Living near long-lived mothers increased the number of offspring their children had..
The researchers looked at 2,800 women living in two 18th and 19th century farming communities in Finland and Canada.
They wanted to see how long women lived after their menopause and what effect that might have on how many babies their own children had.
The data showed women had two extra grandchildren for each extra decade they lived after 50.
The grandmothers not only increased the number of children their children had but also increased the number that survived childhood.
This effectively would have selected for women to live longer.
The research, led by Mirkka Lahdenpera, a professor of human ecology at the University of Turku in Turku, Finland, found that "prolonged post-reproductive longevity in humans is associated with greater grandchild production."
In both Canada and Finland, women gained two extra grandchildren for every decade they lived beyond age 50.
The research also reveals that the declining role of grandmothers in childrearing is one factor among many that have led to the birth rate falling in modern societies.
Because of greater mobility people are less likely to live next to their parents. Therefore they are less likely to have parental babysitting services available.It would be interesting to look at industrial society populations today and measure physical distance between parents and their children and compare that to the number of offspring the children have. In light of this study it seems likely that people who do not live near their mothers have smaller families than those who do.
The effect of living near one's mother might not be as strong on childbearing today for the upper classes in particular since wealthy people can afford nannies and babysitters.
Humans are unusual in the length of time they survive after they become infertile.
By Randall Parker at 2004 March 11 03:27 PM Trends, Human Evolution | TrackBackThe post-reproductive survival of humans—women in particular—is truly unusual. Non-reproductive “helpers” of individuals who are breeding are found in many species. But they are usually young animals that have yet to establish themselves, rather than relics from previous generations. The post-reproductive elderly just die. Chimpanzees, for example, have a similar pattern of fertility to people. A female chimp's fertility peaks in her late 20s, and is more or less extinguished by her mid-40s. But in chimpanzees, mortality rises as fertility declines.
Interesting. There was some research about a decade on a tribe in Africa which found much the same thing. Grandmothers typically foraged for root foods, and the researchers found that children who had 'foraging grandmothers' had a greater chance to survive to adulthood than those who did not. Like this research, the object was to find whether individuals living past the age of fertility conferred a survival advantage on offspring. They found that they did.
I recently wrote a lengthy post over at the Speculist about grandparents:
http://www.speculist.com/archives/000682.html
Keypoints:
"Human children are uniquely defenseless in the wild. If Dad is miles away hunting and Mom is out of earshot gathering, Mammaw and Pappaw are quite valuable. Aunts and Uncles are better employed having kids of their own. And by having babysitters Junior has a better chance of surviving to pass on genes that, incidentally, favor longevity...
Whatever evolutionary pressure there may have been favoring grandparents, that pressure is less for great grandparents... senescence is the product of evolutionary neglect (a lack of pressure to live longer), not part of some grand design to clear the way for future generations. Generally a child is not placed at risk by a great-grandparent's death, but there is no benefit conferred to the child by the death either."
And from the comments:
John Farren: "What about: individuals with longer living parents and grandparents have a competitive edge due to the amounts of cultural data available and time for that to be transferred?"
Stephen Gordon: "John: Good point. Wisdom transfer time.
I can just see some poor family dying of thirst and hunger during a drought when Grandpa speaks up and says, 'This same thing happened when I was a kid. My Dad led us on a five day march to the east where we camped out on a lake at the base of a mountain.'"
>A female chimp's fertility peaks in her late 20s, and
>is more or less extinguished by her mid-40s. But in
>chimpanzees, mortality rises as fertility declines.
This would lend support the the "wisdom transfer time" theory John Farren mentioned in the last comment. Since chimps have less wisdom to tranfer than people (and are less capable of communicating what wisdom they have) there is less of an survival advantage conveyed to the young by having the grandparents around. Since there is no advantage there is no selective evolutionary pressure to engineer long lives for chimps.