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.”
