Useful grandmothers
Scientists have again confirmed the usefulness of grandmothers for the survival of grandchildren
Ekaterina Rusakova, N+1
Scientists have once again confirmed the "hypothesis of grandmothers," according to two articles (1, 2) published in Current Biology. In Finnish families of the XVII-XIX centuries, the presence of a maternal grandmother was associated with a 30 percent increase in the survival rate of their grandchildren under the age of five. But the presence of grandmothers on her father's side did not affect her in any way. In Canadian families of the XVII-XVIII centuries, the presence of maternal grandmothers was associated with a large number of children. However, the "grandmother effect" disappeared if the relatives lived far from each other.
Women live long after menopause. This can be explained by the success of modern medicine, but perhaps the longer life expectancy was due to evolutionary advantages. According to the "grandmother's hypothesis", children who were cared for by elderly women who lived in the family survived more often. The researchers found that the survival rate of grandchildren depended on the age of the child, and on which grandmother took care of them, on the father's side or on the mother's side. However, until now, scientists have not analyzed whether a woman's age affects the "grandmother effect". It was also unclear whether it persists if the grandmother lives far away from her grandchildren.
To analyze whether the "grandmother effect" depends on her age, biologists from the University of Turku, led by Vipri Lummaa, studied the records in the church books of Finland for the years 1731-1895. They looked at the survival rate of children and analyzed whether it depended on the presence of grandmothers and their age. In total, the researchers analyzed 5815 grandchildren.
The Finns who lived at that time had large families, with an average of 5.5 children per family. But a third of children died before they reached the age of five, and almost half died before they reached the age of 15. They mostly died from infectious diseases, including measles and smallpox. The average life expectancy of adults was more than 60 years. At the same time, more than half of the women who had at least one child lived to be 50 years old and managed to raise grandchildren up to 5-10 years of age. Most of the country's inhabitants were engaged in agriculture. Usually the eldest son inherited the family farm, and his siblings lived in the same area. So both grandmothers often lived nearby, and the husband's mother usually lived in the same house with his family.
It turned out that the maternal care of grandmothers aged 50-75 was associated with a lower mortality of their grandchildren. In such families, children 2-5 years old had a 29.5 percent higher chance of survival. The presence of paternal grandmothers in the family was not associated with the health of grandchildren. And the husband's mother over 75 years old, who lived in the family, on the contrary, was associated with the deterioration of the situation. In such families, the survival rate of children under two years of age fell by 37.1 percent.
At the same time, the researchers analyzed the life expectancy of the women themselves. It began to decrease when they were over 60, and most of them had already had all their grandchildren. At 70, the risk of death increased three times, and at 80, it was six times higher than at 50. Presumably, it is the level of modern medicine that has made it possible to significantly increase life expectancy (residents of modern Finland have a life expectancy of 84.2 years).
In the second study, Canadian biologists and anthropologists led by Patrick Bergeron from Bishops University analyzed the Historical Register of the Population of Quebec (Registry de la Population du Québec Ancien), which contained information about French immigrants from 1608 to 1799. Scientists analyzed records from 149 church parishes in the St. Lawrence River Valley about 3,382 grandmothers who had 7,164 married daughters and 56,767 grandchildren.
Scientists have also noticed the "granny effect". Those women whose mothers were alive at the time of the birth of their first child gave birth to their first child four months earlier, and, on average, gave birth to two more children than women whose mothers died. The presence of a maternal grandmother in the family increased the survival rate of children under 15 years of age by an average of 1.14 children (there were an average of 10.2 children in the families of French immigrants). The researchers suggested that the further the mother and daughter lived, the weaker the "grandmother effect" was manifested. And so it turned out. In families who lived 325 kilometers away from maternal grandmothers, there were, on average, 1.75 fewer children, and the survival rate of children was lower (on average, 1.45 children) compared to families who lived next door to grandmothers. If the relatives lived closer, the picture changed. In families living 100 and 25 kilometers from grandmothers, on average, there were 0.58 and 0.14 fewer children, and 0.48 and 0.12 fewer children survived, respectively, than in families living next to grandmothers.
Earlier, anthropologists associated the emergence of the institution of marriage with the influence of the "effect of grandmothers". As shown by the mathematical model that the researchers built, the increase in life expectancy of women led to its increase in the entire population as a whole. Competition for young, fertile partners became higher, and they had to be protected from competitors. As a result, this led to the emergence of a long-term pair relationship.
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