Why Evolution Needs Grandmothers To Stick Around

Study explains why it makes evolutionary sense for women to continue living decades after menopause.

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Evolution loves grandmothers—so the theory goes. Older, infertile matriarchs may not be able to add much to the gene pool, but they can instead focus their energies on caring for children and gathering resources, freeing up their daughters and to make more babies. The theory explains why evolution, a system designed to optimize childbearing and get rid of individuals who cannot reproduce, would allow women to live decades after menopause (indeed, most animal species do not survive long after their childbearing years). Now, a new study suggests that natural selection may have kept post-menopausal women kicking for yet another reason—they help make their grandchildren smarter.

Alas, all they get in return is hot flashes and mood swings. “The Grandmother Hypothesis has long neglected the role of cognitive resources by placing the emphasis on the role of strength in acquiring resources,” coauthor on the study Carla Aimé of the Institute of Evolutionary Sciences of Montpellier, told Fatherly. “This importance of cognitive resources, known as the Embodied Capital Model, has been suggested, but we are first to formally test it.”

Aimé is referring to two well-travelled hypotheses involving older, infertile women. The Grandmother Hypothesis states that natural selection favors women who have nothing better to do than care for existing children, increasing the likelihood that their own genetic material will be successfully transmitted to their progeny. Call it the “nanny hypothesis”. The Embodied Capital Model, on the other hand, holds that women don’t become peak nannies and hunter-gatherers until they reach the age of menopause, so natural selection keeps them around because it takes time for them to become proficient at transmitting wisdom and resources most efficiently. Call it the “tutor hypothesis”.

For the study, Aimé and colleagues ran evolutionary computer simulations to model how resource allocation decisions relate to reproduction in simulated populations. They first confirmed that grandmothers were a product of natural selection, finding that “menopause can be view as a selected trait which has evolved to increase gene transmission,” Aimé says. “Rather than the results of physiological constraints.”  They then tested the two aforementioned hypotheses, along with The Maternal Hypothesis, which states that women become infertile as an evolutionary protection, because each pregnancy increases their risk of dying. They did not find evidence for this third hypothesis, but were surprised to find evidence for both the rejected the Grandmother Hypothesis and the Embodied Capital Model.

In other words, this study demonstrates that grandmothers may exist to function as both tutors and nannies.

The biggest limitation of the study, Aimé says, is that it was a one-sex model that did not look at males. Although sperm quality and quantity decrease over time, men do not technically outlive their own fertility. “This is both the biggest limitation of our study, and the next step of our research,” she says, noting that the findings could have implications for grandfathers as well. “We found that the physiological cost of reproduction, which is higher for women, cannot explain menopause. So there seems to be no reason to expect a different mechanism for men.” Next up: where is man-o-pause?

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