It is not obvious why selection should favor menopause or the continued survival of individuals that can no longer reproduce. The famous Grandmother Hypothesis had been used to explain the evolutionary significance of menopause. A new study conducted on the Ngogo chimpanzees community of wild chimpanzees in Uganda challenges this hypothesis. Science 27 Oct 2023 Vol 382, Issue 6669 DOI: 10.1126/science.add547
A related link was posted in this comm not too long ago.. It tries to address why female chimps would live past reproductive age, to begin with.
The catch here is that adult male chimps stay in the clan of their parents, while the female ones migrate to other clans. And this creates an asymmetry between old vs. newer adult females in the same clan:
In situations where food is short, it’s advantageous for the clan to have less children: every new child spreads the food resources thinner, and puts at risk the lives of the other children. But that pressure to stop having children only affects the older female, because it puts at risk the lives of her grandchildren; for the newer females it’s more like “why would I stop having children? For the sake of my in-laws? Screw them!”.
Evolution solved this through menopause; you got the older females still alive, gathering resources, and taking care of the children of the clan, but they aren’t bearing new children.
I think there is a much simpler explanation: childbearing is an extra effort for the organism, having the highest success rate with individuals in peak physical shape, so in order to maximize the number of food gatherers for the group, it makes sense to prevent older individuals from being exposed to the risk at all, and shutting down the whole mechanism further reduces their energy requirements.
The process only makes sense in species where individuals can live and contribute to a group long enough after the childbearing risk has reached some threshold. Also meaning, the longer and “more expensive” the pregnancy, the earlier that threshold might be reached.
Otherwise, if they don’t live in a group, and/or their average lifespan is not long enough, and/or the pregnancy is relatively short and inexpensive, the evolutionarily beneficial process is to keep trying to reproduce until the effort is too much and the individual dies, stopping to compete for resources.
It becomes the same explanation once you take into account that younger females don’t care about the rest of the clan - because they are not her relatives. Only the older females have some reason to shut down the mechanism.
Kind of weird though that the males don’t feel the same grandchild pressures.
They don’t because the males in a clan are likely all related, as father and sons and grandsons. For them the relationship is mostly symmetric:
But isn’t old male and old female POV the same?
For both of them the new babies are biological grandchildren. So why would only one of them want to stop producing more? Why is there not a male menopause?
What am I missing here?
The competition with the younger generation putting the older females in a disadvantageous position, but not the older males.
Let me put it this way. Imagine the following chimp clan:
Now imagine that the clan has resources to raise exactly one child. Once it has two children, both are likely to starve.
From the male side of the things:
As such, you’ll see fertility going down regardless of age, to adapt themselves to the situation.
From the female side of the things, the picture is different:
As such, Alice shuts off her reproduction through menopause, and Charlotte keeps high fertility.
I feel like the stupidest person in the world because I still don’t see the difference between Bob and Alice and now I also don’t understand this part
How does Bob do this? Why doesn’t he just menopause too? If menopause ensures more descendant survival wouldn’t they both do it?
Why doesn’t Alice just die?
The troupe still have to find enough food for her, how is that an evolutionary advantage to keep a non breeding member around?
If something happens to Charlotte now the troupe cannot reproduce unless they go out and find a new female, but if something happens to Daniel then Bob can still reproduce with Charlotte. What is the advantage in that asymetry?
Edit: I was puzzling over the Charlotte factor. Is it more that somewhere along the line the Charlottes of this world were killing the non-menopausal Alices? Because that kind of would make sense.
Thank you so much for taking the time to try to explain it by the way. If you don’t feel like answering my latest round of questions that’s okay!
Don’t feel stupid - the subject is complex and it took me quite a while to understand it too.
Because both Bob (the old male) and Daniel (the new male, likely Bob’s son) are slightly discouraged from having new children, until they get access to more resources. That results in both ceding a bit, but not too much - with a slight lower fertility for both sides, but they don’t shut off reproduction completely.
The same won’t happen between Alice and Charlotte, because no matter what Alice does, Charlotte will keep pumping out children. So Alice keeps ceding, ceding, ceding, for the sake of her grandchildren, until she has zero fertility (i.e. menopause).
Note how Charlotte and Daniel’s roles are essential to understand why Alice and Bob behave in one or another way. Hypothetically speaking, if Daniel kept pumping out children even if this endangered Bob’s children (i.e. Daniel’s siblings), Bob would eventually be forced to undergo menopause, like Alice. That doesn’t happen though.
Alice is an adult. As such, she likely contributes with more food than the clan needs to provide her. She might not be getting new children, but by hanging around she improves the odds of survival of her grandchildren. (That’s also present in the grandmother hypothesis.)
Give this article a check. It’s explaining menopause for another species (humans), but the reasoning should be identical. There’s also this article about menopause in cetaceans, but take conclusions from it with a bit of salt because the social structure among cetaceans is different from ours (humans and chimps).
Thanks so much, I understand the hypothesis now!!
And that article does show how it could map onto humans. For some reason I had been under the impression that early hominids did not necessarily have the females-as-strangers setup.
It’s interesting to compare with elephants, who are matriarchal. The “Alice” of an asian elephant herd will often stop having kids (though, she biologically still can) so her daughters can have some, even though unlike Charlotte, her daughters are related to her so theoreticly it’s more of a Bob/Daniel situation.