Life Expectancy
Despite enormous gains in life expectancy over the last few centuries, a new study finds that males still lag well behind females, and not just in humans but across the primate family tree.
Worldwide, people are living longer, healthier lives. Now, not surprisingly, a study of mortality patterns in humans, monkeys and apes suggests that the last few generations of humans have enjoyed the biggest life expectancy boost in all of primate history.
"The lives of females tend to be longer and less variable than the lives of males, suggesting deep evolutionary roots to the male disadvantage."
The increases in life expectancy are partly due to advances in medicine and public health that have increased the odds of survival for human infants and reduced the death toll from childhood illness. Yet males still lag behind females -- not just in humans but across the primate family tree, the researchers find.“The male disadvantage has deep evolutionary roots,” said study co-author Susan Alberts, biology professor at Duke University.
For the study, an international research team from the U.S., Germany, Denmark, Kenya and Canada compiled records of births and deaths for more than a million people worldwide, from the 18th century to the present. The data included people in post-industrial societies such as Sweden and Japan, people born in pre-industrial times, and modern hunter-gatherers, who provide a baseline for how long people might have lived before supermarkets and modern medicine.
The researchers combined these measurements with similar data for six species of wild primates that have been studied continuously for three to five decades, including Verreaux's sifaka lemurs, muriqui monkeys, capuchins, baboons, chimpanzees and gorillas.
The data confirm a growing body of research suggesting that humans are making more rapid and dramatic gains than ever before seen in the primate family tree. For example, in the last 200 years life expectancy in Sweden has jumped from the mid-30s to over 80, meaning that a baby born today can hope to live more than twice as long as one born in the early 19th century.
One indicator of healthcare improvement is infant mortality, which strikes fewer than 3 in 1000 babies born in Sweden or Japan today. But it was more than 40 times higher for those born two centuries ago, and is still high among hunter-gatherers and wild primates.
The researchers also studied lifespan equality, a measure similar to income equality that indicates whether longevity is distributed evenly across society, or only enjoyed by a few.
They found that, for both humans and wild primates, every gain in average lifespan is accompanied by a gain in lifespan equality. That is, for a population to be very long-lived, everyone must benefit more or less equally, with fewer individuals left behind.
"Across the primate order and across human populations, the lives of females tend to be longer and less variable than the lives of males, suggesting deep evolutionary roots to the male disadvantage," write the researchers. "Our findings cast fresh light on primate evolution and human history, opening directions for research on inequality, sociality, and aging."
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The researchers were surprised to find that the longevity of human males has yet to catch up with females, and the improvements in males aren’t spread as evenly..The life expectancy gender gap isn’t just true for humans. Females outlived males in almost every wild primate population they looked at.
“It’s puzzling,” Alberts said. “If we can make life last so long, why can’t we shrink the male-female gap?”
Numerous hypotheses have been proposed. Some pin the blame on genetics. Male primates, who carry only one copy of the X chromosome compared with two copies in females, lack a second X chromosome to compensate for any harmful gene variants their single X may have.
Another possibility, Alberts says, is that gender differences in risky behaviors like fighting continue to hold males back, even while deaths from infectious and chronic diseases that impact both sexes have declined.
If we can identify the culprit and intervene, Alberts says, we might be able to help men catch up.
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