A new study challenges a prominent evolutionary theory about why women live long after their childbearing years.

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

  • Lvxferre
    2•1 year ago

    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:

    • young male PoV - the children of the old male are likely his half-siblings (1/4 relatedness), rarely full siblings (1/2 relatedness)
    • old male PoV - the children of the young male are his grandchildren (1/4 relatedness)
    • livEnglish
      1•1 year ago

      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?

      • Lvxferre
        3•1 year ago

        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:

        • Alice - old female
        • Bob - old male
        • Charlotte - young female; unrelated to both above
        • Daniel - young male; Alice and Bob’s son, Charlotte’s mate

        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:

        • If Bob has a new child, Daniel won’t have his own child, to avoid starving his [half-/full ]sibling.
        • If Daniel has a child, Bob won’t have a new child, to avoid starving his grandchild.

        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:

        • If Alice has a new child, Charlotte will still risk it and have her own, even if the chance of the new child surviving is rather small. Because Charlotte doesn’t give a fuck about Alice’s children, they are not Charlotte’s relatives.
        • If Charlotte has a child, Alice won’t have a new child, to avoid starving her grandchild.
        • If Charlotte is likely to have a new child in the future, Alice won’t have a new child either - because it’ll likely die, but it’ll still reduce the odds of her potential new grandchild to survive.

        As such, Alice shuts off her reproduction through menopause, and Charlotte keeps high fertility.

        • livEnglish
          1•1 year ago

          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

          If Daniel has a child, Bob won’t have a new child, to avoid starving his grandchild.

          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!

          • Lvxferre
            2•1 year ago

            Don’t feel stupid - the subject is complex and it took me quite a while to understand it too.

            How does Bob do this? Why doesn’t he just menopause too? If menopause ensures more descendant survival wouldn’t they both do it?

            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.

            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?

            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).

            • livEnglish
              1•1 year ago

              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.