WEDNESDAY, March 10, 2004 (HealthDayNews) -- It was no accident if your grandmother helped raise you: It was a biological and social necessity that helped you survive and helped your mother have more children, researchers report.
While most female animals continue to reproduce until they die, women live long after menopause. The evolutionary explanation may be the so-called "grandmother hypothesis."
This hypothesis says that menopause and female life span beyond the ability to have children evolved because the advantage of helping daughters reproduce and raise their children outweighed the advantages of continuing to give birth.
It suggests natural selection favored menopause, long life and perhaps even close family ties, because only grandmothers who are not busy feeding their own children have time to help with grandchildren.
In a new study, researchers looked at two groups of women -- one in Finland and the other in Canada. In Finland, they collected data on 537 women, following the families from 1702 to 1823.
The Canadian population was made up of 3,290 women born from 1850 to 1879. The research team followed these families through over 130 years.
In both groups they looked at the life span of the women, according to the report in the March 11 issue of Nature.
"Surviving grandmothers may have some effect on the reproduction of their own children," says study co-author Marc Tremblay, director of the Interdisciplinary Research Group on Demographics, Epidemiology and Genetics at the University of Quebec.
This influence is on the number of children that their own children have and how many survive to adulthood, he explains.
"On average, in the families where the grandmother was still living when their children started to have children, there were more children born compared with families where the grandmother was dead," Tremblay says.
Also, more children survived when the grandmother was around than when she wasn't, he adds.
The researchers also found that for these benefits to happen, the grandmother had to be nearby and available to help in raising her grandchildren. If the grandmother was living, but a distance away, the beneficial effect was not seen.
In addition, they found grandmothers were more likely to die when their own children reached menopause and could no longer have children.
This phenomenon appears to be the result of social rather than biological reasons, Tremblay says. There may be a biological component, but that cannot be supported in this study, he adds.
Tremblay's team plans to follow up the study by looking at the role of grandfathers and grandparents from both sides of the family.
"The message is that the presence of grandparents can improve the success of raising children," he says.
Dr. Kristen Hawkes, a professor of anthropology at the University of Utah, comments that "this analysis shows the impact that postmenopausal women have on the reproductive success of their kids and the survival of their grandchildren."
Hawkes points out in her editorial in the same journal issue that humans have an unusually long adult life compared to other primates. It may have something to do with grandmother's role, she says.
A lot of people think the increase in life expectancy seen over the last 160 years is the reason why there are more elders around, but "the fraction of adult women over 45 is essentially indistinguishable across really big shifts in life expectancy over time," Hawkes says.
"While all kinds of things are changing our reproductive rates, it should not blind us to a deeper pattern in our evolution which makes us age much more slowly than other apes and may be the consequence of the key role that grandmothers play," Hawkes says.
More information
The National Women's Health Information Center has more information on menopause, while Becoming Human from Arizona State University can tell you more about human evolution.
