Why Do Women Live Longer Than Men?

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One theory about longevity is that males have bigger bodies in order to win out in the race for breeding opportunities.

Genes in sperm may determine why female mammals live longer than males, according to a Japanese study published on Wednesday in Human Reproduction, a European journal.

Tokyo University professor Tomohiro Kono and Manabu Kawahara of Saga University found that female mice produced from genetic material from two mothers, but not from a father, lived significantly longer than mice with the normal mix of maternal and paternal genes.

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The "bi-maternal" mice were created by manipulating DNA in mouse eggs, so that the genes behaved like those in sperm.

Once modified, this material was implanted into unfertilized adult female mice eggs to create embryos.

These mice lived 841.5 days on average -- 186 days longer than in control mice born with a normal genetic mix, whose lifespan was 655.5 days.

The longest-lived "bi-maternal" mouse lived for 1,045 days, while the oldest control mouse expired after 996 days.

Another intriguing finding was that the "bi-maternal" mice were lighter and smaller than control mice and seemed to have a stronger immune system.

The big difference could lie in a gene called Rasgrf1, the researchers believe.

The gene, located on Chromosome 9, is associated with post-natal growth. It normally expresses from the paternally inherited chromosome.

"The study may give an answer to the fundamental questions: that is, whether longevity in mammals is controlled by the genome of only one or both parents and, just maybe, why women are an advantage over men with regard to the lifespan," Kono said.

One theory about longevity is that males have bigger bodies in order to win out in the race for breeding opportunities and thus scatter their genes.

The price for this, though, is a shorter lifespan.

Females, though, do not have to engage in this genetically costly beauty show, and instead optimize their reproductive output by conserving energy for delivering their offspring, nurturing it, foraging for food and avoiding predators.

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