Also in this week's column: Why do I have an extreme fear of needles? Does TV watching in childhood trigger autism? Is it true that fewer boy babies are born in hard times? Asked by Jack O'Connor of Dublin, Ireland The sex ratio of boy and girl births is affected by severe stressors such as earthquakes, tsunamis, …
Deary, deary me...
Quoth the good doctor: "The theory behind this is that natural selection favours female births when times are hard because, on average, females have a better chance of mating successfully than males. This principle seems to be followed in animals."
Er... what on earth are you talking about? You don't think that, just maybe, it's because it's the females that give birth and thus bolster the population? One male human can impregnate a very large number of females whereas if a large number of males mate with a single female in one breeding cycle there's no advantage whatsoever in terms of additional offspring. Therefore, if a population suffers a sudden drop the ideal scenario is lots of females and only just enough males to impregnate them all.
It's absolutely not about "females having a better chance of mating successfully". A successful mating - i.e. one in which young are conceived and born live - is successful for both the male and the female in the partnership. If a male mates "unsuccessfully" with a female, logically the female must also have mated unsuccessfully! Whether or not a particular gender is more likely to have the *opportunity* to mate very much depends on the gender ratio, so the answer quoted above is either tautological or just plain daft depending on the situation that appertained before the drop in relative supply of males.
In many species, all males are good for is hanging around consuming resources until their gametes are needed for reproduction... This is particularly true of certain primates, where a limited number of males are required for protection and reproduction and the females operate a "creche" system and do most of the hard work. There are far more males in most mammal populations than would seem, on the face of it, to be required. This is almost certainly due to the advantages of maintaining genetic diversity.
I await the inevitable parallels being drawn... ;-)
Yes, the culled cohort theory explains the drop in male births - up to a point - but doesn't necessarily explain the increase in female births, which don't just increase as a proportion of total live births, but also numerically in relation to the number of females births *before* the "shock event" according to several sources.