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Senior Citizens & Sex
Older Men Chasing Younger Women Sheds Light on Human
Longevity
Males much older than 50 have substantial realized
fertility through matings with younger females - likely typical among
early humans
By
Lisa Trei, Sanford News Services
 Sept.
14, 2007 - It turns out that older men chasing younger women contributes
to human longevity and the survival of the species, according to new
findings by researchers at Stanford and the University of
California-Santa Barbara.
Evolutionary theory says that individuals should
die of old age when their reproductive lives are complete, generally by
age 55 in humans, according to demographer Cedric Puleston, a doctoral
candidate in biological sciences at Stanford. But the fatherhood of a
small number of older men is enough to postpone the date with death
because natural selection fights life-shortening mutations until the
species is finished reproducing.
"Rod Stewart and David Letterman having babies in
their 50s and 60s provide no benefit for their personal survival, but
the pattern [of reproducing at a later age] has an effect on the
population as a whole," Puleston said. "It's advantageous to the species
if these people stick around. By increasing the survival of men you have
a spillover effect on women because men pass their genes to children of
both sexes."
"Why Men Matter: Mating Patterns Drive Evolution of
Human Lifespan," was published Aug. 29 in the online journal Public
Library of Science ONE.
Shripad Tuljapurkar, the Morrison Professor of
Population Studies at Stanford; Puleston; and Michael Gurven, an
assistant professor of anthropology at UCSB, co-authored the study in an
effort to understand why humans don't die when female reproduction ends.
Human ability to scale the so-called "wall of
death"—surviving beyond the reproductive years—has been a center of
scientific controversy for more than 50 years, Puleston said.
"The central question is: Why should a species that
stops reproducing by some age stick around afterward?" he said.
"Evolutionary theory predicts that, over time, harmful mutations that
decrease survival will arise in the population and will remain invisible
to natural selection after reproduction ends."
However, in hunter-gatherer societies, which likely
represent early human demographic conditions and mating patterns,
one-third of people live beyond 55 years, past the reproductive lifespan
for women.
Furthermore, life expectancy in today's
industrialized countries is 75 to 85 years, with mortality increasing
gradually, not abruptly, following female menopause.
Grandmother hypothesis
In 1966, William Hamilton, a British evolutionary
biologist, worked out the mathematics describing the "wall of death."
Since then, the most popular explanation for why humans don't die by age
55 has been termed the "grandmother hypothesis," which suggests that
women enhance the survival of their children and grandchildren by living
long enough to care for them and "increasing the success of their
genes," Puleston said. However, Hamilton's work has been difficult to
express as a mathematical and genetic argument explaining why people
live into old age.
Unlike previous research on human reproduction,
this study—for the first time—includes data on males, a tweak that
allowed the researchers to begin answering the "wall of death" question
by matching it to human mortality patterns. According to Puleston,
earlier studies looked only at women, because scientists can reproduce
good datasets for humans entirely based on information related to female
fertility and survival rates.
"People don't like to do two-sex models because
[it's difficult] to look at how [men and women] pair up," he said. "But
men's fertility is contingent on women's fertility—you have to figure
out how they match up. We care about reproduction because that is a
currency by which force of selection is counted. If we have not
accounted for the entire pattern of reproduction, we may be missing
something that's important to evolution."
Men and longevity
In the paper, the researchers analyzed "a general
two-sex model to show that selection favors survival for as long as men
reproduce." The scientists presented a "range of data showing that males
much older than 50 years have substantial realized fertility through
matings with younger females, a pattern that was likely typical among
early humans." As a result, Puleston said, older male fertility helps to
select against damaging cell mutations in humans who have passed the age
of female menopause, consequently eliminating the "wall of death."
"Our analysis shows that old-age male fertility
allows evolution to breach Hamilton's wall of death and predicts a
gradual rise in mortality after the age of female menopause without
relying on 'grandmother' effects or economic optimality," the
researchers say in the paper.
The scientists compiled longevity and fertility
data from two hunter-gatherer groups, the Dobe !Kung of the Kalahari and
the Ache of Paraguay, one of the most isolated populations in the world.
They also looked at the forager-farmer Yanomamo of
Brazil and Venezuela, and the Tsimane, an indigenous group in Bolivia.
"They're living a lifestyle that our ancestors lived and their fertility
patterns are probably most consistent with our ancestors," Puleston said
about the four groups. The study also looked at several farming villages
in Gambia and, for comparison, a group of modern Canadians.
In the less developed, traditional societies, males
were as much as 5-to-15 years older than their female partners. In the
United States and Europe, the age spread was about two years.
"It's a universal pattern that in typical marriages
men are older than women," Puleston said. "The age gaps vary by culture,
but in every group we looked at men start [being sexually reproductive]
later. At the end of reproduction, male fertility rates taper off
gradually, as opposed to the fairly sharp decline in female fertility by
menopause."
Despite small differences based on marriage
traditions, all women and most men in the six groups stopped having
children by their 50s, the researchers found.
But some men, particularly high-status males,
continued to reproduce into their 70s.
The paper noted that the age gap is most pronounced
in societies that favor polygyny, where a man takes several wives, and
in gerontocracies, where older men monopolize access to reproductive
women. The authors also cite genetic and anthropological evidence that
early humans were probably polygynous as well.
Older male fertility also exists in societies
supporting serial monogamy, because men are more likely to remarry than
women.
"For these reasons, we argue that realized male
fertility was substantial at ages well past female menopause for much of
human history and the result is reflected in the mortality patterns of
modern populations," the authors say. "We conclude that deleterious
mutations acting after the age of female menopause are selected against
… solely as a result of the matings between older males and younger
females."
According to Puleston, the "grandmother hypothesis"
may be true, but the real pattern of male fertility extends beyond this
explanation. "The key question is: Does the population have a greater
growth rate if men are reproducing at a later age? The answer is 'yes.'
The age of last reproduction gets pushed into the
60s and 70s if you add men to the analysis. Hamilton's approach was
right, but in a species where males and females have different
reproductive patterns, you need a two-sex model. You can't correctly
estimate the force of selection if you leave men out of the picture. As
a man myself, it's gratifying to know that men do matter."
Grants from the U.S. National Institute on Aging
supported this study.
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