A recent study carried out in Japan warns of yet another unexpected consequence of climate change: the study shows that male human fetuses are more sensitive to temperature differences than female fetuses and are more likely to spontaneously abort when exposed to extremes of heat and cold in utero. While similar studies in New Zealand and Finland have not confirmed this finding, the Japanese team believe they have an explanation for that.
The Japanese team, who published their study in the journal Fertility and Sterility in September 2014, believe the reason they had more striking results than the New Zealand and Finnish teams was because Japan experiences a wider annual temperature range than the other two countries. The Finnish study in fact showed that periods of increased ambient temperature lead to a slight increase in live male births in that country. The Japanese team believe, however, it is the variation that is the significant factor. They 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.”
The Japanese study looked at whether climate change was associated with male:female ratios, or sex ratios, of fetal deaths and births in Japan from 1968 to 2012. They found that the sex ratios of fetal deaths had increased steadily along with temperature differences, whereas the sex ratios of newborn infants had decreased at a steady rate since the 1970s. Most significantly, they observed: “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 nine months later in June 2011 and October 2011 but also statistically significant increases of fetal death rates immediately, in September 2010 and January 2011.”
A study of Swedish birth and mortality data from 1850 to 1915 also found that males seemed to be more sensitive to temperature fluctuations. In that study it was observed that male fetuses who experienced warmer than average temperatures in utero, but who then experienced colder temperatures until age four had a shorter life expectancy than normal. However, it’s important to bear in mind that while the Japanese study shows correlation, it does not show causation. And while the debate regarding the impact of climate change on human fetuses may continue for a while yet, for turtles and other reptiles that have temperature-dependent sex determination, where the gender of the hatchling will be determined by the temperature experienced by the egg at a certain critical stage of development, fluctuations in temperature are going to have significant impacts.