Grandmothers can be great for many reasons. In terms of evolution, however, it’s puzzling why women might live long past their child-bearing years. Having them around may help later generations survive, find two new studies. But gran’s benefit seems to depend on her age and where she lives.
The studies are part of a broader effort to explain why menopause exists. This is the time at which a woman stops being able to have a baby. Throughout the animal world, females in few species live much beyond their reproductive age.
So what’s different about humans?
One idea is that long-living women may help their grandkids survive to adulthood. That would up the chance that gran’s genes would be passed on to future generations. Scientists sometimes refer to this as the “grandmother hypothesis.”
Today, especially in richer countries, relatively few people die in childhood. So it would be hard to tell in modern families whether grandmas affect kids’ survival. Instead, researchers have turned to data from past centuries, when people had larger families and more children died young.
One research team used church records. It reviewed those for 5,815 children born in Finland between 1731 and 1895. (Many churches kept detailed records about members, including births and deaths.) Back then, families had an average of almost six children. About one-third of those kids died before age 5.
Kids between two and five years old were 30 percent more likely to survive if their mom’s mom lived nearby, the data showed — but only if she was 50 to 75 years old. Living near dad’s mom, even if she was the same age,did not aid a child’s survival. Neither did mom’s mother if she was over age 75.
And when a dad’s mom lived past age 75? She might actually hurt her grandkids’ chances of survival, the team found. A child was 37 percent more likely to die before age 2 if their dad’s mom was older than 75.
Simon Chapman works at the University of Turku in Finland. “We said it as a joke when we had the idea for this study: ‘Oh killer grandmothers, wouldn’t that be such a great story?’” But then, this evolutionary biologist says, “We found it.”
Paternal grandmothers typically lived with their sons’ families, notes David Coall at Edith Cowan University in Joondalup, Australia. He studies how family structure can affect health. Parents back then could have found themselves crunched between the competing needs of ailing grandmothers and wailing babies. Young kids may have gotten less care when their parents faced higher demands, he suspects.
In the second study, researchers wanted to know if grandmothers helped even when families lived far apart. This team studied data from 1608 to 1799. It covered 3,382 maternal grandmas and 56,767 grandchildren in Canada’s St. Lawrence Valley. As in the Finnish study, these early settlers had large families. They also had high rates of child deaths. A bit different here, these families tended to move around a lot.
Grandma’s presence made a difference for these families too, the researchers found. The team looked at some families where a woman had died after her older daughter started having children but before her younger daughter’s kids were born. Daughters who started having kids when their moms were still alive had more children than their younger sisters. And the older daughters’ kids were more likely to survive to at least age 15.
But those benefits depended on physical closeness, notes Patrick Bergeron. He is an evolutionary biologist at Bishop’s University in Sherbrooke, Canada. For every 100 kilometers (62 miles) of distance between mothers and daughters, the daughters had an average of 0.5 fewer children, he reports. Once a maternal grandmother lived more than 350 kilometers (220 miles) away, she no longer boosted the number or survival of her grandkids.
Both studies appear in the February 7 Current Biology.
These findings may help explain how menopause evolved. But the trends may not hold up in the modern world. Parents today tend to have fewer kids and to live farther from family, says Chapman. And children face different challenges, such as stress and other mental-health issues. He’d like to see data on whether grandma’s presence might help with these problems.
Both studies provide an interesting peek at life in past eras, says Melissa Melby. She studies menopause and health at the University of Delaware in Newark. But she remains skeptical about the grandmother hypothesis. Menopause may have come about by accident, she says. Maybe evolution favored men who could reproduce into old age. If true, those men would have passed on their longevity genes to both their sons and daughters.
Women in the Canadian study had babies until age 40 or so, Melby notes. So maybe a long life helped those grandmothers rear their own children as well. Post-reproductive life is often defined as starting at menopause, she says. “But it’s not just about getting the baby out. You need to raise that baby.”