Reply: sex ratio of births conceived during wartime

Branko Zorn

Andrology Centre, Reproductive Unit, University Medical Centre, Slajmerjeva 3, 1000 Ljubljana, Slovenia e-mail: branko.zorn{at}kclj.si

Dear Sir,

The implications of ‘proximate’ or accessory and ‘ultimate’ or predominant causes in the explanation of sex ratio changes after wars as proposed by Bisioli might be of interest. The ‘ultimate’ point of view supposes a natural selection to penalize any deviation from the 1:1 ratio. Though this hypothesis has been mentioned by many authors, it is difficult to test it because of insufficient scientific support. Moreover, it cannot be used as a general rule, because it is not applicable in all cases.

Although the advocates of the theory believe that the adaptive equilibrium compensates for the loss of males after wars is a general phenomenon, it is beyond understanding why in certain European countries (Italy, Spain) no changes in sex ratio were observed during and after World Wars (Graffelman and Hoekstra, 2000Go). Moreover, the increase in males during the war stopped at the end of the war, far before the number of males was restored. Because of theses exceptions, the ‘ultimate’ point of view cannot be presented as a general theory, and its role, if present, is reduced.

When comparing sex ratio changes after World War I, the occurrence of less important changes in the Netherlands in comparison with Germany was noted. One explanation might be that during the World War I, the Netherlands did not participate in the conflict. This absence of less important changes was also found in other neutral countries (van den Broek, 1997Go). Furthermore, as indicated above, the rapid increase in sex ratio during a war is followed by a decline just after the war. The most probable common factor that can explain both sex ratio changes related to war, occurring and ending with it, and a gradation of these changes related to the war intensity, might be psychological stress, regardless of birth order, race and age of the parents, the factors supposed to influence sex ratio.

However, a more scientific approach, based on molecular biology, can further elucidate the enigma of sex ratio changes. Since the introduction of IVF techniques our knowledge of human physiology has increased considerably. Why can this not be the case with sex ratio changes?

After extended culture and transfer of embryos at the blastocyst stage, IVF researchers found the sex ratio to be in favour of males (Menezo et al., 1999Go), but did not have a reliable explanation. Moreover, in the general population it has been found that pregnant women carrying boys have a 10% higher energy intake than those carrying girls (Tamimi et al., 2003Go). What is the mechanism common to these two phenomena? The answer to this question might perhaps be found in the work of Jimenez et al. (2003)Go.

Hyperglycaemia is a factor in apoptosis. Under hyperglycaemic conditions, Jimenez et al. found that female bovine blastocysts produced significantly higher amounts of X-linked inhibitor of apoptosis, which resulted in a significant reduction of the number of males born. Moreover, after culturing in hyperglycaemic media, female blastocysts implanted better than male.

In humans, psychological stress is related to endocrine dysregulation with modifications of glucose disposal (Seematter et al., 2002Go).

In conclusion, if there is only one mechanism generating differences in sex, why were contradictory events observed during wars: an increase in sex ratio during the World Wars and decrease after a 10-day war? The explanation for this inverse phenomenon is probably, as suggested by Bisioli, in the duration of the conflicts, or in other words, in the intensity of induced stress. Acute stress is different from chronic stress; the changes of glycaemia and weight gain may be quite opposing.

References

Graffelman J and Hoekstra RF (2000) A statistical analysis of the effect of warfare on the human secondary sex ratio. Hum Biol 72,433–445.[ISI][Medline]

Jimenez A, Madrid-Bury N, Fernandez R, Perez-Garnelo S, Moreira P, Pintado B, de la Fuente J and Gutierrez-Adan A (2003) Hyperglycemia-induced apoptosis affects sex ratio of bovine and murine preimplantation embryos. Mol Reprod Dev 65,180–187.[CrossRef][ISI][Medline]

Menezo YJR, Chouteau J, Torello MJ, Girard A and Veiga A.(1999) Birth weight and sex ratio after transfer at the blastocyst stage in humans. Fertil Steril 72,221–224.[CrossRef][ISI][Medline]

Seematter G, Battilana P and Tappy L (2002) Effects of dexamethasone on the metabolite responses to mental stress in humans. Clin Physiol Funct Imaging 22,139–144.[CrossRef][ISI][Medline]

Tamimi RM, Lagiou P, Mucci LA, Hsieh C-C, Adami H-O and Trichopoulos D (2003) Average energy intake among pregnant women carrying a boy compared with a girl. BMJ 326,1245–1246.[Free Full Text]

van den Broek JM (1997) Change in male proportion among newborn infants. Lancet 349,805.