Offspring Sex Ratio Research Articles

A possible case of offspring sex manipulation as result of a biased adult sex ratio

Although random meiosis should prevent the facultative adjustment of offspring sex ratio, theory predicts that females should produce more of the sex with the higher reproductive value. We reported a case of offspring sex ratio manipulation in grass wrens Cistothorus platensis. Males in better body condition would have higher reproductive value than females due to the potential for social polygyny and extra-pair fertilizations. On the other hand, local demography influences reproductive strategies in grass wrens as male abundance affects both social polygyny and extra-pair paternity frequencies. We evaluated whether females bias their brood sex ratio in response to adult sex ratio and nestling body condition (a proxy for female’s prospects of producing high-quality males). Females raised more male offspring when males were less abundant in the population (female-biased adult sex ratio). However, we found no relationship between nestling body condition and brood sex ratio, suggesting that females did not bias the brood sex ratio towards males when able to raise nestlings in better body condition. Taken together, our results provide the first suggestive evidence that female birds can manipulate their offspring sex ratio in response to the adult sex ratio.

The offspring sex ratio at birth in one of the largest human harems.

The offspring sex ratio at birth in one of the largest human harems.

Oviposition behavior of the quasi-gregarious parasitoid, Ooencyrtus kuvanae (Hymenoptera: Encyrtidae).

The parasitoid wasp, Ooencyrtus kuvanae (Howard) (Hymenoptera: Encyrtidae), is a natural enemy of the spongy moth, a significant forest pest in North America. We investigated the oviposition behavior of O. kuvanae females on spongy moth egg masses by (i) presenting female parasitoids with a single spongy moth egg mass that was replaced every day, 2nd day, 4th day, 8th day, or 16th day (which is the total length of the oviposition period) and (ii) presenting female parasitoids with 1, 2, 4, or 8 egg masses at a time. Offspring developmental length ranged from 18 to 24 days. On average, male offspring exhibited faster developmental times, emerging approximately 1 day ahead of females. The amount of time that adult females spent on an egg mass affected the number of parasitized eggs. Specifically, more offspring emerged in the 4-, 8-, and 16-day treatments than in scenarios involving daily or every second-day egg mass replacement. The percentage of male offspring decreased as the number of egg masses presented to females increased. Interestingly, the total number of female offspring remained constant, but the number of male offspring decreased with an increase in the number of egg masses and time spent by the parent within a patch. The observed sexual dimorphism in development time, the influence of resource availability on offspring sex ratios, and flexible oviposition patterns illustrate the adaptability of O. kuvanae in response to varying conditions. These insights have implications for our understanding of parasitoid-host interactions and their potential role in biological control strategies.

Socioeconomic status and sex ratio in the contemporary Hungarian population

Abstract According to the Trivers–Willard hypothesis (TWH), when the mother's condition around conception influences the future reproductive success of male and female offspring differently, the adjustment of offspring sex ratio (SR) to maternal condition will increase the parents’ fitness. The TWH has been tested in several taxa, including humans where socioeconomic status as an index of condition has been widely used. The results are inconsistent, possibly because the preconditions of the TWH are not always met. To investigate the preconditions and prediction of the TWH in the contemporary Hungarian population, we collected data by an online questionnaire on self-perceived childhood living standard, the number of children and the sex of the respondents’ siblings. We found no sex-specific relationship between reproductive success and childhood living standards, thus the precondition of the TWH was not met. We found no relationship between socioeconomic status and offspring SR when data from the whole country was used, but there was a tendency in the predicted direction when we used data from Budapest and considered the SR of only those family members who were born under similar conditions. Similar approaches should be preferred in the future to avoid noise caused by changing status during the reproductive lifespan.

Hard life for sons in the nest? Sex-dependent offspring mortality in Great Tits in urban and forest areas

Sex-biased mortality can occur in birds during development, for example due to sexual differences in energy requirement and/or environmental sensitivity, or the effects of sex hormones or sex differences in the expression of mutations linked to sex chromosomes. The extent of sex-bias in mortality may also be related to environmental conditions that influence offspring development and survival. Urban areas often provide poorer conditions for nestling development resulting in higher offspring mortality compared to natural areas, which may accelerate sex differences in offspring mortality in cities. To test this hypothesis, we examined the sex ratio of dead offspring in Great Tits (Parus major), using 427 samples of unhatched eggs and dead nestlings collected in two urban and two forest sites between 2013 and 2019. The ratio of males in the whole sample of dead offspring (56.9%) was significantly higher than expected by an 1:1 ratio, and the strongest sex biases were detected in urban areas (57.6% males) and in young nestlings (<14 days old, 59.0% males). However, the sex ratios of dead offspring did not differ significantly among study sites and between offspring developmental stages. 29.3% of unhatched eggs contained a visible embryo, and the proportion of embryo-containing unhatched eggs did not differ significantly between urban and forest study sites. These results suggest male-biased offspring mortality in Great Tits, and highlight the need of large datasets to detect subtle differences between habitats and developmental stages.

Male Feminization of the Common Pillbug Armadillidium vulgare by Wolbachia bacteria

Wolbachia pipientis (Wolbachia) is a maternally inherited endosymbiotic bacterium that infects over 50% of arthropods, including pillbugs, and acts as a reproductive parasite in the host. In the common terrestrial pillbug Armadillidium vulgare (A. vulgare), Wolbachia alters the sex ratio of offspring through a phenomenon called feminization, where genetic males develop into reproductive females. Previous studies have focused on the presence or absence of Wolbachia as a sex ratio distorter in laboratory cultured and natural populations mainly from sites in Europe and Japan. Our three-year study is the first to evaluate the effects of the Wolbachia sex ratio distorter in cultured A. vulgare offspring in North America. We asked whether Wolbachia bacteria feminize A. vulgare isopod male offspring from infected mothers and if this effect can be detected in F1 offspring by comparing the male/female offspring ratios. If so, the F1 offspring ratio should show a higher number of females than males compared to the offspring of uninfected mothers. Over three years, pillbug offspring were cultured from pregnant A. vulgare females and developed into adults. We determined the Wolbachia status of mothers and counted the ratios of male and female F1 progeny to determine feminization effects. In each year sampled, significantly more female offspring were born to Wolbachia-infected mothers than those from uninfected mothers. These ratio differences suggest that the Wolbachia infection status of mothers directly impacts the A. vulgare population through the production of reproductive feminized males, which in turn provides an advantage for further Wolbachia transmission.

The impact of different pairing scheme on female reproductive investment: Research on Japanese Quail (Cortunix japonica)

Determining the sex of the embryo during incubation is crucial for the poultry industry. The primary problem to comprehend in this study is the manipulation of the sex ratio and also the egg production. The study will concentrate on how two distinct methods of pairing parents—one male with one female and another male with three females—affect egg production, the average egg mass, and the sex offspring ratio. As the animal model, we employ the layer strain of the Japanese Quail (Coturnix japonica) since they produce continuous eggs each day and easy to handle. The Japanese quail pairs were housed for seven days prior to the start of egg collection. These eggs were nurtured for up to three weeks after hatching until the sexing could be determined. When compared to a pair of three females, the egg production of a single pair was much higher when alternative pattern schemes were used. However, the mass of their typical eggs is remarkably similar. The male to female chick offspring sex ratio regularly showed to be 1:1 or equal. This finding could serve as the foundation for further research into the impact of partner pairing on another avian layer strain.

Parasitism success of Microplitis manilae (Hymenoptera: Braconidae) on different diet‐fed Spodoptera frugiperda (Lepidoptera: Noctuidae) larvae

AbstractEfficient rearing is necessary for the successful biological control of pests. The braconid wasp, Microplitis manilae, is a natural solitary parasitoid of larval Spodoptera frugiperda. However, the effect of the host diet on the parasitism success of M. manilae on S. frugiperda larvae remains unclear. Here, we determined the parasitism rate, oviposition, eclosion rate, developmental period and offspring sex ratio of M. manilae parasitizing S. frugiperda fed with fresh corn leaves or an artificial diet. M. manilae parasitizing the corn leaf‐fed S. frugiperda larvae exhibited a significantly higher parasitism rate (40%), oviposition (62.50 eggs/female) and eclosion rate (81.38%) than the wasp parasitizing the artificial diet‐fed S. frugiperda larvae (22%, 38.33 eggs/female, and 58.73%, respectively). Furthermore, M. manilae parasitizing the corn leaf‐fed S. frugiperda larvae exhibited a significantly shorter developmental period (16.35 days) and lower sex ratio (1.17) than the wasp parasitizing the artificial diet‐fed S. frugiperda larvae (18.00 and 2.15 days, respectively). Overall, M. manilae exhibited better parasitic success on corn leaf‐fed S. frugiperda larvae than on artificial diet‐fed S. frugiperda larvae. Therefore, using fresh corn leaf‐fed S. frugiperda is the most suitable strategy for mass‐rearing M. manilae for pest biocontrol. Our findings suggest M. manilae as an effective biocontrol candidate for controlling S. frugiperda in corn fields.

The moon cycle influences reproductive and productive traits in guinea pigs (Cavia porcellus) from a tropical Andean area

ABSTRACT There is currently innumerable evidence showing that the lunar cycle affects various reproductive aspects in farm animals. However, there is very little information on the effect of the lunar cycle on productive traits in these species. A retrospective study was conducted (2015–2018) to evaluate the influence of the lunar cycle on some reproductive and productive traits in a guinea pig (Cavia porcellus) production system in the southern Andean region of Ecuador. A total of 7352 productive and reproductive records of guinea pig females housed in 3 m2 cages with a breeding male were analyzed. The following variables were considered: offspring sex ratio, litter size per cage, number of weaned guinea pigs, mortality, individual and litter weaning weight of guinea pigs, and calving frequency. The lunar cycle was split into eight periods of ~3.7-d length each. Data were analyzed by logistic regression and general linear model, and means were compared by the least mean squares method of the SAS. The offspring sex ratio was not correlated to the lunar cycle. The lunar cycle at mating and calving influenced the litter size per cage, number of weaned, and mortality of guinea pigs. The individual and litter-weaning weights were influenced by the lunar cycle at calving. The frequency of calving was greater around the new and full moon than in the remaining periods of the lunar cycle. In conclusion, the lunar cycle influenced several productive and reproductive traits in guinea pigs, such as litter size, mortality, number of pups weaned, and individual and litter weaning weights. This valuable information may have practical applications in management of guinea pig production systems.

Gamma irradiation-induced offspring masculinization is associated with epigenetic changes in female zebrafish

Sex ratio variation is a key topic in ecology, because of its direct effects on population dynamics and thus, on animal conservation strategies. Among factors affecting sex ratio, types of sex determination systems have a central role, since some species could have a sex determined by genetic factors, environmental factors or a mix of those two. Yet, most studies on the factors affecting sex determination have focused on temperature or endocrine-disrupting chemicals (EDCs), and much less is known regarding other factors. Exposure to gamma irradiation was found to trigger offspring masculinization in zebrafish. Here we aimed at deciphering the potential mechanisms involved, by focusing on stress (i.e. cortisol) and epigenetic regulation of key genes involved in sex differentiation in fish. Cortisol levels in exposed and control (F0) zebrafish females’ gonads were similar. However, irradiation increased the DNA methylation level of foxl2a and cyp19a1a in females of the F0 and F1 generation, respectively, while no effects were detected in testis. Overall, our results suggest that parental exposure could alter offspring sex ratio, at least in part by inducing methylation changes in ovaries.

Effects of Glycitein Injections on Native Chickens’ Ability to Reproduce, Sex Ratio in Offspring

The aim of this study was to improve the reproductive ability of native Iraqi chickens with the use of glycitein. The Studie was conducted on a of 120 Iraqi native chickens, consisting of 100 hens and 20 roosters. The chickens were 26 weeks old at the time of the study. The chickens were divided into four treatment groups, with each group consisting of 25 chicks. The experimental design consisted of four groups: the first group served as the non-injection control (referred to as T1), while the remaining groups (T2, T3, and T4) were treated with injections of glycitein at concentrations of 5, 10, and 15 mg/kg body weight, respectively. These injections were given subcutaneously in the neck region, with a frequency of once every 28 days across a span of three periods. Subsequently, an examination was conducted on the percentages of fertility and hatchability, as well as the primary and secondary sex ratios pertaining to female subjects. The results of the study showed that the use of glycitein injection had a beneficial impact on fertility, hatching, as well as primary and secondary sex ratios. Therefore, it can be concluded that the impact of glycitein yields a favourable outcome on both the primary and secondary sexual ratios.

Reproductive Biology of Trichopria anastrephae (Hymenoptera: Diapriidae), a Biological Control Agent of Drosophila suzukii (Matsumura) (Diptera: Drosophilidae).

Hymenopteran parasitoids, like any other insect, employ strategies to ensure their reproduction. Understanding these strategies is important for ecological purposes, but also to improve mass rearing of biological control agents. Here, we describe mating strategies used by the pupal parasitoid Trichopria anastrephae Lima (Hymenoptera: Diapriidae), a potential biocontrol agent, that has been considered for augmentative releases for management of the invasive pest species Drosophila suzukii (Matsumura) (Diptera: Drosophilidae). We studied the emergence pattern of males and females of T. anastrephae and the effects of parental ae on offspring number and sex ratio. Polygamy was also studied and its effects on parasitism and offspring production were described. Adults from this species emerge in the first hours of photophase, and males emerge before females, demonstrating that T. anastrephae is a protandrous species. Parasitoid age when first mated influences the parasitism and sex ratio. Younger females result in a higher number of offspring (8.16 parasitoids/day), while older males result in a more female-biased sex ratio of offspring (64% females). Both males and females are polygamic, and the order in which a female is mated by the male affects parasitism, viability of parasitized pupae, and sex ratio of offspring, with the first female performing the highest parasitism and sex ratio (63.83 and 61% of females, respectively), but the lowest viability (92.92%). Females that are allowed to mate multiple times generate lower numbers of offspring (113.05 parasitoids) when compared to virgin or single-mated females (135.20 and 130.70 parasitoids, respectively), but the highest sex ratio (49% of females). Data present in this study and how it can be used to improve parasitoid rearing and field releases of T. anastrephae, in biological control programs for D. suzukii are discussed.

Efficacy of artificial nest shading as a climate change adaptation measure for marine turtles at Ascension Island

AbstractSuccessful embryonic development and offspring sex ratios of marine turtles are determined by thermal conditions experienced during incubation, rendering them potentially vulnerable to anthropogenic climate change. With the rate of projected temperature rises likely to outpace the adaptive capacity of long‐lived species such as marine turtles, there is growing interest in management interventions aimed at mitigating the effects of climate change at nesting grounds. In this study, we experimentally tested the impacts of artificial nest shading on the incubation temperature, hatching success, and predicted offspring sex ratio of green turtle (Chelonia mydas) clutches at Ascension Island. Clutches (n = 97) were sampled from 2 nesting beaches with naturally contrasting thermal environments (one hot; one cool) and either left as in situ controls or relocated to shaded or unshaded hatcheries on their beach of origin. Compared to unshaded experimental clutches, shading reduced mean incubation temperatures and sex‐determining temperatures (i.e., middle third of embryonic development) by 0.5–0.9°C and 0.5–1.2°C respectively, with the reduction being greater on the hotter beach. Shading also differentially affected hatchling output across the 2 sites: on the hot beach, shading significantly improved hatching success by ~23% but had minimal effects on offspring sex ratio; whereas on the cooler beach, shading did not impact hatching success but resulted in ~12% more male offspring. Interestingly, mean incubation temperatures of in situ controls did not differ significantly from shaded clutches, and were significantly cooler than unshaded experimental clutches, suggesting relocation may have negated some of the benefits of shading. Our results demonstrated that artificial shading may be a viable approach for partially offsetting climate change impacts on nesting marine turtles; however, scalability will be a major challenge in achieving conservation objectives at high‐density nesting sites like Ascension Island.

Identifying Offspring Sex Ratio Skews in Zoological Facilities Using Large Historical Datasets

The Fisher principle states that species should produce offspring at a ratio of 1:1 unless there are sex-specific differences in rearing costs. Research conducted across taxa has found that animals will vary the sex ratio of their offspring so as to maximize personal fitness in response to various ecological and biological variables. This phenomenon has been especially well studied within the Avian class. Professionally managed populations provide a useful framework within which to study the impacts of numerous variables on sex allocation strategies. Zoological facilities may be especially motivated to investigate this phenomenon due to corresponding conservation and welfare implications. We analyzed a large dataset containing the demographic data of over 1 million zoo-housed birds for species-specific offspring sex ratio biases. The offspring sex ratios of 19,867 zoo-hatched dams were calculated, and the median offspring sex ratio of 277 species from 25 orders was calculated across two 20-year time periods. We used a Wilcoxon signed rank tests with a Holm Bonferroni alpha criterion in order to determine if species medians differed from parity. Only one species in one time period displayed a significant species median offspring sex ratio. The limits of our dataset are discussed. We propose that future research should further explore the determinants of sex allocation strategies for species held within zoological facilities.

Repeated Exposure to High-THC Cannabis Smoke during Gestation Alters Sex Ratio, Behavior, and Amygdala Gene Expression of Sprague Dawley Rat Offspring.

Because of the legalization of Cannabis in many jurisdictions and the trend of increasing Δ9-tetrahydrocannabinol (THC) content in Cannabis products, an urgent need exists to understand the impact of Cannabis use during pregnancy on fetal neurodevelopment and behavior. To this end, we exposed female Sprague Dawley rats to Cannabis smoke daily from gestational day 6 to 20 or room air. Maternal reproductive parameters, offspring behavior, and gene expression in the offspring amygdala were assessed. Body temperature was decreased in dams following smoke exposure and more fecal boli were observed in the chambers before and after smoke exposure in dams exposed to smoke. Maternal weight gain, food intake, gestational length, litter number, and litter weight were not altered by exposure to Cannabis smoke. A significant increase in the male-to-female ratio was noted in the Cannabis-exposed litters. In adulthood, male and female Cannabis smoke-exposed offspring explored the inner zone of an open field significantly less than control offspring. Gestational Cannabis smoke exposure did not affect behavior on the elevated plus maze test or social interaction test in the offspring. Cannabis offspring were better at visual pairwise discrimination and reversal learning tasks conducted in touchscreen-equipped operant conditioning chambers. Analysis of gene expression in the adult amygdala using RNA sequencing revealed subtle changes in genes related to development, cellular function, and nervous system disease in a subset of the male offspring. These results demonstrate that repeated exposure to high-THC Cannabis smoke during gestation alters maternal physiological parameters, sex ratio, and anxiety-like behaviors in the adulthood offspring.

Paternal influences over offspring sex ratio in mammals – tested hypotheses and potential mechanisms

Abstract The sex ratio of sexually reproducing animal species is usually close to 1:1 and could be influenced by both parents. Deviations from this ratio occur in nature and can have an adaptive role. Although there are several evolutionary hypotheses explaining this bias, there is a significant lack of knowledge of the mechanisms of sex ratio facultative adjustments. While maternal effects are relatively well studied, the paternal effects on the primary sex ratio remain elusive. This review aims to assemble and summarise the knowledge of male control in sex allocation. In particular, we focused on identifying possible mechanisms through which the adjustment could be achieved at pre‐, peri‐ and post‐fertilisation levels. A systematic literature review was performed. We selected 59 articles that provided a potential proximal explanation of observed offspring sex ratio skews for which mammalian males might be responsible. We outlined potential physiological and genetic mechanisms of male‐driven sex allocation operating in male germ cells and zygote stage in mammals. However, most of the proposed hypotheses of how males can bias the offspring sex ratio concern mostly the production of unequal numbers of Y‐ and X‐chromosome‐bearing spermatozoa, and their differences in morphology and physiology. As males possess a wide range of mechanisms to effectively alter the sex ratio of their offspring, they can play a significant role in this phenomenon and should be studied to reduce the gap in our understanding of their role in sex allocation in comparison to the role of females. More empirical studies on different mammalian models are needed to fully characterise the trends of the adjustment. Predicting the sex ratio of the progeny is of great importance, not only in animal husbandry but also for the conservation of endangered species.

Evidential segregation analysis for offspring sex ratio in rabbit and sheep populations

Offspring sex ratio has been found to be altered by environmental and genetic distortions in multiple species, against the Mendelian inheritance rules. However, little is known in livestock populations where it is essential to validate whether a polymorphic major gene with relevant effects may segregate in the target population. However, the current analytical tool (model FREQ) cannot handle new mutations in non-founder individuals, reducing the chance of detecting them. Our new analytical approach aimed to overcome this limitation in the context of evidential inference, a statistical framework based on the likelihood function as a robust objective measure of the strength of statistical evidence without variation from the sample size. Two field data sets from sheep and rabbit populations were used. Models evaluated environmental and inbreeding effects in both species. Our new approach assumed that the mutation primarily arose in an individual of the analyzed data set (model MUTj). Each sire was individually analyzed to determine the most plausible source for the new mutation, if any. The likelihood ratio (LR) against a reference parametrization without mutations (model NULL) was used to test the statistical relevance of systematic effects (LR ≥ 8) and models (LR ≥ 32). Both species revealed relevant departures for offspring sex ratio along the analyzed time frame with strong evidence for the year (LR = 1.4 × 109 in Ripollesa sheep and LR = 85.7 in MARET rabbits) and season (LR = 12.6 in MARET rabbits), although with a fluctuating pattern. The age of the dam reported weak evidence in both species (LR < 4). Inbreeding had a relevant linear impact on both sheep (LR = 60.7) and rabbit (LR = 780.4) populations, whereas the quadratic effect only showed strong evidence in MARET rabbits (LR = 8.3). Regarding the segregation analysis, most models showed an almost homozygous pattern of weak or lacking evidence of new mutations influencing offspring sex ratio. The only species showing strong evidence for the MUTj model (LR > 32) was the Ripollesa sheep, with a ram three to six generations from the founders as the most likely source for a new mutation increasing the odds of daughters. The additive genetic effect of this model for the mutant allele also had strong evidence (LR = 1,195). Therefore, the MUTj parametrization can be a valuable analytical tool to check for the possibility of new mutations along the pedigree files, not only before the founders.

Sex-specific DNA methylation and transcription of zbtb38 and effects of gene–environment interactions on its natural antisense transcript in zebrafish

ABSTRACT There is increasing evidence for the involvement of epigenetics in sex determination, maintenance, and plasticity, from plants to humans. In our previous work, we reported a transgenerational feminization of a zebrafish population for which the first generation was exposed to cadmium, a metal with endocrine disrupting effects. In this study, starting from the previously performed whole methylome analysis, we focused on the zbtb38 gene and hypothesized that it could be involved in sex differentiation and Cd-induced offspring feminization. We observed sex-specific patterns of both DNA methylation and RNA transcription levels of zbtb38. We also discovered that the non-coding exon 3 of zbtb38 encodes for a natural antisense transcript (NAT). The activity of this NAT was found to be influenced by both genetic and environmental factors. Furthermore, increasing transcription levels of this NAT in parental gametes was highly correlated with offspring sex ratios. Since zbtb38 itself encodes for a transcription factor that binds methylated DNA, our results support a non-negligible role of zbtb38 not only in orchestrating the sex-specific transcriptome (i.e., sex differentiation) but also, via its NAT, offspring sex ratios.

Effects of Food Source Availability, Host Egg:Parasitoid Ratios, and Host Exposure Times on the Developmental Biology of Megacopta cribraria Egg Parasitoids.

Parasitoids forage for hosts in dynamic ecosystems and generally have a short period of time to access hosts. The current study examined the optimal reproductive attributes of two egg parasitoids, Paratelenomus saccharalis Dodd (Hymenoptera: Platygastridae) and Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae), of the kudzu bug, Megacopta cribraria Fabricius (Hemiptera: Plataspidae). The proportion of O. nezarae and P. saccharalis adult offspring that emerged from M. cribraria eggs and the sex ratio of the parasitoid offspring were compared among treatments for the effects of different adult parasitoid food sources, host egg-to-adult parasitoid ratios, and host exposure times. Our results suggest that honey solution as a food source, a 21:7 host-to-parasitoid ratio, and three-to-five days of exposure time optimized the production of female O. nezarae offspring. For P. saccharalis, honey solution as a food source, a 21:7 host-to-parasitoid ratio, and one day were optimal for maximizing female offspring production. These findings provide new information about the biology of these egg parasitoids.

Reproductive plasticity in response to the changing cluster size during the breeding period: a case study in a spider mite

Animals living in clusters should adjust their reproductive strategies to adapt to the social environment. Theories predict that the benefits of cluster living would outweigh the costs of competition. Yet, it is largely unknown how animals optimize their reproductive fitness in response to the changing social environment during their breeding period. We used Tetranychus ludeni Zacher, a haplodiploid spider mite, to investigate how the ovipositing females modified their life-history traits in response to the change of cluster size (i.e., aggregation and dispersal) with a consistent population density (1 ♀/cm2). We demonstrate that (1) after females were shifted from a large cluster (16 ♀♀) to small ones (1 ♀, 5 and 10 ♀♀), they laid fewer and larger eggs with a higher female-biased sex ratio; (2) after females were shifted from small clusters to a large one, they laid fewer and smaller eggs, also with a higher female-biased sex ratio, and (3) increasing egg size significantly increased offspring sex ratio (% daughters), but did not increase immature survival. The results suggest that (1) females fertilize more larger eggs laid in a small population but lower the fertilization threshold and fertilize smaller eggs in a larger population, and (2) the reproductive adjustments in terms of egg number and size may contribute more to minimize the mate competition among sons but not to increase the number of inhabitants in the next generation. The current study provides evidence that spider mites can manipulate their reproductive output and adjust offspring sex ratio in response to dynamic social environments.