A Japanese researcher has taken a look at how temperature fluctuations attributed to climate change affect the sex ratio of infants, and has found that male fetuses are more vulnerable to extreme weather events, resulting in an increased female-to-male ratio.

In research published in the catchily-named Fertile and Sterile journal (with an even more catchy website FertStert.org) Dr. Misao Fukuda, from M&K Health Institute analyzed monthly temperature data in Japan along with national data on births and fetal deaths between the years of 1968 and 2012. Fetal deaths were defined as fetuses spontaneously aborted (miscarried) after 12 weeks of gestation. The researchers paid particular attention to two specific extreme weather events: an extraordinarily hot summer in 2010 (the hottest since records began in 1898 and August being the hottest and month on record ever) a distinctly cold winter in 2011. Live Science reports:

During the hot summer — which was the warmest in the country since 1898 — there was an increase in the number of fetal deaths in September of that year, and nine months later, there was a decrease in the ratio of male to female babies born in the country.

A similar pattern was found as a result of the cold 2011 winter—there was an increase in fetal deaths and a decrease in the male to female baby ratio. The researchers cannot say for sure if this is a direct result of climate change—Fukuda mentions other factors like pollution and earthquakes—but have concluded:

The recent temperature fluctuations in Japan seem to be linked to a lower male:female sex ratio of newborn infants, partly via increased male fetal deaths. Male concepti seem to be especially vulnerable to external stress factors, including climate changes.

While other similar studies in other countries have not found such clear data, these countries like New Zealand and Finland do not have such extreme temperature fluctuations as Japan.

Image via Getty.