The variances of the distributions of the combinations of the sexes within mammalian litters: notes to mark the centenary of the problem

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.

Studies of human sex ratios at birth may lead to the understanding of several forms of pathology

This article deals with the problem of the causes of the variation of sex ratio (proportion male) at birth. This problem is common to a number of areas in biology and medicine, for example, obstetrics, neurology/psychiatry, parasitology, virology, oncology, and teratology. It is established that there are signifi cantly biased, but unexplained, sex ratios in each of these fields. Yet workers in them (with the possible exception of virology) have regarded the problem as a minor loose end, irrelevant to the field's major problems. However, as far as I know, no one has previously noted that unexplained biased sex ratios occur, and thus pose (perhaps similar) problems, in all these fields. Here it is suggested that similar sorts of solutions apply in each. Further research is proposed for testing each solution. If the argument here is substantially correct across this range of topics, it may lead to an improved understanding not only of sex ratio but also of some of the pathologies in these specialties.

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.

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.