A simple model for adaptive variation in the sex ratios of mammalian offspring

Sex Ratio at Birth after Concurrent Events of Earthquakes and the COVID-19 Pandemic in Croatia

The sex ratio at birth (ratio of males to females) has been known to be affected by exogenous shocks such as wars, pollution, natural catastrophes, economic crises, and others. Among these stressful events, both earthquakes and the COVID-19 pandemic have been reported to lower the sex ratio at birth. In this article, a rather unusual situation of two episodes of simultaneous events of COVID-19 lockdown and earthquakes approximately nine months apart (March and December of 2020) is investigated to assess whether they were associated with a bias in sex ratio at birth 3-5 months later (in utero loss) and 9 months later (loss at conception) in Croatia. The monthly time series of sex ratio at birth, total number of births, and total number of both male and female births from January 2010 to December 2021 were analyzed. Seasonally adjusted autoregressive moving-average models were used to estimate the functional form of the time series from January 2010 to February 2020. These results were used to predict the future values of the series until December 2021 and to compare them with the actual values. For all series used, there was no indication of deviation from the values predicted by the models, neither for 3-5 months nor for 9 months after the COVID-19 lockdown and earthquake events. The possible mechanisms of the absence of bias, such as the threshold of the stressful events and its localized reach, as well as the statistical methods employed, are discussed.

Revisiting the Trivers-Willard theory on birth sex ratio bias: Role of paternal condition in a Malagasy primate

Within current theories on potential adaptive manipulation of offspring sex ratio, giving birth to a male or to a female is assumed to depend on the capacity of the mother to invest in offspring to maximize her fitness. The active role of the father in sex ratio bias at birth has been neglected until recently. The human sex ratio at birth is biased towards sons, although in occidental populations, the ratio has decreased regularly for 30 years and could be the consequence of the adverse effects of environmental chemicals on male hormones. In a Malagasy primate, the lesser mouse lemur, the potential effect of paternal testosterone levels on sex ratio bias at birth was tested on 130 litters (278 babies) produced in 52 mixed-sex groups. For each group, social dominance among males was characterized based on aggressive interactions and sexual behaviours. Using a multi correspondence analysis, high testosterone levels in grouped males, particularly those of the dominant male, were significantly correlated with more infants produced in male-biased litters, independent of the female condition. According to these results, predictions for sex ratio bias towards one sex or the other in mouse lemurs were discussed considering the influence of both parents.

Sex Differences in the Prenatal Programming of Adult Metabolic Syndrome by Maternal Androgens

CONTEXT

Growing preclinical evidence suggests that hormonal programming by androgens in utero may contribute to cardiovascular disease risk in adult offspring. However, the effect of prenatal androgens on cardiometabolic outcomes in the human population, especially their potential differential impact on male vs female offspring, has not been well studied.

DESIGN

Adult offspring (n = 274) of mothers enrolled in the New England birth cohorts of the Collaborative Perinatal Project were assessed at ages 39 to 50. Androgen bioactivity was measured in maternal serum during the third trimester using a receptor-mediated luciferase expression bioassay. Metabolic syndrome (MetS) using Adult Treatment Panel III criteria was assessed in adult offspring. Bioactive androgens were analyzed as quartiles, with the lowest quartile (Q1) defined as the reference. Generalized estimating equations were used to evaluate the relationship of maternal bioactive androgens on offspring MetS risk overall and by sex, controlling for potential confounders and intrafamilial correlation.

RESULTS

Mean age and body mass index of adult offspring were 44.7 ± 2.6 years and 29.7 ± 6.7 kg/m2, respectively. Participants born to mothers with the highest quartile (Q4) compared with Q1 of bioactive androgens had higher risk for MetS [adjusted odds ratio (aOR): 2.53(1.07 to 6.02)]. Stratified by sex, this association was found to be significant among women [Q4 vs Q1; aOR: 4.06 (1.10 to 14.93)] but not men [Q4 vs Q1; aOR: 1.67 (0.53 to 5.26)]. Women born to mothers with the highest levels of maternal bioactive androgens also demonstrated a 4.84-fold increased odds for having hypertension [Q4 vs Q1; aOR: 4.84 (1.12 to 20.85)].

CONCLUSION

Higher levels of maternal androgens were associated with increased risk for incident MetS in adult offspring, an effect that was significant in women but not men.

Associations of Maternal Testosterone and Cortisol Levels With Health Outcomes of Mothers and Their Very-Low-Birthweight Infants

OBJECTIVE

Although the roles of testosterone and cortisol in various health problems have been extensively investigated, little is known about their associations with health outcomes in mothers and their very-low-birthweight (VLBW) infants when maternal testosterone and cortisol are examined together during the postpartum period.

METHOD

The 101 mother-VLBW infant pairs were recruited from the neonatal intensive care unit of a tertiary medical center in the southeastern United States. Demographic information, pregnancy and labor complications of mothers, and health and growth outcomes of infants were obtained from medical records and interviews with mothers. Maternal salivary testosterone and cortisol levels were determined using enzyme immunoassay.

RESULTS

Linear regression showed that mothers who were older and had a larger body mass index experienced more pregnancy complications, whereas mothers who were single and had a cesarean section experienced more labor complications. Generalized linear models showed that mothers with high cortisol levels had more antepartum hemorrhage, whereas infants of mothers with high cortisol levels had fewer neurological insults and shorter hospitalizations than other infants. More mothers experienced premature prolonged rupture of membranes (PPROM) than chorioamnionitis, and maternal medical complications were negatively associated with infant health outcomes except PPROM, which was positively associated with infant outcomes.

CONCLUSIONS

High maternal cortisol levels were associated with maternal health problems during pregnancy. Beneficial effects of PPROM and high maternal cortisol levels on infant health outcomes were important findings, and understanding the mechanisms of these relationships may be of practical value for clinicians and researchers.

Preconception stress and the secondary sex ratio in a population-based preconception cohort

OBJECTIVE

To examine the association between preconception parental stress and the secondary sex ratio, defined as the ratio of males to females at birth.

DESIGN

A population-based preconception cohort.

SETTING

Not applicable.

PATIENT(S)

A total of 235 couples who were enrolled before conception in Michigan and Texas between 2005 and 2009 and who had a singleton birth during the follow-up period. Couples were interviewed separately at baseline to obtain information on perceived stress (Cohen's Perceived Stress Scale) and lifetime history of physician-diagnosed anxiety and/or mood disorders. Female partners were also trained to collect basal saliva samples for the measurement of salivary stress markers, alpha-amylase and cortisol.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Birth outcome data including infant sex were collected upon delivery. Modified Poisson regression models were used to estimate the relative risks (RRs) of a male birth for each stress marker.

RESULT(S)

After adjusting for potential confounders, we observed a 76% increase in the risk of fathering a male infant (RR 1.76; 95% confidence interval 1.17-2.65) in men diagnosed with anxiety disorders compared with those who were not diagnosed. When lifetime history of physician-diagnosed anxiety disorders was modeled jointly for the couple, the association was slightly strengthened (RR 2.03; 95% confidence interval 1.46-2.84).

CONCLUSION(S)

This prospective cohort study suggests that paternal lifetime history of physician-diagnosed anxiety disorders may be associated with an increase in the secondary sex ratio, resulting in an excess of male births.

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.

Shared risk aversion in spontaneous and induced abortion

STUDY QUESTION

Does the incidence of spontaneous abortion correlate positively over conception cohorts with the incidence of non-clinically indicated induced abortion as predicted by shared risk aversion?

SUMMARY ANSWER

We find that the number of spontaneous and non-clinically indicated induced abortions correlates in conception cohorts, suggesting that risk aversion affects both the conscious and non-conscious mechanisms that control parturition.

WHAT IS KNOWN ALREADY

Much literature speculates that natural selection conserved risk aversion because the trait enhanced Darwinian fitness. Risk aversion, moreover, supposedly influences all decisions including those that individuals can and cannot report making. We argue that these circumstances, if real, would manifest in conscious and non-conscious decisions to invest in prospective offspring, and therefore affect incidence of induced and spontaneous abortion over time.

STUDY DESIGN, SIZE, DURATION

Using data from Denmark, we test the hypothesis that monthly conception cohorts yielding unexpectedly many non-clinically indicated induced abortions also yield unexpectedly many spontaneous abortions. The 180 month test period (January 1995 through December 2009), yielded 1 351 800 gestations including 156 780 spontaneous as well as 233 280 induced abortions 9100 of which were clinically indicated.

PARTICIPANTS/MATERIALS, SETTING, METHODS

We use Box-Jenkins transfer functions to adjust the incidence of spontaneous and non-clinically indicated induced abortions for autocorrelation (including seasonality), cohort size, and fetal as well as gestational anomalies over the 180-month test period. We use cross-correlation to test our hypothesized association.

MAIN RESULTS AND THE ROLE OF CHANCE

We find a positive association between spontaneous and non-clinically indicated induced abortions. This suggests, consistent with our theory, that mothers of conception cohorts that yielded more spontaneous abortions than expected opted more frequently than expected for non-clinically indicated induced abortion.

LIMITATIONS, REASONS FOR CAUTION

Limitations of our work include that even the world's best registration system will not capture all spontaneous abortions and that results may not generalize beyond Denmark.

WIDER IMPLICATIONS OF THE FINDINGS

Our findings imply that abortion, intentional or 'spontaneous,' follows from a woman's estimate, made consciously or otherwise, of the costs and benefits of extending gestation given characteristics of the prospective offspring, likely environmental circumstances at birth, and maternal resources.

STUDY FUNDING/COMPETING INTERESTS

The Robert Wood Johnson Health and Society Scholars Program funded the research described in this manuscript. None of the authors has any conflict of interest to declare.

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.

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.

Confronting Intrasexual Rivals

The current research sheds light on a physiological mechanism potentially underlying confrontational responses to infidelity. Findings suggest that responses to infidelity threats in adulthood are shaped by hormonally mediated masculinization of the brain in utero. 2D:4D digit ratio (widely regarded as an index of prenatal testosterone exposure) moderated behavioral and endocrinological responses to infidelity threat. After an infidelity prime (but not a control prime), lower (more masculine) 2D:4D was associated with a greater tendency to approach attractive same-sex targets (intrasexual rivals) and with heightened increases in circulating testosterone, a hormone related to a variety of aggressive and confrontational behaviors.

Effects of perinatal testosterone on infant health, mother-infant interactions, and infant development

OBJECTIVE

Many researchers and health care providers have noticed male vulnerability in infant health, mother-infant interactions, and some infant cognitive development, especially among very low birth weight (VLBW) preterm infants. However, factors beyond gender that could explain these observed differences have not been clear. The purpose of this article is to review the literature on the subject and to introduce a conceptual framework relating these factors.

DISCUSSION

According to gender-difference theories, prenatal exposure to high levels of testosterone may influence infant health and mother-infant interactions by negatively affecting infant cognitive/motor/language development. We constructed a conceptual framework based on the associations among biological (perinatal testosterone), stress-related (perinatal and maternal cortisol), and developmental (infant cognitive/motor/language skills) factors. If research establishes these biological, environmental, and developmental associations in mother-VLBW preterm pairs, the results will highlight the importance of addressing gender differences in nursing research and encourage the development of nursing interventions designed to reduce stress among mothers of VLBW preterm infants, particularly male infants.

CONCLUSION

From a psychobiosocial perspective, combining biophysiological factors such as perinatal testosterone and cortisol with socioenvironmental factors such as the quality of mother-infant interactions and infant temperament may provide a broader view of gender differences in infant health and development.

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.

Preconception stress and the secondary sex ratio: a prospective cohort study

OBJECTIVE

To study the association between salivary stress biomarkers and the secondary sex ratio.

DESIGN

Prospective, longitudinal cohort study.

SETTING

Community setting in the United Kingdom.

PATIENT(S)

On discontinuation of contraception for purposes of becoming pregnant, 338 women aged 18-40 years with complete data (90%) were followed until pregnant or up to six menstrual cycles.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Secondary sex ratio.

RESULT(S)

Human chorionic gonadotropin pregnancies were detected in 207 (61%) women of whom 130 (63%) delivered singleton infants with available gender data. The adjusted odds ratio for a male birth was decreased for women in the highest quartile (AOR = 0.26; 95% confidence interval = 0.09, 0.74) of salivary cortisol relative to women in the lowest quartile during cycle 1. An inverse relation was observed between α-amylase and the 2° sex ratio, though not statistically significant.

CONCLUSION(S)

Our findings are consistent with a reversal in the 2° sex ratio with increasing preconception salivary cortisol concentrations. This relation suggests that activation of the hypothalamus-pituitary-adrenal axis may have implications in sex allocation and requires further study.