You are going to read an article about human evolution. For each question, choose the answer which you think fits best according to the text.
When the going gets tough, have daughters
In most species, people included, males and females exist in more or less equal numbers. There is a good reason for this. Suppose females were rare. Lots of males would fail to reproduce. So a genetic mutation that caused parents to have more daughters than sons would be rewarded with extra grandchildren. Thus it would spread until it ceased to bring further advantage, in other words when the sexes were equal in number. Mutatis mutandis, the same argument applies if it is males who are rare. But what applies in general, does not always apply in particular. There are circumstances when it might behove an individual mother to give birth preferentially to sons or to daughters. In a paper just published in the Proceedings of the Royal Society, Mhairi Gibson and Ruth Mace, of University College, London, outline such a case in people.
The mothers concerned are Ethiopian. Some are malnourished. Those that are, seem more likely to give birth to daughters than sons. Dr Gibson and Dr Mace used two measures to assess how well-fed the mothers were. The body-mass index measures fat and muscle, but cannot distinguish between them. The mid-upper-arm muscle-area index is able to make the distinction. In both cases, there was a clear correlation between how malnourished a woman was and how likely her most recent child was to have been a girl. In the case of the muscle-area index, the best-fed women were more than twice as likely to have had a son recently than the worst-fed.
There are two possible explanations for this. One is that the cost to the mother of bearing sons is greater than that of bearing daughters. That is suggested by the fact that women who have borne only sons have shorter lives than those who have borne only daughters. A mother who is well nourished probably tolerates such costs better than one who is not. Better, then, for a starving woman to abort a male fetus early and hope for better times, or for a daughter next time round.
The other explanation is not the prospects of the mother, but those of the child. In most species, people included, the reproductive output of males is more variable than that of females. Successful males have lots of offspring, unsuccessful ones few, or even none. A female’s output is likely to be closer to the average. Since both health and height contribute to male success, and both are reduced by gestating in the womb of a malnourished woman, there may be little point, in terms of grandchildren accrued, to such a woman giving birth to a son.
These explanations are not, of course, mutually exclusive. Indeed, if both were true they could add up to a very powerful effect. Exactly what, in fact, Dr Gibson and Dr Mace observed.
(adapted from The Economist)