Evolutionary Advantage of Menopause?

[Hand Bent Spoon]

I was trying to think of what the advantage of menopuase would be, but can only see the 'downside' of it in the form of such things as osteoporosis.

So why menopause? What's the benefit?

[LucyR]

I think you may be looking at it the wrong way. Reproductive age is reached long before menopause, and so the new generation can be produced without a problem.

[geni]

Originally posted by Hand Bent Spoon
I was trying to think of what the advantage of menopuase would be, but can only see the 'downside' of it in the form of such things as osteoporosis.

So why menopause? What's the benefit?

After a certian age the odds of surviveing childbirth fall so low that the person is unlikly to be able to have more children anyway. On the other hand staying around to look after the kids impoves their servivle chances.

[Hand Bent Spoon]

Originally posted by geni

On the other hand staying around to look after the kids impoves their servivle chances.

Ah, ha! We have a winner here.

Taking into account a woman simply being cut off from further childbearing alone only creates the question of why not let her at least attempt to have one or two more, after all she just might be successful, and if not, well, nothing ventured nothing gained.

But then you take into account the one thing my brain didn't come up with (the port is very good, indeed... ;) ), that the children who's mother didn't die in an unnecessary additional childbirth were more likely to survive themselves, thereby selecting women who went into menopause as being preferred.

Boy, do I love women. Even thier biologies are self-sacrificing!

[Rolfe]

I have thought about this one long and hard. Menopause is just one of several peculiar attributes human women have that are not seen in animals. I have a theory on this one however.

Consider, man is by quite a long way the most long-lived mammal. Certainly for his size. I suspect that an evolutionary precursor in the not-too-distant past had a life-span of maybe about 45 years. Man has evolved to live another 30 or so on top of that, but the reproductive tissue of the female is still as it was inherited from the shorter-lived ancestor.

So, rather than considering evolutionary advantage of the menopause (which I suspect there ain't, not sufficient selection pressure to amount to much anyway), it might be better to consider the evolutionary advantage of getting three-score-years-and-ten/four-score-years as opposed to dying of old age at 45.

Rolfe.

[Brian the Snail]

Originally posted by Rolfe

So, rather than considering evolutionary advantage of the menopause (which I suspect there ain't, not sufficient selection pressure to amount to much anyway), it might be better to consider the evolutionary advantage of getting three-score-years-and-ten/four-score-years as opposed to dying of old age at 45.

Perhaps this is because humans take much longer to reach sexual maturity than other animals, so children need care and support over a much longer period. This is much easier if there are grandparents to help out as well. Plus it's backup in case the parents die.

[Rolfe]

Originally posted by Brian the Snail
Perhaps this is because humans take much longer to reach sexual maturity than other animals, so children need care and support over a much longer period. This is much easier if there are grandparents to help out as well. Plus it's backup in case the parents die. I appreciate that, but I could never see that there was enough advantage there that the women who did go on ovulating until they died were selected out. Even if you do go on ovulating into old age, the idea that this is such a bad thing (because you almost inevitably don't make old age because you die in childbirth, even though you managed to avoid that when you were younger, and therefore your grandchildren don't make it to sexual maturity because without you their parents don't have enough childcare) that these people died out, seems to me relatively far-fetched. I don't think there's enough selection pressure there to make a decisive difference.

I'm much more attracted to the idea that there never were women who ovulated longer, the change has been in the life-span and the reproductive capacity never expanded to match (because it didn't have to, obviously).

Rolfe.

[Denise]

Sure, if you consider reproduction without education of the children the end all be all of the equation then childbirth until death would be a good thing. But considering the amount of knowledge that can be gained from the community, having women who no longer procreate but teach would be considered a good thing. Since humans can live in extended family environments, it's a good thing to not only have the parents, but four grandparents who also want to actively participate in education. Our success depended ( in my opinion) not just on individuals but the group as a whole social structure. I think that having relatives that were no longer bearing offspring but could focus on their offspring's children led much to our success as a species. Not to mention that many animals are dead before reaching menopause.
From this site http://www.sciencenet.org.uk/database/bio/animals/mammals/mammother/b00815d.html Do other mammals go through the menopause?


No, at least not in the wild. This is probably because in the wild, mammals' natural lives aren't long enough to reach the biological stages that cause the menopause.

[Rolfe]

Even kept as pets with all that modern veterinary medicine can do for them, and living until extreme old age, animals don't exhibit anything analagous to the human menopause.

I'm not saying there is no advantage to it. It's debatable. What I do think is that the idea that the species originally went on ovulating until old age but this stopped happening because of evolutionary pressure, is probably incorrect. I suspect that the reproductive benefit gained by having yet more offspring might counterbalance any tendency to increased mortality in the grandchildren. And that in any case any such tendency to increased mortality wouldn't be a strong enough pressure to get major change.

I think humans just evolved this way by chance, because the precursor species was less long-lived, and we got their ovaries. And it's worked just fine, so no pressure to change it.

Rolfe.

[Denise]

Other primates experience menopause. Maybe it's because of our social structure. I googled and found many links. Here's one. http://invention.swmed.edu/cgi-bin/etblast/abstract_local?pmid=12493689&user_id=1073934154

[cbish]

Hand Bent Spoon wrote:

QUOTE]Taking into account a woman simply being cut off from further childbearing alone only creates the question of why not let her at least attempt to have one or two more, after all she just might be successful, and if not, well, nothing ventured nothing gained.[/QUOTE]

This isn't exactly true. Reproduction is terribly "expensive" in terms of energy and process for a female compared to a male. A female egg is much more complex than just a DNA bag the male produces. Females develop a finite number of eggs prenatally. I don't recall exactly but I believe it's around 500-600. Evolutionarily speaking, menopause is probably more of the result. The question would be, why only 600 eggs? Why not 1000? Perhaps it's a balance of quality vs. quantity in which it's best to produce 600 "good" eggs vs. 1000 "weaker" ones.

[cbish]

One more thought.

Why does there have to be any advantage/disadvantage to something like menopause? Basically, life is nothing more than a vessel to pass on DNA. If that's been done, who cares? Game over. Apparently, for now, our reproductive cycles are "good enough".

[Suezoled]

*hordes the dark chocolate and leftover Cadbury creme eggs under her seat*

[neutrino_cannon]

I invoke the Evolutionary Theory of Aging .

Basically, organism capable of long life, notably those well protected from predators by either flight or hard shells, will live longer, and will have genes optimized for such life. Tortoises, which are very hard to kill, live quite a while for an animals of such size (even the small ones will keep trucking for seventy or so years) and are actually quite virile rigt up to the end, unlike us unlucky synapsids which are forced to endure a dropoff in *ahem* the rigidity of our reproductive abilities.

Animals that on average get killed at an early age by predators, like mice and the like, however suffer no penalty for random mutations that cuase debilitation at late ages, since few live long enough to encounter these effects. It doesn't matter if you have a gene that causes blindness, flatuence and joint troubles at age five if your species' lifestyle usually dictates reproduction and death before age four.

We are saddled, I would posit, with the genetic legacy of having been a relatively short lived species within our recent evolutionary past. Our purpose is to spawn some young, invest lots of energy in them to ensure their sucess, and then die and get out of the way so they can use the car.