I'm currently brainstorming the rules for some Tolkien-esque elves. I'm trying to do some research to make them as realistic as possible, but all the science-y things go right over my head. I've progressed much further on the cultural side of things.

Some elements I've decided on:

  • Very little sexual dimorphim and notable rate of intersex individuals (they don't really have a concept of gender, only that there are two common types of sexual organs and you need one of each to reproduce)
  • Commonplace same-sex and non-reproductive relationships (there is no cultural expectation to have children)
  • Ability to reproduce with humans and dwarves (which are like different 'breeds' of the same species - vastly different characteristics but biologically the same, like a miniature poodle and a Newfoundland)
  • 100-200 year lifespan, wherein they age slowly and are still vulnerable to disease and accidental death (humans and dwarves have a 60-80 year lifespan)
  • Magical abilities that help maintain this lifespan (healing) but do not guarantee immortality
  • A stable or slowly declining population, keeping them at a rate of about 1 elf : 1 dwarf : 2 humans

My questions are: What would be a plausible life cycle, how often and how many children would elves be having, and what could be the lifespan and life cycle of an elf-human or elf-dwarf hybrid?

  • $\begingroup$ Does the 100-200 year mean that some elves literally age twice as fast as others? Or do you mean the average lifespan is somewhere between 100-200 and you haven't decided exactly where yet? $\endgroup$
    – Kys
    Sep 23 '16 at 20:20
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    $\begingroup$ @Kys Not OP but guessing this is a probabilistic lifespan range rather than an average. IE some large portion (80%?) falls within the provided age range upon death. $\endgroup$
    – Ranger
    Sep 23 '16 at 20:57
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    $\begingroup$ What do you mean when you say "Tolkienesque"? I'm guessing you mean that as a physical description. That about right? $\endgroup$
    – The Nate
    Sep 24 '16 at 20:42
  • $\begingroup$ I'm not sure what your 6th point means. A small population of humans can become a large population of humans in a very few generations. Unless you have something that is deliberately monitoring/controlling human population growth, there is nothing that would bind a stable human/elf ratio, especially with a non-growth/declining elf population. $\endgroup$ Jan 24 '18 at 14:23

It Depends on your Definition of "Commonplace"

...a statement not to be confused with Bill Clinton's "it depends on what the definition of 'is' is".

On Reproduction Minimums

Let's start with Elves, since I think this going to set you up for the rest of your species.

You state in your premise that same-sex and non-reproductive relationships are "commonplace". Without knowing just how common we can't say what their lifecycle would look like. I would strongly suggest however that "commonplace" not be very common at all or there would be no possibility for a stable population or even a slow decline; in fact it would seem very unlikely that the population would be particularly old at all with such values.

Consider for example that for a population to remain stable every 2 elves must produce 2 offspring in their lifetime. The implications of this statement are considerable when you stop and consider that for every elf that dies before producing a matching offspring someone else needs to pick up the slack.

If "Common" were to mean "as often as any other type" the implication is that that 33% of the population was engaged in same-sex relationships and another 33% was non-reproductive, leaving just 33% in a reproductive relationship. In that case just to break even the reproductive relationships would need to have 6 children each - and that's before discussing the mortality rate! Realistically you probably end up needing to pop out 12+ kids each. So this definition seems to suggest that you either have no children or A LOT of children. Anything less would be sharp, noticeable decline that would suggest a race with a relatively short history. Of course if only a third of your population reproduced it would not be hard to see how that would quickly create social or physical evolutionary changes that would push the group towards an increase in the number of reproductive relationships as only the breeders pass along their genes and are responsible for the majority of the passed-on values/preferences.

If "Common" were to mean "not rare" we could maybe call it 10% same-sex/10% non-reproductive/80% reproductive. This is still far from an ideal, but it's not as bad. In this model the reproductive relationships need to have 2.25+ children each before mortality is considered. Realistically you're still talking 5+.

Hopefully you see where this is going, and I'll let you do the math yourself to suggest what your definition of "common" is. When you do though, please consider that in the middle ages the child mortality rate was around 30%. That means you need to overcome a loss that size just to break "even", and that doesn't count an adult mortality rate.

If I may, I would personally suggest that your definition of "common" be that same-sex and non-reproductive account occur less often than 10%... less than 5% would be even better. At that rate your neighbor might not be likely to fit one of those roles, but you would certainly have met a number of people that do.

These numbers also do not account for when elves interbreed with Humans or Dwarves instead of other Elves. Any time a female elf is impregnated by another race is effectively a lost opportunity to produce another elf, which adds up if it's done with any frequency at all. If the males stay with their mates they are effectively removed from the reproductive pool as well.

On Child Spacing & Old Age

The next area of clarity that is needed is why elves seem to be "very old" for 100 years. You give both Humans and Dwarves lifespans of 60-80 years - an average of 70 years give or take 10 (~14%). Elves on the other hand have an average of 150 years plus or minus 50 years (~30%). That just seems like too much of a swing. I don't know whether to consider a 100-year-old elf "very old for up to 100 more years" or "middle aged".

If an elf is "very old" at 100 they are presumably not going to have children anymore, but if they can reasonably expect to live another 50-80 years (50-80% of their life to-date) and are still properly mobile then why not have some more kids?

I would suggest that unlike Humans and Dwarves, in your scenario Elves probably wouldn't want to start having children until at least their 40's. If they are very long-lived and there is no cultural rush, they might as well wait until they have had a chance to fully stabilize their mental, social, and economic/political positions (NOTE: I am stating as occurring in their 40's, but you could certainly have it come sooner). At the same time there would be no incentive to juggle multiple children at once, so you could reasonably expect them to wait ~16-20 years between births. That would suggest that if they did stop at 100 the maximum number of baby elves a couple could be expected to produce under normal circumstances to be 4 (more if they continue reproducing beyond that).

On Hybrid Life-Cycles

This is really up to you. There's no reason you couldn't say that the life-extending biology of elves transfers 100%... or that the life-limiting biology of Humans/Dwarves nullifies any elven benefit. Or just say "it's half the difference" and call it a day. Because the mechanism is undetermined so far (and without genetic testing, likely never to be determined) you could claim there is a science-based reason for any result and be equally correct.

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    $\begingroup$ Some great points. I would add that the capability and psychological inclination to reproduce would be evolutionarily selected for, which would also lead to a larger percentage of the population reproducing as GrinningX suggests. $\endgroup$
    – Kys
    Sep 23 '16 at 20:35
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    $\begingroup$ Also might be worth considering if relationships are strictly monogamous or not. $\endgroup$
    – Dan
    Sep 24 '16 at 8:01

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