The “grandmother hypothesis” — the evolutionary proposition that women live long past their reproductive years in order to help their grandchildren survive and pass on their DNA — has received support from two new studies.
But the studies, published in the journal Current Biology, also found that the hypothesis comes with a couple of important limitations. Grandmothers who are younger and who live closer to their grandchildren tend to have the greatest influence on their grandchildren’s survival.
At least, that appears to have been the case in the two preindustrial 17th– and 18th-century Finnish and Canadian populations on which the new studies are based.
“Grandmother help is central to human families all around the world, but we find that the opportunity and ability to provide help to young grandchildren declines with grandmother age,” said Virpi Lummaa, a co-author of one of the studies and a biological anthropologist at the University of Turku, Finland, in a released statement.
“In our study, women whose mothers were alive had more children and more of those children lived to the age of 15,” added Patrick Bergeron, the lead author of the second study and an evolutionary biologist at Bishop’s University in Canada.
“Interestingly,” he continued,” the grandmother effects decreased as the grandmother daughter geographic distances increased, suggesting that the potential for help may be related to geographic proximity.”
An age limitation
For the Finnish study, Lummaa and her colleagues used a database of all births, marriages and deaths recorded in Finland between 1731 and 1895.
Child mortality was high during that period. One-third of Finnish children died before their fifth birthday, and half died by age 15, usually from infectious diseases (especially tuberculosis, smallpox and measles), but also from accidents.
For those individuals who reached adulthood, life expectancy exceeded 60 years, and more than half of women with children survived at least to the beginning of menopause (around age 50). Most women were grandparents for the first time by their 40s and for the last time in their late 60s.
It was common during the study period for grandmothers to reside near — or with — their children and grandchildren, although typically only paternal grandmothers lived in the same house.
Using all that data, the researchers sought to identify any association between the age of the grandmothers and the survival of their grandchildren. They found that grandchildren, particularly infants, were more likely to survive when maternal grandmothers were around — a finding that supports the grandmother hypothesis.
But once the grandmothers passed the age of 70, their presence became decreasingly important for their grandchildren’s survival. In fact, having a paternal (but not maternal) grandmother over the age of 75 living in the house was actually detrimental to the survival of infant (but not older) grandchildren.
That last finding may have been due to “some competition between grandchildren and co-residence grandmothers for parental resources,” the researchers speculate.
A geographical limitation
For the second study, Bergeron and his colleagues analyzed data on the first French settlers who came to Quebec between 1608 and 1799. These families were typically quite large. The women gave birth, on average, to 10 children — the first when they were around 22 and the last when they were around 41. They also averaged 50 grandchildren each.
The typical lifespan of these women was 69 years.
The researchers combed through the data to determine if the grandmothers among these French settlers had any effect on the number of children their daughters had. They did this by comparing the reproductive history of sisters who lived with or without their mother. That enabled the researchers to control for the potential effects of family genetics on family size.
The data revealed that daughters with living mothers had, on average, about two more children than daughters whose mothers had died. They also had, on average, about one more child who survived to the age of 15.
Both of those findings support the grandmother hypothesis.
But, the further away the mother lived from a daughter, the fewer children the daughter was likely to have. In addition, the daughter tended to have her first child at a later age.
Specifically, a woman who lived 200 miles from her mother had, on average, 1.75 fewer children than her sister who lived in the same community (or house) as the mother.
So, the effect of having a living grandmother was primarily beneficial to the daughter’s reproductive success if the grandmother lived nearby.
“In investigating geographic distances, we have empirically shown another mediating factor of grandmother help, adding another piece to the complex puzzle of post-reproductive lifespan,” the researchers conclude.
Both teams of researchers say they plan to continue studying the role of grandmothers in different populations to get a clearer understanding of the evolutionary puzzle of why humans are one of the few species in which females have a post-reproductive life.
FMI: You’ll find both studies on the Current Biology website.