Variation In Sex Ratio Research Articles

Sex-Biased Mortality and Sex Reversal Shape Wild Frog Sex Ratios

Population sex ratio is a key demographic factor that influences population dynamics and persistence. Sex ratios can vary across ontogeny from embryogenesis to death and yet the conditions that shape changes in sex ratio across ontogeny are poorly understood. Here, we address this issue in amphibians, a clade for which sex ratios are generally understudied in wild populations. Ontogenetic sex ratio variation in amphibians is additionally complicated by the ability of individual tadpoles to develop a phenotypic (gonadal) sex opposite their genotypic sex. Because of sex reversal, the genotypic and phenotypic sex ratios of entire cohorts and populations may also contrast. Understanding proximate mechanisms underlying phenotypic sex ratio variation in amphibians is important given the role they play in population biology research and as model species in eco-toxicological research addressing toxicant impacts on sex ratios. While researchers have presumed that departures from a 50:50 sex ratio are due to sex reversal, sex-biased mortality is an alternative explanation that deserves consideration. Here, we use a molecular sexing approach to track genotypic sex ratio changes from egg mass to metamorphosis in two independent green frog (Rana clamitans) populations by assessing the genotypic sex ratios of multiple developmental stages at each breeding pond. Our findings imply that genotypic sex-biased mortality during tadpole development affects phenotypic sex ratio variation at metamorphosis. We also identified sex reversal in metamorphosing cohorts. However, sex reversal plays a relatively minor and inconsistent role in shaping phenotypic sex ratios across the populations we studied. Although we found that sex-biased mortality influences sex ratios within a population, our study cannot say at this time whether sex-biased mortality is responsible for sex ratio variation across populations. Our results illustrate how multiple processes shape sex ratio variation in wild populations and the value of testing assumptions underlying how we understand sex in wild animal populations.

Population, Reproductive and Fishery Dynamics of Spotted Box Crab (Calappa philargius), a New Claw-Only Fishery Species, in the Southern Taiwan Strait, China

The spotted box crab Calappa philargius (Calappidae) is an increasingly consumed species in China, mainly sourced from a claw-only fishery. Being a not well-characterized species in the literature, this study is warranted to inform sustainable management approaches. Here we report on the first in-depth overview on biology and fishery dynamics of C. philargius in the southern Taiwan Strait of China. Whole body crabs (N = 1,009) were collected monthly from January to December 2019 from trawlers, operating in the southern Taiwan Strait; sample collection was absent from May to July because of the national fishing moratorium regulation. Sex ratio, growth pattern, size at maturity and fecundity were estimated. The overall male: female ratio was 1: 1.47, significantly differing from the 1: 1 (p < 0.01); monthly sex ratio variations were also significant (p < 0.01), suggesting possible seasonal sexual segregation. We found the carapace widths (CW) of males were significantly larger than those of females (p < 0.01). Based on the occurrence and percentages of berried females, we identified twin spawning events in January–April and August–October with the overall peak occurring in February. Size at 50% female maturity was 11.47 cm CW. Female absolute fecundity was significantly related to CW via a power function relationship (N = 14, p < 0.01). The estimated average claw yield was 36.28 ± 3.07% of the whole body weight (N = 95), irrespective of sex and size (p > 0.5). Capture and trade data of C. philargius using trawl and trap fishing gears were documented to characterize claw-only fishery dynamics for future research and sustainable use.

Sex ratio effects on reproductive success of male and female Daphnia.

The intensity of mating competition and the opportunity for sexual selection are thought to depend on the operational sex ratio, the ratio of sexually active males to fertilizable females. Cyclic parthenogens, organisms that alternate between sexual reproduction and female-only parthenogenesis, show particularly high variation in sex ratios in natural populations but the effects of this variation on mating competition and reproductive success of each sex are poorly understood. In a series of experiments with Daphnia magna, we experimentally imposed five sex ratio categories, varying from one male per 81 females to an even sex ratio. We found that, in males, reproductive success strongly and monotonically decreased with decreasing number of females per male. In females, in contrast, mating success and reproductive success were reduced only at the most female-biased sex ratio (1:81), when many females remained unmated and unfertilized, and then again at equal sex ratios, probably due to negative effects of high density or stress induced by numerous males. Our results suggest that females experienced male limitation at heavily female-biased sex ratios below one male to about 50 females. As this is well within the sex ratio variation observed in natural Daphnia populations, we conclude that mating competition and the opportunity for sexual selection may exist not only in males but, at least periodically, also in females.

Is It a Boy or a Girl? Testing Hypotheses to Explain Variable Sex Ratios in Columbian Ground Squirrels

Is It a Boy or a Girl? Testing Hypotheses to Explain Variable Sex Ratios in Columbian Ground Squirrels

Pedigree accumulation analysis: Combining methods from community ecology and population genetics for breeding adult estimation

Abstract Estimates of the number of successfully breeding adults (NS) in a population can predict levels of recruitment. However, assessments of NS are often difficult to obtain because encounters with adults are limited due to life‐history characteristics, low abundance or other constraints associated with access to critical habitats. Alternatively, efforts to sample individuals at earlier ontogenetic stages can be more tractable, resulting in more comprehensive samples. To estimate NS, we describe, evaluate and apply two nonparametric species richness estimators to information associated with genetic pedigree reconstruction. Simulations compared bias and precision associated with Chao and Jackknife methods when estimating NS. We also evaluated NS estimation sensitivity to two sources of variation associated with species reproductive ecology (variance in reproductive success and sex ratio skew) and genetic pedigree assignment error. Finally, the application of our novel method was demonstrated in two different species and systems (Chinook Salmon in Oregon, USA and Lake Sturgeon in Michigan, USA). We found unbiased NS estimates were generated across a broad range of offspring sample sizes using the Chao method. Empirical results corroborated simulation‐based expectations and highlighted applications where parents and offspring are sampled, and when only offspring are sampled. When offspring sample sizes are adequate and pedigree reconstruction errors are low, the combination of established methods from community ecology and genetic pedigree reconstruction provides an accurate alternative method to estimate NS that can facilitate population assessments.

Sexing first-calendar-year Carrion Crows Corvus corone from biometrics reveals variation between years in post-fledging sex ratio

ABSTRACT Sex is an important parameter to consider when studying population dynamics and movement ecology, for example, though sex determination is often difficult in young birds of sexually monomorphic species, with large biometric overlap between sexes. We aimed at determining the sex of first-calendar-year (1cy) Carrion Crows, in order to study temporal trends in sex ratio. We performed molecular sexing of a sample of 48 females and 45 males, creating a statistical framework to confidently discriminate sexes using four morphometric variables. Although biometrics overlapped between sexes, we developed a discriminant function which separated the sexes with high accuracy (88%). Then we applied this discriminant function to biometrics obtained from all 1cy individuals captured, to determine their sex. We found a female-biased overall sex ratio of the captures: female Carrion Crows might therefore be more numerous than males, or more prone to enter traps. There were significant variations in sex ratio between years, but not between months.

Relationships between litter size, sex ratio and within-litter birth weight variation in a sow herd and consequences on weaning performance.

Routine selection for litter size has resulted in an increase in the proportion of lightweight piglets. There is a need to balance prolificacy with litter uniformity to maximize profit. A total of 3,465 piglets from 310 litter records obtained from 2016 until 2019 at the Pig Industry Board research unit, Arcturus, Zimbabwe were used to determine the relationships between litter size, sex ratio, and within-litter birth weight variation in the sow herd and consequences on performance at weaning. The regression procedure of SAS was used to determine the relationships between litter size, sex ratio, and within-litter birth weight variation. The regression procedure was also used to determine the relationships between number born alive, within-litter birth weight variation, and sex ratio, and litter performance traits at weaning. Parity of sow, year, and month of farrowing did not affect sex ratio (P > 0.05). The number born alive and number of piglets born had no relationship with sex ratio (P > 0.05). As the sex ratio increased, percent survival of piglets at weaning also increased linearly (P < 0.05). As the proportion of males in litters increased, within-litter birth weight variation and within-litter weaning weight variation increased reaching maximum as the proportion of males in litters approached 0.5 and then decreased onwards. As the proportion of males in litters approached 1, within-litter birth weight variation and within-litter weaning weight variation reached their least values. In conclusion, within-litter sex ratio does not vary with parity, year, and month of farrowing. Within-litter weight variation is highest in litters with equal number of male and female piglets and lowest in unisex litters. This implies that the production of unisex litters can help to reduce the variation in the weight of pigs at birth, weaning, and marketing which is one of the biggest economic challenges faced by pork producers.

Responses of the pest Drosophila suzukii population to environmental variables in southern Spain (Granada, Andalusia) – results of a pilot study

Abstract In this study conducted in Granada province in southern Spain, we analysed the flying activity and sex-ratio variability of the invasive pest Drosophila suzukii (the spotted wing drosophila) in the context of environmental factors and compared it with co-occurring Drosophilidae in a village garden. The studied D. suzukii population showed overwintering capabilities and successful maintenance in village garden with non-crop plants. Its flight period showed two peaks: female-biased, in November, and male-biased, in May. The sex ratio varied throughout the year and was mostly female-biased. We observed the effect of temperature on Drosophilidae and rainfall and temperature on the D. suzukii. Catches of the studied species were minimal at the minimal temperatures below 0 °C, and absent or near absent when the maximal temperature exceeded 30 °C. Moreover, we did not observe D. suzukii in samples when the minimum temperature below 0 °C was combined with heavy rainfall. The spotted wing drosophila seems to be more sensitive to precipitation than native Drosophilidae, as its proportion in the samples decreased with increasing rainfall. This pest also shows more pronounced short peaks in which it makes an important proportion of the biocoenosis of in comparison with other Drosophilidae.

Deadly discrimination: Implications of “missing girls” for workplace safety

We examine an indirect but potentially deadly consequence of the “missing girls” phenomenon. A shortage of brides causes many parents with sons of marriageable age to work harder and seek higher-paying but dangerous jobs. In response, employers invest less in workplace safety, which in turn increases work-related mortality. Drawing from a broad range of data sets and taking advantage of large regional and temporal variations in sex ratios in China, we demonstrate that in areas with more severe shortages of young women, the cohort of parents with sons of marriageable age suffers a higher incidence of accidental injuries and workplace deaths.

Sex ratios and natal origins of green turtles from feeding grounds in the Southwest Atlantic Ocean

Abstract Potential effects of climate change on living species are a widely debated topic. Species with temperature-dependent sex determination can be particularly impacted by warmer temperatures because unbalanced sex ratios could threaten population viability. In sea turtles, sex ratio estimates have highlighted the potential feminization of current populations, which tends to increase since warmer temperatures would generate more females. Here, we evaluated temporal variation in sex ratios of green turtles from feeding grounds of the Southwest Atlantic Ocean (SWA) using data from a 7-year time frame, from 2010 to 2016. We also evaluated natal origins of female and male green turtles from SWA based on mitochondrial DNA. Sex ratios of juvenile and adult green turtles were generally female-skewed across collection years. We identified 11 haplotypes in northeast SWA, and the haplotype composition of females and males was slightly different. Likewise, the estimated the natal origins of females and males were divergent. Ascension Island was estimated to be the main source of females while Guinea Bissau was estimated to be the main source of males. Studies evaluating natal origins of females and males independently are rare, this study provides one of the first assessments of the kind for green turtles in the SWA.

Sex-specific deaths in Norway and Sweden during the COVID-19 pandemic: Did mandates make a difference?

To determine whether differences between Norway's and Sweden's attempts to contain SARS-CoV-2 infection coincided with detectably different changes in their all-cause mortality sex ratios. Measuring temporal variation in the all-cause mortality sex ratio before and during the pandemic in populations exposed to different constraints on risky behavior would allow us to better anticipate changes in the ratio and to better understand its association with infection control strategies. I apply time Box-Jenkins modeling to 262 months of pre-pandemic mortality sex ratios to arrive at counterfactual values of 10 intra-pandemic ratios. I compare counterfactual to observed values to determine if intra-pandemic ratios differed detectably from those expected as well as whether the Norwegian and Swedish differences varied from each other. The male to female mortality sex ratio in both Norway and Sweden increased during the pandemic. I, however, find no evidence that the increase differed between the two countries despite their different COVID-19 containment strategies. Societal expectations of who will die during the COVID-19 pandemic will likely be wrong if they assume pre-pandemic mortality sex ratios because the intra-pandemic ratios appear, at least in Norway and Sweden, detectably higher. The contribution of differences in policies to reduce risky behavior to those higher ratios appears, however, small.

Deadly Discrimination: Implications of "Missing Girls" for Workplace Safety

We examine an indirect but potentially deadly consequence of the “missing girls” phenomenon. A shortage of brides causes many parents with sons of marriageable age to work harder and seek higher-paying but potentially dangerous jobs. In response, employers invest less in workplace safety, which in turn increases work-related mortality. Drawing from a broad range of data sets and taking advantage of large regional and temporal variations in sex ratios in China, we demonstrate that in areas with a more severe shortage of young women, the parents with unmarried sons suffer a significantly higher incidence of accidental injuries and workplace deaths. Institutional subscribers to the NBER working paper series, and residents of developing countries may download this paper without additional charge at www.nber.org.

Sex ratios and mental health: Evidence from China

While sex ratios (i.e., relative numbers of men and women) have been linked to various economic and social outcomes, how sex ratios affect mental health is underexplored. Using nationally representative data from the China Family Panel studies (CFPS) and Population Census, we evaluate the impact of sex ratios on mental health among Chinese men and explore potential mechanisms. Employing the instrumental variables (IV) approach where the One-Child Policy’s mandated fertility limits and implementation are used as exogenous variations in local sex ratios, we find that higher local sex ratios increase depressive symptoms and probability of depression among Chinese men. The impact is stronger for men with lower levels of education and living in rural areas. Analyses of potential mechanisms show that higher sex ratios increase the likelihood of marriage delay and unemployment for men, and prolong working hours for the employed men. The findings are of direct relevance to the health and population policy in China.

Trends in missing females at birth in India from 1981 to 2016: analyses of 2·1 million birth histories in nationally representative surveys

Half of the world's missing female births occur in India, due to sex-selective abortion. It is unknown whether selective abortion of female fetuses has changed in recent years across different birth orders. We sought to document the trends in missing female births, particularly among second and third children, at national and state levels. We examined birth histories from five nationally representative household surveys (National Family Health Surveys 1-4 and District Level Household Survey 2) to compute the conditional sex ratio (defined as the number of girls born per 1000 boys depending on previous birth sex) in India during 1981-2016. We estimated decadal variation in conditional sex ratio for 1987-96, 1997-2006, and 2007-16, and quantified trends in the numbers of missing female births for the states constituting >95% of India's population, as well as in 5-year intervals for each survey round. We used multivariate logistic regression to calculate the odds ratio of a second (or third) girl depending on the sex of the earlier child (or children), adjusting for education, wealth, religion, caste, and place of residence. We assessed 2·1 million birth histories across the five surveys. Applying the conditional sex ratios from the surveys to national births, we found that 13·5 million female births were missing during the three decades of observation (1987-2016), on the basis of a natural sex ratio of 950 girls per 1000 boys. Missing female births increased from 3·5 million in 1987-96 to 5·5 million in 2007-16. Contrasting the conditional sex ratio from the first decade of observation (1987-96) to the last (2007-16) showed worsening for the whole of India and almost all states, among both birth orders. Punjab, Haryana, Gujarat, and Rajasthan had the most skewed sex ratios, comprising nearly a third of the national totals of missing second-born and third-born females at birth. From about 1986, the conditional sex ratio for second-order or third-order births after an earlier daughter or daughters diverged notably from that after an earlier son or sons. From 1981 to 2016, the sex ratio for second-born children after an earlier daughter decreased from 930 (99% CI 869-990) to 885 (859-912), and that for third-born children after two earlier daughters decreased from 968 (866-1069) to 788 (746-830). The probability of missing girls was mostly determined by earlier daughters, even after considering wealth quintile and education levels. The conditional sex ratio among the richest and most educated mothers was most distorted compared with lower wealth and education groups, and generally decreased with time, until a modest improvement in 2007-16. In contrast to the substantial improvements in female child mortality in India, missing female births, driven by selective abortion of female fetuses, continues to increase across the states. Inclusion of a question on sex composition of births in the forthcoming census would provide local information on sex-selective abortion in each village and urban area of the country. None. For the Hindi translation of the abstract see Supplementary Materials section.

Genetic parameter estimates and identification of SNPs associated with growth traits in Senegalese sole

Breeding programs are essential for aquaculture. The present study was conducted to investigate the genetic parameters of four growth traits in Senegalese sole. Families were produced by mass spawning (n = 2171 offspring) from an industrial broodstock. Weight, total area, standard length and width (the two latter measured directly on the fish and by image analysis) were determined before the on-growing stage (age ~ 400 d) and at harvest (~800 d) in RAS. Phenotypic data are presented and discussed. Females grew faster than males and a high variation of sex ratios by family was observed. Heritabilities were high for all traits ranging from 0.568 to 0.609 at 400 d and from 0.424 to 0.500 at 800d. The genetic correlations between traits were very high (>0.94) at both ages. Genetic estimates for traits measured in vivo and by digital image analysis were similar. To investigate genomic regions associated with these growth traits, a low-density array using a set of 60 single nucleotide polymorphisms (SNPs) distributed in 17 linkage groups was designed. Using samples from wild fish, a total of 49 assays for SNP analysis were validated. The association analysis was carried out using two fast-growth families and two slow-growth families (n = 279). Two significantly-associated SNPs with most of the traits at both ages were detected. They were located in the general transcription factor 3C polypeptide 4 (gtf3c4) and the mitochondrial fission process protein 1 (mtfp1). Results are of high relevance for genetic breeding programs in this species.

Male-biased effective sex ratio across populations of the threatened Zamia boliviana (Zamiaceae)

Populations of dioecious plants commonly exhibit dissimilarities to the equilibrium expectation of a 1:1 sex ratio. Differential expenditure for reproduction between genders is cited as the primary mechanism responsible for a male-biased sex ratio, with increased effects on long-living species, but these mechanisms are still poorly understood. We explore the sex ratio in the endemic gymnosperm Zamia boliviana (Zamiaceae) populations from the Brazilian savanna (the Cerrado). We aim to investigate what the Z. boliviana sex ratio is, and whether population density and ecological correlates lead to variation in the sex proportion among Cerrado habitats. The study was conducted on ten in situ populations of Z. boliviana at sexual maturity (tertiary sex ratio). We estimated the populations’ sex ratio and performed a redundancy analysis to assess the relationship between biotic traits, such as sex ratio, and associated environmental features. Soil texture classes were used to classify the cycad habitats and were expressed in a ternary phase diagram. The results show a significant male-biased sex ratio in seven of the ten populations surveyed. Environmental factors did not explain the redundancy in the reproductive characteristics. However, the cycad occurs in different habitats in their endemic zone. Our study provides new biological data for Z. boliviana, suggesting that the differential reproductive expenditure of sexes in reproduction is governing the mechanisms of sex ratio variation, compared to local environmental factors in this cycad. The pattern of effective sex ratio found here improves our understanding of mechanisms causing biased sex ratios in cycads and other dioecious species from tropical ecosystems.

Are human natal sex ratio differences across the world adaptive? A test of Fisher's principle

Fisher's principle states that natural selection favours an equal number of male and female births at the population level, unless there are sex differences in rearing costs or sex differences in mortality before the end of the period of parental investment. Sex differences in rearing costs should be more pronounced in low- than in high-resource settings. We, therefore, examined whether human development index and sex differences in child mortality contribute to the natural variation in human sex ratio at birth across the globe. As predicted by Fisher's principle, the proportion of male births increased with both increasing male-biased childhood mortality and level of development of each country. However, these relationships were absent after accounting for spatial autocorrelation in the residuals, which our inference is conditioned on. This work shows how the failure to account for residual spatial autocorrelation can lead to incorrect conclusions regarding support for predictions from sex allocation theory.

Natural variation in social conditions affects male mate choosiness in the amphipod Gammarus roeselii.

The extent of male mate choosiness is driven by a trade-off between various environmental factors associated with the costs of mate acquisition, quality assessment and opportunity costs. Our knowledge about natural variation in male mate choosiness across different populations of the same species, however, remains limited. In this study, we compared male mate choosiness across 10 natural populations of the freshwater amphipod Gammarus roeselii (Gervais 1835), a species with overall high male mating investments, and evaluated the relative influence of population density and sex ratio (both affecting mate availability) on male mate choosiness. We investigated amplexus establishment after separating mating pairs and presenting focal males with a novel, size-matched female from the same population. Our analysis revealed considerable effects of sex ratio and (to a lesser extent) population density on time until amplexus establishment (choosiness). Male amphipods are able to perceive variable social conditions (e.g., sex ratio) and modify their mating strategy accordingly: We found choosiness to be reduced in increasingly male-biased populations, whereas selectivity increases when sex ratio becomes female biased. With this, our study expands our limited knowledge on natural variations in male mate choosiness and illustrates the importance of sex ratio (i.e., level of competition) for male mating decisions in natural environments. Accounting for variation in sex ratios, therefore, allows envisioning a distinctive variation of choosiness in natural populations and highlights the importance of considering social background information in future behavioral studies.

Vector preference and heterogeneity in host sex ratio can affect pathogen spread in natural plant populations.

Vector-borne diseases threaten human and agricultural health and are a critical component of the ecology of plants and animals. While previous studies have shown that pathogen spread can be affected by vector preferences for host infection status, less attention has been paid to vector preference for host sex, despite abundant evidence of sex-specific variation in disease burden. We investigated vector preference for host infection status and sex in the sterilizing "anther-smut" pathogen (Microbotryum) of the alpine carnation, Dianthus pavonius. The pathogen is transferred among hosts by pollinators that visit infected flowers and become contaminated with spores produced by infected anthers. The host plant has a mixed breeding system with hermaphrodites and females. In experimental floral arrays, pollinators strongly preferred healthy hermaphrodites over both females and diseased plants, consistently across different guilds of pollinators and over multiple years. Using an agent-based model, we showed that pollinator preferences for sex can affect pathogen spread in populations with variable sex ratios, even if there is no preference for infection status. Our results demonstrate that vector preferences for host traits other than infection status can play a critical role in pathogen transmission dynamics when there is heterogeneity for those traits in the host population.

Paucity and Disparity of Publicly Available Sex-Disaggregated Data for the COVID-19 Epidemic Hamper Evidence-Based Decision-Making.

COVID-19 has joined the long list of sexually dimorphic human disorders. Higher lethality in men, evident in the first reports from China, was confirmed in the subsequent Italian outbreak. Newspapers and scientific journals commented on this finding and the preexisting conditions, biological processes, and behavioral differences that may underlie it. However, little appeared to be released about sex differences in severity of disease, comorbidities, rate of recovery, length of hospital stay, or number of tests performed. Systematic analysis of official websites for 20 countries and 6 US states revealed a wide disparity in sex-disaggregated data made available to the public and scholars. Only a handful reported cases by sex. None of the other characteristics, including deaths, were stratified by sex at the time. Beyond suboptimal sex disaggregation, we found a paucity of usable raw data sets and a generalized lack of standardization of captured data, making comparisons difficult. A second round of data capture in April found more complete, but even more disparate, information. Our analysis revealed a wide range of sex ratios among confirmed cases. In countries where a male bias was initially reported, the proportion of women dramatically increased in 3 weeks. Analysis also revealed a complex pattern of sex ratio variation with age. Accurate, peer-reviewed, analysis of harmonized, sex-disaggregated data for characteristics of epidemics, such as availability of testing, suspected source of infection, or comorbidities, will be critical to understand where the observed disparities come from and to generate evidence-based recommendations for decision-making by governments.Electronic supplementary materialThe online version of this article (10.1007/s10508-020-01882-w) contains supplementary material, which is available to authorized users.