Even as babies, females survived more during famine and epidemics than males.
In 1846, 22 members of the Donner family headed west to California from Illinois. They picked up more travelers along the way, in Utah and Wyoming, until their group—known as the Donner Party—grew to 87. As the infamous tale goes, they ran into harsh winter weather on the eastern side of the Sierra Nevada and had to make camp. Freezing temperatures, starvation, and cannibalism ensued.
Besides being a symbol of western emigration hardship, the Donner Party also reveals a biological curiosity about the survival of men and women: women are generally better at it. The men in the Donner Party died at nearly twice the rate of the women. Thirty out of 53 men died, compared to 10 out of 34 women, all presumably of cold, starvation, illness, or some combination of each.
We’ve long known that women live longer than men do, even when they’re not stranded. In every country in the world, women have longer life expectancies. In the US, women will live almost 5 years longer than men.
But we don’t really know why. Researchers who study aging have pointed to potential biological factors, like hormones and genetics, to try and explain why men die more from respiratory cancer, cirrhosis of the liver, emphysema, and heart disease. Still, there has always been a confounding factor: gendered behavior. Many believe that aside from basic biological differences, men don’t live as long because they take bigger physical risks, have more dangerous jobs, go to war, smoke and drink more, have higher suicide rates, are less health conscious, and go to the doctor less than women.
Behavioral differences haven't been able to explain it all, though. When researchers looked to see if the sex differences in life expectancy went away in Mormon populations—where men partake in less “risky” behavior than the general population —they found the differences remained. Same, when life expectancies of nuns and monks were compared. In a study comparing non-smoking women to non-smoking men, women still had a survival advantage.
“Despite the fact that there is a lot of literature about differences in survival, it’s not clear whether it’s biology or behavior,” says Virginia Zarulli, an assistant professor in biodemography at the University of Southern Denmark. “It’s really two opposite sides, and when they come to debate, the answer is not clear at all.”
The answer, of course, is never purely one or the other—biology or behavior. But in a new study, published online last week in PNAS, Zarulli offers new evidence that fundamental biological differences, not behavior, are at the root of sex survival differences. The study took a closer look at situations like the Donner Party, when people were faced with extreme conditions, like famines or epidemics.
“When you want to analyze what’s going on with survival of a species, usually you put an organism in the laboratory and you subject them to extreme conditions and see what happens,” Zarulli says. “So we looked for data among humans that would somehow simulate such an extreme mortality shock and we found these populations.”
Zarulli and her collaborators examined seven periods in history when people had extremely low life expectancies—20 years or less—that also had credible documentation for death rates and gender. A few tragic examples: Freed Liberian slaves from the early 1800s who migrated back to Africa had mortality rates of 1.68 years for men and 2.23 years for women. Following the collectivization of agriculture in 1933 in Ukraine, a famine caused life expectancy to drop to 7.3 years for men and 10.9 years for women, down from 41.58 and 45.93 years, respectively.
In these and other groups, women had lower mortality rates across almost all ages, with the exception of one Trinidadian slave population. (In that case, Zarulli thinks the young adult male slaves were considered more valuable.)
While women survived better across the board, they found that it was the infant girls who survived the best. It was this difference that created most of the total gender gap in survival. Since boy and girl babies have minimal behavioral differences, Zarulli says, and don’t yet partake in gendered behavior, she thinks their findings can finally show that biology plays the driving role in female survival, not behavior.
She says they were surprised at their findings about infants because, in historical contexts, sons were prioritized over daughters. “Despite this potential discrimination, the fact that girls still survive better than boys is striking,” she says. “It points strongly that there is very likely a deep biological root in the survival advantage of women, in particular, girls.”
Zarulli says the biological explanation could be both hormones or genetics. Women have a double X chromosome, and men have X and Y. In very simple terms, if a negative mutation occurred on one of the two X chromosomes, women had the possibility to cope or partially compensate for it. Over the course of evolutionary time, this might have given them extra genetic material to deal with immune challenges or a changing environment. Men may also have, evolutionarily speaking, genetically prioritized reproduction over life span.
Mary Schooling, a professor at the CUNY Graduate School of Public Health, thinks that hormones could have a large part to play. She studies evolutionary biology and public health, and has been researching the effects of testosterone on disease for years, particularly testosterone's relationship with heart disease. She says estrogen has been shown to provide a wide variety of health benefits in the brain, heart, and veins. Some studies show that testosterone might, instead, increase the mortality risk for disease. Estrogens could enhance immune defenses, while testosterone may have immunosuppressive effects, meaning estrogen might bolster a woman's immunes system's fight against disease.
Schooling says the idea that life span differences could be mostly biological has not always been received well. “It’s not really very flattering to men, particularly if you say it’s testosterone,” she says.
Because biological theories haven't always been popular, it may have even led to less research says Steven Austad, a biogerontologist at the University of Alabama, Birmingham who studies aging in animal models. That means we still know very little about the nitty-gritty of how these differences can lead to longer lives. “I think very few people really believe it’s biological," he says. "As a consequence, the biological basis has been woefully understudied. Even fewer people think about it as being biological in very early life.”
He’s always thought it was at least half biology, he says. But this new finding, from historical situations where we know girls had it worse, has shifted his thinking. “This makes the point in a really robust way," he says. "What this is saying is that girls are better designed for survival. They’re better designed for survival in good times, obviously, but also in these extreme times. Biology really emerges as the only viable explanation.”
He hopes that this paper will lead other researchers to study these differences at a mechanistic level, to understand the biology further. In his own work, which has usually focused on old age to understand life expectancy, he's inspired to look at infancy and early life.
Even with their results, Zarulli still doesn’t want to exclude the role of social factors. Ultimately, biology and behavior influence each other, but she hopes that while also encouraging more biological research, her findings might have some cultural implications as well.
“We should probably stop considering women the ‘weaker’ sex because maybe it’s the other way around,” she says. “Often when we see that women live longer, we hear it's because they have had an easier life, because they had lighter jobs, they worked less, and so on. But it could be that it's simply biology.”
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