From an evolutionary perspective, human menopause doesn’t make a lot of sense. Searching for an explantion for this somewhat uniquely human process, a group of Canadian researchers used computer models to show that menopause is an unintended outcome of natural selection — and men's sexual preferences might be to blame.
No doubt, menopause, or reproductive senescence, is a strange biological phenomenon. Virtually every female creature on Earth remains reproductive until the day she dies. Among mammals, whales have been observed to give birth into their 80s, and elephants into their 60s. Other than humans, the only animals to experience menopause appear to be some whales and captive chimpanzees. The question that scientists have been asking, therefore, is why humans have such a pronounced postmenopausal survival stage.
Biologists have come up with a bunch of theories to explain menopause, including the grandmother theory — the idea that women evolved age-induced infertility to help them assist with the rearing of their grandchildren. At a certain level this makes sense; it’s a process that improves the chances of their kin surviving. Interestingly, the grandmother hypothesis may also be responsible for human longevity.
There are at least nine other theories that try to explain human menopause, including the lifespan-artifact hypothesis (it’s a consequence of human longevity) and the patriarch hypothesis (the origin of menopause allowed men to mate with younger women, resulting in increased longevity (for both men and women) and increased status in society).
But this last hypothesis fails to explain why menopause evolved in the first place — and this is where the new study, which was published today in PLOS Computational Biology, comes in.
According to computer models set up by Rama Singh, Richard Morton, and Jonathan Stone of McMaster University in Hamilton, Ontario, competition among men of all ages for younger women left older females with a diminished chance of reproducing. Over time, natural selection protects fertility in women — but only when they are most likely to reproduce. The researchers’ simulations showed that male mating preference for younger females resulted in the accumulation of deleterious mutations that negatively impacted fertility among older females. The net result: the advent of menopause in the human species.
To run the simulation, the researchers relied on two primary assumptions. First, that there was a shift of male preference toward younger females. And second, that there are female-specific mutations with detrimental effects on fertility in older women (e.g. hormonal changes, oocyte depletion and ultimate loss, ovulation cessation, menstruation termination, and so on).
After running the simulation over a hundred times, the researchers consistently saw that the older female simulants eventually acquired enough mutations to result in the evolutionarily acquired condition of menopause.
Interestingly, according to this theory, if men didn’t prefer younger females, women of all ages would still be able to reproduce. Also, if the tendency were reversed, and it was women who preferred younger males, then fertility among older males, instead, would have been compromised.
Needless to say, the use of computers to understand evolution, while interesting, is a wholly inadequate approach. Yes, the conditions set up in the simulation resulted in the menopause effect, but that doesn’t necessarily mean that real life evolution occurred in the exact same way.
For example, Singh’s theory is contingent upon the assumption that males, for the most part, prefer younger females. The paper does very little to explain this assumption aside from references to two other papers, both written by Singh. This is problematic because the model assumes the existence of male-driven sexual selection — and the computer models don’t account for female preference of mates. Nor does it account for cultural factors (which, when describing human evolution, has to be considered as a major contributor; memes can oftentimes direct the evolution of genes).
At the same time, given tough competition among males for females, it’s not unreasonable to suggest that many males would still look for older females (including, and especially, older males). Why older females should have been simply been left out doesn’t make a whole lot of sense. The only thing I can think of is that females might have been the ones to prefer older males (i.e. female driven sexual selection), or that the older males held back the younger males from mating.
Moreover, the researchers have not satisfactorily dismissed the other prevailing theories, including the lifespan-artifact hypothesis and the senescence hypothesis. Humans do in fact live extraordinarily long lives, and it’s quite possible that fertility is simply one of the first things to go; life goes on, but fertility does not. At least in Homo sapiens. It is, after all, a rather complex biological process. More work needs to be done in this area.
Lastly, given that menopause has been documented in whales and chimps, it would be worth doing a study on those animals to see if the males of those species also prefer younger females.
Read the entire study at PLOS: “Mate Choice and the Origin of Menopause.”
Top image: It's Rude to Stare.