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New Study Looks into Why Females Live Longer than Males

Zlatica Hoke

   WASHINGTON - Women live longer than menacrossthe worldand scientists
   haveby and largelinked the sex differencesin longevitywithbiological
   foundation to survival. A new studyof wild mammalshasfoundconsiderable
   differences inlife span and aging in various mammalian species.

   Among humans, women's life span isalmost 8%on
   averagelongerthanmen'slife span.But among wildmammals,females in 60%of
   thestudiedspecieshave, on average,18.6%longer lifespans.Theratiois
   considerablydifferentfor different groups of mammals.

   An international team of scientistsled by Jean-FrançoisLemaître,
   fromtheUniversityLyoninFrance,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," saidLemaître.

   "For example, lionesses live at least 50% longer in the wild than male
   lions," saidTamásSzékely, 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 supportthis,"saidSzékely.

   Scientists havefoundthateven though females consistently livelonger
   than males, the risk of mortalitydoes 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 conditionsin
   which the animals live andsex-specificgrowth, survival andreproduction
   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 bylocal
   environmental conditions witha trade-offbetween reproduction and
   survival. In some species, males allocate more resources to sexual
   competition and reproduction, which, scientists say,couldlead 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
   parentalcare,"saidSzékely. "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 mammalswith the data on mammals kept in the zoo,where they do
   not have to fight with predators or competefor foodand mates.

   Scientists hope thefindingswillcontribute to betterunderstanding ofwhat
   affects human longevity.In the past 200 years, the average life
   expectancy of humans has more than doubleddue to improved living
   conditions and advances in medicine. Yet women continue to live longer
   than men, suggesting the biological differencesalsohavea role.

   According to the U.S. Centers for DiseaseControl,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. Expertsshave saidthegapis duetoa combination
   ofbiologicalandsocial differences.

   Men's hormone testosterone is linked to a decrease in theirimmune
   systemand risk of cardiovascular diseasesas they age. It is also linked
   to riskybehavior:smoking, drinking andunhealthy eating habits.If
   diagnosed, men are less likely than women to follow doctor's
   advice.Statistics show thatmen are more likely to take life-threatening
   risks and to die in car accidents, or gun fights.

   Authors of the new studysay the differencesbetween male and female
   longevityare shaped by complex interactions between local environmental
   conditions and sex-specific reproductivebiology.They
   saythatmoreresearchis likelytoprovide"innovative insights into the
   evolutionary roots and physiology underlying aging in both sexes."

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