Climate change will affect gender ratio among newborns, scientists say
Global warming will have a variety of effects on our planet, yet it may also directly impact our human biology, research suggests.
Specifically, climate change could alter the proportion of male and female newborns, with more boys born in places where temperatures rise and fewer boys born in places with other environmental changes, such as drought or wildfire caused by global warming.
A recent study in Japan found a link between temperature fluctuations and a lower male-to-female sex ratio at birth, with conceptions of boys especially vulnerable to external stress factors, wrote Dr. Misao Fukuda, lead study author and founder of the M&K Health Institute in Hyogo.
Last summer, Fukuda and his colleagues published a separate study looking at births in areas hit by environmental events that caused extreme stress. They included Hyogo Prefecture after the Kobe earthquake of 1995; Tohoku after the Great East Japan Earthquake of 2011 (and subsequent nuclear disaster at the Fukushima Daichii power plant); and Kumamoto Prefecture after the 2016 earthquakes.
Nine months after these disasters, the proportion of male babies born in these prefectures declined by between 6% and 14% from the previous year.
This data supports the idea that major stress affects gestation, which in turn alters the newborn sex ratio, Fukuda and his co-authors wrote. Stress stemming directly from “climate events caused by global warming” might also affect the sex ratio, Fukuda wrote in an email. Though scientists do not know how stress affects gestation, Fukuda theorizes that the vulnerability of Y-bearing sperm cells, male embryos and/or male fetuses to stress is why “subtle significant changes in sex ratios” occur.
Scientists believe that the sex ratio is equal at conception, explained Steven Orzack, president and senior research scientist of the Fresh Pond Research Institute in Cambridge, Massachusetts. But more than half of all human conceptions die during gestation, and this results in a sex imbalance at birth.
“Overall, more females die during pregnancy than do males. So that’s why there’s an excess number of males at birth,” said Orzack, who has published research on this issue.
Ray Catalano, a professor in the school of public health at the University of California, Berkeley, explained that the process of natural selection in utero is why deaths occur during gestation.
A mother’s biology spontaneously aborts some conceptions in utero but not others. The factors that filter out who “gets through” from conception to birth include chromosomal or genetic abnormalities of the fetus or the mother’s stress response to changes in her environment, Catalano said.
Because the ovaries of a female fetus carry all the eggs she will ever possess, the possibility of genetic defects being found in a female (and her eggs, which represent her potential children) are greater than the possibility of defects in a male fetus, which carries only his own genes.
Worldwide, the newborn sex ratio averages 103 to 106 males born for every 100 females, Catalano explained. In part, this is a result of the fact that a male infant are “a relatively frail organism,” he said.
“For every society, for every year, the human being most likely to die [prematurely] is male infants. And that’s true for every society that we have data for,” Catalano said.
The reasons why are not understood, but some scientists believe that boys are biologically weaker WEand more susceptible to diseases and premature death.
The general theory as to why the sex ratio is not equal at birth is that if you want the sex ratio to be 50:50 by reproductive age, you want a few more males than females at birth because more males than females will probably die in early childhood, Catalano said.
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