As if there's not enough conflict and friction between mothers-in-law and daughters-in-law already, Michael A. Cant of the University of Exeter and Rufus A. Johnstone of the University of Cambridge, have added a new wrinkle to this relationship. In a paper published in the Proceedings of the National Academy of Sciences [vol 105, p 5332], Cant and Johnstone contend that the presence of daughters-in-law may have an adverse impact on the number of reproductive years of their mothers-in-law.
The "premature" onset of menopause in human females has been a puzzle to evolutionary biologists. The age of menopause is typically in the 40's and seems to be invariant with respect to life expectancy. Why should there be any reproductive advantage in foregoing offspring, when a woman may have several years left in her lifespan and her ovaries may still have as many as 1100 eggs on an average?
There have been a couple of theories to explain this puzzle. One, "the mother theory", hypothesizes that it may be advantageous to expend resources on existing children, rather than on a new baby near the end of the mother's life cycle "a bird in the hand is worth two in the bush" sort of argument. Such a strategy may be particularly optimal in humans, who have a long gestation period, and therefore, are high-maintenance, until they reach maturity [our political leaders are, of course, extreme outliers in this respect sorry, couldn't resist the temptation :)].
The second, "the grandmother theory", hypothesizes that a healthy grandmother without dependent children of her own, may divert her resources to the grand-children. The grand-children carry, on an average, 25% of her genes. It may be in her self-interest, therefore, to ensure their survival, the expected gains offsetting the expected losses from foregoing her own children.
Neither the mother hypothesis, nor the grandmother hypothesis has held up well against data. The numbers simply do not add up. Cant and Johnstone argue that the missing variable in the calculus of reproductive advantage from menopause, may be the daughter-in law.
In certain species, daughters and mothers cohabitate long after the former reach maturity. The younger females in those species have been observed to postpone reproduction while other older females who share their genes remain reproductive a sort of voluntary, quasi-menopause. Not so among humans. Historically, in many human cultures around the world, point out Cant and Johnstone, daughters have migrated from their maternal homes to their husbands' families. The daughter-in-law typically has no genetic relationship with her mother-in-law. She has no interest in propagating her mother-in-law's gene. The mother-in-law, through her son's genes, has an interest in her grandchild's survival, though. If we reworked the calculus of reproductive advantages and disadvantages, taking into account the cultural aspects of marriage and migration of human females, the early menopause strategy may gain better support.
Makes sense, I think, in the Indian context. In this predominantly patriarchal society, where joint family is still the norm, the mean age of menopause is 44.3, well below the world-wide average of 51. I don't know if the difference is significant, but it's in the expected direction. In contrast, should we expect the mean age of menopause in matriarchal communities to be higher than word-wide average? Cant and Johnstone pose this question elsewhere:
If stable patterns of matrilocality and male transfer persisted for long enough, we would expect the behavioural decision about when to stop breeding to be delayed, and, eventually, the evolution of a slower rate of reproductive senescence. Thus we would predict that among societies that have been strictly matrilocal for an extended period (many generations), cultural restrictions on reproduction by grandmothers should be less strict than among patrilocal societies, and mean age at last birth should be older in the former than the latter.
The Nair community of Kerala may provide an answer to this question. Apparently, the Nairs are matrilineal and matrilocal. They have historically followed a system called Marumakkathayam, where the family lived together in a Tharavadu, which comprised of a mother, her brothers and younger sisters, and her children. Matrilocal residence implies visiting marriage, meaning that husband and wife are living apart in their separate families, the couple visiting each other in their respective homes occasionally. The children of such marriages are raised by the mother's extended matrilineal clan in the Tharavadu. The practice of Marumakkathayam, apparently, is on the decline within the Nair community, with the cultural and legal changes that have been taking place in the rest of the society. Nevertheless, considering that evolutionary strategies are likely to be stickier than cultural strategies, the Nair community may provide a nice experimental setting to test the Cant and Johnstone hypothesis.
A more intriguing question, however, is: which is evolutionarily more stable matrifocal system + a higher age of menopause, or patrifocal system + a lower age of menopause and why?