For Longer Male Lifespans, Let It Snow

UC Berkeley researchers have found that if a pregnant woman who lives in a cold environment has a male child, he will probably live longer than his sun-reared peers.

The study, published earlier this month in the Proceedings of the National Academy of Sciences, used precise Scandinavian vital statistics and temperature data to conclude that natural selection culls fetuses growing in the womb during colder years, increasing the frequency of female births and the lifespan of male offspring.

Though it was known that pregnant women tend to have fewer male children during times of stress, the study was the first of its kind to show that environment influences male longevity, said environmental health sciences professor Kirk Smith, who contributed to the research.

In times of stress, organisms attempt to simultaneously increase the probability of their offspring breeding and keep their own energy expenditures low. Females are generally more likely to have children and take less energy to raise than males, he said.

This evolutionary strategy, a remnant of human hunter-gatherer ancestry, causes pregnant women to differentially abort male fetuses during stressful periods like extreme cold, he said.

To examine the extent of the climate's influence on birth rates, Smith said he and public health professor Ralph Catalano compared Scandinavian birth, death and temperature records from 1878 to 1914. The data was the most complete and accurate set they could find.

While the researchers confirmed women tended to have fewer males during colder years, they also discovered something new.

"Women tended to have less males, but they were longer-lived," Smith said. "This is evidence that women are differentially aborting weaker male fetuses (in times of stress). This is completely new."

In periods of stress, women's bodies would differentially abort fetuses until a female or a healthy male was produced.

Males gestated during colder years survived two weeks longer on average than males gestated in climates warmer by one degree Celsius.

While two weeks does not seem long when considering the average lifespan of a human being, Smith said the effect was dose-dependent. If the Earth were suddenly thrown into an ice age, for instance, the differences might become more prominent.

But the actual mechanism by which cold temperatures affect fetal development is still a mystery.

Cold temperatures could manifest themselves through less sunlight, poorer food, greater indoor pollution, or even increased alcohol consumption, Smith said.

He also said uncomfortably hot environments could have the same effects on gestation, though a suitable data set for such a climate has not yet been found.

Data from a warmer environment would be more relevant considering the trend of global climate change, he added.

"This (study) shows that humans are closely attuned to the climate, that even subtle changes in the climate affect us," he said.

Tags: CHILDREN, WEATHER