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Shifting sex ratio may be caused by PCBs, but how?
More girls than boys are being born in certain Inuit villages in the Arctic, and scientists say man-made chemicals are to blame. Women were tested for the level of PCBs (pervasive hormone mimicking chemicals) in their bodies, and the results showed that women with extremely high levels of the chemicals were more likely to give birth to girls. High levels of hormone mimics, the scientists said, “were capable of triggering changes in the sex of unborn children in the first three weeks of gestation.”
A little background: PCBs are endocrine disruptors, which become more concentrated in the bodies of animals as they move up the food chain. Big top-feeders have a lot more of the chemicals. Those big animals are a staple of the Inuit diet, so when they eat polar bear or walrus they’re getting extremely high doses of chemicals that act like estrogen. That definitely could have an effect on an adult's body, and for a developing fetus there’s little reason to believe it wouldn’t have some effect. It's also true that a study released this year by the National Institute of Environmental Health Sciences found that there has been a shift towards more females in the sex ratio in the US and Japan.
So basically, we’ve got a study that looks like it says that PCBs are turning male fetuses into female fetuses. Sounds like it should be right up our ally, doesn’t it? But let’s look at some complications.
- The scientists, who work with the Arctic Monitoring and Assessment Programme, haven’t actually released the study yet – they just released the results at a symposium in Greenland. The study won’t be released until next year. That means there's no way to verify the details of media reports or find out more about the research. Also, it appears that the study only took the mother's PCB load into account, not the father's.
- The three articles I’ve read about the study (in the Guardian The Independent and The Times) have been short on details and fuzzy on reasoning.
- There's no why in any of the media reports. Although we're probably better off without wild speculation, the media is making it sound like male embryos are mysteriously turning into female embryos, and that's unlikely. Think way back to high school bio. Remember sex chromosomes? Females have two Xs (XX) and males have an X and a Y (XY). One of the chromosomes comes from the mother, the other from the father (and since women don’t have Y chromosomes to pass on, technically it’s the father’s gamete that decides a fetus’s sex). The sex of the fetus is decided when the two gametes fuse.
Are the scientists proposing that the load of endocrine disruptors in mothers’ bodies is actually changing the sex chromosomes of the fetuses they carry? Probably not. So what else might be going on? Here are a couple of possibilities:
- Male embryos are more fragile. It's possible that X and Y gametes are fusing in the mothers' bodies and simply not implanting, or that PCBs are causing a lower survival rate for male embryos in early pregnancy.
- Endocrine disruptors can throw off fathers' bodies, too, lowering Y gamete survival and making them more likely to contribute girl-sperm than boy-sperm.
- Male embryos are more fragile. It's possible that X and Y gametes are fusing in the mothers' bodies and simply not implanting, or that PCBs are causing a lower survival rate for male embryos in early pregnancy.
It may be true that endocrine disruptors are to blame for the shifting sex ratio – in fact, I’d say there’s a good chance they’re at least partly to blame. How and why is still an open question, and it will be interesting to see the hypothesis proposed by AMAP's researchers when the study is released.
I have an idea why/how this would work. Several recent studies, including one of my own (Bonier et al. Behavioral Ecology 2007), has shown that experimentally increasing some hormone levels, such as corticosteroids, or stress hormones, can skew offspring sex ratios in birds. If PCBs are mimicking the effects of these hormones, they could easily have the same effect. It seems likely that increased susceptibility of XY embryos or of Y-bearing sperm, to the hormonal effects is the cause, as you suggest.