Adult sex ratio (ASR) exhibits immense variation in nature, although neither the causes nor the implications of this variation are fully understood. According to theory, the ASR is expected to ...influence sex roles and breeding systems, as the rarer sex in the population has more potential partners to mate with than the more common sex. Changes in mate choice, mating systems and parental care suggest that the ASR does influence breeding behaviour, although there is a need for more tests, especially experimental ones. In the context of breeding system evolution, the focus is currently on operational sex ratios (OSRs). We argue that the ASR plays a role of similar importance and urge researchers to study the ASR and the OSR side by side. Finally, we plead for a dynamic view of breeding system evolution with feedbacks between mating, parenting, OSR and ASR on both ecological and evolutionary time scales.
Preference for sons and smaller families and, in the case of China, a one-child policy, have contributed to missing girl births in India and China over the last few decades due to sex-selective ...abortions. Selective abortion occurs also among Indian and Chinese diaspora, but their variability and trends over time are unknown. We examined conditional sex ratio (CSR) of girl births per 1000 boy births among second or third births following earlier daughters or sons in India, China, and their diaspora in Australia, Canada, United Kingdom (UK), and United States (US) drawing upon 18.4 million birth records from census and nationally representative surveys from 1999 to 2019. Among Indian women, the CSR in 2016 for second births following a first daughter favoured boys in India (866), similar to those in diaspora in Australia (888) and Canada (882). For third births following two earlier daughters in 2016, CSRs favoured sons in Canada (520) and Australia (653) even more than in India (769). Among women in China outside the one-child restriction, CSRs in 2015 for second order births somewhat favoured more girls after a first son (1154) but more heavily favoured boys after a first daughter (561). Third-birth CSRs generally fell over time among diaspora, except among Chinese diaspora in the UK and US. In the UK, third-birth CSRs fell among Indian but not among other South Asian diasporas. Selective abortion of girls is notable among Indian diaspora, particularly at higher-order births.
Cooperative breeding is a form of breeding system where in addition to a core breeding pair, one or more usually non-breeding individuals provide offspring care. Cooperative breeding is widespread in ...birds, but its origin and maintenance in contemporary populations are debated. Although deviations in adult sex ratio (ASR, the proportion of males in the adult population) have been hypothesized to influence the occurrence of cooperative breeding because of the resulting surplus of one sex and limited availability of breeding partners, this hypothesis has not been tested across a wide range of taxa. By using data from 188 bird species and phylogenetically controlled analyses, we show that cooperatively breeding species have more male-biased ASRs than non-cooperative species. Importantly, ASR predicts helper sex ratio: in species with more male-biased ASR, helper sex ratio is also more male biased. We also show that offspring sex ratios do not predict ASRs, so that the skewed ASRs emerge during the period when individuals aim to obtain a breeding position or later during adulthood. In line with this result, we found that ASR (among both cooperatively and non-cooperatively breeding species) is inversely related to sex bias in dispersal distance, suggesting that the cost of dispersal is more severe for the further-dispersing sex. As females usually disperse further in birds, this explains the generally male-biased ASR, and in combination with benefits of philopatry for males, this probably explains why ASR is more biased in cooperatively breeding species. Taken together, our results suggest that a sex bias in helping in cooperatively breeding species relates to biased ASRs. We propose that this relationship is driven by sex-specific costs and benefits of dispersal and helping, as well as other demographic factors. Future phylogenetic comparative and experimental work is needed to establish how this relationship emerges.
This article is part of the themed issue ‘Adult sex ratios and reproductive decisions: a critical re-examination of sex differences in human and animal societies’.
The sex ratio at birth (SRB; ratio of male to female live births) imbalance in parts of the world over the past few decades is a direct consequence of sex-selective abortion, driven by the ...coexistence of son preference, readily available technology of prenatal sex determination, and fertility decline. Estimation of the degree of SRB imbalance is complicated because of unknown SRB reference levels and because of the uncertainty associated with SRB observations. There are needs for reproducible methods to construct SRB estimates with uncertainty, and to assess SRB inflation due to sex-selective abortion. We compile an extensive database from vital registration systems, censuses and surveys with 10,835 observations, and 16,602 country-years of information from 202 countries. We develop Bayesian methods for SRB estimation for all countries from 1950 to 2017. We model the SRB regional and national reference levels, the fluctuation around national reference levels, and the inflation. The estimated regional reference levels range from 1.031 (95% uncertainty interval 1.027; 1.036) in sub-Saharan Africa to 1.063 1.055; 1.072 in southeastern Asia, 1.063 1.054; 1.072 in eastern Asia, and 1.067 1.058; 1.077 in Oceania. We identify 12 countries with strong statistical evidence of SRB imbalance during 1970–2017, resulting in 23.1 19.0; 28.3 million missing female births globally. The majority of those missing female births are in China, with 11.9 8.5; 15.8 million, and in India, with 10.6 8.0; 13.6 million.
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. Mostwomen (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.
Background: Child sex ratio (CSR) is defined as the number of females consistent with thousand males in the age group of zero to six years. This serves as a valuable indicator for measuring ...socio-economic development in a particular region. Objective: To understand community perspectives on gender preference at birth and its potential influence on the decreasing Child Sex Ratio in a selected urban primary care setting in Puducherry, aiming to identify key determinants contributing to the observed declining trend. Methodology: A qualitative study was conducted among residents of the 4 wards under the care of JIPMER Urban Health Centre, Puducherry. Participants were purposively selected among mothers, family members attending well baby clinic at the centre and frontline health workers. Two FGDs involving 8-10 participants were conducted till saturation of information is achieved. Collected data was transcribed in verbatim format. Descriptive manual content analysis was conducted to draw the categories and themes. Results: The qualitative study on Child Sex Ratio revealed that there is a prevailing inclination towards expecting female children. Moreover, the study found no evidence of gender selection practices at present. Participants mentioned awareness of schemes such as “CM Cares for the empowerment of newborn girl child” as contributing factors to these attitudes and behaviors. Gender revealment is found inadvertently in government and private hospitals in the studied population. Conclusion: The study region exhibits a declining CSR, and there is no evidence of gender selection practices at present in the selected region. While improving female literacy and economic standing is essential, it alone is insufficient for enhancing the overall life prospects of female children.
Parental allocation of resources into male or female offspring and differences in the balance of offspring sexes in natural populations are central research topics in evolutionary ecology. Fisher ...(Fisher, R. A. 1930. The genetical theory of natural selection, Clarendon Press, Oxford, UK) identified frequency-dependent selection as the mechanism responsible for an equal investment in the sexes of offspring at the end of parental care. Three main theories have been proposed for explaining departures from Fisherian sex ratios in light of variation in environmental (social) and individual (maternal condition) characteristics. The Trivers–Willard model (Trivers, R., and D. Willard. 1973. Natural selection of parental ability to vary the sex ratio of offspring. Science 179:90–92) of male-biased sex allocation by mothers in the best body condition is based on the competitive ability of male offspring for future access to mates and thus superior reproduction. The local resource competition model is based on competitive interactions in matrilines, as occur in many mammal species, where producing sons reduces future intrasexual competition with daughters. A final model invokes advantages of maintaining matrilines for philopatric females, despite any increased competition among females. We used 29 yr of pedigree and demographic data to evaluate these hypotheses in the Colombian ground squirrel (Urocitellus columbianus), a semisocial species characterized by strong female philopatry. Overall, male offspring were heavier than female offspring at birth and at weaning, suggesting a higher production cost. With more local kin present, mothers in the best condition biased their offspring sex ratio in favor of males, and mothers in poor condition biased offspring sex ratio in favor of females. Without co-breeding close kin, the pattern was reversed, with mothers in the best condition producing more daughters, and mothers in poor condition producing more sons. Our results do not provide strong support for any of the single-factor models of allocation to the sexes of offspring, but rather suggest combined influences of relative maternal condition and matriline dominance on offspring sex ratio.
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