Studies add weight to "grandmother hypothesis" as reason humans evolved menopause
For women, menopause is a completely normal and natural part of aging. Losing fertility between the ages of 40 and 50 simply marks a new phase in a woman's life. However, menopause is an incredibly uncommon phenomena in the animal kingdom. Only five species of mammals have been clearly shown to commonly undergo menopause and live significantly past their reproductive prime. Alongside humans, the other four species are all types of whale. Some primates and great apes have displayed characteristics of menopause, although generally these animal's lifespan ends around the same time as their reproductive period.
So why did humans evolve menopause when the vast majority of other animals on the planet are reproductively capable until the end of their lives? One of the more common hypotheses put forward to explain why women significantly outlive fertility is called the "grandmother hypothesis." The idea being that at a certain point in a woman's lifespan it becomes more valuable for her to stop having new children and turn her energies toward supporting her subsequent generations of offspring.
Two intriguing new studies set out to attempt to empirically quantify the effect a grandmother has on the survival of her grandchildren. To do this, researchers examined 17th and 18th century datasets from several locations around the world.
In the first study researchers examined pre-industrial Finnish church registers to assess how the lifespan of a grandmother affected survival rates of her grandchildren. This research revealed the presence of maternal grandmothers between the ages of 50 and 75 significantly increased both the amount of grandchildren born and their rates of survival. But, most interesting, was the finding that past the age of 75 the presence of a grandmother turned out to be detrimental to a grandchild's survival.
"Our work implies that, whilst post-reproductive lifespan could indeed have evolved at least partly due to beneficial grandmother effects, such benefits wane with age as the opportunities and ability to provide help decline, leading to limits to the evolution of even longer lifespan," says Simon Chapman, first author on this Finnish study. "As lifespan in modern industrialized nations is much longer than in the past, it may be that medicine has allowed us to overcome the 'natural' limit on longevity."
The second study looked at 150 years of incredibly detailed birth and death records from French Catholic priests in the St. Lawrence Valley, Quebec. This research looked at whether geographic distance between grandmothers and their daughters affected the health and lifespan of their grandchildren. The results were comprehensive, revealing the local presence of a grandmother resulted in the number of grandchildren surviving to the age of 15 increasing by an average of one per family. The further a grandmother lived from her daughter, the fewer grandchildren she had.
This study presents one of the clearest indications of the grandmother hypothesis to date, demonstrating that as geographical distance between the mother and grandmother increased, the number of grandchildren and their survival rates decreased. These results add evidentiary weight to the grandmother hypothesis, offering a granular insight into the local presence of a grandmother potentially influencing the survival of her grandchildren.
"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," says Patrick Bergeron, lead author on the geographic grandmother study.
Of course, this new research doesn't definitely prove the grandmother hypothesis as the primary evolutionary explanation behind menopause. There are a number of competing ideas out there attempting to explain the mystery of menopause and why humans are one of the few animals where females continue to live past reproductive age. However, these two new compelling studies offer interesting evidence suggesting a strong evolutionary benefit to having your grandmother around.