Rising Temperatures Lead To A Higher Proportion Of Girls Being Born Than Boys

by Stephen Luntz

Photo credit: A 19th Century school class. Future versions may have more girls and fewer boys.

Rising temperatures in Japan lead to a higher proportion of girls being born than boys, according to a study in Fertility and Sterility. The observation has led the researchers to suggest that climate change could alter the mix of men and women.

Certain species, particularly reptiles, engage in temperature dependent sex selection (TDSS), in which the sex of offspring is determined by the warmth of the environment in which eggs are incubated. Humans rely on genes, but even for us there are slightly more women conceived in tropical regions than at the poles. Nevertheless, things can get complex when it comes to reproduction. It is only four years since the discovery of a species that uses TDSS in some circumstances and genetics in others. 

Dr. Misao Fukuda of the M&K Health Institute in Japan found evidence to support the possibility that human sex ratios may also be influenced by temperature, although in a more subtle way and through a different mechanism.
In 1968, 1.07 boys were born in Japan for every girl. By 2012, that was down to 1.05. A similar shift has been observed in other places, but there is debate about how widespread the trend is, as well as the causes. Fukuda noted that while the trend is clear, there have been some bumps on the trendline and compared these to the fluctuations in temperature.

Moreover, Fukuda also looked at data on the ratio of male to female “spontaneous fetal deaths” -- miscarriages after the first twelve weeks of pregnancy over the same period. Here the trend was much steeper, beginning at 1.3 and reaching 2 by the end of the study period.

“Two climate extremes, a very hot summer in 2010 and a very cold winter in January 2011, showed not only statistically significant declines in sex ratios of newborn infants 9 months later in June 2011 and October 2011, but also statistically significant increases of fetal death rates immediately in September 2010 and January 2011,” Fukada reports. 

The increased number of male fetuses dying, relative to females, does not fully account for the decline in male births, but maybe indicative of a similar trend earlier in pregnancy, for which data is not available.

“Male conception seems to be especially vulnerable to external stress factors, including climate changes,” the paper concludes.

The fact that female fetuses are more likely to survive is well-known, and there is longstanding evidence that periods of stress affect male fetuses more severely. However, it's unexpected for temperatures to have such a strong effect in a country with as much air conditioning as Japan.

Changes to sex ratios for humans are so small that, unlike for reptiles, there is no threat to our survival. Nevertheless, an increase in miscarriages for all fetuses may be one more effect of rapidly changing climates.

H/T Mother Jones