present data from a long-term feeding experiment in black-legged kittiwakes (Rissa tridactyla) that allowed us to increase investment capacity for some breeding pairs. Previous findings showed that these pairs then overproduced sons compared with control parents. Here, our aim was to test the underlying assumptions of the 2 appropriate sex allocation models for our context: the “cost of reproduction hypothesis” and the “Trivers–Willard hypothesis.” The former assumes a sex difference in rearing costs, whereas the latter assumes a difference in fitness returns. 1) Independent of feeding treatment, rearing sons was energetically more demanding for parents (as revealed by higher energy expenditure and higher baseline corticosterone levels) than rearing daughters, thereby corroborating the underlying assumption of the “cost of reproduction hypothesis.” 2) Evidence supporting the assumptions of the “Trivers–Willard hypothesis” was less convincing. Overall, our results suggest that drivers of parental sex allocation decisions are probably more related to offspring sex-specific energetic costs than to their future reproductive success in our study species. Assessing the adaptive value of sex-ratio biases requires precise investigation of the assumptions underlying theoretical models, particularly as long as the mechanisms involved in sex-ratio manipulation remain largely unknown.

Homogenous genetic structure in populations of <i>Taxus baccata</i> with varied proportions of male and female individuals

English yew( L.) is a strictly outcrossing and dioecious species whose populations are small and isolated. It is known that sex ratios may vary in natural populations due to local environmental conditions or stochastic events. However, unbalanced sex ratios may have negative impacts on genetic diversity through enhanced genetic drift and inbreeding. The present study represents one of the first attempts to compare the genetic variation at microsatellite loci within and between populations with different gender proportions. Our results indicated that there were no significant correlations between sex ratio and the extent of genetic variation in different populations. All populations exhibited high levels of genetic diversity. Additionally, the genetic structure was characterized separately in male and female individuals. Statistical analyses of the set estimators describing the genetic structure of male and female individuals of revealed no significant differences between the two groups. Molecular analysis verified that microsatellite nuclear loci neutrality developed for as there were no significant differences in the genetic variation between males and females and no evidence for any outlier loci using coalescent and hierarchical Bayesian simulations. The results demonstrate that ignoring biased sex ratios in populations had no effect on the assessment of genetic differentiation and genetic diversity within and between populations of this species. These results are discussed with regards to the practical application of molecular markers in conservation programs. Taxus baccataT. baccataT. baccata,T. baccata

Simultaneous courtship and parenting in males and sex role reversal in females of the haremic bluebanded goby, Lythrypnus dalli

While males typically compete for females, species with female biased sex ratios and/or large male investment in offspring care often exhibit reversed sex roles. Here we investigated, in a haremic fish species, the bluebanded goby, Lythrypnus dalli, the impact of male and female courtship behaviour on male reproductive success, measured as the total number of eggs in the nest and total number of developed eggs. Reproductive success was not associated with rates of male behaviour, such as parenting, approaching and courtship, but was associated with rates of female courtship. Consistent with predictions for a role-reversed reproductive strategy, only males demonstrated nest care and females exhibited high rates of courtship and intrasexual competition, such that alpha females interrupted courtship solicitations by beta females. Overall, these data are consistent with sex role reversal in L. dalli and show that the expression of male courtship behaviour does not interfere with paternal care.

Nesting biology of an Oriental carpenter bee, Xylocopa (Biluna) nasalis Westwood, 1838, in Thailand (Hymenoptera, Apidae, Xylocopinae)

The biological study of wild non- Apis bees can provide useful information that may help with the pollination of food crops and native plants in areas where the keeping of honey bee colonies is restricted or affected by CCD. Here, we describe the nesting biology of the Oriental large carpenter bee, Xylocopa (Biluna) nasalis Westwood, 1838. An aggregation of more than 80+ bamboo nests of X. nasalis was discovered in Suan Pheung district, Ratch Buri province, Thailand on the 25 th of May 2012. We collected 27 nests from the site to dissect, measure the external and internal nest architecture, and analyze the pollen composition of the pollen masses. X. nasalis constructs linear unbranched nests with nest entrance mostly located at the open-end of the bamboo culms. The nest length and the branch diameter of the nest entrance (excluding nesting edge) are 25.40 ± 6.95 cm and 17.94 ± 6.00 mm, and the maximum number of provisioned cells is 8. A biased sex ratio of 8♀: 1♂ is reported, with up to 7 adults inhabiting in a single nest. 29 pollen types were identified from 14 pollen masses using an acetolysis method and visualization under both light microscope and scanning electron microscope. 13 pollen types were considered as major pollen sources (contribute ≥ 1% in total pollen volume); however, only 10 can be identified to family and generic levels. The dominant pollen sources are of the families Elaeagnaceae ( Elaeagnus cf. latifolia), Euphorbiaceae (Croton), Fabaceae (Senna siamea and Cassia), Fagaceae (Lithocarpus and Castanopsis), and Lythraceae (Trapa) which are mostly native to the region of Southeast Asia. The nesting architectural details should prove to be beneficial to beekeepers and researchers who are interested in trapping and studying X. nasalis, and the polylectic behavior of X. nasalis can be highly valuable for future crop pollination strategies, particularly for plants that require sonication of their poricidal anthers.

Characterization of a subtropical hawksbill sea turtle (Eretmocheyles imbricata) assemblage utilizing shallow water natural and artificial habitats in the Florida Keys.

In order to provide information to better inform management decisions and direct further research, vessel-based visual transects, snorkel transects, and in-water capture techniques were used to characterize hawksbill sea turtles in the shallow marine habitats of a Marine Protected Area (MPA), the Key West National Wildlife Refuge in the Florida Keys. Hawksbills were found in hardbottom and seagrass dominated habitats throughout the Refuge, and on man-made rubble structures in the Northwest Channel near Cottrell Key. Hawksbills captured (N = 82) were exclusively juveniles and subadults with a straight standard carapace length (SSCL) ranging from 21.4 to 69.0cm with a mean of 44.1 cm (SD = 10.8). Somatic growth rates were calculated from 15 recaptured turtles with periods at large ranging from 51 to 1188 days. Mean SSCL growth rate was 7.7 cm/year (SD = 4.6). Juvenile hawksbills (<50 cm SSCL) showed a significantly higher growth rate (9.2 cm/year, SD = 4.5, N = 11) than subadult hawksbills (50–70 cm SSCL, 3.6 cm/year, SD = 0.9, N = 4). Analysis of 740 base pair mitochondrial control region sequences from 50 sampled turtles yielded 12 haplotypes. Haplotype frequencies were significantly different compared to four other Caribbean juvenile foraging aggregations, including one off the Atlantic coast of Florida. Many-to-one mixed stock analysis indicated Mexico as the primary source of juveniles in the region and also suggested that the Refuge may serve as important developmental habitat for the Cuban nesting aggregation. Serum testosterone radioimmunoassay results from 33 individuals indicated a female biased sex ratio of 3.3 females: 1 male for hawksbills in the Refuge. This assemblage of hawksbills is near the northern limit of the species range, and is one of only two such assemblages described in the waters of the continental United States. Since this assemblage resides in an MPA with intensive human use, basic information on the assemblage is vital to resource managers charged with conservation and species protection in the MPA.

Inter-Annual Variability of Fledgling Sex Ratio in King Penguins.

As the number of breeding pairs depends on the adult sex ratio in a monogamous species with biparental care, investigating sex-ratio variability in natural populations is essential to understand population dynamics. Using 10 years of data (2000–2009) in a seasonally monogamous seabird, the king penguin (Aptenodytes patagonicus), we investigated the annual sex ratio at fledging, and the potential environmental causes for its variation. Over more than 4000 birds, the annual sex ratio at fledging was highly variable (ranging from 44.4% to 58.3% of males), and on average slightly biased towards males (51.6%). Yearly variation in sex-ratio bias was neither related to density within the colony, nor to global or local oceanographic conditions known to affect both the productivity and accessibility of penguin foraging areas. However, rising sea surface temperature coincided with an increase in fledging sex-ratio variability. Fledging sex ratio was also correlated with difference in body condition between male and female fledglings. When more males were produced in a given year, their body condition was higher (and reciprocally), suggesting that parents might adopt a sex-biased allocation strategy depending on yearly environmental conditions and/or that the effect of environmental parameters on chick condition and survival may be sex-dependent. The initial bias in sex ratio observed at the juvenile stage tended to return to 1∶1 equilibrium upon first breeding attempts, as would be expected from Fisher’s classic theory of offspring sex-ratio variation.

Conflict over condition-dependent sex allocation can lead to mixed sex-determination systems.

Theory suggests that genetic conflicts drive turnovers between sex-determining mechanisms, yet these studies only apply to cases where sex allocation is independent of environment or condition. Here, we model parent–offspring conflict in the presence of condition-dependent sex allocation, where the environment has sex-specific fitness consequences. Additionally, one sex is assumed to be more costly to produce than the other, which leads offspring to favor a sex ratio less biased toward the cheaper sex in comparison to the sex ratio favored by mothers. The scope for parent–offspring conflict depends on the relative frequency of both environments: when one environment is less common than the other, parent–offspring conflict can be reduced or even entirely absent, despite a biased population sex ratio. The model shows that conflict-driven invasions of condition-independent sex factors (e.g., sex chromosomes) result either in the loss of condition-dependent sex allocation, or, interestingly, lead to stable mixtures of condition-dependent and condition-independent sex factors. The latter outcome corresponds to empirical observations in which sex chromosomes are present in organisms with environment-dependent sex determination. Finally, conflict can also favor errors in environmental perception, potentially resulting in the loss of condition-dependent sex allocation without genetic changes to sex-determining loci.

Suitability of five mealybug species (Hemiptera, Pseudococcidae) as hosts for the solitary parasitoid Anagyrus sp. nr. pseudococci (Girault) (Hymenoptera: Encyrtidae)

Anagyrus sp. nr. pseudococci is an endoparasitoid which has been used as a biological control agent of mealybug pests. In this study, we compared the suitability of five mealybugs species with different phylogenetic relationships and geographical origins as hosts of this parasitoid. The selected mealybugs were: (1) a Mediterranean-native species, Planococcus ficus, sharing a long co-evolutionary history with the parasitoid; (2) three exotic species, the Afrotropical Planococcus citri, the Australasian Pseudococcus calceolariae and the Neotropical Pseudococcus viburni, with a recent history; and (3) the Neotropical Phenacoccus peruvianus, with no previous common history with the parasitoid. Host suitability was assessed based on different fitness parameters, such as body size, developmental time, emergence rate and sex ratio. The parasitoid was able to complete development in all mealybug species. Nevertheless, its emergence rate significantly varied among mealybug species, with the highest values observed in Pl. ficus and Pl. citri, intermediate values in Ps. calceolariae and the lowest ones in Ps. viburni and Ph. peruvianus. The body size of adult wasp females varied with host suitability and was positively correlated with other measures of parasitoid fitness, including the emergence rate and the sex ratio. The parasitoid developmental time differed among mealybug species but did not correlate with any other measure of fitness. A female biased sex ratio was found in the parasitoid progeny emerged from all mealybug species, except in Ps. viburni and Ph. peruvianus. There was a direct relationship between the proportion of females in the parasitoid progeny and the emergence rate.

Small-scale variation in sexual size dimorphism and sex ratio in the aquatic moth Acentria ephemerella Denis and Schiffermüller, 1775 (Lepidoptera: Crambidae)

The aquatic herbivorous and capital-breeding moth Acentria ephemerella Denis and Schiffermüller, 1775 feeds on submerged pondweeds, Potamogeton spp., and is highly preyed upon by fish in the littoral zone. We studied the spatiotemporal within-lake variability of length, sexual size dimorphism (SSD) and sex ratio of A. ephemerella pupae and of larval population densities. Population densities at three sampling sites strongly increased from July to August and were significantly higher at the Reichenau site in July. Acentria ephemerella sex ratio was male-biased at the Güttingen and Hagnau sites, but showed unbiased or slightly biased sex ratios at Reichenau. The SSD was strongly female-biased. Female size and SSD declined during summer, possibly due to reduced food quantity/quality. The SSD was highest at Reichenau, with little to no differences between Hagnau and Güttingen. At Reichenau, the high population size in July coincided with an unbiased sex ratio, and large SSD/female size due to multiple, possibly interacting factors, including fish predation.

Sex ratio distortion in the Nesolynx thymus (Hymenoptera: Eulophidae), an ecto-pupal parasitoid of uzifly, Exorista sorbillans (Diptera: Tachinidae)

The reproductive alterations induced by maternally inherited α-proteo-bacteria Wolbachia to their hosts is a well-document- ed phenomenon. In Nesolynx thymus, a gregarious hymenopterous ecto-pupal parasitoid of the uzifly, Exorista sorbillans, diagnostic PCR assay using specific primers revealed the presence of Wolbachia. Following genetic crossing experiments, we observed a female biased sex ratio of 1 : 9.5 at 25°C and 1 : 3 male to female ratio when the populations were exposed to heat shock 33°C for six hours. Furthermore, we found infection polymorphism, where female parasitoids are infected by Wolbachia but males are not infected. In- fected eggs develop into females, whereas uninfected eggs develop parthenogenetically into males. The results are discussed in the context of the possible mechanism of sex-ratio bias caused by Wolbachia.

Different male vs. female breeding periodicity helps mitigate offspring sex ratio skews in sea turtles

The implications of climate change for global biodiversity may be profound with those species with little capacity for adaptation being thought to be particularly vulnerable to warming. A classic case of groups for concern are those animals exhibiting temperature-dependent sex-determination (TSD), such as sea turtles, where climate warming may produce single sex populations and hence extinction. We show that, globally, female biased hatchling sex ratios dominate sea turtle populations (exceeding 3:1 in >50% records), which, at-a-glance, reiterates concerns for extinction. However, we also demonstrate that more frequent breeding by males, empirically shown by satellite tracking 23 individuals and supported by a generalized bio-energetic life history model, generates more balanced operational sex ratios (OSRs). Hence, concerns of increasingly skewed hatchling sex ratios and reduced population viability are less acute than previously thought for sea turtles. In fact, in some scenarios skewed hatchling sex ratios in groups with TSD may be adaptive to ensure optimum OSRs.

Mutually beneficial host exploitation and ultra-biased sex ratios in quasisocial parasitoids.

Selfish interests usually preclude resource sharing, but under some conditions collective actions enhance per capita gains. Such Allee effects underlay early explanations of social evolution but current understanding focusses on kin selection (inclusive fitness). We find an Allee effect that explains unusual quasisociality (cooperative brood care) among parasitoid wasps without invoking or precluding kin selection effects. In Sclerodermus harmandi, individual females produce most offspring when exploiting small hosts alone. However, larger hosts are more successfully exploited by larger groups of females, with the per-female benefits outweighing the costs of host sharing. Further, the extremely biased sex ratios (97% female) are better explained by mutually beneficial female–female interactions that increase the reproductive value of daughters (local resource enhancement), rather than by the usually invoked local mate competition between males. Thus, atypical quasisocial behaviour in a parasitoid wasp directly enhances reproductive success and selects for very extremely female-biased sex ratios.