You could argue that life is all about cheating death and having enough sex to pass on your genes to the next generation, as many times as possible. From this dispassionate viewpoint, human reproduction is very perplexing for our reproductive potential has an early expiry date. At an average age of 38, women start becoming rapidly less fertile only to permanently lose the ability to have children some 10 years later during menopause.
From an evolutionary point of view, this decline is bizarre. Other long-lived animals stay fertile until close to the end of their lives, with elephants breeding until their 60s and the great whales doing so in their 90s. In comparison, a human woman is exceptional in losing her child-bearing potential years or decades before losing her life. Even in hunter-gatherer societies that lack our access to modern medicine and technology, women who pass through menopause can expect to live well into their sixties.
Now, a pair of scientists have proposed a new model to explain the origins of menopause. Michael Cant from the University of Exeter and Rufus Johnstone from the University of Cambridge suggest that the loss of fertility helps to lessen reproductive conflicts between successive generations of women.
A few theories have already been put forward to resolve this conundrum. I've previously blogged about one of these, which suggests that the menopause reduces the health risks that repeated childbirth brings to both mother and child. This idea complements the most popular theory, known as the "grandmother hypothesis", which suggests that older, infertile women can still boost their reproductive legacy by feeding, teaching and caring for their existing children and grandchildren.
The basic idea makes sense and while some studies have backed it up, it's clearly not the whole story. Some analyses of hunter-gatherer populations have found that the indirect advantages of helping your family don't outweigh the potential benefits of having more children yourself. Alone, the grandmother hypothesis can explain why women continue to live past the menopause, but not why they go through it in the first place.
A new model
Cant and Johnstone believe that the current picture is incomplete because previous studies have ignored the fact that new children affect not just their mothers, but other members of the community too. The children of all the fertile women within any group draw upon the same pool of food, resources and attention from other adults, which effectively leads to a form of "reproductive competition" between mothers.
Cant and Johnstone suggest that menopause serves to minimise this conflict and cite the timing of menopause as evidence for their theory. In humans, there is remarkably little overlap between the reproductive periods of different generations. In hunter-gatherer societies, mothers tend to stop being fertile at more or less the same time that their daughters become sexually mature.
This degree of separation is truly exceptional among other primates, which, as the graph below shows, mostly become fertile while their mothers are still more than capable of conceiving. For example, the fertile periods of successive generations of Japanese macaques overlap by about 12 years, which is about 70% of their total reproductive lifespan. Based on the trends shown by other primates, human women would be expected to keep on bearing children till the ripe old age of 70, rather than the much earlier cut-off in their 50s.
From birth, a woman is outfitted with a lifetime supply of follicles, shells of cells that contain immature egg cells, and these are used up as she goes through more and more menstrual cycles. The stocks are gradually worn away but the process accelerates dramatically at about the age of 38, in a way that doesn't happen in chimps, monkeys or rodents. If this acceleration never happened, the earlier and slower rate of follicle loss would lead to menopause at around the age of 70, the same age predicted by trends in other primates.
Old vs. new
If older and younger women do indeed experience reproductive conflict, why is it the older generation who cedes ground by becoming infertile, and not the younger one? After all, in most mammals that cooperate to raise young, it's the other way round - the older generation continues to breed and suppresses the fecundity of the younger generation. Cant and Johnstone believe that it all boils down to how the social groups of our ancestors mingled with one another.
In all social mammals, groups exchange members to some extent and in most cases, it's the males that strike out. But for ancestral humans, there's plenty of evidence to suggest that the females were more often the ones who left home and found new groups. Genetic evidence, along with the behaviour of hunter-gatherers and our close relatives, the chimps and bonobos, all support this idea.
This simple fact changes how related different females are to other members of their group and that shifts the balance of conflict in favour of the young newcomers. A young female entering a group is better off raising children of her own, for she is completely unrelated to the group's existing members and gains no indirect benefits from helping to raise their children. On the other hand, an old female can benefit from either having more children herself or helping to raise any grandchildren that her sons father with the young newcomers.
Using a simple model to simulate these interactions, Cant and Johnstone found that these asymmetric benefits skew the results of the competition towards the younger females. The competition resolves itself in a stable way if the older females stop reproducing when the younger ones begin.
The duo freely admit that their hypothesis will need to be tested further and suggest ways of doing so. For a start, they note that if they are correct, scientists should be able to show that young mothers experience drawbacks if they have children alongside older grandmothers who are still doing the same, as is the case in some polygamous societies.
Finally, Cant and Johnstone note that their new hypothesis is not meant to be an alternative to existing ones, but a complement to them. They hope that it will help us to more fully understand the origins of menopause if we view it as a reflection of the "ghost of reproductive competition past".
Images from PNAS and Petercantfail
Reference: doi:10.1073/pnas.0711911105. This paper will be published in PNAS and I will upload a citation when it is.
I may have missed something but that doesn't really explain why elephants and whales don't have a reproductive competition and why their females continue to reproduce. Don't they have the same competition for resources amongst herd members?
According to the paper, the difference lies in the degree of resource sharing in humans and the extent to which children depend on adult help. It matters that hunted and gathered resources are shared extensively among the rest of the group. Also, human children are born fairly helpless and are heavily dependent on the help of adults to survive. These two factors combine to generate competition for both resources and help from adults. That makes sense to me but I wonder if anyone else has anything to add to this?
Assuming that is indeed the young women who are the newcomers (captives, perhaps), most existing members of the group are more related to the offspring of the older women. So if there's only enough food for some of the kids, I would expect the newbie and her husband to lose out.
Thanks for the follow-up Ed, I did miss something, apparently.
Ford - I think the model assumes that the newbie's husband was the son of one of the group members. The model's more sophisticated than I managed to describe in the post and it's probably worth checking out in full.
There are 3 human characteristics that may add to the evolutionary advantage of menopause: uprightness, brain size and hidden ovulation.
1) I've read that the biological cost of building and operating larger brains requires a trade-off and that we have smaller guts than chimps. Might that also create a drawdown from a woman's reproductive organs?
2) uprightness exacts a toll on bones and joints. After a certain age, maybe the resource cost of supporting a woman and her child are greater than the likely success of raising the child plus the woman's later contribution--due to the added damage to bones & joints from pregnancy and carrying infants. That's the social group's weighing in. For the individual woman, maybe the physical toll causes there to be an advantage to supporting her grandchildren over gambling she can successfully have more of her own.
3) Maybe the biological cost of hidden ovulation takes a toll. Maybe the biological cost of always being available for intercourse takes a toll. Maybe a reproductive system can't survive too many years of constant, "unfruitful" use. (Maybe the Catholics are right--it ain't for entertainment. Our original sin occurred 5 million years ago with hidden ovulation.)
Add to the mix that human males pose one of the greatest threats to female health with their jealous rages, forced obedience, and "kicking the dog--or wife" after unsuccessful status fights. And their attractiveness as targets of this violence is greatest during child-bearing years. Maybe females that had children, survived to raise them, and then became unavailable for mating had a survival advantage over females that continued to be available for mating and beating.
I'm a recovering lawyer, working as a programmer. Should I keep my nose out of a science I have no training in?
"I'm a recovering lawyer, working as a programmer. Should I keep my nose out of a science I have no training in?"
Absolutely not! You have posited three hypotheses for other people to consider and done so in a modest suggestive way. I think that sort of thing is to be encouraged not sniffed at.
I think that probably all three of your points and certainly 2 and 3 are dealt with by the earlier theory I blogged about - i.e. that repeated bouts of childbirth take a physical toll on a woman's body that brings costs to both her health and that of her children. That's supported by some evidence but I'm not sure as to the reasons behind those costs.
One of the things i enjoyed about practicing law was the challenge of quickly gaining enough understanding of a technical area to make my expert look smarter than their expert. So if any reader has thought about law school but wonders if they'd lose the payoff they get from technical studies--go to law school.
Your summary of the article and new hypothesis only seems to identify the differences in individual costs/gains in competing for group resources. In other words, you introduce the group's interest in maximizing resource application, but only discuss the individual costs/gains.
If the inquiry into reasons for menopause advantage focus on individuals, there is no reason to introduce the group's interests. The focus simply remains upon the costs to the individual in continuing to bear vs. advantages in stopping to aid grandchildren. Which is the "grandmother hypothesis" without the new hypothesis.
In summary: does the new hypothesis propose that uniquely-human social structure caused the group's resource allocations to favor young mothers over old? And if yes, the group's evaluation would have to be based on more than the likelihood of successful bearing and maturation of offspring--or it is just a gloss on the grandmother theory.
I think the place to look might be in the uniquely-human monogamy of parents. He wants to know that he's feeding only his children. She wants to know that he's feeding only her children. Of course monogamy is not ubiquitous among humans, but it is the prevalent behavior. Thus, it is not the group's resource allocation decisions as much as it is the 2-parent dynamic that gives an advantage to a couple that shifts focus from their children to their grandchildren.
Women... you'll never figure them out.
From the post: 'In hunter-gatherer societies, mothers tend to stop being fertile at more or less the same time that their daughters become sexually mature.'
Which societies are these? If young women become fertile by 15, the same age as their mothers did, these mothers will be 30. Does menopause start that early? Am I missing something?
If young women become fertile by 15, the same age as their mothers did, these mothers will be 30. Does menopause start that early? Am I missing something?
Puberty generally started later in hunter-gatherer societies, so fertility probably began a bit later than 15. Menopause was also earlier. Our society has early puberty and late menopause for a number of reasons, including better nutrition, but also including artificial hormones and the like.