WASHINGTON - Women live longer than men across the world and scientists have by and large linked the sex differences in longevity with biological foundation to survival. A new study of wild mammals has found considerable differences in life span and aging in various mammalian species.
Among humans, women’s life span is almost 8% on average longer than men’s life span. But among wild mammals, females in 60% of the studied species have, on average, 18.6% longer lifespans. The ratio is considerably different for different groups of mammals.
An international team of scientists led by Jean-François Lemaître, from the University Lyon 1 in France, collected information on age-related mortality for 134 populations of 101 wild mammalian species.
“It was surprising to observe that this gender gap in lifespan often exceeds the one observed in humans and is, at the same time, extremely variable across species,” said Lemaître.
"For example, lionesses live at least 50% longer in the wild than male lions,” said Tamás Székely, from the University of Bath, one of the authors of the study.
“We previously thought this was mostly due to sexual selection - because males fight with each other to overtake a pride and thus have access to females, however our data do not support this,“ said Székely.
Scientists have found that even though females consistently live longer than males, the risk of mortality does not increase more rapidly in males than in females across species. Therefore, they say, there must be other, more complex factors at play, such as environmental conditions in which the animals live and sex-specific growth, survival and reproduction through the history of the species.
For example, the authors of the study say, roaming males could be exposed to more environmental pathogens. This was noticed in three populations of the bighorn sheep.
The magnitude of the lifespan gap could also be shaped by local environmental conditions with a trade-off between reproduction and survival. In some species, males allocate more resources to sexual competition and reproduction, which, scientists say, could lead to bigger sex differences in lifespans.
"Another possible explanation for the sex difference is that female survival increases when males provide some or all of the parental care,“ said Székely. “Giving birth and caring for young becomes a significant health cost for females and so this cost is reduced if both parents work together to bring up their offspring."
In order to measure the extent to which biological differences between the sexes affect life expectancy, scientists plan to compare the data on wild mammals with the data on mammals kept in the zoo, where they do not have to fight with predators or compete for food and mates.
Scientists hope the findings will contribute to better understanding of what affects human longevity. In the past 200 years, the average life expectancy of humans has more than doubled due to improved living conditions and advances in medicine. Yet women continue to live longer than men, suggesting the biological differences also have a role.
According to the U.S. Centers for Disease Control, the average American man will live to age 76, while the average woman in America will live to age 81. Women can also expect to be healthier than men in their senior years. Experts shave said the gap is due to a combination of biological and social differences.
Men’s hormone testosterone is linked to a decrease in their immune system and risk of cardiovascular diseases as they age. It is also linked to risky behavior: smoking, drinking and unhealthy eating habits. If diagnosed, men are less likely than women to follow doctor’s advice. Statistics show that men are more likely to take life-threatening risks and to die in car accidents, or gun fights.
Authors of the new study say the differences between male and female longevity are shaped by complex interactions between local environmental conditions and sex-specific reproductive biology. They say that more research is likely to provide “innovative insights into the evolutionary roots and physiology underlying aging in both sexes.”