Old Men/Young Women, A Good Thing?

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.

"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 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.