The evolution of the menopause was 'kick-started' by a fluke of nature, but then boosted by the tendency for sons and grandsons go on living close to home, a new study by Liverpool scientists suggests. The full paper, titled 'Patterns of philopatry and longevity contribute to the evolution of post-reproductive lifespan in mammals' is published in the journal Biology Letters here.
When you think about it from a biological point of view....Menopause doesn't make that much sense, does it? All life is an embroidery of the fierce competition each living thing finds itself in in order to pass on its genes (for the most part). And yet, the females of some mammal species (including humans) spend up to one third of their lifespan unable to conceive offspring. So what's the point in slamming the breaks early on one's reproductive ability?
In an effort to understand why this happens, University of Liverpool and Liverpool John Moores University researchers applied phylogenetic principles to see how the most common theories of why menopause evolved stand up to scrutiny. They used data from 26 different mammal species (including three distinct human populations) to test the effects lifespan, group size and each sex's tendency to remain within family groups have on post-reproductive lifespan (PRLS)
This study, published in Biology Letters, used data from 26 different mammal species, including three different tribal or historical human populations, to test for the effects of lifespan, group size and male and female philopatry (the tendency to remain within a family group) on post-reproductive lifespan (PRLS).
First was the so-called "Grandmother hypotheses," probably the most widely-believed explanation for the advent of menopause. According to it women outlive their reproductive period to help with raising their grandchildren. This way, they increase their chances to reach maturity and have children of their own -- making the grandmother's genes more likely to be passed on.
Another theory they looked into is that evolution didn't actually make menopause happen -- we did. Because menopause offers no obvious advantage to the female, some hold that it's just a 'mismatch' that stems from the desynch between our long lifespans in the modern world compared to what we were likely to get in the wild.
The team determined that no one hypothesis could, by itself, adequately explain why menopause sets in. They suggest a new scenario, in which "non-adaptive origins followed by evolutionary tinkering" would bridge the two theories and finally explain menopause.
"Our results suggest that the menopause arose through a non-adaptive 'mismatch' between lifespan and reproductive span. Subsequently we think that in populations where males remained at home and females dispersed to reproduce, an adaptive benefit drove the extension of this post-reproductive period," said evolutionary biologist, Dr Kevin Arbuckle, from the University of Liverpool.
"This adaptive benefit could have come from grandmothers looking after their sons and grandsons at home. As females tend to reproduce more reliably than males, this additional family support could have made it more likely that their grandsons successfully reproduced. "
Co-author Dr Hazel Nichols, from Liverpool John Moores University, added: "Conflicting views in science can be challenging to reconcile, but our study suggests that both adaptive and non-adaptive ideas may be correct - it's just that they apply to different parts of the evolution of this most unusual reproductive trait."