JOIN USWhat do women, whales and elephants have in common? Answer: The irreversible cessation of fertility that occurs well in advance of the end of the average adult life span.
Or in plain English: menopause. In all the animal kingdom, only some kinds of whales, Asian elephants and women go through menopause. Naturally, scientists have long pondered over why this might be. And funnily enough, your grandmother might have a lot to do with this mystery.
Exactly why women go through menopause is still not known. None of the other long-lived species, such as elephants or tortoises, experience it. Neither do females of closely linked species. In chimpanzees, for example, more than half of the females die before they reach the maximum age of reproduction. On the other hand, the average woman spends at least a third of her life in a non-reproductive state, and a significant number spend half their lives or more post-menopause. Nor is menopause an artefact of greater longevity since the 20th century: scientists have shown that pre-industrial societies had roughly the same age distribution as today.
The hypothesis
Many of the explanations for menopause consider it an adaptation, implying that menopause and a significant post-reproductive life somehow confer a fitness advantage, meaning that they lead to greater reproductive success. The most influential amongst these is the so-called grandmother hypothesis, which has been around in some form or the other for a few decades but has received more attention in recent years.
The essence of this hypothesis is that by ceasing reproduction well before the end of their lifespan, women have the energy and resources to care for their grandchildren and so give their descendants a better chance at survival. In other words, a woman’s extended post-menopausal lifespan is beneficial not only for her children’s survival but also her grandchildren’s survival.
What this implies, among other things, is that grandmas have been the most motivated and experienced helpers mothers could have in raising their children through most of human history. And humans could do with help raising kids. Not only are human babies unable to fend for themselves for far longer than primate babies, we also bear children more often: Chimpanzees have a child every 4-5 years, orangutans every 6-9 years, while humans in natural fertility populations (which do not use contraceptives) have a child every three years.
Anthropologist Kristen Hawkes, from the University of Utah, was one of the first to articulate the hypothesis in its present form. In the late 1980s, Hawkes’ early research on the Hadza, a small hunter-gatherer tribe in Tanzania, was one of the first tests of the hypothesis on a living population. She showed how young Hadza children depend on older adult females, most often their grandmothers, for about half their caloric intake.
Since then, several studies have been done to test the hypothesis. Among the most compelling is a 2004 study published in the journal Nature. Using church and civil records of births, deaths and marriages, Mirkka Lahdenpera from Finland’s University of Turku, and her colleagues, analysed the survival and reproductive patterns of more than 500 women who lived in Finland in the 1700s and 1800s, and more than 3000 women who lived in Canada in the 1800s. In both these populations, they found that the longer a grandmother lived after her reproductive years, the more grandchildren she had: on average, two extra grandchildren for every ten menopausal years that she lived.
Crucially, more of them survived: having a grandmother more than doubled the probability of the grandchild living to the age of 15. Lahdenpera writes in her 2010 doctoral dissertation, “This work suggests that the post-reproductive lifespan is actually a fitness enhancing trait and not an evolutionary enigma.”
Yet, support for the grandmother hypothesis has not been universal. Grandmas, it seems, can run the gamut from having a positive effect, no effect or even a negative effect on survival of their grandchildren. And interestingly, paternal grandmothers often have none or even negative effects on survival of their grandchildren.
The genetic question
A clutch of recently published papers might explain at least some of these discrepancies. The grandmother hypothesis relies on the fact of genetic relatedness: grandmothers help their grandchildren because they carry at least some of their genes. But are grandmothers equally genetically related to all their grandchildren?
Enter the ‘X-linked grandmother hypothesis.’ In 2010, Cambridge biological anthropologist Leslie Knapp and her student Molly Fox and others proposed that because grandmothers and grandchildren differ in the degree of X-chromosome relatedness, so does the grandmothers’ investment in the grandchildren. Why particularly the X-chromosome? Because this one chromosome makes up as much as 8% of all our genes, including some which determine our fertility.
All women carry two X-chromosomes. Males have one X and one Y-chromosome. A son inherits his solitary X-chromosome from his mother and passes it on to his daughters.
This means paternal grandmothers are 50% X-chromosome related to their granddaughters. And because a son does not pass his X-chromosome to his male progeny, paternal grandmothers have zero X-chromosome relatedness to their grandsons. Similarly, maternal grandmothers are likely to be 25% X-chromosome related to grandsons and granddaughters equally.
This means that overall genetic relatedness between grandmothers and grandchildren varies from 23 to 31%.
X-chromosome relatedness
Re-analysing data from seven populations across four continents, Fox and her colleagues found that grandmothers’ effects on grandchildren varied according to their X-chromosome relatedness: paternal grandmothers, who are 50% X-related to granddaughters, were most beneficial to the survival of their granddaughters but least beneficial to the survival of their grandsons, with whom they share zero X-chromosomes.
In all seven populations, boys survived better with a maternal rather than a paternal grandmother. And in six of the seven populations, the paternal grandmother had more of a beneficial effect on survival of girls than boys. Maternal grandmothers showed an intermediate effect on survival of grandsons and granddaughters.
This paper, published in the Proceedings of the Royal Society, Series B, Biological Sciences, became one of the ten most downloaded articles in the journal that year.
And yet, one can’t help but wonder: do just genes and gender determine how much grandma loves you? Anyone who has ever been indulged by a doting grandmother will feel something amiss.
Along with the rest of your chromosomes, perhaps culture has a role to play. A few months ago, Yasuyuki Fukukawa, an associate professor in Waseda University, Tokyo, presented a test of the grandmother hypothesis in modern Japan. Using survey data on 3,168 women, he and his colleagues found that the only grandparent who had any effect on the birth of grandchildren was the paternal grandmother, who positively influenced the earlier birth of a grandchild and reduced the time between the first and second child.
They write, “In Japan, it isn’t unusual for a daughter-in-law to live with her mother-in-law at the same time she gets married. The implication, therefore, is that it is necessary to take any cultural context into consideration when testing the grandmother hypothesis in a modern society.”