为什么女性比男性更长寿？

Researchers already know that women live longer than men in all parts of the world. Scientists have largely linked the difference in life expectancy between the sexes to biological survival factors.

Now, a new study of wild mammals has found great differences in length of life and aging in many species of mammals.

The researchers found that, in humans, women live almost 8 percent longer than men. But among wild mammals, females in 60 percent of the studied species live, on average, 18.6 percent longer. The percentages are very different for different groups of mammals.

Jean-Francois Lemaitre led the study. He is from the University Lyon 1 in France. He and his research team collected information on age-related deaths for 134 groups of 101 wild species of mammals.

Lemaitre said, "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."

Tamás Székely, from the University of Bath, was one of the writers of the study. He told VOA that female lions in the wild live at least 50 percent longer than male lions.

"We previously thought this was mostly due to sexual selection, he said. "However our data do not support this."

Scientists say the risk of death does not increase more quickly in males than in females across species. They say there must be other, more complex reasons. These include things such as environmental conditions in which the animals live, sex-specific growth, survival and reproduction through the history of the species.

Székely suggests that one reason for the differences between the sexes is that "female survival increases when males provide some or all of the parental care."

He added that "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."

Scientists plan to compare the data on wild mammals with the data on mammals kept in zoos. They want to measure how much biological differences between the sexes affect life expectancy. In zoos, animals do not have to fight with predators or fight for food and mates.

Scientists hope the findings will give them a better understanding of what affects length of life in humans. In the past 200 years, the average life expectancy of humans has more than doubled. The main reasons for the increase are improved living conditions and developments in the field of medicine. However, women continue to live longer than men. This suggests biological differences also play a part.

The U.S. Centers for Disease Control says the average American man will live to age 76. The average woman in America will live to age 81. Women can also expect to be healthier than men in their older years. Experts said this is because of a combination of biological and social differences.

A hormone in men called testosterone is connected to a decrease in their immune system and risk of heart-related diseases as they age. It is also connected to risky behavior, such as smoking, drinking and unhealthy eating habits.

Men are also less likely than women to follow suggestions and warnings from a doctor. And research shows that men are more likely to take life-threatening risks and to die in car accidents or gun fights.