Update: Comment from Chris Surridge of PLOS One:
Just a quick note. The paper is now formally published on PLoS ONE. The citation is:
Tuljapurkar SD, Puleston CO, Gurven MD (2007) Why Men Matter: Mating Patterns Drive Evolution of Human Lifespan. PLoS ONE 2(8): e785. doi:10.1371/journal.pone.0000785
As it is PLoS ONE you can rate the paper, annotate and discuss it there too.
There's a new preprint posted (PDF) on PLOS One titled Why Men Matter: Mating Patterns Drive Evolution of Human Lifespan. The basic question is this: why do humans live beyond the lifespan of the post-menopausal female, about ~55 at the outer bound? You might ask, "Why not?" As alluded to in the paper there is the problem of antagonistic pleiotropy, mutations which favor fertility early in life with a trade off of heightened mortality past reproductive age should always be favored. Over time these mutations would build up and there should be a "Wall of Death" past the age of 50 as these accumulated mutants manifest themselves.
And yet we observe this doesn't happen. So what gives? One model is the grandmother hypothesis. Roughly, this model posits that post-menopausal females aid in the survival of their daughter's offspring. In fact, this hypothesis purports to explain the physiological fact of menopause, which is after all a interlocking set of biochemical processes which "shut down" the female reproductive cycle. That is, it isn't a gradual breakdown, but a positive clamp down on potential fertility. The theory is that an older female gains more in investing her time and energy in increasing her daughter's fitness than she does in producing another child.
But of course this is a totally gynocentric view. The authors above claim that a model which includes the "long tail" of male reproduction can also explain why humans don't exhibit deleterious mutations which result in death rapidly after 50. Roughly, the model offers that polygyny, serial monogamy and the pairing of older males (past the age of 50) with younger females has been a pervasive selective pressure which favors alleles which allow one to survive past the age of 50. In fact, there is some data which points to the possibility that very long-lived individuals carry the same deadly mutants as shorter-lived individuals, but they also carry alleles which modify or suppress deleterious affects. In other words, modifier genes might have arisen in the genetic background due to the selective pressure of the long tail of male reproduction.
Of course it may be that both the hypotheses are correct. Both the grandmother effect as well as selective pressures due to the reproduction of older males as simply complementary forces in pushing the natural human life span further than the typical mammal. After all, the long tail of male reproduction does not explain menopause. But there is also the reality that a disproportionate number of societies do not simply discard their old males, even though the data does not suggest any grandfather effect. Rather, there are polygynous gerontocracies where older males live off the labor and muscle of the youth.
There is demographic literature about the "black-white" crossover in mortality: at older ages, 70 or 75+, the hazard of death of blacks crosses that for whites so the risk of death at any age is lower.
Demographers have gone through all sorts of gymnastics to talk this away, to explain it as some sort of sampling or data artifact. But they can't quite get rid of it, and it shows up in African censuses and data sets.
Maybe it is time for another look: Africa is certainly the continent where old males reproduce a lot.
Henry
One model is the grandmother hypothesis. Roughly, this model posits that post-menopausal females aid in the survival of their daughter's offspring...The theory is that an older female gains more in investing her time and energy in increasing her daughter's fitness than she does in producing another child.
But how does menopause increase her daughter's fitness? On any real evolutionary timescale where the combined effects of antagonistic pleiotropy would develop a "reproductive wall" (for some reason that idea bugs me, because it's so...gene-expresso-centric), would grandmothers confer any real survivability benefits to their progeny or grand-progeny? This genetic theory assumes a sociological viewpoint that may not be valid.
A case could be made that menopause prevents reproductive competition with her own progeny (imagine that this developed when life expectancies were much shorter, and assume that menopause still occurred at roughly past mid-life, and the resulting reproductive window suddenly is much curtailed), or could prevent the accumulation of deleterious alleles by accidental mating with her own progeny.
Menopause only really occurs in long-lived mammals (does it occur in elephants, I wonder?), so I'm wondering if it has more to do with that long life span than anything else.
Well, you can rock me to sleep tonight :-D
But how does menopause increase her daughter's fitness? On any real evolutionary timescale where the combined effects of antagonistic pleiotropy would develop a "reproductive wall" (for some reason that idea bugs me, because it's so...gene-expresso-centric), would grandmothers confer any real survivability benefits to their progeny or grand-progeny?
follow the link above, they do confer a benefit. grandmothers are there to aid their offspring, and obviously if they keep having pregnancies that does result in a risk of death due to the stress of pregnancy or childbirth.
Just a quick note. The paper is now formally published on PLoS ONE. The citation is:
Tuljapurkar SD, Puleston CO, Gurven MD (2007) Why Men Matter: Mating Patterns Drive Evolution of Human Lifespan. PLoS ONE 2(8): e785. doi:10.1371/journal.pone.0000785
As it is PLoS ONE you can rate the paper, annotate and discuss it there too.
I'm still not sold on the idea that there might not be a grandfather effect also, aside from the fact that males can continue to reproduce into old age. In hunter gatherer groups it's the older men who are the weapon makers and teachers. When they're too old to be effective hunters they teach hunting skills to the young boys. Considering the materials and tools they were working with, the weapons were not crude things, they took a long time and a lot of patience to make, and required acquired knowledge and fine craftsmanship developed from practice.
And the grannies are not just minding the girls, they're teaching them important survival stuff too. There had to be some selection for all that knowledge. Then when people get too old to be useful or too slow to keep up with the group, well there are traditions where they just get left behind to die.
I'm not trying to paint some noble picture, there has to be a reason for longevity and group support of the less physically able, and these seem like good reasons.
The other thing that is interesting is that long before menopause, females continuing to reproduce becomes a progressively more bad idea; the chances of defective or less able children increases pretty rapidly past the early 30s, and there has to be a reason for that too. As women get older they want sex more, but the offspring have progressively decreasing chance of survival. Over 40 the odds start getting really nasty. You know all this stuff.
As women get older they want sex more, but the offspring have progressively decreasing chance of survival.
No. The whole 'women hit their sexual peak at 40' thing is folklore, as the evolutionary logic of your comment alone should tell you. The horny hormones peak after puberty for both males and females. In reality, nearly half of women over age 40 don't even continue having sex. And this number continues to fall at a faster clip for females than for males (presumably because older men are still carrying on our ancestral tradition of sex with younger females!)
Obligatory picture for this post. :)
Thanks Jason. It did seem illogical.
I wonder where that "women get hornier as they get older" thing came from. You figure it came from some academic -- who was surrounded by contrary evidence, in the form of undergrads vs. faculty colleagues. More amazing stupidity.
One version of the folktale is that it happens due to women losing the Victorian inhibitions of their youth, becoming more comfortable with their sexuality and body, etc. -- despite the fact that young people in recent times are the opposite of Victorians, slutting it up every weekend, often with girls making out with girls in front of an audience at a party! How do you grow more uninhibited than that?
And obviously women grow more insecure and uncomfortable with the sexual aspects of their bodies as they age: that's why cosmetics, fashion, & surgery sell so well. Younger girls don't need to rely on "butt-sculpting" jeans -- they're high and tight on their own.
Some of this is clearly ego-protection from aging women, but a lot of it must be wishful thinking on the part of aging male losers, like "Finally, I'm going to be rolling in it, now that females are getting really horny." Yeah right.
Could it be that as women's bodies age, it becomes more physically stressful to bear and nurse children, riskier, etc.
The older body just doesn't handle it as well. Men have no such concerns.