Menopause: blame the daughter-in-law

Humans are one of the few social species in which females far outlive their reproductive lifecycle. This has been explained in part by the grandmother hypothesis, first put forward by American evolutionary biologist George Williams in 1957.

This hypothesis suggests that natural selection has favoured survival of older, post-reproductive women because they can assist their offspring to survive and reproduce.

However, while this hypothesis can account for the continued survival of grandmothers, it doesn't explain why women stop breeding in the first place.

Now, two UK researchers - Michael Cant from the University of Exeter and Rufus Johnstone of the University of Cambridge - say they have an answer: menopause is an evolutionary adaptation that minimises reproductive conflict between generations of women in the same family unit.

In a paper published in the April 1 edition of the US journal Proceedings of the National Academy of Sciences, Cant and Johnstone suggest that early reproductive cessation is due to reproductive competition between generations - namely, mothers and "daughters-in-law".

Other primates, such as our closest relatives, the chimpanzees, do not seem to go through the menopause at all, although their reproductive abilities do decline somewhat with age. A 2007 study by Melissa Emery Thompson from Harvard University, found no evidence that wild chimps experience menopause and that elderly chimps - those over 40 and even 50 - still reproduce.

The UK researchers note that in most primate societies there is reproductive competition but that it is the older females that retain breeding rights while younger females are repressed.

Our closest primate relatives do have something in common with ancestral humans but uncommon for most mammals - it is the female of the species who usually leaves home to find a mate (female-biased dispersal) while males stay in their own territory (philopatry).

Their hypothesis is that the rapid senescence of reproductive ability after age 38 in women coincides with the age at which they encounter reproductive competition from breeding-age females of the next generation, and in our ancestors this was typically comprised of immigrant females - "daughters-in-law".

These younger females would have an evolutionary advantage as they would have no genetic relatives and because a mother-in-law would have more interest in helping her daughter-in-law's reproductive success than vice versa.

The researchers stress that their hypothesis complementary, not an alternative, to the grandmother hypothesis.

DOI:10.1073/pnas.0711911105