Study Number One:
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.
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.
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.
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.
Study Number Two:
In the study, published in the journal Evolutionary Psychology, the researchers looked at 400 lonely hearts ads in the Swedish newspapers Göteborgs-Posten and Aftonbladet and on the Websites Spraydate and match.com.
Men in all categories prefer younger partners. Of a total of 97 men who mentioned age in their ads, only three were looking for an older partner – among men aged 40 to 59, only one out of 67.
Younger women, on the other hand, prefer older men: fully 14 of 16 women aged 20-39 were looking for an older partner. Among women over 60, however, the majority were looking for a younger man.
Credits:
Why Men Matter: Mating Patterns Drive Evolution of Human Lifespan Shripad Tuljapurkar, the Morrison Professor of Population Studies at Stanford; Puleston; and Michael Gurven, an assistant professor of anthropology at UCSB Published online 2007 August 29. doi: 10.1371/journal.pone.0000785. http://www.pubmedcentral.nih.gov/articlerender.fcg… http://timesofindia.indiatimes.com/articleshow/379…
