Apr 28 2008
Researchers from the universities of Exeter and Oxford in England have published a study that they claim shows that a woman’s diet around the time of conception can influence the sex of their child. At first glance this claim sounds implausible, since in mammals the male sperm entirely determines the sex of a child, not the female egg. But the story is more complicated than it at first may appear.
The study involved having 740 women who were trying to get pregnant for the first time keep track of their daily diet. What they found is that those women who had a higher calorie diet overall, and those who ate at least one bowl of breakfast cereal per day, had a 24% greater chance of having a boy than those who skipped breakfast or consumed lower total calories.
What this type of study shows is a potential correlation. Before we can conclude that the correlation is reliable the study should be replicated. Also, correlation does not equal causation – it may be due to causation, but we cannot be sure. There may be some other factor involved that was not measured or controlled for in the study. So, in addition to replication, the hypothesis that diet can affect the sex of a child should be tested prospectively. This means that women who are trying to get pregnant can be randomized to various diets and then followed prospectively to see if there is any difference in the chance of conceiving a boy or a girl.
What about the plausibility of the claim? As I said, the female eggs always contain an X chromosome. The male sperm are 50% X and 50%Y. If an X sperm fertilizes the egg, the resultant child is female, and if a Y sperm wins the race, then the child will be male. So how can the mother’s diet affect the outcome?
Well, there are a two main hypotheses: either the female environment is more friendly to Y sperm, giving them an advantage, or it is more friendly to XY embryos, allowing them to survive better. There is evidence to suggest that the latter case may be true. In vitro, XY embryos tend to survive better in high glucose environments. Women who skip breakfast may have low blood sugar, which may in turn lower the survival of male embryos.
There is also some indirect evidence from animal husbandry for a dietary effect on male births. Well fed animals are more likely to give birth to males than females. This observation does not necessarily translate to humans, but it shows that such an effect is plausible.
Why would this be the case from an evolutionary point of view? Well, first, we must listen to the warning of Stephen J. Gould against hyperadaptationalism. We do not necessarily need to find a specific evolutionary advantage for every single trait. Many traits may be epiphenomena – they occur for no particular reason, or as a side consequence of some other trait that was selected for. So it may be that male sperm of male embryos do better when women have a superior diet, but for some idiosyncratic reason not specifically selected for.
However, it may also be that there is an advantage to birthing slightly more males when food is plentiful, and fewer males (or more females) when food is scarce. When it comes to reproduction, and slight statistical advantage will be subject to direct and powerful Darwinian selective pressures. So it is plausible from this perspective that there would be mechanisms to tweak the male-female ratio in response to the environmental situation.
This study is an excellent example of science at work. The initial study was well designed and executed but has limitations on what it can actually tell us. Scientists are in the process of debating the merits and implications of the study – but all agree that follow up experiments are necessary to answer the questions raised by these data. Meanwhile, there is also sharp debate about the plausibility and possible mechanisms of such an effect in the context of multiple disciplines within biology. And in the end, evidence will rule the day.
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