Could maternal testosterone levels govern mammalian sex ratio deviations?

Observable variations in human sex ratio at birth

The human sex ratio at birth (SRB), defined as the ratio between the number of newborn boys to the total number of newborns, is typically slightly greater than 1/2 (more boys than girls) and tends to vary across different geographical regions and time periods. In this large-scale study, we sought to validate previously-reported associations and test new hypotheses using statistical analysis of two very large datasets incorporating electronic medical records (EMRs). One of the datasets represents over half (∼ 150 million) of the US population for over 8 years (IBM Watson Health MarketScan insurance claims) while another covers the entire Swedish population (∼ 9 million) for over 30 years (the Swedish National Patient Register). After testing more than 100 hypotheses, we showed that neither dataset supported models in which the SRB changed seasonally or in response to variations in ambient temperature. However, increased levels of a diverse array of air and water pollutants, were associated with lower SRBs, including increased levels of industrial and agricultural activity, which served as proxies for water pollution. Moreover, some exogenous factors generally considered to be environmental toxins turned out to induce higher SRBs. Finally, we identified new factors with signals for either higher or lower SRBs. In all cases, the effect sizes were modest but highly statistically significant owing to the large sizes of the two datasets. We suggest that while it was unlikely that the associations have arisen from sex-specific selection mechanisms, they are still useful for the purpose of public health surveillance if they can be corroborated by empirical evidences.

The human sex ratio at birth (SRB), usually slightly greater than 1/2, have been reported to vary in response to a wide array of exogenous factors. In the literature, many such factors have been posited to be associated with higher or lower SRBs, but the studies conducted so far have focused on no more than a few factors at a time and used far smaller datasets, thus prone to generating spurious correlations. We performed a series of statistical tests on 2 large, country-wide health datasets representing the United States and Sweden to investigate associations between putative exogenous factors and the SRB, and were able to validate a set of previously-reported associations while also discovering new signals. We propose to interpret these results simply as public health indicators awaiting further empirical confirmation rather than as implicated in (adaptive) sexual selection mechanisms.

Effects of global warming on species with temperature-dependent sex determination: Bridging the gap between empirical research and management

Global warming could threaten over 400 species with temperature-dependent sex determination (TSD) worldwide, including all species of sea turtle. During embryonic development, rising temperatures might lead to the overproduction of one sex and, in turn, could bias populations' sex ratios to an extent that threatens their persistence. If climate change predictions are correct, and biased sex ratios reduce population viability, species with TSD may go rapidly extinct unless adaptive mechanisms, whether behavioural, physiological or molecular, exist to buffer these temperature-driven effects. Here, we summarize the discovery of the TSD phenomenon and its still elusive evolutionary significance. We then review the molecular pathways underpinning TSD in model species, along with the hormonal mechanisms that interact with temperatures to determine an individual's sex. To illustrate evolutionary mechanisms that can affect sex determination, we focus on sea turtle biology, discussing both the adaptive potential of this threatened TSD taxon, and the risks associated with conservation mismanagement.

Predator odor exposure increases social contact in adolescents and parental behavior in adulthood in Brandt's voles

Research suggests that predation risk during adolescence can program adult stress response and emotional behavior; however, little is known about the short-term and lasting residual effects of this experience on social behavior. We explored this concept in social Brandt's voles (Lasiopodomys brandtii). Adolescent male and female voles were exposed to distilled water, rabbit urine (as a non-predator stimulus), and cat urine for 60 min daily from postnatal day (PND) 28-49. Social play tests were conducted immediately following exposure on PND 28, 35, 42, and 49. In the social play test, repeated cat odor (CO) exposure enhanced the contact behavior of voles with their cagemate. Adolescent exposure to CO did not affect behavioral responses toward unrelated pups in the alloparental behavior test or same-sex individuals in the social interaction test. However, exposure to CO significantly enhanced the licking/grooming behavior of voles towards their own pups in the home cage parental behavior test. Repeated CO exposure significantly inhibited weight gain in male voles during adolescence. This effect was transmitted to the next generation, with lower weight gain in offspring before weaning. Following repeated CO exposure, males tended to have more female offspring whereas females produced more offspring, suggesting an adaptive strategy to increase inclusive fitness under predatory risk. These findings demonstrate that adolescent exposure to predatory risk augments adolescent social contact and adult parental behavior and suggest a role for improved inclusive fitness in mediating long-term outcomes.

Hair cortisol levels in pregnancy as a possible determinant of fetal sex: a longitudinal study

Stress during pregnancy has been widely studied and associated to different variables, usually with negative results for the health of the mother and the newborn, such as having a higher risk of suffering postpartum depression, premature birth, obstetrics complications or low birthweight, among others. However, there are not many lines of research that study the role that the sex of the baby plays on this specific stress and vice versa. Thus, the main objective was to analyse the relationship between the sex of the offspring and the stress of the mothers in the first trimester of pregnancy. In order to achieve this, 108 women had their biological stress measured (trough hair cortisol levels) and psychological stress evaluated (the Prenatal Distress Questionnaire (PSS), the Perceived Stress Scale (PDQ) and the Stress Vulnerability Inventory (IVE)). The results revealed significant differences in maternal hair cortisol levels in the first trimester based on the sex of the baby they had given birth to (t = -2.04; P < 0.05): the concentration of the hormone was higher if the baby was a girl (164.36:54.45-284.87 pg/mg) than if it was a boy (101.13:37.95-193.56 pg/mg). These findings show that the sex of the future baby could be conditioned, among many other variables, by the mother´s stress levels during conception and first weeks of pregnancy. Further research is needed in this area to support our findings.

In utero exposure to organochlorine pesticide residues and their potential impact on birth outcomes and fetal gender

Being the largest agriculture country in the Arab world, Egypt was one of the major consumer of organochlorine pesticides (OCPs) in this area, continued to have a heavy burden of OCPs in the environment. There is growing concern that OCPs could pass from the maternal circulation through the placenta to the fetal circulation and pose several health risks to their fetuses. The current study was intended to identify OCPs residue exposure in healthy pregnant women and to justify the potential impacts of these residues on their fetuses. In this study, the prevalence of 18 OCPs was estimated in 81 maternal and cord blood samples, using Agilent 7890, gas chromatograph equipped with micro-electron capture detector (GC-μECD). Our data signposted that the heptachlor epoxide has the highest detection rate among all residues in both maternal (32%) and cord blood serum (27.16%). DDTs were still quantifiable, but with the lowest quantifiable percentage. More than 85% of mothers' serum with detectable residues transfer OCPs residue to their fetuses in a statistically significant manner (x = 42.9, p value < 0.001). The present findings showed no significant growth retardation, or preterm delivery induced by in utero exposure to the most abundant residues. There is growing evidence that exposure to OCPs residue has profound impact on sex ratio. Methoxychlor, in this study be deemed as testosterone triggers which yields high boys ratio (x = 4.37, p < 0.05). In conclusion, Egypt continued to have a heavy burden of OCPs residues, and fetuses and infants are especially the most vulnerable groups to their adverse health effects. Exposure to OCPs may disrupt the maternal hormones, which regulate the offspring gender, but these results need to be validated in larger sample sizes.

Body size at birth in babies born during World War II: The evidence from Poland

OBJECTIVES

The objective of the study was to determine whether exposure of pregnant women to stresses caused by World War II (WWII) negatively affected pregnancy and perinatal outcomes.

METHODS

Individual medical documents deposited in the Gynaecology and Obstetrics Clinic of Medical University, Poznań (1934-1943; N = 7058) were evaluated. These were divided into two birth cohorts: before WWII and during it. Frequency tables were constructed for the numbers of pregnancy outcomes: miscarriages, stillbirths, live births, and neonatal deaths, according to the period of birth and sex of a child. The numbers of recorded days were standardized and the numbers of cases per day were computed. Statistical differences in the averages (medians) between periods and years under study were tested. Birth weight, length, and body mass index (BMI) were compared according to the periods related to WWII.

RESULTS

Significant differences in proportions of males, females, and subjects with unknown sex were found between the periods: a higher proportion of males and different structure of/within negative outcomes were found during WWII. Children born during WWII were heavier and longer than those born before it.

CONCLUSIONS

As an explanation, adverse conditions of WWII, related to the psychological stress and food shortages, could have influenced greater elimination of fetuses and neonates of male sex during pregnancy and shortly after delivery. Higher average body size in newborns recorded during WWI could be explained by a hidden process of increased early prenatal mortality of weaker individuals, differences in average gestation length between the periods, differences in parity, or some undocumented differences in social/ethnic composition of the sample.

Maternal prenatal stress phenotypes associate with fetal neurodevelopment and birth outcomes

Maternal prenatal stress influences offspring neurodevelopment and birth outcomes including the ratio of males to females born; however, there is limited understanding of what types of stress matter, and for whom. Using a data-driven approach with 27 variables from questionnaires, ambulatory diaries, and physical assessments collected early in the singleton pregnancies of 187 women, 3 latent profiles of maternal prenatal stress emerged that were differentially associated with sex at birth, birth outcomes, and fetal neurodevelopment. Most women (66.8%) were in the healthy group (HG); 17.1% were in the psychologically stressed group (PSYG), evidencing clinically meaningful elevations in perceived stress, depression, and anxiety; and 16% were in the physically stressed group (PHSG) with relatively higher ambulatory blood pressure and increased caloric intake. The population normative male:female secondary sex ratio (105:100) was lower in the PSYG (2:3) and PHSG (4:9), and higher in the HG (23:18), consistent with research showing diminished male births in maternal stress contexts. PHSG versus HG infants were born 1.5 wk earlier (P < 0.05) with 22% compared to 5% born preterm. PHSG versus HG fetuses had decreased fetal heart rate-movement coupling (P < 0.05), which may indicate slower central nervous system development, and PSYG versus PHSG fetuses had more birth complications, consistent with previous findings among offspring of women with psychiatric illness. Social support most strongly differentiated the HG, PSYG, and PHSG groups, and higher social support was associated with increased odds of male versus female births. Stress phenotypes in pregnant women are associated with male vulnerability and poor fetal outcomes.

Non-model species deliver a non-model result: Nutria female fetuses neighboring males in utero have lower testosterone

Neighboring fetuses may impact their siblings in various respects, depending on their in utero location and sex. The effects of the intrauterine position (IUP) are widely studied in model organisms, especially laboratory bred murine strains that are characterized by short gestations and altricial offspring. In some species, the proximity to a male fetus and its higher circulating testosterone masculinizes neighboring female fetuses. In utero testosterone exposure might be manifested as higher testosterone concentrations, which contribute to a variation in morphology, reproductive potential and behavior. In this study, we examined the influence of neighboring an opposite sex fetus on testosterone levels in a feral animal model characterized by a long gestation and precocious offspring. Using necropsies of culled nutria (Myocastor coypus), we accurately determined the IUP and quantified testosterone immunoreactivity in fetal hair. We found that as expected, both male and female fetuses neighboring a male in utero had longer anogenital distance. However, females adjacent to males in utero showed lower testosterone levels than male fetuses, while testosterone levels of females without a male neighbor did not differ from those of males. This surprising result suggests an alternative mode by which local exogenous steroids may modify the local fetal environment. Our study emphasizes the importance of examining known phenomena in species with different life histories, other than the traditional murine models, to enhance our understanding of the evolutionary mechanisms that are driving sexual differentiation.

Sexual conflict in action: An antagonistic relationship between maternal and paternal sex allocation in the tammar wallaby, Notamacropus eugenii

Sex ratio biases are often inconsistent, both among and within species and populations. While some of these inconsistencies may be due to experimental design, much of the variation remains inexplicable. Recent research suggests that an exclusive focus on mothers may account for some of the inconsistency, with an increasing number of studies showing variation in sperm sex ratios and seminal fluids. Using fluorescent in-situ hybridization, we show a significant population-level Y-chromosome bias in the spermatozoa of wild tammar wallabies, but with significant intraindividual variation between males. We also show a population-level birth sex ratio trend in the same direction toward male offspring, but a weaning sex ratio that is significantly female-biased, indicating that males are disproportionately lost during lactation. We hypothesize that sexual conflict between parents may cause mothers to adjust offspring sex ratios after birth, through abandonment of male pouch young and reactivation of diapaused embryos. Further research is required in a captive, controlled setting to understand what is driving and mechanistically controlling sperm sex ratio and offspring sex ratio biases and to understand the sexually antagonistic relationship between mothers and fathers over offspring sex. These results extend beyond sex allocation, as they question studies of population processes that assume equal input of sex chromosomes from fathers, and will also assist with future reproduction studies for management and conservation of marsupials.

Cortisol advantage of neighbouring the opposite sex in utero

Population sex ratios naturally fluctuate around equality. It is argued that the production of an equal number of male and female offspring by individual parents should be favoured by selection, if all costs and benefits are equal. Theoretically, an even sex ratio should yield the highest probability for a fetus to be adjacent to a fetus of the opposite sex in utero. This may cause developmental costs or benefits that have been overlooked. We examined the physiological and developmental parameters associated with in utero sex ratios in the nutria (Myocastor coypus), an invasive wildlife species with a strong reproductive output. Using hair testing, we found that litters with even sex ratios had the highest average cortisol levels. Fetuses neighbouring the opposite sex exhibited longer trunks than those neighbouring the same sex, which might imply better lung development. Our results are the first, to our knowledge, to link intra-utero sex ratios and fetal cortisol and suggest that fetal cortisol might be a mechanism by which even sex ratios are maintained via developmental advantages.

The sex ratio of a gilt’s birth litter can affect her fitness as a breeding female

Lifetime reproductive performance and behaviour are established in utero driven by genetics and the steroidal milieu of the uterine environment. Developing males synthesise androgens that can be absorbed by females in the litter. Consequently, the proportion of males in a litter has the potential to affect both lifetime reproductive performance and behaviour of female littermates. Although reproductive potential is understandably important for gilt selection, behaviour is becoming progressively more important as group housing of sows increases. Aggression can be a cause for removal from the breeding herd, and can also negatively affect reproductive performance and herd longevity. This review covers existing evidence of masculinisation in rodents and sheep, the mechanism causing masculinisation, and the knowledge gaps surrounding masculinisation in pigs. Premature culling of gilts is an economic and animal welfare issue in the pork industry resulting in low retention of gilts and reduced productivity. The selection of gilts needs to be improved to incorporate both lifetime reproductive potential and behaviour. We determined that there is cause to investigate the effect of the birth litter sex ratio of a gilt on her suitability for selection into the breeding herd.

Chances to Have A Boy after Gender Selection by Pre-Implantation Genetic Screening Are Reduced in Couples with only Girls and without A Boy Sired by The Male Partner

BACKGROUND

Gender selection and family planning have their roots in human history. Despite great interest in these fields, very few scientific propositions exist which could explain why some family do not attain the desired sex. Therefore, the aim of this study was to evaluate whether sex of previous child or children could affect the outcomes of pre-implantation genetic screening (PGS).

MATERIALS AND METHODS

This historical cohort study including 218 PGS cases referring to Isfahan Fertility and Infertility Center (IFIC). Couples were grouped as those who their male child passed away or her husbands' has a son(s) from their previous marriage (n=70) and couples who just have daughter (n=148). Male normal blastocysts were transferred for both groups. The outcomes of PGS including pregnancy, implantation and abortion rates, along with possible confounding factors were compared between the two groups.

RESULTS

Significant differences in pregnancy, implantation and abortion rates were observed between couples whose their male partner had/has one boy (n=70) compared to those who have just girl(s) (n=148) despite similar number and quality of male normal blastocyst transferred in the two groups. Confounding factors were also considered.

CONCLUSION

The Ybearing spermatozoa in male partners with no history of previous boy have lower ability to support a normal development to term, compared to male partners with previous history of boy requesting family balancing.

Developmental sexual dimorphism and the evolution of mechanisms for adjustment of sex ratios in mammals

Sex allocation theory predicts biased offspring sex ratios in relation to local conditions if they would maximize parental lifetime reproductive return. In mammals, the extent of the birth sex bias is often unpredictable and inconsistent, leading some to question its evolutionary significance. For facultative adjustment of sex ratios to occur, males and females would need to be detectably different from an early developmental stage, but classic sexual dimorphism arises from hormonal influences after gonadal development. Recent advances in our understanding of early, pregonadal sexual dimorphism, however, indicate high levels of dimorphism in gene expression, caused by chromosomal rather than hormonal differences. Here, we discuss how such dimorphism would interact with and link previously hypothesized mechanisms for sex-ratio adjustment. These differences between males and females are sufficient for offspring sex both to be detectable to parents and to provide selectable cues for biasing sex ratios from the earliest stages. We suggest ways in which future research could use the advances in our understanding of sexually dimorphic developmental physiology to test the evolutionary significance of sex allocation in mammals. Such an approach would advance our understanding of sex allocation and could be applied to other taxa.

Gestational experience alters sex allocation in the subsequent generation

Empirical tests of adaptive maternal sex allocation hypotheses have presented inconsistent results in mammals. The possibility that mothers are constrained in their ability to adjust sex ratios could explain some of the remaining variation. Maternal effects, the influence of the maternal phenotype or genotype on her developing offspring, may constrain sex allocation through physiological changes in response to the gestational environment. We tested if maternal effects constrain future parental sex allocation through a lowered gestational stress environment in laboratory mice. Females that experienced lowered stress as embryos in utero gave birth to female-biased litters as adults, with no change to litter size. Changes in offspring sex ratio was linked to peri-conceptual glucose, as those females that had increasing blood glucose peri-conceptionally gave birth to litters with a higher male to female sex ratio. There was, however, no effect of the lowered prenatal stress for developing male embryos and their sperm sex ratio when adult. We discuss the implications of maternal effects and maternal stress environment on the lifelong physiology of the offspring, particularly as a constraint on later maternal sex allocation.

Does the Mother or Father Determine the Offspring Sex Ratio? Investigating the Relationship between Maternal Digit Ratio and Offspring Sex Ratio

OBJECTIVE

In mammals, high parental testosterone levels present around the time of conception are thought to skew offspring sex ratio toward sons. The second to fourth digit ratio (digit ratio) is now widely accepted as a negative correlate of prenatal testosterone. Thus, we investigated the association between digit ratio and offspring sex ratio.

METHODS

A total of 508 Korean patients (257 males and 251 females) less than 60 years old who had one or more offspring were prospectively enrolled. The lengths of the 2nd and 4th digits of the right hand were measured by a single investigator using a digital vernier calliper. Next, the patients' lifetime offspring birth sex ratios were investigated.

RESULTS

Maternal (rather than paternal) digit ratio was significantly associated with the number of sons (r = -0.153, p = 0.015), number of daughters (r = 0.130, p = 0.039), and offspring sex ratio (r = -0.171, p = 0.007). And, the maternal digit ratio was a significant factor for predicting offspring sex ratio (B = -1.620, p = 0.008) on multiple linear regression analysis. The female patients with a lower digit ratio (< 0.95) were found to have a higher offspring sex ratio (0.609 versus 0.521, p = 0.046) compared to those with a higher digit ratio (≥ 0.95). Furthermore, females in the low digit ratio group have a probability 1.138 greater of having sons than females in the high digit ratio group.

CONCLUSIONS

Maternal digit ratio was negatively associated with offspring sex ratio. Females with a lower digit ratio were more likely to have more male offspring compared to those with a higher digit ratio. Thus, our results suggest that the sex of offspring might be more influenced by maternal rather than paternal factors.

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

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

Economic stress or random variation? Revisiting German reunification as a natural experiment to investigate the effect of economic contraction on sex ratios at birth

BACKGROUND

The economic stress hypothesis (ESH) predicts decreases in the sex ratio at birth (SRB) following economic decline. However, as many factors influence the SRB, this hypothesis is difficult to test empirically. Thus, researchers make use of quasi-experiments such as German reunification: The economy in East, but not in West Germany, underwent a rapid decline in 1991. A co-occurrence of a decline in the East German SRB in 1991 has been interpreted by some as support for the ESH. However, another explanation might be that the low SRB in 1991 stems from increased random variation in the East German SRB due to a drastically reduced number of births during the crisis. We look into this alternative random variation hypothesis (RVH) by re-examining the German case with more detailed data.

METHODS

Our analysis has two parts. First, using aggregate-level birth register data for all births in the period between 1946 and 2011, we plot the quantum and variance of the SRB and the number of births and unemployment rates, separately for East and West Germany, and conduct a time series analysis on the East German SRB over time. Second, we model the odds for a male birth at the individual level in a multiple logistic regression (1991-2010, ~13.9 million births). Explanatory variables are related to the level of the individual birth, the mother of the child born, and the regional economic context.

RESULTS

The aggregate-level analysis reveals a higher degree of variation of the SRB in East Germany. Deviations from the time trend occur in several years, seemingly unrelated to economic development, and the deviation in 1991 is not statistically significant. The individual-level analysis confirms that the 1991-drop in the East German SRB cannot directly be attributed to economic development and that there is no statistically significant effect of economic development on sex determination in East or West Germany.

CONCLUSION

Outcomes support the RVH but not the ESH. Furthermore, our results speak against a statistically significant effect of the reunification event itself on the East German SRB. We discuss the relative importance of behavioral and physiological responses to macro-level stressors, a distinction that may help integrate previously mixed findings.

Maternal gestational cortisol and testosterone are associated with trade-offs in offspring sex and number in a free-living rodent (Urocitellus richardsonii)

The adaptive manipulation of offspring sex and number has been of considerable interest to ecologists and evolutionary biologists. The physiological mechanisms that translate maternal condition and environmental cues into adaptive responses in offspring sex and number, however, remain obscure. In mammals, research into the mechanisms responsible for adaptive sex allocation has focused on two major endocrine axes: the hypothalamic pituitary adrenal (HPA) axis and glucocorticoids, and the hypothalamic pituitary gonadal (HPG) axis and sex steroids, particularly testosterone. While stress-induced activation of the HPA axis provides an intuitive model for sex ratio and litter size adjustment, plasma glucocorticoids exist in both bound and free fractions, and may be acting indirectly, for example by affecting plasma glucose levels. Furthermore, in female mammals, activation of the HPA axis stimulates the secretion of adrenal testosterone in addition to glucocorticoids (GCs). To begin to untangle these physiological mechanisms influencing offspring sex and number, we simultaneously examined fecal glucocorticoid metabolites, free and bound plasma cortisol, free testosterone, and plasma glucose concentration during both gestation and lactation in a free-living rodent (Urocitellus richardsonii). We also collected data on offspring sex and litter size from focal females and from a larger study population. Consistent with previous work in this population, we found evidence for a trade-off between offspring sex and number, as well as positive and negative correlations between glucocorticoids and sex ratio and litter size, respectively, during gestation (but not lactation). We also observed a negative relationship between testosterone and litter size during gestation (but not lactation), but no effect of glucose on either sex ratio or litter size. Our findings highlight the importance of binding proteins, cross-talk between endocrine systems, and temporal windows in the regulation of trade-offs in offspring sex and number.

Anogenital distance and condition as predictors of litter sex ratio in two mouse species: a study of the house mouse (Mus musculus) and mound-building mouse (Mus spicilegus)

The Trivers - Willard hypothesis (1973) suggests that the maternal condition may affect the female's litter size and sex ratio. Since then other factors had been found. Previous findings revealed in the case of some mammalian species, that females with larger anogenital distance have smaller litters, while the sex ratio is male-biased. That has only been demonstrated in laboratory animals, while the genetic diversity of a wild population could mask the phenomenon seen in laboratory colonies. We examined the connection between morphological traits (weight and anogenital distance) and the reproductive capacity of two wild mice species, the house mouse and the mound-building mice. We showed in both species that anogenital distance and body weight correlated positively in pre-pubertal females, but not in adults. Neither the house mouse nor the mound-building mouse mothers' weight had effect on their litter's size and sex ratio. Otherwise connection was found between the mothers' anogenital distance and their litters' sex ratio in both species. The results revealed that females with larger anogenital distance delivered male biased litter in both species. The bias occurred as while the number of female pups remained the same; mothers with large anogenital distance delivered more male pups compared to the mothers with small anogenital distance. We concluded that a female's prenatal life affects her reproductive success more than previously anticipated.

Hormone-mediated adjustment of sex ratio in vertebrates

The ability to adjust sex ratios at the individual level exists among all vertebrate groups studied to date. In many cases, there is evidence for facultative adjustment of sex ratios in response to environmental and/or social cues. Because environmental and social information must be first transduced into a physiological signal to influence sex ratios, hormones likely play a role in the adjustment of sex ratio in vertebrates, because the endocrine system acts as a prime communicator that directs physiological activities in response to changing external conditions. This symposium was developed to bring together investigators whose work on adjustment of sex ratio represents a variety of vertebrate groups in an effort to draw comparisons between species in which the sex-determination process is well-established and those in which more work is needed to understand how adjustments in sex ratio are occurring. This review summarizes potential hormone targets that may underlie the mechanisms of adjustment of sex ratio in humans, non-human mammals, birds, reptiles, and fishes.

Winning the genetic lottery: biasing birth sex ratio results in more grandchildren

Population dynamics predicts that on average parents should invest equally in male and female offspring; similarly, the physiology of mammalian sex determination is supposedly stochastic, producing equal numbers of sons and daughters. However, a high quality parent can maximize fitness by biasing their birth sex ratio (SR) to the sex with the greatest potential to disproportionately outperform peers. All SR manipulation theories share a fundamental prediction: grandparents who bias birth SR should produce more grandoffspring via the favored sex. The celebrated examples of biased birth SRs in nature consistent with SR manipulation theories provide compelling circumstantial evidence. However, this prediction has never been directly tested in mammals, primarily because the complete three-generation pedigrees needed to test whether individual favored offspring produce more grandoffspring for the biasing grandparent are essentially impossible to obtain in nature. Three-generation pedigrees were constructed using 90 years of captive breeding records from 198 mammalian species. Male and female grandparents consistently biased their birth SR toward the sex that maximized second-generation success. The most strongly male-biased granddams and grandsires produced respectively 29% and 25% more grandoffspring than non-skewing conspecifics. The sons of the most male-biasing granddams were 2.7 times as fecund as those of granddams with a 50∶50 bias (similar results are seen in grandsires). Daughters of the strongest female-biasing granddams were 1.2 times as fecund as those of non-biasing females (this effect is not seen in grandsires). To our knowledge, these results are the first formal test of the hypothesis that birth SR manipulation is adaptive in mammals in terms of grandchildren produced, showing that SR manipulation can explain biased birth SR in general across mammalian species. These findings also have practical implications: parental control of birth SR has the potential to accelerate genetic loss and risk of extinction within captive populations of endangered species.

Revisiting a sample of U.S. billionaires: how sample selection and timing of maternal condition influence findings on the Trivers-Willard effect

Based on evolutionary theory, Trivers & Willard (TW) predicted the existence of mechanisms that lead parents with high levels of resources to bias offspring sex composition to favor sons and parents with low levels of resources to favor daughters. This hypothesis has been tested in samples of wealthy individuals but with mixed results. Here, I argue that both sample selection due to a high number of missing cases and a lacking specification of the timing of wealth accumulation contribute to this equivocal pattern. This study improves on both issues: First, analyses are based on a data set of U.S. billionaires with near-complete information on the sex of offspring. Second, subgroups of billionaires are distinguished according to the timing when they acquired their wealth. Informed by recent insights on the timing of a potential TW effect in animal studies, I state two hypotheses. First, billionaires have a higher share of male offspring than the general population. Second, this effect is larger for heirs and heiresses who are wealthy at the time of conception of all of their children than for self-made billionaires who acquired their wealth during their adult lives, that is, after some or all of their children have already been conceived. Results do not support the first hypothesis for all subgroups of billionaires. But for males, results are weakly consistent with the second hypothesis: Heirs but not self-made billionaires have a higher share of male offspring than the U.S. population. Heiresses, on the other hand, have a much lower share of male offspring than the U.S. average. This hints to a possible interplay of at least two mechanisms affecting sex composition. Implications for future research that would allow disentangling the distinct mechanisms are discussed.

Parental status and gender preferences for children: is differential fertility stopping consistent with the trivers-willard hypothesis?

Based on evolutionary reasoning, Trivers & Willard (1973) predicted status-biased sex composition and parental investment with son-preferencing effects in higher, and daughter-preferencing effects in lower status groups. Previous research shows mixed results. This study uses event-history methods and Swedish register data to study one possible mechanism in isolation: do parents in different status groups vary in their proclivities to continue fertility based on the sex composition of previous offspring? The results show no support for the Trivers-Willard hypothesis on a wide range of different status indicators. Future research on the stated hypothesis should focus on physiological rather than behavioural mechanisms.

The relevance of the epidemiology of human sex ratios at birth to some medical problems

The human sex ratio (proportion male at birth) shows considerable variation which is central to reproductive epidemiology. For example, it has reportedly shown significant secular trends and wartime variation, and it has shown replicated variation with parental exposure to several deleterious chemicals. However, scientific progress in identifying the causes of these forms of variation has been very slow and, as described here, this has elicited some scepticism. Benefits may be expected from identifying these causes. Two (non-competing) hypotheses have emerged, the first relating to the hormonal regulation of sex ratio at conception, and the second to the sex-selective effects of stressors during pregnancy. It is shown here that if the first of these hypotheses was substantially correct, it would illuminate a number of scientific and medical problems, for example, the causes of autism and of selected sex-related congenital malformations and obstetric pathologies.

Anogenital distance as a predictor of attractiveness, litter size and sex ratio of rabbit does

In the case of some mammalian species, females with larger anogenital distance (AGD) have smaller litters, where the sex ratio is male-biased. The first aim of the present study was to test whether this phenomenon exists in the Lagomorph group, especially in rabbits. The results revealed that does with large AGD have significantly smaller and lighter litters with a male biased sex ratio; with fewer females but not more males. The second aim of the present study was to test whether males differentiate between females differing in their AGD and are thus capable of choosing the more fertile and fecund does. Males showed a stronger response to the chin marks of females with small AGD than to the marks of females with large AGD. Our results suggest that variation in the prenatal hormonal environment, reflected through variation in AGD, could have long-term consequences on mate choice and population dynamics.

Periconceptional influences on offspring sex ratio and placental responses

Maternal diet and secondary factors can strikingly influence fetal outcomes, including biasing offspring sex ratio and altering the molecular biological responses of the conceptus, namely within the placenta. Alterations in the in utero environment might also lead to profound developmental origin of health and disease (DOHaD) outcomes into adulthood, including increased risk for cardiovascular disease, obesity and cancer, with males in general being at greater risk for these diseases. Female mice maintained on a very high fat (VHF) diet birth more sons than those on a chow-based and low fat (LF), high carbohydrate diet, with the latter group producing more daughters. However, neither the underlying mechanisms that contribute to this shift in offspring sex ratio nor when they occur during pregnancy have been resolved. In this review, we consider the evidence that maternal diet and other factors influence secondary sex ratio in a variety of species, including humans, and discuss when this skewing might occur. Additionally, we examine how fetal sex and maternal diet influences gene expression patterns in the mouse placenta, which serves as the primary nutrient acquisition and communication organ between the mother and her developing pups. These adaptations to diet observed as changes in gene expression are likely to provide insight into how the placenta buffers the fetus proper from environmental shifts in nutrient availability during pregnancy and whether male and female conceptuses respond differently to such challenges.

Older mothers follow conservative strategies under predator pressure: the adaptive role of maternal glucocorticoids in yellow-bellied marmots

When the maternal environment is a good predictor of the offspring environment, maternal glucocorticoid (GC) levels might serve to pre-program offspring to express certain phenotypes or life-history characteristics that will increase their fitness. We conducted a field study to assess the effects of naturally occurring maternal GC levels on their offspring in yellow-bellied marmots (Marmota flaviventris) subjected to different predator pressures. Maternal fecal corticosteroid metabolites (FCM) were positively correlated with predator pressure. Predators had both direct and indirect effects on pups. We found that older mothers with higher FCM levels had smaller and female-biased litters. Moreover, sons from older mothers with high FCM levels dispersed significantly more than those from older mothers with low FCM levels, whereas the opposite pattern was found in pups from younger mothers. These age-related effects may permit females to make adaptive decisions that increase their pups' fitness according to their current situation.

Can mammalian mothers influence the sex of their offspring peri-conceptually?

Although controversial, growing evidence from evolutionary biology suggests that the mammalian mother may have a role in influencing the sex of her offspring. However, there is competing information on the molecular mechanisms by which such influence could be manifested. The new initiatives are based on hypotheses from evolutionary biology: the 'good condition' hypothesis, which suggests that post conception, higher levels of maternal glucose may differentially promote the development of male embryos; and the 'maternal dominance' hypothesis, which proposes that before conception, higher follicular testosterone may influence the development of the ovum so that it emerges already adapted to receive an X- or a Y-chromosome-bearing spermatozoon. Now, it seems these hypothesised mechanisms could be operating in synchrony, each complementing and reinforcing the other. On the other hand, there are continuing problems in identifying a precise sequence of mechanisms as evidenced from research in sperm-sorting. Research on high-fat diets and the sex ratio in polytocous species may indicate important differences in proximate mechanisms for sex allocation between polytocous and monotocous mammals.

Programming of offspring sex ratios by maternal stress in humans: assessment of physiological mechanisms using a comparative approach

Sex ratio adjustment has become a hot topic in ecology and evolutionary biology, as documentations of sex ratio skews are numerous, and include examples in diverse animal species. Over the past several decades, scientists have repeatedly debated whether human sex ratios also significantly deviate toward one sex or the other based on environmental or social conditions. An increasing number of studies supports the idea that exposure to stressful conditions can influence the sexes of offspring produced by humans, a majority of which document significantly fewer males after exposure to adverse conditions such as severe life events, economic disruption, or natural disasters. From a comparative standpoint, these findings are similar to studies in non-human mammals and other vertebrate species showing a bias toward females during times of stress. However, the mechanisms by which stress-related biases in the offspring sex ratio may occur remain elusive, and the involvement of glucocorticoids indicating a true influence of stress itself remains unstudied. Here, I review the evidence that stressful events induce sex ratio adjustment in humans. Additionally, I discuss the possibility for glucocorticoid mediation of sex ratio adjustment and the potential reproductive stages during which stress-induced sex ratio adjustment may occur in humans and other mammals.

Prenatal synthetic glucocorticoid exposure alters hypothalamic-pituitary-adrenal regulation and pregnancy outcomes in mature female guinea pigs

Preterm delivery occurs in approximately 10% of all pregnancies. Prenatal exposure to synthetic glucocorticoids (sGCs) reduces the incidence of respiratory distress syndrome (RDS) in these babies. Therefore, administration of multiple courses of sGCs became common practice. Animal and human studies have demonstrated that multiple courses of sGCs can have long-term effects. While the majority of animal studies have been undertaken in male offspring, it is emerging that there are profound sex differences in the consequences of prenatal sGC exposure. To our knowledge, no studies have determined the effects of prenatal sGC exposure on hypothalamic-pituitary-adrenal (HPA) axis function in female offspring while accounting for reproductive cycle status, or determined if there are effects on pregnancy parameters. Pregnant guinea pigs were administered three courses of betamethasone (Beta), dexamethasone (Dex) or vehicle on gestational days 40/41, 50/51 and 60/61. In adulthood (age range: postnatal days 126-165), basal and activated HPA axis function were assessed at various stages of the reproductive cycle. The female offspring were then mated and underwent an undisturbed pregnancy. Females were killed in the luteal phase of the reproductive cycle following litter weaning, and molecular analysis undertaken. In the luteal phase, Beta-exposed females exhibited significantly lower basal salivary cortisol levels (P < 0.05). Dex-exposed females also exhibited significantly lower basal salivary cortisol levels during the luteal phase (P < 0.05), but increased basal salivary cortisol levels during the ostrous phase (P < 0.01). The Beta-exposed females exhibited increased glucocorticoid receptor (GR) mRNA expression in the CA1/2 region of the hippocampus (P < 0.05) and MC2R mRNA in the adrenal cortex (P < 0.05). The Dex-exposed animals exhibited higher hippocampal GR and mineralocorticoid receptor (MR) mRNA levels (P < 0.05). Beta-exposed females showed reduced fecundity (P < 0.05). In Dex-exposed females there was a lower male to female sex ratio. In conclusion, prenatal sGC exposure affects HPA axis activity, in a cycle-dependent manner, and long-term reproductive success. The clinical implications of the findings on endocrine function and pregnancy in females are profound and further follow-up is warranted in human cohorts. Furthermore, we have shown there are considerable difference in phenotypes between the Beta- and Dex-exposed females and the specific endocrine and maternal outcome is contingent on the specific sGCs administered during pregnancy.

Intrafollicular testosterone concentration and sex ratio in individually cultured bovine embryos

Recent studies have suggested a relationship between bovine follicular fluid testosterone concentration and the likelihood of the oocyte being fertilised by an X- or Y-bearing spermatozoon; however, this theory has been challenged. To further test this hypothesis, follicles were dissected from the ovaries of slaughtered heifers, measured and carefully ruptured. The cumulus–oocyte complex (COC) was removed and the follicular fluid collected and testosterone concentration determined by radioimmunoassay. COCs were matured, fertilised and cultured in an individually identifiable manner; all cleaved embryos (2- to 4-cell stage, n = 164) had their sex determined by PCR. Testosterone concentrations were positively skewed. There was no significant difference between follicular fluid testosterone concentrations in male and female embryos (mean ± s.e.m. 51.5 ± 5.59 and 49.5 ± 7.42 ng mL–1, respectively). Linear, quadratic and cubic logistical regression showed that follicular testosterone concentration could not reliably predict the sex of the embryo with odds ratios of 1.001, 1.013 and 1.066, repectively, and coefficient of determination (R2) values of 0.0003, 0.0126 and 0.0567, respectively. Follicular size and testosterone concentration were not related (R2 = 0.087). Finally, follicular size had no influence on embryo sex determination (P = 0.70). In conclusion, under the conditions of the present study, the likelihood of an oocyte being fertilised by an X- or Y-bearing spermatozoon was not affected by the size of the follicle from which it was derived, nor by the testosterone concentration in the follicular fluid.

Maternal exposure to triclosan impairs thyroid homeostasis and female pubertal development in Wistar rat offspring

Although the effects of triclosan have been examined in male reproductive functions, it is unknown whether this potent antibacterial agent affects pregnancy and female pubertal development. Effects of maternal exposure to triclosan on thyroid homeostasis (TH) and reproductive-tract development in female Wistar rats were thus studied. Dams were exposed daily to triclosan (0, 1, 10, or 50 mg/kg/d) from 8 d before mating to lactation day 21. Offspring were also exposed after weaning. In vivo triclosan estrogenic activity was screened by uterotrophic assay and vaginal opening (VO), with first estrus and uterus and ovarian weight determined in offspring. Dam blood samples were taken during pregnancy and lactation to examine the effect of triclosan on TH. No apparent external signs of toxicity or differences in mean numbers of implantation sites were observed in treated rats. Triclosan treatment decreased total serum T(4) and T(3) in pregnant rats and also lowered sex ratio, lowered pup body weights on postnatal day (PND) 20, and delayed VO in offspring. In addition, the highest dose of triclosan significantly reduced the live birth index (percentage) and 6-d survival index. Data indicate that triclosan impairs thyroid homeostasis and reproductive toxicity in adult rats and produces fetal toxicity in offspring exposed in utero, during lactation, and after weaning.