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Mammal study explains why females live longer

A new study that looks at lifespan in wild mammals shows that females live substantially longer than males.

The research finds that, on average, females live 18.6% longer than males from the same species.

This is much larger than the well-studied difference between men and women, which is around 8%.

The scientists say the differences in these other mammals are due to a combination of sex-specific traits and local environmental factors.

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In every human population, women live longer than men, so much so that nine out of 10 people who live to be 110 years old are female.

This pattern, researchers say, has been consistent since the first accurate birth records became available in the 18th Century.

While the same assumption has been held about animal species, large-scale data on mammals in the wild has been lacking,

Now, an international team of researchers has examined age-specific mortality estimates for a widely diverse group of 101 species.

In 60% of the analysed populations, the scientists found that females outlived the males - on average, they had a lifespan that's 18.6% longer than males.

"The magnitude of lifespan and ageing across species is probably an interaction between environmental conditions and sex-specific genetic variations," said lead author Dr Jean-Francois Lemaître, from the University of Lyon, France.

He gives the example of bighorn sheep for which the researchers had access to good data on different populations.

Where natural resources were consistently available there was little difference in lifespan. However, in one location where winters were particularly severe, the males lived much shorter lives.

"Male bighorn sheep use lots of resources towards sexual competition, towards the growth of a large body mass, and they might be more sensitive to environmental conditions," said Dr Lemaître.

"So clearly the magnitude of the difference in lifespan is due to the interaction of these sex-specific genetics, the fact that males devote more resources towards specific functions compared with females, and to the local environmental conditions."

Even if females lived longer than males, the team found that it did not mean that the risks of dying are increasing more in males than females as they get older. The expected male mortality is always higher, but the rate of mortality is about the same in both sexes as they age.

One recent study in this field suggested that the genetic differences between males and females were key.

In humans, our cells contain different chromosomes, depending on sex. Females have two X chromosomes while males have an X and a Y. The theory is that the extra X in women has a protective effect against harmful mutations and that this holds true in other species.

The author of the new study on mammals says that both pieces of research are complementary.

"They show that in XX or XY systems, the XX, or the female, lives longer, so clearly there is an effect of sex chromosomes," said Dr Lemaître.

"What we show in our paper is that the difference is very variable across species, meaning there are other factors that need to be considered to explain this variability."

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