Different in degree or kind: only humans have menopause (Introduction)

by David Turell , Thursday, February 14, 2019, 01:34 (1893 days ago) @ dhw
edited by David Turell, Thursday, February 14, 2019, 01:41

The so-called Grandma theory in Darwinism is challenged:

https://www.sciencenews.org/article/grandmothers-survival-evolution

"Grandmothers are great — generally speaking. But evolutionarily speaking, it’s puzzling why women past their reproductive years live so long.

"Grandma’s age and how close she lives to her grandchildren can affect those children’s survival, suggest two new studies published February 7 in Current Biology. One found that, among Finnish families in the 1700s–1800s, the survival rate of young grandchildren increased 30 percent when their maternal grandmothers lived nearby and were 50 to 75 years old. The second study looked at whether that benefit to survival persists even when grandma lives far away. (Spoiler: It doesn’t.)

"The studies are part of a broader effort to explain the existence of menopause, a rarity in the animal kingdom. The so-called “grandmother hypothesis” stipulates that, from an evolution standpoint, women’s longevity is due to their contributions to their grandkids’ survival, thus extending their own lineage. (my bold)

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"The team found that when maternal grandmothers living nearby were aged 50 to 75, their 2- to 5-year-old grandchildren had a 30 percent higher likelihood of survival than children whose maternal grandmothers were deceased. Similarly aged paternal grandmothers and maternal grandmothers aged past 75 did not affect children’s overall survival.

"But when paternal grandmothers lived past age 75, their grandchildren’s odds of dying before age 2 was 37 percent higher than a child with a deceased paternal grandmother.

***

"In the second study, researchers wanted to know if the grandmother boost persisted even when families lived far apart. The team used data from 1608 to 1799, encompassing 3,382 maternal grandmothers and 56,767 grandchildren in Canada’s St. Lawrence Valley. As with the Finnish population, those early French settlers had large families and high child mortality, but they also moved around a lot.

"For every 100 kilometers of distance between mothers and daughters, the daughters had 0.5 fewer children, the researchers found. Older sisters whose moms were alive when the women started having children had more children, and those children were more likely to survive to age 15, compared with younger sisters who started having children after their mother’s death.

"Mathematically speaking, as grandma moved farther away, those survival and reproduction rates began to resemble those of the younger sisters with deceased moms. Once a maternal grandmother moved 350 kilometers away or more, her benefits ceased, says study coauthor Patrick Bergeron, an evolutionary biologist at Bishop’s University in Sherbrooke, Canada.

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"Both studies provide an interesting peek at life in these North American and European communities, says Melissa Melby, a medical anthropologist at the University of Delaware in Newark. But she remains skeptical about the grandmother hypothesis because menopause may well have come about by accident. Maybe, she says, women live past their reproductive years because evolution favored men who could reproduce into old age, who then passed on those longevity genes to their sons and daughters. "

Comment: Followers of Darwin can make up all sorts of just-so stories. There is no clear evidence that grand motherhood evolved to keep another generation alive. Note my bold. Menopause in another big way that humans differ from all other animals.