Urine samples collected from wild chimpanzees in Uganda over decades have revealed older female chimps undergo hormonal changes much like those in menopausal humans.
One older female named Marlene, for instance, outlived her eldest, a male named Dolphy, with whom she was very close […] she took up a solitary existence and moved away into the northern part of their territory. […] Marlene would show up within 15 minutes, begging for meat. Even into her very old age, she’d almost always get meat.
And then Marlene shares half of her meat with a bunch of servals. And spends most of her day cleaning serval poop around her. (I’m joking.)
Resource scarcity: trying to raise too many children at the same time, in a given clan, puts at risk the likelihood of all those children to survive until adulthood.
Asymmetric relatedness: the children of an older female, in a chimp clan, are mostly unrelated to the incoming females (they’re basically in-laws). However, those children of the incoming young females are likely related to the older female (they’re likely her grandchildren).
Because of those two things, it might be advantageous to keep the older female alive, gathering resources for children, but not having children of her own. …that’s what menopause does.
Some maths with made up numbers illustrating this pointLet’s say that Alice has a son called Bob. Bob mates with Charlotte. And since they’re chimps, Charlotte went to live with Alice and Bob. They’re now a clan, with shared resources. Including the fridge. If the clan raises a new child, there’s a [made up] 90% chance that it’ll reach adulthood. But if the clan tries to raise two children, they’ll both become malnourished, and each will only have 20% odds of surviving. From Alice’s PoV, her own children share 1/2 of her genes. However, Charlotte’s children share 1/4, as they’re Alice’s grandchildren. From Charlotte’s PoV, her own children share 1/2 of her genes. However, Alice’s children share zero of their genes with Charlotte - they’re Charlotte’s in-laws, not relatives! As such, Charlotte will never give up having her own children for the sake of Alice’s. So Charlotte will always have a child. In this situation, Alice can choose between having another child or not. If Alice decides to give up having children, she’ll get a grandchild from Bob and Charlotte 90% of the time. So on average Alice shares (1/4)*(90%) = 22.5% of her genes with the newer gen. If Alice insists having yet another child, four things can happen: * only Alice’s child survive. This will happen 16% of the time. Alice shares 1/2 of her genes with her new child. * only Charlotte’s child survive. This will happen 16% of the time. Alice shares 1/4 of her genes with her new grandchild. * both children die. This will happen 64% of the time. Alice shares nothing with the kids because they’re both dead. * both children survive. This will happen 4% of the time. Alice will share 1/2 + 1/4 = 3/4 of her genes with both kids. If you do the maths for the above, Alice should expect to share, on average 15%, of her genes with the (0, 1 or 2) child[ren] of the newer gen. It’s actually less than the 22.5% that she would, if she gave up having children. As such, it’s evolutionarily advantageous for Alice to not have further children. And, if the clan had the resources to raise two children instead of one, the same reasoning still applies - it’s better for Alice to leave Charlotte to give her two grandchildren than to put their resources at risk by trying to get her own new kids.
It is both things - statistics on random events. Einstein would argue that God doesn’t play dice with the universe, but it seems that the gods play poker with living things instead.
At first I thought it was just a general mammal thing, and it probably is, but the question really is why only some mammals have evolved to survive after the reproductional age.
The article briefly mentions food availability as a key, and there are many other external causes, so it’s still not understood how or if longevity is inherited. In humans it’s generelly accepted that a long lifespan is partly inherited and partly due to society.
The similarities to other species is interesting to confirm, because it shows that the construction of a society can maybe influence the genetic evolution. In short, it’s (maybe) a sort of indirect feedback loop, where if a society can afford to have old individuals, it will also evolve to more individuals getting old. I realise that evolution itself can not do this alone, but it can apparently take advantage of happening in a society that enables this.
Anyway that’s just my take on it, I’m not a biologist.
Add comment