There’s a new paper out in PNAS (available here) by Ronin Institute Board member and Research Scholar Steven Orzack. It’s also been covered briefly by NPR (listen here). Steven is the founder of the Fresh Pond Research Institute, which is one of the inspirations/models for Ronin, and he played a critical role in helping to get Ronin up and running. So congratulations to him!
But also, it’s a cool paper, where they (Orzack and colleagues from a few different institutions) assembled a crap-ton of data on the sex ratio at various stages of pregnancy, which has allowed them to chart out how that sex ratio changes over time (due to differential mortality of males and females).
So why is this interesting? Well, if you know one thing about sex ratios, you know that there are equal numbers of boys and girls. In humans, sex is determined by the sex chromosomes (X and Y). If you get two X chromosomes, you’re female, but if you get an X and a Y, you’re male. At reproduction, you inherit an X from your mom, and either an X or a Y from your dad. The 50/50 sex ratio follows naturally from the fact that half of his sperm carry his X, while the other half carry his Y.
But if you know two things about sex ratios, you know that the first thing you knew about sex ratios is wrong. In humans, about 51% of the babies that are born are male. (Naturally, that is. In places where there is a lot of sex-specific abortion, like China, the fraction of boys born is higher.) That fact has been known for over a hundred years. It has also been known for a long time that near the end of pregnancy, male fetuses suffer higher mortality than female fetuses. This combination of facts led many people to conclude that at the time of conception, the sex ratio is highly skewed, as high as 56% male, and that higher rates of male mortality during pregnancy bring that ratio down to 51% by birth. Like, maybe sperm carrying Y chromosomes swim faster than sperm with X chromosomes. This is the story that you’ll find circulating around biology departments and in a lot of textbooks.
What this paper shows, however, is that at the time of conception, the ratio is probably very, very close to 50/50. But during the week of pregnancy, there is a higher male mortality rate, so that the sex ratio actually skews female. This seems to be linked to chromosomal abnormalities (like having too many or too few chromosomes), which cause mortality in males earlier than in females. Then, for the next ten to fifteen weeks, there is a higher rate of female mortality, so that the sex ratio correct itself and then becomes male biased. Here’s a figure from their paper, which shows both the timecourse of the change in sex ratio and gives a sense of the diversity of data that was integrated for the analysis:
The dip at the end results from the fact that boys tend to be born earlier than girls, so the individuals that are still fetuses start to skew female.
So what does it mean? Hard to say. There’s a lot of theory out there about the evolution of sex ratios, but much of it is hard to test without a lot of data that is hard to get, like what the timecourse of parental provisioning looks like for boys and girls. In fact, it may be impossible to get that data for the social systems relevant to human evolution (depending on how you feel about the use of contemporary hunter-gatherer populations as proxies for the human Environment of Evolutionary Adaptation).
But what it does mean is that all of that theory was trying to explain the wrong phenomenon. Now that we know what the pattern actually looks like, there are a whole lot of new mechanistic and evolutionary questions that are suddenly relevant and interesting.