Curiosity is a curse sometimes. It strikes in the most unusual circumstances and often with little warning. Some little thing will set it off and bang, you’re hooked. I’m an obstetrician, so procreative issues are constantly surfacing in my life. Environmental stressors and reproductive failure also seem to be de rigeur in the social media nowadays so there’s no escaping it. The worry may have started with animal data -animals are the easiest to study so we often look at what evidence they provide and then extrapolate. I’m thinking of those dark mysterious star-filled nights at summer camp when there is howling in the distance, and everybody huddles together with questions.
And worst of all are those important things that don’t have ready answers, or the answers have different explanations each time you look for them. Different causes. The secondary sex ratio has always been that type of enigma for me: why isn’t the ratio exactly 1:1 in humans? Well, first of all, some definitions. The primary sex ratio is the ratio between the sexes at fertilization, and the secondary sex ratio is their ratio at birth. There’s even a tertiary ratio -the sex difference in mature organisms.
In the past, the gender ratio at conception was unknowable, so the only useful ratio was the one at birth -and that seemed to favour males (1.1 males for every 1.0 female). So did that mean that male sperm somehow outswam the female ones or damaged them on the way to the egg? Did it speak to the quality of the gametes or merely suggest that to balance tertiary sex ratios (the ratio in sexually mature organisms, remember) more males were needed because, unlike females, they were less able to make it through childhood..? Until recently, as I mentioned, there was no way to measure the primary sex ratio, so it remained a mystery. Now it seems there is, and, surprise surprise, there would appear to be an equality of sexes -at conception at least: http://www.pnas.org/content/112/16/E2102.full.pdf This fascinating study tracks gendered mortality during development in the uterus. There is a theory (the Trivers-Willard hypothesis) which posits that more males are born in a favourable environment and more females in an unfavourable one because just one sex will be better at ultimate reproduction under those differing conditions.
So what conditions might effect the secondary sex ratio? Well, amongst other things, there is some evidence that major stressors may influence it. Large disasters have certainly been implicated -earthquakes, for example: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3881738/ And then, of course, there were the changes in secondary sex ratio immediately after the 9/11 World Trade Center attack in 2001… One explanation that has been offered to explain how this could occur is that males typically attain a critical fetal weight earlier than do females (the average weight of newborn males, for example, is ∼100 g greater than females) and this might exert a higher metabolic demand on mothers. So, depending on the gestational age and the extent of the stress, the mothers may be able to abort the male fetuses, but maintain the less physiologically demanding female ones. In other words, evolution would seem to have selected for those females that can regulate the sex of their offspring… Really?
That explanation seems rather contrived to me. Exactly how would the mother accomplish this feticide? And avoiding direct maternal involvement by referring it back to changes in placental function merely pushes the question back another layer. Of course, some have tried other approaches -for example citing the epigenetic environment (factors influencing the functionality of genes): http://humrep.oxfordjournals.org/content/20/9/2662.full But even when I force myself through the commentator’s words, the explanation still seems a little strained.
And yet, statistically, there does seem to be reason to believe that something is happening that relates to stress.
Of course pregnancy itself is a stress -levels of stress hormones increase as pregnancy unfolds: (http://www.jogc.ca/abstracts/full/201505_Editorial_1.pdf) -although, as the editorialist explains, ‘as a pregnant woman approaches term, environmental stress has less effect in triggering the usual response in the hypothalamic-pituitary-adrenal axis, and she becomes less responsive to the effects of stress’. Uhmm… So, pick your answer from a hat?
Well, in the rubble of destroyed answers and ever blossoming questions, what are we left with? Is there something special about violence that triggers it? Or does any stress threaten the ratio? And what constitutes a stress anyway? All imponderables, I suppose, but at least a recent article in the JOGC (Journal of Obstetrics and Gynaecology of Canada) brings it closer to home: http://www.jogc.ca/abstracts/full/201505_WomensHealth_1.pdf And in an ‘Only in Canada, eh?’ fashion it demonstrates that we, too, can participate in the secondary sex ratio debate -on our own terms, of course. I mean, who would have thought that our two referenda on Quebec secession from Canada could provoke such a response? I’m almost proud that it did –it shows how involved we are in our country. How much it matters. And how we don’t need earthquakes, either.
And maybe the slight increase in female births that the worries about the referenda caused says something about our growing appreciation of women in Canadian society as well… I live in hope. But you gotta love this stuff, eh?