The hypothesized hormonal control of mammalian sex ratio at birth--a second update

Placental abruption, offspring sex, and birth outcomes in a large cohort of mothers

OBJECTIVE

To investigate stillbirth, neonatal, and perinatal death outcomes in pregnancies complicated by placental abruption, according to fetal sex.

METHODS

We utilized maternally linked cohort data files of singleton live births to mothers diagnosed with placental abruption during the period 1989 through 2005 (n = 10,014). Logistic regression models were employed to generate adjusted odd ratios and their 95% confidence intervals. Male babies served as the referent category.

RESULTS

The sex ratio at birth was 1.18. The overall prevalence of stillbirth, neonatal mortality, and perinatal mortality was 7.2%, 4.5%, and 11.8%, respectively. Placental abruption was less likely to occur in mothers carrying female pregnancies than mothers of male infants (adjusted odds ratio [95% confidence interval] = 0.89 [0.86-0.93]). There were no significant sex differences with regards to stillbirth, neonatal mortality, and perinatal mortality. Similar findings were observed for preterm and term infants.

CONCLUSIONS

Although a preponderance of male infants was discernable among mothers with placental abruption, no sex difference in fetal survival was observed among the offspring of the mothers affected by placental abruption.

THE CATEGORIES OF EVIDENCE RELATING TO THE HYPOTHESIS THAT MAMMALIAN SEX RATIOS AT BIRTH ARE CAUSALLY RELATED TO THE HORMONE CONCENTRATIONS OF BOTH PARENTS AROUND THE TIME OF CONCEPTION

This note categorizes the evidence for the hypothesis that mammalian offspring sex ratios (proportions male) are causally related to the hormone levels of both parents around the time of conception. Most of the evidence may be acknowledged to be correlational and observational. As such it might be suspected of having been selected; or of having been subject to other forms of bias or confounding; or, at any rate, of being inadequate as a firm basis for causal inference. However, there are other types of evidence that are not vulnerable to these types of criticism. These are from the following sources: (1) previously neglected data from Nazi Germany and Soviet Russia; (2) fulfilled predictions; (3) genetics; and (4) a network of logically (mathematically) related propositions, for some of which there is overwhelming empirical evidence. It is suggested that this variety of evidence confers greater overall credibility on the hypothesis than would be the case if all the evidence were of the same observational/correlational status. This observational/correlational evidence is tabulated to illustrate its consistency.

Could maternal testosterone levels govern mammalian sex ratio deviations?

Although maternal dominance and good condition are frequently associated with raised offspring sex ratios in mammals, the key factor may be female testosterone, which not only underpins the behavioural indicators but could also provide a pathway to a possible proximate mechanism for sex determination. By taking into account the fact that female testosterone levels rise in response to environmental stressors, it is possible to re-interpret the findings of atypical sex ratios in mammals in a way that reconciles seemingly conflicting results and reveals instead what could be a coherent, adaptive system of sex allocation in mammals.

Offspring sex ratios at birth as markers of paternal endocrine disruption

There is good evidence that paternal (and maternal) hormone levels at the time of conception are associated with offspring sex ratios (proportions male) at birth. The mechanisms underlying this association (pre- or postzygotic) are not of primary relevance here. When people are exposed to endocrine-disrupting agents, these agents may have different hormonal effects on men and women. So, if endocrine disruption is to be revealed by offspring sex ratios, it is necessary to categorize the sexes of subsequent offspring by the four possible parental mating classes, viz. exposed/unexposed mothers/fathers. In general, substantially altered sex ratios may reveal endocrine disruption, but the tiny (admittedly significant) secular meanderings of national live birth sex ratios across the 20th Century (and before) are not now readily interpretable.

A safety study of oral xanthohumol administration and its influence on fertility in Sprague Dawley rats

Xanthohumol (XN) is a prenylated chalcone, which has been shown to possess a broad range of potential cancer preventive and additional biological activities. In the present study, we have determined the subchronic 4-wk toxicity of XN and monitored its influence on fertility and development of offspring in two fertility studies. Four-week-old female Sprague Dawley (SD) rats were treated with 0.5% XN in the diet or with 1,000 mg XN/kg body weight (b.w.) per day by gavage for 28 days. No remarkable treatment-related changes in general appearance and b.w. occurred during the study. After autopsy, liver, kidney, lung, heart, stomach, and spleen were examined macroscopically and histopathologically. Relative liver weights of animals in both treatment groups were significantly reduced by 30--40% in comparison with the control group, indicating weak hepatotoxicity. Also, mammary glands of treated rats appeared less developed compared to the controls. Consequently, we investigated the influence of XN on rat reproduction. In two fertility studies, XN (100 mg/kg b.w. per day), given either for 4 wk prior to or during mating, gestation, and nursing, did not cause any adverse effects on female reproduction and the development of offspring. Noteworthy, treatment of male rats prior to mating significantly (p=0.027) increased the sex ratio of male to female offspring. Overall, lifelong treatment at a daily dose of 100 mg/kg b.w. in a two-generation study did not affect the development of SD rats.

Possible constraints on adaptive variation in sex ratio at birth in humans and other primates

There is general agreement that adaptive variation of sex ratio at birth has not been decisively demonstrated in primates (including human beings). So some workers have questioned whether it actually exists. Others have conjectured that it exists but is subject to as yet unidentified 'constraints' (factors opposing the modifying influences of selection in the phenotype). Meanwhile though most workers have called for research to reveal the proximate causes of sex ratio variation, few (if any) have directed studies toward that end. Here it is argued that hormonal action is responsible both for much adaptive and non-adaptive sex ratio variation, and for constraints on the adaptive variation. My hypothesis proposes that levels of steroid hormones (testosterone and oestrogen) of both parents around the time of conception are positively associated with offspring sex ratio (proportion male at birth) of mammals including man. Testosterone in men and oestrogen in women are also known to be positively associated with the health, attractiveness and fertility of individual human beings. However, high levels of testosterone in women are frequently associated with adverse medical conditions. It is suggested that for these reasons (and contrary to some adaptive theory) some classes of people (particularly women) in suboptimal health ("condition") produce excesses of sons. It seems that gonadal hormones are responsible for adaptive variation; and that maternal adrenal hormones are responsible for maladaptive variation. In evolutionary terms, gonadal hormones precede adrenal hormones.

Relationship between urinary estrogen levels before conception and sex ratio at birth in a primate, the gray mouse lemur

BACKGROUND

In primates, including humans, bias of sex ratio at birth has been reported to depend on maternal condition at conception. In a Malagasy primate, the gray mouse lemur, male-biased sex ratio at birth occurred in captive parous females. The male bias was previously demonstrated to be pre-conceptual and independent of the female's nutritional state. It was hypothesized to be related to changes in hormonal state at ovulation.

METHODS AND RESULTS

The relationship between urinary estradiol (E2) levels during the follicular phase until estrus and litter production (number and sex of newborns) was assessed in 91 females mated under controlled conditions. Changes in E2 levels prior to ovulation followed the typical primate pattern characterized by a gradual rise during the 10 days preceding the sharp increase at estrus. A clear decline in E2 levels occurred with ageing. Direction of the sex ratio bias was unrelated to E2 levels at ovulation time but was significantly dependent on E2 levels during the follicular phase. Reduced E2 levels prior to estrus led to male-biased litters.

CONCLUSION

This study suggests that hormonal stimulation during the follicular phase plays a role in shifting sex ratio at conception through changes in the local environment of the ova. This hypothesis deserves testing by assessing estrogen levels throughout the follicular phase in other primate species including humans.

Sex ratio in the offspring of male radiologists

RATIONALE AND OBJECTIVES

The purpose of this study was to determine if male radiologists predominantly father daughters and, if so, to investigate the association between this skewed sex ratio of offspring and radiation exposure.

MATERIALS AND METHODS

Questionnaires completed by 586 male radiologists in Japan provided data regarding the radiologist's age, length of employment in radiology, if he had ever received radiation doses higher than that recommended by the International Commission on Radiological Protection, the sex of each child fathered, and the birth date of each child.

RESULTS

As a group, male radiologists tended to father a lower proportion of boys (48.47%) compared with the control group (51.46%), and the relative risk was 1.13 (95% confidence interval [CI]: 1.00, 1.27). Offspring of highly irradiated radiologists, however, had a reduced proportion of males (34.48%), with a significantly (P = .002) increased relative risk of 2.01 (95% CI: 1.29, 3.13).

CONCLUSION

X-ray exposure may explain the reduced percentage of sons in the offspring of male radiologists.

Hormonal alterations in rheumatoid arthritis, including the effects of pregnancy

A number of hormonal abnormalities are present in RA patients. A major theme of these abnormalities seems to be that deficiencies in the production or action of gonadal (estrogens and androgens) and adrenal (corticosteroids and DHEA) hormones may be involved in regulating the onset, severity, and progression of RA. Differences in RA incidence and activity in the pregnant and postpartum periods provide the strongest support for this view. Hormonal changes during these periods clearly have the potential to exert profound effects on RA incidence and activity. The effect of pregnancy on RA activity is actually greater than the effect of some of the newer therapeutic agents. The striking increase in corticosteroids, estrogen, and progesterone during pregnancy may suppress RA onset or activity through the regulation of production or action of cytokines such as TNF alpha, IL-1, IL-6, IL-12, and IL-10. The relative adrenal- and gonadal-deficient environment of the postpartum period further supports the view that hormonal deficiencies predispose to the development or increased activity of RA. These observations justify the search for hormonal abnormalities in RA patients outside the pregnancy and postpartum periods. In particular, further studies on the period before the onset of disease are needed. Additional evidence does exist that a functional abnormality in the adrenal glands in RA patients results in dysregulation of corticosteroid and DHEA production. These abnormalities seem to be linked to aging and disease activity. It is still not established whether these abnormalities are primary or secondary, although data indicating adrenal hypofunction before the development of RA or within the first year of disease activity suggest a primary abnormality. Several hormonal abnormalities seem to be restricted by gender and age, particularly around perimenopause and menopause. These age- and gender-influenced effects may be the cause of some of the contradictory data reviewed here. Studies in the future should make greater efforts to segregate study populations by age, gender, and reproductive status. The identification of the specific hormonal abnormalities and patient populations that are at risk is important, because these factors may allow new therapeutic approaches that are less toxic than current regimens.

Effect of timing of artificial insemination on sex ratio

For a number of years, the time of insemination or mating during estrus has been believed to influence the sex ratio of offspring, with early insemination resulting in more females and late insemination, more males. Possible mechanisms of altering the sex ratio include facilitating or inhibiting the transport of either X- or Y-chromosome-bearing sperm through the reproductive tract, preferential selection of sperm at fertilization, or sex-specific death of embryos after fertilization. In livestock species, there is evidence for preferential selection of X- or Y-bearing sperm, based on the maturational state of the oocyte at fertilization. In deer and sheep, early and late insemination appears to skew the sex ratio toward females and males, respectively. In cattle, conflicting reports on the effect of time of insemination on sex ratio make the premise less clear. Many of the published studies lack adequate observations for definitive conclusions and/or are based on infrequent observations of estrus, making it difficult to assess the effect of time of insemination on sex ratio. It is likely that any effect of time of insemination on sex ratio in cattle is relatively small. Evidence is accumulating that treatments used for synchronization of estrus or ovulation in cattle may influence the sex ratio.

Serum androgen-anabolic hormones and the risk of rheumatoid arthritis

OBJECTIVE

It has been hypothesised, mainly on the basis of indirect evidence, that low serum concentrations of androgen-anabolic hormones would play a causal part in the aetiology of rheumatoid arthritis (RA).

METHODS

A case-control study was nested with a Finnish cohort of 19,072 adults who had neither arthritis nor a history of it at the baseline examination during 1973-1977. Pre-illness serum specimens for the assay of testosterone and dehydroepiandrosterone sulphate (DHEAS) were available from 116 cases who had developed RA by late 1989. Three controls per each incident case were individually matched for sex, age, and municipality.

RESULTS

The mean testosterone concentration was 1.4 nmol/l in those 84 women who developed RA and 1.4 nmol/l in their controls; the corresponding figures for DHEAS were 5.2 mumol/l and 5.5 mumol/l, respectively. Mean testosterone concentration in the 32 male cases was 26.1 nmol/l and 26.4 nmol/l in their controls; the corresponding figures for DHEAS were 11.2 mumol/l and 10.1 mumol/l, respectively. Analysis by subgroups (rheumatoid factor positive and negative disease, pre-menopausal and postmenopausal women) and by hormone distributions showed no differences.

CONCLUSION

The findings are not in line with the contention that low concentrations of testosterone and DHEAS play a part in the aetiology of RA.

Adaptive sex ratio variation in pre-industrial human (Homo sapiens) populations?

Sex allocation theory predicts that in a population with a biased operational sex ratio (OSR), parents will increase their fitness by adjusting the sex ratio of their progeny towards the rarer sex, until OSR has reached a level where the overproduction of either sex no longer increases a parent's probability of having grandchildren. Furthermore, in a monogamous mating system, a biased OSR is expected to lead to lowered mean fecundity among individuals of the more abundant sex. We studied the influence of OSR on the sex ratio of newborns and on the population birth rate using an extensive data set (n = 14,420 births) from pre-industrial (1775-1850) Finland. The overall effect of current OSR on sex ratio at birth was significant, and in the majority of the 21 parishes included in this study, more sons were produced when males were rarer than females. This suggests that humans adjusted the sex ratio of their offspring in response to the local OSR to maximize the reproductive success of their progeny. Birth rate and, presumably, also population growth rate increased when the sex ratio (males:females) among reproductive age classes approached equality. However, the strength of these patterns varied across the parishes, suggesting that factors other than OSR (e.g. socioeconomic or environmental factors may also have influenced the sex ratio at birth and the birth rate.

Sex ratio, coital rate, hormones and time of fertilization within the cycle

In this paper I shall consider the relationships between coital rates, timing of fertilization within the cycle, and sex ratios of the resulting offspring. It will be suggested that there is: (1) Analytic (mathematical) evidence for a correlation between coital rate and time of fertilization. (2) Empirical evidence for a correlation between time of fertilization and sex ratio of offspring. Whence there arises: (3) A correlation between coital rate and sex ratio secondary to correlations of both with time of fertilization. These variables will be considered in these pairs in order. Section 4 will be devoted to the magnitude of the correlation between coital rate and sex ratio. Lastly it will be noted that the data on these two variables cohere not only with that on timing of insemination, but with the overall hypothesis that parental hormone levels at conception are causally associated with sex ratios at birth.

Sex ratios are multiply determined: a reply to James

No abstract

Birth order and sibling sex ratio in two samples of Dutch gender-dysphoric homosexual males

Two studies were undertaken to confirm the previous findings that homosexual men in general tend to have a later than expected birth order and that extremely feminine homosexual men also tend to have a higher than expected proportion of brothers (i.e., a higher sibling sex ratio). Subjects in Study 1 were Dutch, adult and adolescent, biological male patients with gender dysphoria (persistent and recurrent desires to belong to the opposite sex), who were undergoing treatment with feminizing hormones. These comprised 83 patients who reported sexual attraction to other males (the homosexual group) and 58 who reported sexual attraction to females or equal attraction to males and females (the non-homosexual group). Subjects in Study 2 were Dutch adolescent male patients at another hospital. The homosexual group consisted of 21 gender-dysphoric homosexual teenagers referred to a gender identity clinic for children and adolescents. The control group were 21 adolescent males referred to the child psychiatry department of the same hospital for reasons other than gender identity disorder, homosexuality, or transvestism. These were individually matched to the homosexual subjects on age and sibship size. In both studies, the homosexual group had a significantly later average birth order than the comparison group. In Study 1, the homosexual group had a significantly elevated sibling sex ratio; this was not tested in Study 2 because of its small sample size. These studies add to the mounting evidence that late birth orders are common to all homosexual samples and that elevated sibling sex ratios are an additional characteristic of extremely feminine ones.

A re-examination of the sex ratios of families with a neurodevelopmentally disordered child

It has been claimed that mothers with certain hormonal imbalances and/or immunological disorders are more likely to give birth to male than female children and that they are more likely to give birth to a neurodevelopmentally disordered (ND) child. Liederman and Flannery (1995) reported that the sex ratio of families with an index ND child did not significantly differ from the sex ratio of families without an index ND child. The data were derived from the National Collaborative Perinatal Project (NCPP). James (in the current issue) challenged us to reconsider the standard against which we judged the sex ratio of siblings of ND children, since the 1.10 sex ratio that we obtained for the siblings of non-ND children is much higher than the sex ratio of children born during that period according to the U.S. Census Bureau. In the current paper, the NCPP data were reexamined and it was demonstrated that the high sex ratio of siblings in our non-ND sample was not due to various retention or recruitment biases. Thus, we suggest that use of the sex ratio of siblings of non-ND children was the proper standard against which to judge the sex ratio of ND families, and that our data provide no evidence to support the notion that these families are male biased.

Temperature-dependent sex determination: the interplay of steroid hormones and temperature

Sex determination is the product of coordinated gene expression. Mutational analyses have yielded great progress in our understanding of mammalian sex determination, and insight into the evolution of this sex chromosome system would be valuable. Mammals arose from turtle-like reptiles, and in many turtles the incubation temperature of the egg determines the sex of the offspring, a process known as temperature-dependent sex determination. There is mounting evidence that sex steroid hormones are the physiological equivalent of incubation temperature and serve as the proximate trigger for male and female sex determination. Temperature appears to accomplish this end by acting on genes coding for steroidogenic enzymes and sex steroid hormone receptors. The ability to manipulate sex determination in turtles both by temperature and by sex steroid hormones extends our understanding of the evolution as well as the physiology and molecular biology of sex determination.

Might patients with HLA-B27 related diseases benefit from antiandrogenic treatment?

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Potential mechanisms for sex ratio adjustment in mammals and birds

Sex ratio skews in relation to a variety of environmental or parental conditions have frequently been reported among mammals and, though less commonly, among birds. However, the adaptive significance of such sex ratio variation remains unclear. This has, in part, been attributed to the absence of a low-cost physiological mechanism for sex ratio manipulation by the parent. It is shown here that several recent findings in reproductive biology are suggestive of many potential pathways by which gonadotropins and steroid hormones could interfere with the sex ratio at birth. And these hormone levels are well-known to be influenced by many parameters which have been invoked in correlating with offspring sex ratios. Hence, it is argued that the significant, but inconsistent sex ratio biases reported in mammalian and avian populations are coherent with current knowledge on reproductive physiology in those species. However, whether such variations can be viewed at as a consequence of physiological constraint or as adaptive sex ratio adjustment, has still to be determined.

What stabilizes the sex ratio?

It is suggested that the human sex ratio at birth is stabilized only to a minor extent by the direct processes of natural selection. Instead the major factors stabilizing sex ratio seem to be behavioural (coital rates) and psychological (parental perceptions of adult sex ratios). It is suggested that parental hormone levels are (a) a consequence of perceived adult sex ratios, and (b) a cause of sex ratio in the next generation, thus providing the basis for a negative feedback process stabilizing the sex ratio.

The sex ratios of families with a neurodevelopmentally disordered child

It has been conjectured that mothers who give birth to neurodevelopmentally disordered (ND) children may have hormonal or immunological characteristics that bias them toward giving birth to male children. We examined this hypothesis in an epidemiological sample of 2,080 ND children drawn for the National Collaborative Perinatal Project (NCPP) who had one of nine kinds of NDs. No assumptions were made regarding the sex ratio of non-ND children since this could be computed from NCPP data for 11,213 families. The sex ratio of families with ND children was not significantly biased toward boys as compared to the sex ratio of families with non-ND children, provided that the index ND child was excluded from the analysis.

Embryonic growth and the evolution of the mammalian Y chromosome. I. The Y as an attractor for selfish growth factors

The fitness of a mammalian zygote is affected by its probability of implantation and of postimplantation maintenance as well as the level of transplacental and transmammary uptake of resources. As with paternally expressed imprinted genes, in a species in which females are not obligately monogamous, a Y-linked sequence that can positively alter any of the above parameters could spread in a population even if it harms the prospects of other embryos. Such a selfish Y-linked gene could act as a sex ratio distorter. In contrast to autosomal imprinted loci, the patrilineal inheritance of the Y ensures that selfish Y-linked growth-promoting genes need not evolve a means to ensure correct parent-dependent expression rules. Thus, as the conditions for both their initial evolution and spread are relatively relaxed, the mammalian Y chromosome is expected to be an attractor for growth-promoting genes. Data from mice and humans indicate that, as expected and in contrast to the Y of flies, the mammalian Y harbours growth factors, sex ratio factors and multiple foetally expressed genes. The accumulation of Y-linked genes may also be explained in terms of sexual antagonism. Sexual antagonism and the model presented here are not mutually exclusive.

The sex ratios of dyslexic children and their sibs

There is a male excess in many samples of patients with dyslexia and related conditions. Various suggestions have been made to account for this, including: (1) ascertainment bias, in that boys are more likely to be referred than girls; (2) a biological propensity of parents of dyslexic children to produce a high proportion of boys (compared with parents of controls); and (3) greater susceptibility among boys than girls to these conditions. These explanations are not mutually exclusive, and the first undoubtedly applies to some clinic samples. This paper offers equivocal evidence for the second explanation, and introduces new evidence for the third.

The current status of Weinberg's differential rule

A reanalysis of the data presented in a recent large twin study suggests that opposite-sexed (OS) pairs may be not as exhaustively tested as same-sexed (SS) pairs on genetic markers. This is contrary to an assumption I made in estimating that there are about 8 SS dizygotic pairs to every 7 OS pairs (thus impugning Weinberg's differential rule). If this assumption is false also in regard to the samples I discussed, then that estimate is unsound and Weinberg's rule is unscathed by empirical data. However, regardless of such considerations, there are strong theoretical reasons for questioning the status of Weinberg's rule. It is based on two assumptions, namely that p (the probability that a dizygotic twin zygote is male) is equal and independent for all dizygotic twin zygotes. Data are adduced here to suggest that both assumptions are false. The upshot is that, at present, without testing, we cannot know, of any given population, whether the rule holds or not. Otherwise, though Weinberg's rule may be useful (like Hellin's law) as a rule-of-thumb, it cannot be assumed as a basis for serious scientific argument.