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There is a humanoid species whose natural lifespan/period of senescence is very short compared to other sapients of the setting(around 20 years). They have similar levels of intelligence to humans and have sapience but due to them only living long enough to reach adulthood and reproduce but not long enough to properly raise and educate their offspring most members are either civilized and relatively educated orphans in the cities or savage and/or feral groups in the wild.

Despite their short lifespan being widely known about there are nonetheless individuals of the species who seem to live far longer than is normal for their kind, even longer than humans in some cases. The secret behind these extended lifespans are not discussed or well known about even among their own kind but to summarize it all boils down to raw(uncooked) cannibalism. They can eat members of their own kind to extend their own lives, the amount of time gained depending on the mass they consume to a maximum of three years per adult eaten(bones and marrow excluded).

While the setting is planned to have some sort of magic I want most things to exist as independently from magic as possible.

Are there any natural biological mechanisms that would allow an organism to extend their lifespans via cannibalism?

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    $\begingroup$ Your species might need a pretty high reproduction rate unless you can guarantee everyone eating everyone all the time is not an uncommon occurrence, lest they'll drive themselves to extinction through sheer inbreeding. $\endgroup$ Commented Nov 12, 2021 at 13:22
  • $\begingroup$ Yes metabolism, everything living has it, you don't eat you die, cannibalism works as well as any other source of food so if it's to avoid premature death by starvation there's your answer. $\endgroup$
    – Pelinore
    Commented Nov 12, 2021 at 21:42
  • $\begingroup$ @ProjectApex as I understand, not everyone in this species is a cannibal, just a few of them. $\endgroup$ Commented Nov 13, 2021 at 0:09
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    $\begingroup$ Are there any natural biological mechanisms that would allow an organism to extend [its average natural lifespan via the ingestion of a substance not ordinarily consumed]? So, basically the fountain of (the) youth. Not that I know of, lemme know when you find out, +1 $\endgroup$
    – Mazura
    Commented Nov 13, 2021 at 1:35
  • $\begingroup$ Cannibalism pigeon holes this question into a world that doesn't even have birds. $\endgroup$
    – Mazura
    Commented Nov 13, 2021 at 1:42

7 Answers 7

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How about an inverted/slightly altered version of Kuru disease?

Kuru disease is a transmissible spongiform encephalopathy (TSE), which is famous for being transmissable and becoming deadly due to cannibalism (Wiki here: 1)

Inspired by KEY_ABRADE's answer when he mentioned the immune system: What if your species makes a form of antibodies that are stored in a similar way as the Kuru infected prions? They make the antibodies abundantly whilst they are growing up. But they stop making antibodies in their system when they come around the age of 18-20 and die as a result of a multitude of normal diseases. The only way they could gain more of these antibodies would then be via cannibalism.

Even though body size (BMI) can vary wildly between humans, brain size is more or less the same (on average) per person. If, just like the infected prions with Kuru disease, the antibodies are mostly stored in the brain for some reason, this would also yield roughly the same amount per eaten person, no matter their body size. Consuming a normal adult would yield roughly 3 years of antibodies, after which they would need to cannibalise again. Eating children would therefore be less efficient, as they do not have the same amount of antibodies.

Ofcourse these antibodies (and their entire immune system for that matter) would need to work differently than the ones we as normal humans have, as we continuously make new antibodies to replenish the dead ones.

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    $\begingroup$ This is better than my answer. $\endgroup$
    – KEY_ABRADE
    Commented Nov 12, 2021 at 13:55
  • $\begingroup$ Of course, a lot of research would be spent on synthesizing the antibodies in an artificial manner... or just finding any other lifeform that synthesizes it or could be GMO'd into synthesizing it. $\endgroup$ Commented Nov 13, 2021 at 10:30
  • $\begingroup$ @JohnDvorak Only once they'd reached a certain level of scientific aptitude (e.g. equivalent to within the last 200 or so years of Human civilisation), and only if the culture that developed for this species decided that it was worth doing. Anyone who suggested it might be cast out as a social pariah! $\endgroup$ Commented Nov 13, 2021 at 17:33
  • $\begingroup$ Here's an interesting modification: instead of antibodies to "a range of normal diseases", it's only a disease that's native to this part of the world? Or something else local, like the rocks and ground are high in radioactive substances, just enough to have long-term effects to this race of yours? Then, at some point, one of these natives shows up after living on another continent, and he's 40 years old even without the benefit of eating brains? $\endgroup$ Commented Nov 13, 2021 at 22:08
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I do not believe that any such mechanisms exist in nature in real life. However, let me make one up.

Speciesus lemminginus has an exceptionally weak immune system, and members of this species tend to die of disease frequently. This is why their average lifespan is around twenty years. However, they are exceptionally robust in every other aspect of their biology; without this crippling tendency to die of disease, they can easily live longer than a human.

Individuals of speciesus lemminginus are capable of integrating memory T cells from other members of its species into their own immune systems, thereby increasing their immune system's knowledge of various types of pathogens, parasites, and bodily invaders of all kinds. Basically, the body of a member of this species sees a memory T cell show up inside it and goes "okey-dokey, that's-a mine now". They will have some special biochemical mechanism for extracting these cells from food without lysing them.

The problem is that these T cells are inside other members of its own species.

The solution to that is for members of this species to eat other members of this species; preferably, ones they doesn't like, or ones that they're incapable of reproducing with. On the bright side, this means that this trait will likely stick around, as members that are capable of this are not only more immune to diseases than other members, but they're also good at killing off reproductive competition.

The reason that this cannibalism is "uncooked" is because cooking destroys the memory T cells. They have to be taken directly from the source.

I'm not sure how to make it "a maximum of three years per adult consumed", since that seems oddly specific, in my opinion, but I think I got the rest of it.

By the way, this is personally how I explain vampires.

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    $\begingroup$ @MolbOrg You'll have to define "rationality, science, and expertise"; that could be anything from "the answer above works" to "anything without sources and the hard-science tag is invalid". Also, Lemming is allowed to use this even for commercial purposes under CC BY-SA 4.0: see creativecommons.org/licenses/by-sa/4.0 for details. Even if it somehow meant that they weren't allowed to reproduce my idea - which it doesn't - licensing should not be a reason for a -1. This is a place for writing and sharing answers, not a place for writing things that fall within licensing requirements. $\endgroup$
    – KEY_ABRADE
    Commented Nov 12, 2021 at 13:24
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    $\begingroup$ @MolbOrg I don't see how either of the other answers is more based in fact than mine is. There isn't a species in real life that lives longer via cannibalism, nor is there a biological mechanism for doing so. Both of the other answers are also making things up, but, like mine, there's some kind of tie-in to a real thing: kuru, memory T-cells, enzymes required for life - see taurine, for instance. I just said it out loud. $\endgroup$
    – KEY_ABRADE
    Commented Nov 12, 2021 at 18:55
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    $\begingroup$ @Molb0rg What are you on about? This is world building. The logo on the side is a fictitious traveler on an alien planet when their cyborg companion projecting a holographic map. The whole point of this SE is to make stuff up. You aren't building a world without some creativity. I'm dumbfounded by your expectations. The only thing you've said I can agree with is the first acronym (idk) - that seems right. $\endgroup$
    – TCooper
    Commented Nov 12, 2021 at 20:24
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    $\begingroup$ Hi, @TCooper, nice to see you here, also you may refresh the course tour and also help center - I periodically do it myself, so there is no shame. It's not super well written/informative, but it gives some hints it is not about making stuff up, scifi.SE is a better place for made-up stuff, I guess. As I said it is very easy to make stuff up, there is no fun and no challenge and the most fun part is that in many cases it is not required - Clarke tech in action, as long as it does not break physics. $\endgroup$
    – MolbOrg
    Commented Nov 12, 2021 at 23:22
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    $\begingroup$ @MolbOrg There is a tag called “science based”. It’s based on science but guess what, everything but the base is made up. It’s creating a new world and that involves creating things. Yes the things should be based on reality, within known laws of physics, etc. it’s still novel ideas being made up. You’re being silly in how you pick and choose word definitions. $\endgroup$
    – TCooper
    Commented Nov 14, 2021 at 19:43
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Biological? close enough.

A long, long time ago the world was inhabited by a race of eternal immortals.
These immortals achieved their immortality by mean of their advanced technology. Specifically, by microscopic nanobots that swarmed through their bodies, repairing damage at a cellular and even sub-cellular level. These nanobots were almost indestructible, could not replicate themselves, and were very, very, very expensive to make thus the immortals used only enough of them to keep themselves immortal.

Then the immortals died off. (hey, it happens!) Who knows why, maybe they ascended to something more? Maye they got tired of life? Maybe they forgot how to make new nanobots?
Regardless of the cause, their passing released the super-healing nanobots into the environment.

Where the nanobots now inhabit your species of interest.
But there are too few of the nanobots to make them immortal. Or even to extend their lifespan much. And their natural lifespan is so miserably short. (see footnote)

Unless they get some more nanobots!
But they are completely unaware of the little things.
All they know is that if they eat the raw flesh of their own species, especially that of one that is unnaturally healthy and strong and old, then they inherit this vitality. (because they ingested 1 or 2 nanobots from the other, but they have no way of knowing this)

Thus the cannibal gains strength from the act of cannibalism.

Footnote:
It is quite likely that your species is the same species as the original immortals, but their biology became so dependent on the support of the nanobots that their unassisted health and lifespans were badly compromised.

It takes many thousands of the little nanobots to achieve immortality, and most of the species only have 1 or 2 in their bodies. (Never zero, because they do not even survive gestation/birth without the assistance!) It will take a lot of cannibalism to achieve the heights of the Old Ones.

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If you think just a little adjacent to what you're describing, there're tons of examples of sort-of-similar things in nature that could be used.

Start with reproduction. There're tons of biological organisms that can only reproduce in very specific environments - including the parasite that causes malaria for example. That reproduces only in the gut of a mosquito! Maybe your species has some symbiotic organism that works this way; they can only reproduce in your species' gut, and they help your species live longer in some way (fight off some other parasite, provide necessary elements/proteins, etc.).

On a similar bent, you might have an enzyme that can't be synthesized by the species, but is necessary for survival (say a DNA repairing enzyme) - which is produced in the gut by a symbiotic organism (such as gut bacteria). This is, again, entirely normal for life on earth - except for the "cannibalism required" part, but that's not too hard to imagine; either the source material the enzyme acts on would need to be of that organism, or else the way they get the organism into the gut would be cannibalism (we do this, today, with stool transplants - you'd just have to have a reason that this wouldn't work via the stool).

Finally, you have life cycle transition as an option. Many species go through various cycles - think caterpillar -> pupae -> butterfly - and it's possible that the life cycle transition could be caused by cannibalism. Perhaps there is some horomone produced by a pre-transitioned adult that extends their life. This has the added advantage that it explains how this works practically speaking: the species can produce as many "children" as possible, but many fewer adults, since they have to eat the children. (I didn't say it was nice!)

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You could just have and enzyme or mineral that occurs in the environment in exceedingly minuscule quantities. Evolution has enabled this species to sequester any excess of this essential substance in fatty deposits throughout the creatures body. The very young would need smaller amounts, but ever increasing as they approach adulthood. I would pay careful attention to balancing need for the substance from an evolutionary standpoint. Cannibalisms outside of reducing competition for an individual organism to increase its chances to reproduce its genes inside the success of the species is quite rare.

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I can't imagine how evolution wouldn't quickly fix it, but:

Your ephemeral species has a hormone that is essential for it's continued existence. Unfortunately, sexual maturity repurposes the system that produces it, from that point on the body relies on stores and when they're gone it dies. Unlike most such complex molecules this one can be obtained from food.

Note that this means optimum cannibalism is of someone at the cusp of sexual maturity, not an adult. And it means you only need to eat the part that contains the storage, not the whole body. (However, it's possible that due to the covert nature of such actions that nobody has discovered that it's really only the one body part that matters, or perhaps the storage is dispersed somehow.)

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The novel Kren of the Mitchegai describes a species that practises cannibalism.

Juvenile Mitchegai are herbivorous, but if they don't get eaten by their carnivorous seniors, they switch to being carnivorous themselves.

A juvenile or young adult Mitchegai isn't all that smart, but when an adult eats another Mitchegai, it adds its meal's brain mass to it's own rather than just digesting it. Eating a juvenile just makes the adult a little smarter, but if a young adult eats an old adult with a much greater brain mass, the old adult's greater brain mass dominates the younger's, and the older's mind effectively takes over the young adult's body.

So, while Mitchegai bodies have a limited lifespan, by being eaten by a much younger, dumber individual, they achieve a significantly extended lifespan. The Mitchegai's brain cells are motile, and have a longer lifespan than the rest of the body. Once eaten, they travel through the body of the Mitchegai which ate them to the devourer's braincase, which is capable of great expansion.

As a matter of interest, if a young Mitchegai eats only a little of an older, smarter Mitchegai's brain - possibly even leaving the older alive - their own mind doesn't get taken over, but they do gain some of the older's skills and knowledge. It isn't even entirely to the disadvantage of the older Mitchegai either... the older loses the skills/knowledges, but relearning them makes them more likely to achieve breakthrough discoveries in the field.

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