Natural Gender Selection: Starving Women Seem to Give Birth to More Daughters

By Sarah Zhang | March 30, 2012 9:06 am

The disastrous economic policies of Mao’s Great Leap Forward caused the single deadliest famine in the history of the world. Between 1958 and 1961, an estimated 15 to 45 million people died of malnutrition in China. And during this period, according to a new study, a strangely high proportion of babies born were female.

China has had a long cultural tradition of favoring sons over daughters, and boys outnumber girls every year in this data from 1940 to 1980. But from one year after the famine’s beginning to two years after its end, the proportion of males drops sharply, as you can see in the graph above.

This study may bolster the Trivers-Willard hypothesis, which suggests that poor maternal condition, such as famine, would favor giving birth to more girls. Since the reproductive success of males tend to be more variable—a reproductively successful male can father many children, whereas a unfit one fathers none—girls are a “safer” evolutionary investment in risky times. The birth sex ratios of mammals such as ground squirrels and red deer follow this pattern. Lab experiments where the male blastocytes of cows survive better in glucose-rich environments identified a possible mechanism.

For reasons both obvious and fortunate, data on birth sex ratio of humans during famine is difficult to come by. Previous analyses of data from the Dutch Hunger Winter (1944-45) and the Leningrad Seige (1942) have yielded all possible inconsistent results: male biased, female biased, and no bias at all. The author of this paper suggested that those famines, as awful as they were, were too short at six or seven months to have an effect on sex ratios. The Chinese famine lasted for three years.

But this observed trend doesn’t show why girls outnumber boys in times of famine. There could confounding social reasons, especially as the data is based on a retrospective survey of 300,000 women, but it seems that the Chinese bias toward having sons is more likely to drive the ratio in the other direction. (China’s one-child policy was not enacted until 1978, so it has no bearing on the famine data either.) The study’s author hopes to follow up by looking at specific region of China that were struck by famine at different times.

[via Nature News]

Image courtesy of S. Song / Proc. R. Soc. B.

CATEGORIZED UNDER: Health & Medicine
  • Lyndell

    I think the thing missing in this discussion, and perhaps the paper, is that the impacts of the famine are also going to impact men in the population. Famine would affect both parents. Perhaps what we’re really seeing is the effect of decreased nutrition on quality/count of sperm. Sperm carrying the X chromosome and sperm carrying the Y chromosome are not identical; perhaps decreased nutrition favored survival of X chromosome sperm. We also see a female bias in children of males who are subjected to higher than normal radiation or physical forces (fighter pilots, astronauts, etc.). The emphasis of this article makes it sound as though women are responsible for the sex of the child they carry; we know that is not the case. We should be careful that we do not appear to further this old misconception.

    However, it is very interesting to consider the possibility that female blastocytes and zygotes are hardier when developing under extreme nutrition and stress conditions. I hope this study triggers further research along those lines.

  • http://misscellania.blogspot.com/ Miss Cellania

    Over the years, I’ve read studies that talk about how more males are born, but more females survive their childhoods. Also, adult men tend to die younger than adult women, so the usual gender spread is skewed at both ends of the age axis, but is usually equivalent around the age of reproduction. At least in a healthy population. Am I remembering this right?

    That may speak to females being tougher at surviving all around. During a famine, wouldn’t it make sense that even if the same number of boys and girls are conceived, that a female fetus has a better chance of surviving until birth because of this toughness? That’s the picture I get here.

  • AG

    Life is too good and produce more guys to kill each others. Life is too hard and produce more girls to help out family chores?

  • Sarah Zhang

    @Lyndell — you’re completely right that the famine will affect both parents, and we don’t know where the effect on sex ratios is actually coming from. I don’t know of any research on X:Y sperm ratios and starvation though–please share if you do–and there has been research simulating nutrition depletion of XX versus XY embryos in the womb. Sex is determined by the chromosome on the male sperm, but the female has final control over whether to accept or reject the fertilized embryo. (A surprisingly high proportion of pregnancies are terminated before women are even aware they are pregnant, possibly in part to weed out embryos undesirable for whatever reason.) My conjecture is that’s why a lot of this research has focused on females.

    Of course, that doesn’t mean we shouldn’t be looking at what happens to male sperm in starvation. You bring up a great point.

  • floodmouse

    Quote: “Lab experiments where the male blastocytes of cows survive better in glucose-rich environments identified a possible mechanism.” Translation to vernacular: “Gimme some sugar, baby.” (Mmmm, glucose. Yum.)

  • http://discovermagazine.com Iain

    Perhaps this is a trait shared by all mammals. Seems like it would be a natural selection thing to me because famine is anything but new. It only takes 1 male to fertilize many, many females and should you have many, many males and only one female you are limited to about 1 baby/yr.

  • Debra

    In my opinion there is a lot about the female body, menstruation and reproduction that isn’t fully understood. For one I believe that there is a direct link between blood sugar levels and estrogen.

  • Kevin N

    I’m with Lyndell. To often, biologists assume a phenomenon must be a result of evolutionary pressures. Much of the time, these phenomena are simply the result of poor design or conditions that were not factors during the evolutionary process. My laptop may behave differently at very high altitudes. But that’s not because the designers planned it that way. They didn’t even consider it.

  • TheCritic

    @Miss Celania

    That hypothesis does follow a logical path. However, as goes commonly with evolution, it is not the “toughest” that survive. It is the most fit. In times of famine, energy in the form of food is obviously scarce. As science knows, being male requires a higher energy expenditure in terms of development and daily needs by a significant margin. The mechanism that is more likely is that females are more fit to survive in times of famine by virtue of not needing as much energy to survive. If there is a famine and food is split equally. A 210 lbs, 75” male would never survive off 1000 calories a day whereas a 5′ tall, 100 lb female could do so fairly well. I think if it were possible to test the hypothesis, a continued famine that would last long enough to exert evolutionary pressure on the human race as a whole would eventually select for those humans that were smaller in size and weight because they needed less to survive. This would give them better fitness. Whereas toughness is an attribute only selected for when competition winners produce better fitness.

  • S. Lenox

    Boys still outnumber girls even in the times of famine, only less so. Is the normal ratio for China markedly different from the normal ratio for the world as a whole? There are all sorts of innuendos unspoken here. It could just be that the normal hanky-panky breaks down somewhat in times of famine. There are questions that have to be asked, that can’t be skirted around, if appropriate sense is to be made of this data.

  • http://discovermagazine.com Iain

    Well I suspect that in tough times more females are born because they are tougher and they have the babies.
    Proposition
    We know what the m/f ratio is in a rat breeding.
    Put some males and some females on 3/4, 2/3, 1/2, 1/3 and 1/4 necessary food value diet.
    Breed the 5 groups with normal diet rats. Also cross combine the fractional food value breeding groups and analyze.
    I figure (not calculate) that there would be a few decades of study here. Then when the well is almost dry, breed the second generations together to see if there is a lasting effect.
    Hell, with a little good prose this experiment could occupy a full career and have handsome rewards.

  • Sunny D

    Y’all need to keep in mind the scale of the percent proportion of males. It’s relatively small, and the best we can actually say is that famine can increase the number of daughters, but only by an insignificant amount.

  • James

    Women are smaller, have less muscle, and require less calories.

  • Heather

    Perhaps a fetus which is of a different sex than the mother’s would not survive as easily because the hormones are not accurately proportioned to correctly strengthen the boy. Perhaps it just makes sense that a female would be able to nourish another female with her own body? Or perhaps the female carrying the male baby can’t keep both alive and thus self preservation kicks in?

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