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I thank Dr Jongbloet for thoughtful response to our article. According to the SPrOO hypothesis, human females have a fundamental seasonal variation in ovulatory pattern, one of several factors explaining differences in sex ratio (SR). The hypothesis presumes inherent ovulatory and anovulatory seasons and suggests that secondary SR varies over the year from the femininity, or an approach to gender equality, coinciding with the zenith of birth frequency, to the excess of male births concentrating at the beginning and the end of this optimum. The question is what are these seasons in Siberia?
An analysis of our data by month yielded the seasonal pattern for SR variation (p<0.1, Edwards’ test). According to the hypothesis, the birth optimum should occur in February–March when the small trough in SR is observed. This period of gender equality corresponds to the “ovulatory” season in May–June. This trough in turn was preceded by a sharp peak in SR occurring in January and followed by the major peak in April–June. These two peaks might seem to reflect “transitional stages”.
What are the arguments against the hypothesis?
Huntington’s 1938 report of the seasonal variation in both the SR and the total number of births in seven countries, based on analysis of about 52×106 births, showed an inverse relation between the SR and the number of births.1 Just the contrary is apparent in Siberia. These two curves agree closely at least during the first half of the year. In other words, the total number of births and the number of male births vary correspondingly. This direct relation between SR and birth number seems to be a characteristic feature of the Siberian population.
An analysis of 1989 Novosibirsk census data shows that January born males and females comprise 20.8% and 24.2%, respectively, of men and women aged more than 80 years, whereas the expected proportions according to the uniform distribution would be 100/12 = 8.3%. This means that January as a month of birth and April as a month of conception are strong predictors of longevity in Siberia. However, in accordance with the hypothesis, April ought to be a month of conceptopathology, associated with “preferential fertilisation of non-optimally matured oocytes by Y-bearing sperm” and would consequently not seem to be associated with the surprisingly long span of life seen in this cohort.
In summary, while I agree that some aspects of the Siberian data are “in line with” the SPrOO hypothesis, I do not find this concept to be the most satisfying explanation of our findings. While the hypothesis has been supported by animal studies, its reliance on mechanisms such as ovopathy and differential pregnancy loss makes it difficult to establish (or refute) on the basis of studies such as our own. We remain persuaded that A Lerchl’s hypothesis of the temperature dependence of SR better explains our peak of total and male births in January.2
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