Male-skewed Sex Ratios Research Articles

Baby Bonus, Fertility, and Missing Women

This paper presents novel causal evidence on the effects of pro-natalist financial incentives onbabies. I exploit rich spatial and temporal variation in the generosity of cash transfers providedto families with newborn babies and the universe of birth, death, and migrant registry recordsin South Korea. I find that the total fertility rate in 2015 would have been 3% lower withoutthe cash transfers. These cash transfers were particularly effective among working mothersand encouraged them to have second and third children. This selection of working mothers intochildbearing led to a decrease in gestational age, which in turn led to an overall reduction in birthweight, but no change in early mortality. The cash transfers had an unintended consequence ofcorrecting the unnaturally male-skewed sex ratio closer to its natural level.

Deciphering sex-specific miRNAs as heat-recorders in zebrafish

In the last decade, a plethora of microRNAs (miRNAs) has been reported in a wide variety of physiological processes, including reproduction, in many aquatic organisms. However, miRNAome alterations occurred by environmental cues due to water temperature increment have not yet been elucidated. With the aim to identify epigenetic regulations mediated by miRNAs in the gonads in a climate change scenario, the animal model zebrafish (Danio rerio) were subjected to high temperatures during sex differentiation, a treatment that results in male-skewed sex ratios in the adulthood. Once the fish reached adulthood, gonads were sequenced by high-throughput technologies and a total of 23 and 1 differentially expressed miRNAs in ovaries and testes, respectively, were identified two months after the heat treatment. Most of these heat-recorder miRNAs were involved in human sex-related cancer and about 400 predicted-target genes were obtained, some with reproduction-related functions. Their synteny in the zebrafish genome was, for more than half of the predicted target genes, in the chromosomes 7, 2, 4, 3 and 11 in the ovaries, chromosome 4 being the place where the sex-associated-region (sar) is localized in wild zebrafish. Further, spatial localization in the gonads of two selected heat-recorder miRNAs (miR-122-5p and miR-146-5p) showed exclusive expression in the ovarian germ cells. The present study expands the catalog of sex-specific miRNAs and deciphers, for the first time, thermosensitive miRNAs in the zebrafish gonads that might be used as potential epimarkers to predict environmental past events.

Sex ratios and union formation in the historical population of the St. Lawrence Valley.

The ratio of men and women in the adult population, i.e. sex ratio, has long been recognized as a key demographic constraint for partnering behavior. However, the literature remains contradictory on how sex ratio imbalances influence partnering behavior, suggesting either higher or lower rates of male marriage being associated with male-skewed sex ratios. These contradictory findings are likely due to data limitations. Cross-sectional data or limited observation periods preclude studies from distinguishing sex ratio effects on timing from effects on the overall likelihood of marriage. In this paper, we use historical family reconstitution data to study the association of sex ratios with marriage patterns in the French colony of the St. Lawrence Valley in North America (1680-1750). The population experienced a substantial male-skew from sex-selective immigration during the early period of the colony. The long-running observation period allow for differentiating the timing and overall likelihood of marriage. Finally, the data enable us to study the effects of male-skews on the population-level as well as the regional and parish level. Cox proportional hazard models reveal that while male-skewed sex ratios are associated earlier marriage for women, the association with men's marital biographies is less clear-cut. We find that men marry later when sex ratios are more male-skewed, yet we do not find a substantial reduction in the overall likelihood of marriage for men. Our findings reveal that male-skewed sex ratios do not necessarily result in an increase of never married men. We discuss the implications of our findings for the sex ratio literature.

Are skewed sex ratios associated with violent crime? A longitudinal analysis using Swedish register data

There is widespread concern in both the popular and academic literature that a surplus of men in a population intensifies mating competition between men, particularly unpartnered men, resulting in increased violence towards both men and women. Recent contributions challenge this perspective and argue that male mating competition and levels of violence will be higher when sex ratios are female-skewed. Existing empirical evidence remains inconclusive. We argue that this empirical ambiguity results from analyses of aggregate-level data, which put inferences at risk of ecological fallacies. Our analysis circumvents such problems by using individual-level, longitudinal demographic register and police data for the Stockholm metropolitan area, Sweden (1990–2003, n = 758,498). These data allow us to investigate the association between municipality-level sex ratios and violent offending (homicide, assault, threat, and sexual crimes) while adjusting for sociodemographic factors. Results suggest that aggregated offending rates are negatively associated with male-skewed sex ratios, whereas individual-level violent offending correlates positively with male-skews. We find that the more-men-more-violence association holds particularly for male violence against other men , but is insignificant for violence against women . Moreover, the association is significant among childless men, but not among fathers. However, robustness checks question the causality of these associations. Female violent offending is positively, albeit due to a low number of cases, insignificantly associated with male-skews. Moreover, both male and female non-violent offending is higher in male-skewed municipalities. We discuss the implications with regard to the theoretical debate and problems of unobserved heterogeneity in the sex ratio literature.

The Behavioral Ecology of Sex Tourism: The Consequences of Skewed Sex Ratios

The operational sex ratio (i.e., the ratio of reproductive-age females to males in a population) shapes both animal and human behavior in important ways. Drawing on research in evolution and ecology, the author proposes that a local male-skewed sex ratio (i.e., a surplus of males) influences local men’s attitudes toward sex tourism. Analyzing historical field (study 1) and experimental data (study 2), the author demonstrates that male-skewed sex ratios increase men’s sex tourism rationalization and intent, while women’s predispositions are not sensitive to sex ratios. Sex tourism is explained as a subconscious ecological plasticity in response to perceived increased intensities of same-sex competition for mates, signaled by male-skewed sex ratios. The findings demonstrate a link between mating ecologies and sex tourism, with the latter serving as a compensatory behavior of same-sex mating competition. This research contributes a novel, biological perspective on sex tourism with implications for future research and practice.

Mating Affects Lifespan Differently in Two Strains of Pseudo-female Caenorhabditis elegans

Mating is vital for sexually reproducing species, yet the ideal mating strategy for males and females can differ. The ensuing conflict between the sexes – namely, sexual conflict – results in a decrease in population level fitness. The degree of sexual conflict can be affected by the behavior, physiology, and life history of a population. Previous studies in the nematode Caenorhabditis elegans have shown that mating causes lifespan to decrease in pseudo-females and hermaphrodites, which was interpreted as evidence of sexual conflict. However, it is still an open question whether variations in mating condition and strain type can affect the degree of sexual conflict and lifespan decrease. Here, I investigate whether lifespan is affected by mating in conditions other than sex- skewed individual mating scenarios used in previous work. I conducted population-based mating assays in two different strains of C. elegans using both natural and male-skewed sex ratios. Counter to expectations, I found no effect of mating on lifespan in a wild isolate of C. elegans, while virgins from a canonical laboratory strain had a decreased lifespan relative to their counterparts mated in groups. My data offers a counterpoint to the literature, which agrees that mating universally decreases the lifespan of C. elegans pseudo-females and hermaphrodites. These results highlight the flexibility of reproductive costs and the importance of life histories in experimental populations.

Exposure of zebrafish to elevated temperature induces sex ratio shifts and alterations in the testicular epigenome of unexposed offspring

Accumulating evidence shows that environmental changes can affect population sex ratios through epigenetic regulation of gene expression in species where sex depends on both genetic and environmental cues. Sometimes, altered sex ratios persist in the next generation even when the environmental cue is no longer present (a multigenerational effect). However, evidence of transgenerational effects (i.e., beyond the first non-exposed generation), which tend to be paternally transmitted, is scarce and a matter of debate. Here, we used the AB strain of zebrafish, where sex depends on both genetic and environmental influences, to study possible multi- (to the F1) and transgenerational (to the F2) effects of elevated temperature during the critical period of sex differentiation. From eight initial different families, five were selected in order to capture sufficient variation between the sex ratio of the control group (28 °C) and the group exposed to elevated (35 °C) temperature only at the parental (P) generation. Results showed a consistent increase in the proportion of males in the P generation in all five families as a result of heat treatment. Sex ratios were then determined in the F1 and F2 offspring derived from both above groups, which were all raised at 28 °C. A persisting male-skewed sex ratio in the 35°C-derived, unexposed offspring of the F1 generation was observed in three families, denoting family-dependent multigenerational effects. However, no transgenerational effects were observed in the F2 generation of any family. DNA methylation was also assessed in the testis of P, F1 and F2 males derived from exposed and non-exposed fathers and grandfathers. DNA methylation was significantly decreased only in the testis of the 35°C-derived males in the F1 generation but not of the F2 generation and, surprisingly, neither in the 35°C-exposed males of the P generation. Taken together, our results show great interfamily variation, not only in sex ratio response to elevated temperature, but also on its multigenerational effects, denoting a strong influence of genetics. Alterations in the testicular epigenome in F1 males calls for attention to possible, previously unnoticed, effects of temperature in the unexposed offspring of heat-exposed parents in a global warming scenario.

A turtle population study in an isolated urban wetland complex in Ontario reveals a few surprises

We report on the results of the first mark–recapture survey of freshwater turtles in an isolated urban wetland complex in one of Canada’s fastest growing municipalities. Although we found turtles in every surveyed wetland, the density and assemblage of turtles in smaller wetlands were significantly different than in larger wetland bodies. We also documented two species of turtles that were thought to be absent from this wetland complex, the Northern Map turtle and Eastern Musk turtle. We noted that a wetland that was bisected by a high-traffic road showed a male-skewed sex ratio in the population of Midland Painted turtles but not in the population of Eastern Snapping turtles. As a whole, the sex ratios inside the wetland complex were not skewed. These results reinforce the conclusions of a previous study of a single wetland within this same complex that had found a correlation between road mortality and a male-skewed sex ratio in Midland painted turtles. We discuss population sources and sinks within the complex and the importance of protecting the overland corridors that support the safe turtle movements within this provincially significant wetland complex.

Time for a change: patterns of sex expression, health and mortality in a sex-changing tree.

The ability of individuals to change sex during their lifetime is known as environmental sex determination (ESD). This represents a unique life history trait, allowing plants to allocate resources differentially to male and female functions across lifetimes, potentially maximizing fitness in response to changing environmental or internal cues. In this study, Acer pensylvanicum, a species with an unconfirmed sex determination system, was investigated to see what patterns in sex expression existed across multiple years, if there were sex-based differences in growth and mortality, and whether this species conformed to theoretical predictions that females are larger and in better condition. Patterns of sex expression were documented over 4 years in a phenotypically subdioecious A. pensylvanicum population located in New Jersey, USA, and data on size, mortality, health and growth were collected. A machine-learning algorithm known as a boosted classification tree was used to develop a model to predict the sex of a tree based on its condition, size and previous sex. In this study, 54 % of the trees switched sex expression during a 4-year period, with 26 % of those trees switching sex at least twice. Consistently monoecious trees could change relative sex expression by as much as 95 %. Both size and condition were influential in predicting sex, with condition exerting three times more relative influence than size on expressed sex. Contrary to theoretical predictions, the model showed that full female sex expression did not increase with size. Healthy trees were more likely to be male; predicted female sex expression increased with deteriorating health. Growth rate negatively correlated with multiple years of female sex expression. Populations maintained similar male-skewed sex ratios across years and locations and may result from differential mortality: 75 % of dead trees flowered female immediately before death. This study shows conclusively that A. pensylvanicum exhibits ESD and that femaleness correlates with decreased health, in contrast to prevailing theory. The mortality findings advance our understanding of puzzling non-equilibrium sex ratios and life history trade-offs resulting from male and female sex expression.

Male-skewed sex ratio in Myrica esculenta: a dioecious tree species

The population of M. esculenta tends to deviate from the equilibrium of expected 1:1 primary sex ratio predicted by Fisherian theory. Populations of dioecious plants may often exhibit biased sex ratios, which generally favour males. A study on reproductive potential, i.e., pollen production and female flower production, and reproductive success in Myricaesculenta was assessed in three diameter (diameter at breast height: dbh at 1.37 m) classes, i.e., small (10–20 cm), medium (20–30 cm), and large (30–40 cm). Population structure and sex ratio was determined in transects in four major associations of Myricaesculenta, i.e., (i) with mixed Oak-Rhododendron forest (ii) with Chir Pine–Oak forest, (iii) with pure Oak forest, and (iv) with pure Deodar forest. The sex ratio of trees was male-biased. Male frequency was higher in lower diameter classes and maximum on resource-poor site. More female individuals were recorded in higher-diameter classes and resource rich location, indicating that the total reproductive cost in M. esculenta is greater in female trees than that of the male trees. The population of M.esculenta tends to deviate from the equilibrium of expected 1:1 primary sex ratio predicted by Fisherian theory. The male trees demonstrated greater survival, especially in resource-poor conditions, which was reflected strongly biased sex ratios within the lower size classes. The study would be helpful for formulating appropriate silvicultural and genetic management strategies for this species.

Inter- and Intrasexual Interactions in Anoplophora glabripennis (Coleoptera: Cerambycidae) and the Impact of Different Sex Ratios.

The behaviors of Anoplophora glabripennis (Motschulsky) (Coleoptera: Cerambycidae) during inter- and intrasexual interactions between both solitary and paired individuals at different sex ratios were observed on Acer platanoides L. (Sapindales: Sapindaceae) branches in the laboratory. Intrasexual contact was generally ignored by females, but between males resulted in battles from which one male ultimately retreated. When male size differed, larger males won battles, whereas equally sized males split the wins and battled longer. When males initiated intersexual contact, they quickly found and mounted the female. The female apparently determined if and how long the male could copulate with her by controlling access to her genital opening and males would eventually dismount if the female remained unreceptive. After successful copulation, males ignored female intruders and fought off male challengers, generally without dismounting. Some males dismounted females without attempting to copulate. Individual age and size were not predictive of either female or male choice in a mate. Mating duration was affected by both female receptivity and sex ratios present. Male interruptions of the pair shortened time in copula. Male-skewed sex ratios (1F:2M) significantly shortened the time a male would stay with an unreceptive female. Female-skewed sex ratios (2F:1M) did not impact mating duration. Probability of dispersal both from and within the branch was greater for smaller individuals of both sexes and for males when sex ratios were male-skewed. This information demonstrates how reproductive activity might be impacted as population densities decline during efforts at eradication of this species in North America.

Marry Rich, Poor Girl: Investigating the Effects of Sex Selection on Intrahousehold Outcomes in India

Sex ratios at birth have risen steadily over the last three decades across much of the developing world. Many attribute this rise to improved access to sex selection technologies such as ultrasound since 1980. This study seeks to understand the effect of access to sex selection technologies such as ultrasound, and consequently skewed sex ratios, on the marriage market and intrahousehold outcomes of females in India. Existing economic theory and literature view male-skewed populations as a boon to the marital prospects of females. However, Edlund (1999) proposes an (as yet untested) theory that, in environments where hypergamy is practiced and parents derive utility from married children, a male-skewed sex ratio can generate a permanent female underclass. I extend this theory to argue that if sex ratios are skewed disproportionately amongst the rich, as the evidence suggests, then poorer matching in the marriage market will in turn lead to weaker bargaining positions for females. I test this theory and examine its implications for later life outcomes using India-wide household level data on ultrasound use and bargaining power. I present evidence that village-level ultrasound is an exogenous source of variation for access to sex selection technology, demonstrate that parents are indeed considering the sex ratio of their unborn child's future marriage market when determining the sex composition of their own family, and utilize a difference-in-difference approach to identify the effect of ultrasound access on intrahousehold outcomes of affected women. I find evidence that greater parental access to sex selection technology at a child's birth is related to poorer outcomes in her marriage: greater marriage age gaps, increased marital abuse, lower reported autonomy, and poorer health. My results are robust to a TS2SLS specification employing distance to a major health center as my instrument. As the first cohort of females affected by ultrasound at birth have only recently entered the marriage market, this study provides timely and compelling empirical evidence of the unintended consequences on later life outcomes of sex selection in India.

Sexual segregation of spiny dogfish (Squalus acanthias) off the northeastern United States: Implications for a male-directed fishery

Female spiny dogfish experienced high fishing pressure on the US east coast during the 1990’s. This led to a skewed population sex-ratio of 7 males per female which later declined to 4:1 as the stock was rebuilt. The current fishery still targets mature females, resulting in artificially high abundances of male spiny dogfish. Members of the fishing industry have proposed the creation of a male-directed fishery to sustainably harvest more of the available yield. To explore the potential for such a fishery, we investigated environmental factors associated with the spatiotemporal distribution of male spiny dogfish and male-skewed sex ratios. We analyzed data from 454,721 spiny dogfish caught in 2063 trawl samples during the Northeast Fisheries Science Center (NEFSC) annual spring and autumn bottom trawl surveys from 2009 to 2014. Consistent with previous research, there was clear spatial and temporal sexual segregation within the population. Male spiny dogfish could be harvested with little female bycatch along the continental shelf from Chesapeake Bay to Long Island at depths of 70–80m and 300–330m, approximately 90–150km or 240–270km from shore during spring. High proportions of male spiny dogfish in the catch were found in the western Gulf of Maine and on Georges Bank in the fall at depths 80–250m. Using boosted regression trees, we identified depth as the best environmental predictor of high (>75%) proportions of males in the catch, followed by distance from shore and temperature, for spring and fall season respectively. A new male-directed fishery appears to be feasible, and if responsibly managed, could potentially help return sex-ratios to natural levels (2:1), reduce fishing pressure on the female portion of the stock, and benefit local fishing communities that have struggled with restricted fishing opportunities in recent years.

The Roots of Modern Sex Ratios

While most measures of female empowerment have improved with development, sex ra- tios in many countries have become increasingly male. We exploit countries’ prior history of plough-based agriculture to identify cultural variation in patriarchal norms following Boserup (1970) and Alesina, Giuliano, and Nunn (2013). Using detailed birth records from 76 countries between 1970 and 2010, we show that the cultural legacy of plough use explains a large portion of variation in modern sex ratios, and present evidence that plough countries’ male-skewed sex ratios are achieved through a mix of in-utero sex-selection, son-based stopping rules, and increased mortality suggestive of neglect or infanticide. This cultural bias intensifies with lower fertility, even when controlling for a suite of economic and historical controls, a pattern that is not found in non-plough countries.

Population size assessment of the Endangered red-billed curassowCrax blumenbachii: accounting for variation in detectability and sex-biased estimates

AbstractThe Endangered red-billed curassowCrax blumenbachiiis endemic to the lowland Atlantic Forest of Brazil and is extinct across most of its range as a result of habitat loss and hunting pressure. Global population estimates are unreliable as the species has never been surveyed systematically. During March 2012–February 2013 we used line transects to estimate the density and size of one of the most important populations, in the Vale Nature Reserve. UsingDistancewe estimated a density of 1.3 individuals per km2; results stratified by sex indicated a male-skewed sex ratio. Data collected from motion-activated cameras confirmed that the population was male skewed but to a lesser degree than suggested by transect data. Sex-specific behavioural responses to the presence of an observer probably contributed to the difference between camera and transect data. Using the camera-derived sex ratios we corrected transect estimates to 1.6 individuals per km2and a population of 325 individuals. Our systematic survey approach has revealed that Vale Nature Reserve has a larger population than previous estimates of the global population. We show that behavioural differences between the sexes in cryptic species can mislead population estimates, and we highlight the importance of motion-activated cameras as a tool for studying cryptic forest species.

The Roots of Gender Inequality in Developing Countries

Is the high degree of gender inequality in developing countries—in education, personal autonomy, and more—explained by underdevelopment itself? Or do the societies that are poor today hold certain cultural views that lead to gender inequality? This article discusses several mechanisms through which gender gaps narrow as countries grow. I argue that although much of the GDP/gender-inequality relationship can be explained by the process of development, society-specific factors are also at play: Many countries that are poor today have cultural norms that exacerbate favoritism toward males. Norms such as patrilocality and concern for women’s “purity” help explain the male-skewed sex ratio in India and China and low female employment in India, the Middle East, and North Africa, for example. I also discuss why the sex ratio has become more male-skewed with development. Finally, I lay out some policy approaches to address gender inequality.

Extreme male-skewed sex ratios on spawning grounds for Atlantic cod Gadus morhua with typical coastal cod signatures of the Pan I (pantophysin) locus

Large offshore and small inshore populations of Atlantic cod Gadus morhua L. display differences in migratory patterns and settling regimes, but little is known about possible differences in spawning behaviour. Cod presumably of the Norwegian coastal cod type were therefore sampled during 7 spawning seasons (2002�2013) in Malangen, northern Norway. A spawning site in neighbouring Balsfjord was sampled during 5 consecutive spawning seasons (2002�2006). Length, weight, sex and maturity stage were recorded for each individual fish (n = 995). To verify their population assignment, frequencies of the 2 different Pan I (pantophysin) alleles were measured in 6 sampling years in Malangen and 3 in Balsfjord (n = 728). During all sur- vey years there was an extreme skewed sex ratio at the spawning sites. In 6 of 7 seasons in Malan- gen, 8-9 of 10 fish were males, and in Balsfjord 9 of 10 fish were males on average, indicating some form of lekking behaviour among the spawning fish. Average size of the fish varied among years and also between sexes in individual years, although not consistently. The smallest proportion of mature fish was seen in Malangen in the year with the earliest sampling date (23 February). This was also the sample with the least skewed sex ratio (6.5 of 10 fish were males). The frequency of the Pan I A allele of the Malangen cod was highly stable among years at close to 90%, showing that this spawning ground is exclusively used by coastal cod. In Balsfjord, frequencies of the Pan I A allele were lower (67�78%). This fjord is suggested to be more penetrable for the migratory NE Arctic cod with their high frequencies of the Pan I B allele, making it a less exclusively coastal cod residence. In both fjords, length of mature males but not females varied among the Pan I genotypes.

Climate change and pollution speed declines in zebrafish populations

Endocrine disrupting chemicals (EDCs) are potent environmental contaminants, and their effects on wildlife populations could be exacerbated by climate change, especially in species with environmental sex determination. Endangered species may be particularly at risk because inbreeding depression and stochastic fluctuations in male and female numbers are often observed in the small populations that typify these taxa. Here, we assessed the interactive effects of water temperature and EDC exposure on sexual development and population viability of inbred and outbred zebrafish (Danio rerio). Water temperatures adopted were 28 °C (current ambient mean spawning temperature) and 33 °C (projected for the year 2100). The EDC selected was clotrimazole (at 2 μg/L and 10 μg/L), a widely used antifungal chemical that inhibits a key steroidogenic enzyme [cytochrome P450(CYP19) aromatase] required for estrogen synthesis in vertebrates. Elevated water temperature and clotrimazole exposure independently induced male-skewed sex ratios, and the effects of clotrimazole were greater at the higher temperature. Male sex ratio skews also occurred for the lower clotrimazole exposure concentration at the higher water temperature in inbred fish but not in outbred fish. Population viability analysis showed that population growth rates declined sharply in response to male skews and declines for inbred populations occurred at lower male skews than for outbred populations. These results indicate that elevated temperature associated with climate change can amplify the effects of EDCs and these effects are likely to be most acute in small, inbred populations exhibiting environmental sex determination and/or differentiation.

Sexual Dimorphism and Seasonal Variation of Reproductive Hormones in the Pascagoula Map Turtle,Graptemys gibbonsi

The Pascagoula Map Turtle (Graptemys gibbonsi) is a narrowly endemic species found only in the Pascagoula River drainage in Mississippi. It is among the most poorly known turtle species because of research taxonomic biases and this species' relatively recent recognition as a unique taxon. A recent petition requested protective status for G. gibbonsi under the U.S. Endangered Species Act. We describe population parameters, quantitatively assess sexual dimorphism of G. gibbonsi, and document hormone secretion patterns from the Chickasawhay and Leaf rivers in Mississippi. We demonstrate a significant male-skewed sex ratio and a female-biased size dimorphism in both carapace length and height. Males showed a bimodal peak of plasma testosterone in fall and spring, consistent with the pattern shown by many other southeastern turtles with late summer-fall spermatogenesis and mating during spring and fall. Females did not show seasonal variation in estradiol secretion, an unexpected result that was possibly due to our small sample size of females, none of which were gravid when captured. Although this observation may be due to our limited capacity to sample females, given the reproductive issues reported for Graptemys flavimaculata from the same drainage (e.g., reproductive hormone abnormalities, low nesting frequency and success), this finding warrants concern and necessitates additional research. Finally, in order to put our hormone data in context, we briefly review hormone and reproductive patterns in southeastern U.S. turtles. Our review includes the timing of follicular enlargement, ovulation and nesting, clutch frequency, and estradiol cycles. The review for male turtles includes details on the spermatogenic cycle, spermiation, and the timing and frequency of testosterone peaks.

A Wright-Fisher model with indirect selection.

We study a generalization of the Wright-Fisher model in which some individuals adopt a behavior that is harmful to others without any direct advantage for themselves. This model is motivated by studies of spiteful behavior in nature, including several species of parasitoid hymenoptera in which sperm-depleted males continue to mate despite not being fertile. We first study a single reproductive season, then use it as a building block for a generalized Wright-Fisher model. In the large population limit, for male-skewed sex ratios, we rigorously derive the convergence of the renormalized process to a diffusion with a frequency-dependent selection and genetic drift. This allows a quantitative comparison of the indirect selective advantage with the direct one classically considered in the Wright-Fisher model. From the mathematical point of view, each season is modeled by a mix between samplings with and without replacement, and analyzed by a sort of "reverse numerical analysis", viewing a key recurrence relation as a discretization scheme for a PDE. The diffusion approximation is then obtained by classical methods.