I've talked about menopause a fair amount on this blog, usually in relation to the Grandmother Hypothesis. So I thought I'd pass along this article, Eusociality, menopause and information in matrilineal whales, along. I know that many think that menopause is something that will naturally happen if a mammal lives long enough, as opposed to being an adaptation. I'm generally skeptical of this. The one physical anthropologist who I've talked to and who has explored the topic kept reiterating to me how contingent and interlocking the physiological cascades which shut down the reproductive cycle were. In contrast males tend to exhibit less fertility over time as their body just breaks down with age. Finally, of course it seems that even if there was some physiological process which would result in menopause if life history was pushed far enough down the line, over time adaptations should mask such enforced sterility (e.g., a new genetic variant which masks this phenotype).
Comments
I've long been curious about the menopause thing in women. From what I've read (As mentioned above) it's yet another unique thing about human beings in the animal kingdom. The theories that have been postulated about why women experience menopause haven't really worked for me. The grandmother hypothesis in partucular. I can't see how the value of grandmothers in a primitive human society, a social behavorial trait, could have affected human beings biologically in such a profound way. This almost smacks of Lamarickism. Also the theory seems to hinge on the idea that the grandmothers would be around to help their daughters raise their children. If we assume that a primitive hunter-gather woman in the days of yore had a daughter when she was 16 or 18; assuming the human lifespan is similar to what it is today, she'd still be able to have children when her daughter starts having babies. From what I understand we shouldn't even be able to live as long as we do, for mammals our size, isn't the age range only about 30 years or so? The only thing that makes sense to me is, for some reason, maybe it was our big brains, maybe some mutant gene, our potential life span was more then doubled. Only our reproductive organs or specifically a woman's reproductive organs, haven't caught up yet.
Posted by: Susan | August 30, 2007 4:05 AM
Re the grandmother hypothesis smacking of Lamarckism, I don't see this. As I understand it, the idea is simply that women whose potential life spans extend considerably beyond their fertile period will on average be able to devote more time to help raise their grandchildren, and as a result of such help will on average have more of those grandchildren survive and reproduce.
This increases the grandmothers' own fitness, and thus there is natural selection for genetic changes to shut down female fertility at an earlier age. As that age gets earlier, at some point the benefits to fitness of having more time to help grandchildren will be outweighed by the decrease in fitness due to having less time to have one's own children. Thus the age of menopause will stabilize at some point between the onset of fertility and average age at death.
As for the idea that women's reproductive capabilities simply haven't yet caught up with increased lifespan, I think Razib's point is that this is contradicted by the coordinated nature of the menopausal changes: If there were increases in average lifespan and no selection pressure for early termination of fertility, then we would see a gradual drop-off in fertility extending to the end of women's lives, as we do for men. But this is not the case.
Posted by: dev | August 30, 2007 9:13 AM
From what I've read (As mentioned above) it's yet another unique thing about human beings in the animal kingdom.
actually, the whole post is about menopause in matrilineal whales. it isn't uniquely human.
the rest i leave to dev ;-)
Posted by: razib | August 30, 2007 11:30 AM
Why do humans live to be as old as they do?
Because old people carry cultural memory.
Why do women go through menopause, but not men?
1. Fertility is immensely more costly for women than for men. Not merely childbearing itself, but just carrying around fertile organs has a big cost for women.
2. Speculation: as human bodies decline with age (attrition, entropy, etc.), perhaps the egg-production and sperm-production functions only decline a little, but the carrying-to-term function declines a lot. With modern medicine and nutrition this may have changed, though, just by reducing and repairing attrition.
Posted by: John Emerson | August 30, 2007 1:40 PM
Speculation: as human bodies decline with age (attrition, entropy, etc.), perhaps the egg-production and sperm-production functions only decline a little, but the carrying-to-term function declines a lot.
women have all the eggs they'll ever have at birth.
Posted by: razib | August 30, 2007 1:43 PM
So it would be purely (according to my hypothesis) that older mothers weren't as good an environment for the fetus.
IIRC, sperm are continuously produced and old-men's sperm is less viable. Or maybe I've just assumed that.
Posted by: John Emerson | August 30, 2007 3:07 PM
both sperm and eggs become less viable. the risk of down syndrome starts going up at 35, and shoots up at 40. so late children from "old" eggs might be very unfit.
Posted by: razib | August 30, 2007 3:11 PM
John: Speculation: as human bodies decline with age (attrition, entropy, etc.), perhaps the egg-production and sperm-production functions only decline a little, but the carrying-to-term function declines a lot.
But simply pointing to more reproductive problems with age does not explain why a menopause should evolve. If old females weren't contributing to anyone else's fitness, they might as well give it one last shot.
Are you familiar with Craig Packer's theory that "menopause" exists in all mammals, and females are only likely to stick around for the time that it takes to raise the last kid successfully? I think it resembles your scenario a bit. He says this is enough to explain why a human female hunter-gatherer is likely to survive to 55 instead of 40, assuming that children are dependent until their late teens. I am skeptical about this: he talks about lions a lot, they might survive for a couple of years after last reproduction, and that seems like a far cry from an actual menopause. And his theory does not rule out humans having a novel adaptation for shutting down the reproduction to be able to concentrate on raising that last kid.
razib: both sperm and eggs become less viable. the risk of down syndrome starts going up at 35, and shoots up at 40. so late children from "old" eggs might be very unfit.
But even if it shoots up (to a few percent?) is the risk so great that reproduction should be avoided altogether? The child mortality rate in the first year might be up to 20% in hunter-gatherers - even assuming every Down baby as extra mortality on top of that, I think they might not be a deal-breaker?
Posted by: windy | August 30, 2007 6:55 PM
But even if it shoots up (to a few percent?) is the risk so great that reproduction should be avoided altogether?
obviously depends on potential trade offs. if you live that long, as john stated, you might be a wisdom bank that the tribe (and your offspring) could use.
Posted by: razib | August 30, 2007 7:47 PM
The culture-carrier function has to be considered separately from the birthing / childraising function. It wouldn't be limited to childraising -- during all periods women had valuable knowledge in areas like weaving, horticulture, etc.
Posted by: John Emerson | August 30, 2007 10:28 PM
A really big one for H-Gs had to be what stuff is safe to eat, or what you have to do to poisonous stuff to make it safe. Some of the techniques adopted by Oz Aboriginals to render highly poisonous plants edible were really quite complex and non-intuitive, and I have to wonder how the hell they were ever devised. It's hard to avoid the semi-humorous mental image of a series of self-sacrifing experimenters, a string of Aboriginal J. S. Haldanes, doing increasingly weird things to poisonous plants and consuming the products until one of them managed to survive the experience. Other populations elsewhere had some similar processes.
Likewise medicinal use of plants - a lot of the traditional Aboriginal practices on this are only just being documented now, and they are surprisingly extensive, even given that some of that knowledge has already been lost, having died out with the last people who had it.
Posted by: Sandgroper | August 31, 2007 7:57 AM