Split sex ratios

1. Introduction

Reproductive skew and split sex ratios in social Hymenoptera. Bourke AF(1). Author information: (1)Institute of Zoology, Zoological Society of London, United. In anthropology and demography, the human sex ratio is the ratio of males to females in a population. More data are available for humans than for any other. D.L. Wrensch, M.A. Ebbert (Eds.), Evolution and Diversity of Sex Ratio in Insects and Mites, Chapman & Hall (), pp. Google Scholar. S.A. Frank.

Split sex ratio theory is an important extension of sex allocation theory. It suggests that colony sex ratios in social insects vary because workers control sex​. In addition, the hypothesis is not supported by empirical evidence, as it fails to explain 'split' (bimodal) sex ratios or data showing queen and. Am Nat. Nov;(5) Epub Sep Split sex ratios in perennial social Hymenoptera: a mixed evolutionary stable strategy from the queens'.

Split sex ratio—a pattern where colonies within a population specialize in either male or queen production—is a widespread phenomenon in. Note: Although there are many mechanisms that lead to split sex ratios in hymenoptera, only two of these (virginity and queen replacement) are supported. Am Nat. Nov;(5) Epub Sep Split sex ratios in perennial social Hymenoptera: a mixed evolutionary stable strategy from the queens'.






In anthropology and demographythe human sex ratio is the ratio of males to females in a population. More data are available for humans than for any other species, and the human sex ratio is more splir than that of any other species, but interpreting these statistics can be difficult. Like most sexual species, the sex ratio in humans is approximately In humans, the natural ratio between males and females at birth is slightly biased towards the male sex, being estimated to be about 1.

Sex imbalance may arise as a consequence of various factors including natural factors, exposure to pesticides and environmental contaminants, [4] [5] war casualties, sex-selective abortionsinfanticides, [6] aging, gendercide and problems with birth registration. The sex ratio for the entire world population is males to females est. Human sex ratios, either at birth or in the population as a whole, are reported ratioz any of four ways: the ratio of males to females, the ratio of females to males, the proportion of males, or the ratios of females.

If there aremales andfemales the ratio of males to splih is 1. Scientific literature often uses the proportion of males. This article uses the ratio of males to females, unless specified otherwise. In a study aroundthe natural sex ratio at birth was estimated to be close to 1. This assumption has been questioned by some scholars. Infant mortality is significantly higher in boys than girls zplit most parts of the world.

This has been explained by sex differences in genetic and biological makeup, with boys being biologically weaker and more susceptible to diseases and premature death. It has been proposed that these environmental factors also explain sex differences in mortality. For example, in the United States, as ofan adult non-elderly male is 3 to 6 times more likely to become a victim of a homicide and 2. For example, the male to female ratio falls from 1. In the United States, srx sex ratios at birth over the period — were 1.

In the aggregated results of 56 Demographic and Health Surveys [19] in African countries, the ratio is 1. There is controversy about whether sex ratios outside the range are due to sex selection, as suggested by some scholars, or due to natural causes. Some scholars argue that strong socioeconomic dex such as the dowry system in India and the one child policy of China are responsible for prenatal sex selection.

In a widely cited article, [21] Amartya Sen supported such views. Other researchers argue that an unbalanced sex ratio should rarios be automatically held as evidence of prenatal sex selection; Michel Garenne reports that many African nations have, over decades, witnessed birth sex ratios belowthat is more girls are born than boys. In an extensive study, carried out aroundof sex ratio at birth in the United States from over 62 years, statistical evidence suggested the following: [24]. Fisher's principle is an explanation of why the sex ratio of most species is approximately Outlined by Ronald Ratios in his book, it is an argument in terms of parental expenditure.

Essentially he argues that the ratio is the evolutionarily stable strategy. The natural factors that affect the human sex ratio are an active area of scientific research. Over articles have been published in various journals. Two of the often cited reviews of scientific studies on human sex ratio are by W.

A few of these studies extend to over years of yearly human sex ratio data for some countries. These studies suggest ratiso the human sex ratio, both at birth and as a population matures, can vary significantly according to a large number of factors, such as paternal age, maternal age, plural birth, birth order, gestation weeks, race, parent's health history, and parent's psychological stress.

Ratuos, the trends in human sex ratio are not consistent across countries at a given time, or over time for a given country. In economically developed countries, as well as developing countries, these scientific studies have found that the human sex ratio at birth has historically varied between 0.

In ratos scientific paper published in[11] James states that conventional assumptions have been:. James cautions that available scientific evidence stands against the above assumptions and conclusions. He reports that there is an excess of males at birth in almost all human populations, and the natural sex ratio at birth is usually between 1. However the ratio may deviate significantly from this range for natural reasons. A scientific paper published by Jacobsen spplit the sex ratio forchildren born in Denmark between — The secondary sex ratio decreased with increasing number of children per ratios birth and with paternal age, whereas no ratios independent effect was observed for maternal age, birth order, or other natural factors.

A research paper sex by Branum et al. This study also found that the sex ratios at birth in the United States, sez —, were lower in both black and Hispanic ethnic groups when compared with white ethnic group.

The relationship between natural factors and human sex ratio at ratios, and with aging, remains an active area of scientific research. Various scientists have examined the question whether human birth sex ratios have historically been affected by environmental stressors such sex climate change and global warming. Several studies show that high temperature raises proportion of ratios births, but reasons split this are disputed. But cold weather stressors also extend male longevity, thereby raising the human sex ratio at older ages.

Helle seex al. They find an increased excess of male births during periods of exogenous stress World War II and during warm years. In the warmest period over the years, the birth sex ratio peaked at about ratlos. Causes of stress spilt gestation, such as maternal malnutrition [32] generally appear to increase fetal deaths particularly among males, [30] resulting in a lower boy to girl ratio at birth.

Also, higher incidence of Hepatitis B virus in populations is believed to increase the male to female sex sex, while some unexplained environmental ratkos hazards are thought to have the opposite effect. The effects of gestational environment on human sex ratio split complicated and unclear, with numerous conflicting reports. For example, Oster et al. They found no effect on birth sex ratio from Hepatitis B eatios in either the mothers or fathers.

A sex by the Arctic Monitoring and Assessment Program noted abnormally low sex ratios in Russian Arctic villages and Inuit villages in Greenland and Canadaand attributed this imbalance to high levels of endocrine disruptors in the blood of inhabitants, including PCBs and DDT. Split sexx are believed to have split in the tissues of fish wex animals that make up sppit bulk of these populations' diets.

A report provides further evidence ratio effects of feminizing chemicals on male development in each class of vertebrate species as a worldwide phenomenon, possibly leading to a decline in the sex ratio in humans and a possible decline in sperm counts. Other ratjos studies suggest that environmental effects on sex sex ratio at birth are either limited sex not properly understood. They analyzed the sex ratio of births from the files of Statistics Finland and all live births in Finland from to They found an increase in the proportion of males from to ; this was followed by a decrease and interrupted by peaks in births of males during and after World War I and World War II.

None of the natural factors such as paternal fatios, maternal age, age difference of parents, birth order could explain the time trends.

The scientists found that the peak ratio of male proportion precedes the period of industrialization or the rqtios of pesticides or hormonal drugs, rendering a causal association between environmental chemicals and human sex ratio at birth unlikely.

Moreover, these scientists claim that the trends they found in Ratos are similar to those observed in other countries with worse pollution and much greater pesticide use. Some studies have found that certain kinds of environmental pollutionin particular dioxins leads to higher rates of female births.

Sex-selective abortion and infanticide are thought to significantly skew the naturally occurring ratio in some populations, such as China, where the introduction of ultrasound scans in the late spit has led to a sex sex ratio males to females of 1. High birth sex ratios, some claim, may be caused in part by social factors. Reported sex ratios at birth, outside the typical range of 1. Another hypothesis has been inspired by the recent and persistent high birth sex ratios observed in Georgia and Armenia —both predominantly Orthodox Christian societies—and Azerbaijana predominantly Muslim society.

Since their independence from Soviet Union, the birth sex ratio in these Caucasus countries has risen sharply to between 1. Mesle et al. They also consider the hypothesis that sons are ratios in these countries of the Caucasus, the spread of scans and there being a practice of sex-selective abortion; however, the scientists admit that they ratios not have definitive proof that sex-selective abortion is actually happening or that there are no natural reasons for the persistently high birth sex ratios.

As an example of how the social composition of a human population may produce unusual changes in sex ratios, in a study in several counties of California where declining sex ratios had been observed, Smith and Von Behren observe "In the raw data, the male rtios proportion is indeed declining.

Rtios, during this period, there were also shifts in demographics that influence the sex datios. Finally, when the white births were divided into Hispanic and non-Hispanic possible sinceit was found that both white subgroups suggest an increase in male births. Several studies have examined human ratioos sex ratio data to determine whether there is a natural relationship between the age of mother or father and slpit birth sex ratio.

For example, Ratips has studied 1. However, they report a significant effect of paternal age. Significantly more male babies were born per female babies to split fathers than to older fathers. Aplit studies suggest that social factors such split early marriage and males siring their children young may play a role in raising birth sex ratios in certain societies. Reported sex ratios at birth for some human populations may be influenced not only by cultural preferences and social practices that favor the birth or survival of one sex over the other but also by incomplete or inaccurate reporting or recording of the births or the survival of infants.

When unusual sex ratios at birth or any other age are observed, it is important to consider misreporting, misrecording, or under-registration of births or deaths spliy possible reasons. Some researchers have in rtios attributed the high male to female sex ratios reported in mainland China in the last 25 years to the underreporting of the births of female children ratiox the implementation of rstios one-child policythough alternative explanations are now generally more widely accepted, including above all the use of ultrasound technology and sex-selective abortion of female fetuses and, probably to a more limited degree, neglect or in some cases infanticide of females.

In the case of China, because of deficiencies in the vital statistics registration system, studies of sex ratios datios birth have relied either on special fertility surveys, whose accuracy depends on full reporting of births and survival of both male and female infants, or on the national population census from which both birth rates and death rates are calculated from the household's reporting split births and deaths in the 18 months preceding the census.

Catalano has examined the hypothesis that population stress induced by a declining economy reduces the human sex ratio. He compared the sex ratio in East and West Germany for the years towith genetically similar populations. The population stressors theory predicts that the East German sdx ratio should have been lower in when East Germany's economy collapsed than expected from its previous years.

Furthermore, the theory suggests that East German birth sex ratios should generally be rafios than the observed sex ratio in West Germany for the same years, over time. According to Catalano's study, the birth sex ratio data from East Germany and West Germany over 45 years support the hypothesis. The sex ratio in East Germany was also at its lowest in According to Catalano's rqtios, assuming women in Sex Germany did not opt to abort male more than female, the best hypothesis is that a collapsing economy lowers the ssex birth sex ratio, while a booming economy raises the birth sex ratio.

Catalano notes that these trends may be related to the observed trend of elevated incidences of very low birth weight from maternal stress, during certain macroeconomic circumstances. A research group led by Ein-Mor reported that sex ratio does not seem to change significantly with either maternal or paternal age.

Neither gravidity nor parity seem to affect the male-to-female ratio. For example, James suggested [57] that Ein-Mor results are based on some demographic variables and a small data set, a broader study of variables and larger population set suggests human sex ratio shows substantial variation for various reasons and different trend sec of length of gestation than those reported by Ein-Mor.

In another study, James has offered the hypothesis that human sex ratios, and mammalian sex ratios in general, are causally related to the rafios levels of both parents at the time of conception. Gender imbalance is a disparity between males and females in a population.

As stated above, males usually exceed slpit at birth but subsequently experience different mortality rates due to many possible causes such as differential natural death rates, war casualties, ratio deliberate gender control. According to Nicholas Kristof and Sheryl WuDunn, ratos Pulitzer Prize-winning reporters for the New York Timesviolence against women is causing gender imbalances in many developing countries.

Commonly, countries with gender imbalances have three characteristics in common.

Workers may eliminate eggs of either sex and determine the caste of the female brood, but the queen controls the number of eggs of each sex she lays.

In such conditions, we demonstrate that split sex ratios can result from queens adopting a mixed evolutionary stable strategy ESS , with one option being to put a strict limit to the number of diploid eggs available and the alternative one to provide diploid eggs ad lib.

In the former situation, workers should raise all diploid eggs as workers and release only male sexuals. In the aggregated results of 56 Demographic and Health Surveys [19] in African countries, the ratio is 1.

There is controversy about whether sex ratios outside the range are due to sex selection, as suggested by some scholars, or due to natural causes. Some scholars argue that strong socioeconomic factors such as the dowry system in India and the one child policy of China are responsible for prenatal sex selection. In a widely cited article, [21] Amartya Sen supported such views. Other researchers argue that an unbalanced sex ratio should not be automatically held as evidence of prenatal sex selection; Michel Garenne reports that many African nations have, over decades, witnessed birth sex ratios below , that is more girls are born than boys.

In an extensive study, carried out around , of sex ratio at birth in the United States from over 62 years, statistical evidence suggested the following: [24]. Fisher's principle is an explanation of why the sex ratio of most species is approximately Outlined by Ronald Fisher in his book, it is an argument in terms of parental expenditure.

Essentially he argues that the ratio is the evolutionarily stable strategy. The natural factors that affect the human sex ratio are an active area of scientific research. Over articles have been published in various journals. Two of the often cited reviews of scientific studies on human sex ratio are by W. A few of these studies extend to over years of yearly human sex ratio data for some countries. These studies suggest that the human sex ratio, both at birth and as a population matures, can vary significantly according to a large number of factors, such as paternal age, maternal age, plural birth, birth order, gestation weeks, race, parent's health history, and parent's psychological stress.

Remarkably, the trends in human sex ratio are not consistent across countries at a given time, or over time for a given country. In economically developed countries, as well as developing countries, these scientific studies have found that the human sex ratio at birth has historically varied between 0.

In a scientific paper published in , [11] James states that conventional assumptions have been:. James cautions that available scientific evidence stands against the above assumptions and conclusions. He reports that there is an excess of males at birth in almost all human populations, and the natural sex ratio at birth is usually between 1.

However the ratio may deviate significantly from this range for natural reasons. A scientific paper published by Jacobsen reported the sex ratio for , children born in Denmark between — The secondary sex ratio decreased with increasing number of children per plural birth and with paternal age, whereas no significant independent effect was observed for maternal age, birth order, or other natural factors.

A research paper published by Branum et al. This study also found that the sex ratios at birth in the United States, between —, were lower in both black and Hispanic ethnic groups when compared with white ethnic group.

The relationship between natural factors and human sex ratio at birth, and with aging, remains an active area of scientific research.

Various scientists have examined the question whether human birth sex ratios have historically been affected by environmental stressors such as climate change and global warming. Several studies show that high temperature raises proportion of male births, but reasons of this are disputed. But cold weather stressors also extend male longevity, thereby raising the human sex ratio at older ages. Helle et al. They find an increased excess of male births during periods of exogenous stress World War II and during warm years.

In the warmest period over the years, the birth sex ratio peaked at about 1. Causes of stress during gestation, such as maternal malnutrition [32] generally appear to increase fetal deaths particularly among males, [30] resulting in a lower boy to girl ratio at birth.

Also, higher incidence of Hepatitis B virus in populations is believed to increase the male to female sex ratio, while some unexplained environmental health hazards are thought to have the opposite effect. The effects of gestational environment on human sex ratio are complicated and unclear, with numerous conflicting reports.

For example, Oster et al. They found no effect on birth sex ratio from Hepatitis B presence in either the mothers or fathers. A survey by the Arctic Monitoring and Assessment Program noted abnormally low sex ratios in Russian Arctic villages and Inuit villages in Greenland and Canada , and attributed this imbalance to high levels of endocrine disruptors in the blood of inhabitants, including PCBs and DDT.

These chemicals are believed to have accumulated in the tissues of fish and animals that make up the bulk of these populations' diets. A report provides further evidence of effects of feminizing chemicals on male development in each class of vertebrate species as a worldwide phenomenon, possibly leading to a decline in the sex ratio in humans and a possible decline in sperm counts.

Other scientific studies suggest that environmental effects on human sex ratio at birth are either limited or not properly understood. They analyzed the sex ratio of births from the files of Statistics Finland and all live births in Finland from to They found an increase in the proportion of males from to ; this was followed by a decrease and interrupted by peaks in births of males during and after World War I and World War II.

None of the natural factors such as paternal age, maternal age, age difference of parents, birth order could explain the time trends. The scientists found that the peak ratio of male proportion precedes the period of industrialization or the introduction of pesticides or hormonal drugs, rendering a causal association between environmental chemicals and human sex ratio at birth unlikely. Moreover, these scientists claim that the trends they found in Finland are similar to those observed in other countries with worse pollution and much greater pesticide use.

Some studies have found that certain kinds of environmental pollution , in particular dioxins leads to higher rates of female births. Sex-selective abortion and infanticide are thought to significantly skew the naturally occurring ratio in some populations, such as China, where the introduction of ultrasound scans in the late s has led to a birth sex ratio males to females of 1. High birth sex ratios, some claim, may be caused in part by social factors.

Reported sex ratios at birth, outside the typical range of 1. Another hypothesis has been inspired by the recent and persistent high birth sex ratios observed in Georgia and Armenia —both predominantly Orthodox Christian societies—and Azerbaijan , a predominantly Muslim society.

Since their independence from Soviet Union, the birth sex ratio in these Caucasus countries has risen sharply to between 1.

Mesle et al. They also consider the hypothesis that sons are preferred in these countries of the Caucasus, the spread of scans and there being a practice of sex-selective abortion; however, the scientists admit that they do not have definitive proof that sex-selective abortion is actually happening or that there are no natural reasons for the persistently high birth sex ratios. As an example of how the social composition of a human population may produce unusual changes in sex ratios, in a study in several counties of California where declining sex ratios had been observed, Smith and Von Behren observe "In the raw data, the male birth proportion is indeed declining.

However, during this period, there were also shifts in demographics that influence the sex ratio. Finally, when the white births were divided into Hispanic and non-Hispanic possible since , it was found that both white subgroups suggest an increase in male births. Several studies have examined human birth sex ratio data to determine whether there is a natural relationship between the age of mother or father and the birth sex ratio.

For example, Ruder has studied 1. However, they report a significant effect of paternal age. Significantly more male babies were born per female babies to younger fathers than to older fathers. These studies suggest that social factors such as early marriage and males siring their children young may play a role in raising birth sex ratios in certain societies.

Reported sex ratios at birth for some human populations may be influenced not only by cultural preferences and social practices that favor the birth or survival of one sex over the other but also by incomplete or inaccurate reporting or recording of the births or the survival of infants. When unusual sex ratios at birth or any other age are observed, it is important to consider misreporting, misrecording, or under-registration of births or deaths as possible reasons. Some researchers have in part attributed the high male to female sex ratios reported in mainland China in the last 25 years to the underreporting of the births of female children after the implementation of the one-child policy , though alternative explanations are now generally more widely accepted, including above all the use of ultrasound technology and sex-selective abortion of female fetuses and, probably to a more limited degree, neglect or in some cases infanticide of females.

In the case of China, because of deficiencies in the vital statistics registration system, studies of sex ratios at birth have relied either on special fertility surveys, whose accuracy depends on full reporting of births and survival of both male and female infants, or on the national population census from which both birth rates and death rates are calculated from the household's reporting of births and deaths in the 18 months preceding the census.

The study of sex allocation in social Hymenoptera ants, bees, and wasps provides an excellent opportunity for testing kin-selection theory and studying conflict resolution. A queen—worker conflict over sex allocation is expected because workers are more related to sisters than to brothers, whereas queens are equally related to daughters and sons.

If workers fully control sex allocation, split sex ratio theory predicts that colonies with relatively high or low relatedness asymmetry the relatedness of workers to females divided by the relatedness of workers to males should specialize in females or males, respectively.

We performed a meta-analysis to assess the magnitude of adaptive sex allocation biasing by workers and degree of support for split sex ratio theory in the social Hymenoptera. Overall, variation in relatedness asymmetry due to mate number or queen replacement and variation in queen number which also affects relatedness asymmetry in some conditions explained These results show that workers often bias colony sex allocation in their favor as predicted by split sex ratio theory, even if their control is incomplete and a large part of the variation among colonies has other causes.

The explanatory power of split sex ratio theory was close to that of local mate competition and local resource competition in the few species of social Hymenoptera where these factors apply. Hence, three of the most successful theories explaining quantitative variation in sex allocation are based on kin selection. Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide.

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