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The “grandmother effect”: a crucial role in demographic evolution

Photo courtesy of Krystel V. Morin/Bishop’s University.

From a biological perspective, menopause in women is puzzling: in comparison to what is observed in other species, it occurs much earlier, and long before the end of women’s life expectancy. This led researchers from the Université de Sherbrooke and Bishop’s University to wonder whether the relatively early end of reproductive capacity for women might have advantages with regard to gene transmission, by helping their daughters to establish larger families.

Their findings, published in Current Biology, constitute new supporting evidence for the “grandmother hypothesis.”

By studying the exceptionally well-detailed demographic data available on the first French settlers in Québec between 1608 and 1799, first author Sacha Engelhardt, a postdoctoral fellow at the Université de Sherbrooke, whose work was co-supervised by Dr. Fanie Pelletier, Canada Research Chair (CRC) in Evolutionary Demography and Conservation at the Université de Sherbrooke, and Dr. Patrick Bergeron, of Bishop’s University’s Biology Department, found that having a grandmother living close to her daughters was statistically associated with more grandchildren being born and with an increased likelihood that those grandchildren would reach maturity.

“Research results suggest grandmothers played a critical role in Québec’s preindustrial population,” Dr. Engelhardt explains. “We were very interested in looking at the geographical effect on humans’ life events, such as the age of first reproduction, how many children were born and how many children reached the age of 15, for instance.”

“In our study, women whose mothers were alive had more children, and more of those children lived to the age of 15,” explains Dr. Bergeron. “Interestingly, the ‘grandmother effect’ decreased as the grandmother-daughter geographic distances increased, suggesting that the potential for help may be related to geographic proximity.”

“The results show that daughters with geographically close living mothers on average were able to have two more children, and that the number of children still alive by the age of 15 increased by about one on average, compared to families where the maternal grandmother had passed away,” Dr. Pelletier explains. “This is a significant evolutionary advantage, especially considering that, in parts of that period, sometimes up to about a third of children born did not survive their first year.”

To access the vast amount of data on which their conclusions are based, researchers collaborated with Dr. Alain Gagnon and Dr. Lisa Dillon, of the Université de Montréal’s Programme de recherche en démographie historique. The study also benefited from using the Mammoth supercomputer of the Université de Sherbrooke’s Centre for Scientific Computing to analyze the data.

The research was supported by the Fonds de recherche du Québec – Nature et technologies, the CRC in Evolutionary Demography and Conservation, the Quebec Centre for Biodiversity Science, the Social Sciences and Humanities Research Council (SSHRC), the Canada Foundation for Innovation (CFI) and the Natural Sciences and Engineering Research Council of Canada (NSERC).

This article was adapted with permission from This link will take you to another Web site Bishop’s University.

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