# What MATH could I use to conclude my World's Elf prime pregnancy years?

So, let's be real ... I totally suck at math.

If a Human female can live to be 100 and her prime pregnancy years are from 20 years old to 40 years old AND assuming that Elves in this world have the same general biology, but lower birth rates ... what would be the Prime Pregnancy Years for an Elf female who could live to be 750 years old (or more)?

I'm just looking for a check on my own math and a math equation to give me the human equivalent age of a 440 year old Elf ... but I love to hear other Worldbuilders ideas on Elves Longevity and birth cycles :-)

My math:

Human = 20% to 40% of 100 = 20 years to 40 years

Elf = 20% to 40% of 750 = 150 years to 300 years ... does this look right?

Next ... I have an Elf that I want to have pregnant at 440 years old ... I don't know the math to see how old that would be to a comparable Human ... what would that be?

Edit

I got the Human prime pregnancy years from a Google search on the subject and reading various article results ... so I'm not saying it's a fact, just my starting basis to use and it seems logical from what I've read and from my own cultural heritage.

• Your elf woman would be 440/750*100 = 58.9 year old. (in case of direct proportionality) But nothing forces you to hold to these percentages. Elves might mature just as fast as humans or have menopausa only at 700. May 9 '18 at 10:43
• Although not appropriate in western culture, I think biological the prime pregnancy years are more something like 15-30 May 9 '18 at 11:52
• @J.J.Hartly before you swear off SE for good, take note that SE is taking note that people are not enjoying their first experience, and are trying to change that. Realize that most people are actually trying to help, and that SE is not your typical corner of the internet. There are rules here that you might not expect, and they are grounds for immediate deletion of some posts. THAT'S NOT BECAUSE WE DON'T LIKE YOU. We're trying to help here, and the rules are there to make sure we can... May 9 '18 at 22:11
• ... help as many people as possible with every question. It may seem unwelcoming at first, but stick around for a while and you'll understand why the rules are in place. May 9 '18 at 22:12
• All that being said, I'm astounded that no one has written this yet, because it's typically standard for all first time posters - Welcome to WorldBuilding! Part of your question can be seen as opinion based, because the biology of elves isn't set in stone. Luckily we have a [creature-design] tag that you can use to hammer out the question of how long elves might be fertile (in a separate question please!). When you get the chance please take the tour and visit our help center to get the rules of our site. May 9 '18 at 22:16

b.Lorenz sniped in a comment (shame on you, you know the rules ;) ), but I'll go ahead and explain how he got there. You worked with percentages in your post, but it's a bit easier (for me at least) to switch to proportions. Proportions work on the following principle:

$\frac{x_1}{y_1}=\frac{x_2}{y_2}$, where $x_1$ and $x_2$ are comparable, as are $y_1$ and $y_2$. In the case of your question, we have several comparable values. The lifespan of an elf to the lifespan of a human and the age of a particular elf and the age of particular human.

$\frac{age_{human}}{lifespan_{human}}=\frac{age_{elf}}{lifespan_{elf}}$

$\frac{age_{human}}{100}=\frac{440}{750}$

Multiplying each side by both the denominators gives us:

$age_{human}*750=100*440$

Finally divide through by 750 to get $age_{human}$ by itself:

$age_{human} = \frac{100*440}{750}=58.666...$

Does this sound reasonable? Maybe... It makes sense to me that elves would have much longer fertile years before their bodies start to fail. Maybe their menopause lasts just as long as humans, but starts at the age of 730.

• I am really sorry. (upvoted) It just feelt too simple to write an aswer about. May 9 '18 at 20:08
• @b.Lorenz if I came across as angry, I meant it as a joke May 9 '18 at 20:10
• @bendl thank you so much for your answer and for being polite in the comments on my question and helping me out. You've changed my mind for me to give SE/Worldbuilding one more chance. Plus I had an hour nap after work and was able to compose myself better online :-) May 9 '18 at 23:45

Menopause is actually quite rare in the animal kingdom so there is not really any other species where we can compare. There seems to be one good reason for menopause: grandmas.

Grandmas are great because they massively improve the survival of kids without being as vulnerable as pregnant moms. So biologically speaking you'd want fertility to decline roughly at the time the first daughters have their first kids.

So for your assumption to be valid, you could just raise the begin of childbearing age a bit, say, to 200, and then your 440-year-old is quite plausible.

(by the way, you are a bit pessimistic with your ages, in humans the range is roughly 12-45ish although fertility declines quite a bit at about 40)

• Thanks for the answer! I did see some articles relating about 12-45 being fertile years in human females ... I was just going with 20-40 due to my own cultural background ... but maybe I'll expand it for my Worldbuilding. Thanks :-) May 9 '18 at 23:48
• 12 is too early to be "prime years". But 20 is also quite late, most natural is 16-18 as a start (women who give their first birth in their twenties may have problems with giving birth, But this might be just a myth. I would like if somebody could confirm) these years were also considered standard to have your first child in the previous century. Having your child in your forties can also bring medical problems. Giving births is naturally a dangerous process, so choosing just the years that are "optimal" is the point here, I believe. May 9 '18 at 23:58

Let's take another angle on this: Eggs

Human females (and likely every animal species) have a finite number of eggs. When they're gone, the potential for breeding is over.

It is true that menopause often comes before all the eggs are gone. It's also true in human females that the viability of eggs (and/or conception, depending on who you ask) decreases with age.

Traditionally, Elves are a very slow breeding species due to their longevity. Assuming that the average "generation" is 25 years and the average human lifespan is 80 years then at any given time you have 3 generations hanging around.

But Elves... If the fertility rates and duration were the same as humans you'd have 30 generations hanging around at any given time. Assuming no more than 2 kids per generation, you're talking about 1.07 BILLION elves when the first elf dies.

Um.... no...

As you design your species (especially from an evolutionary perspective) you need to realize that there's a give-and-take involved. While not at all a "rule," you can nevertheless reasonably assume that long life spans result in few children and short life spans result in tons of offspring. It's an issue of balancing the resources — and "survival of the fittest" pretty much dictates that a billion elves after only 30 generations means you literally can't keep up resource-wise. Remember, fewer people at the top of the geneation pyramid than the bottom. That rapid expansion cannot be supported.

So we need to slow down fertility something awful or we'll all drown in starving elves

Comparing elves to humans is a bit on the apples-and-oranges side. You need to think of it this way:

• The longer the fetility period, the harder it is to coneive, or...

• The more fertile the elf, the shorter the fertility period.

Add to this the possibility of enormously long gestation. For example, 9 years rather than 9 months. Well... let's not go there yet...

So, either young elves have a very narrow window of opportunity or they have to work hard at it for a long time. Those two choices aren't as silly as they might seem. It will have a dramatic effect on your story.

Because it will affect social mores

A narrow window of opportunity means the family is together a very short time, perhaps 20-30 years out of 750. Families are a huge driving force for communal organization, and this minimizes that association. Elves may spend 600 years in substantial solitude becuase there isn't a reason beyond lonliness to build a companionship.

A long, hard-fought fight for the family, on the other hand, would completely swing the pendulum. Families would be ultra tight — especially since siblings may be born hundreds of years apart. But this isn't without its complications. It's easy for siblings to not marry when they grow up together, but separate their births by a couple of hundred years....

There's that third option where the gestational period is very long, but it needs a reason. What about Elves is so complex that the growth process needs to be slowed down that much? For elephants, the issue is the much larger size of the baby — but for elves? Maybe their brains are much more complex, therefore slower to develop in the womb, and that explains their magical abilities.

Conclusion

It didn't sound like it, but this really is all about the eggs. How many are there? How long do they last? How often do elves ovulate? How long do they gestate?

At this point we're crossing over into story-based solutions, and that's your territory. You need to decide what it is about elves that justifies the long lifespan but not world-overpopulation in only a few thousand years. That decision will affect how you bring that limitation to bear. But in the end, I expect it'll come down to the eggs.

First the answer: prime pregnancy years are shortly after he becomes female.

Bear with me. Clownfish (and many other types of fish) are sequential hermaphrodites. All are born male, fighting it out with the many other males, doing male things. The lucky ones who get old and large can become females. There are fewer of them than of the males, and the females are the bosses. To earn the right to be female they must survive the male years.

So too your elves. I am thinking of a matriarchal society like the D&D Drow. All elves are born male and they stay that way - possible for centuries. They do the fighting and the grunt work. Many die.

At a given time (possibly decided by the dominant female) a successful male makes the transition. I am sure there would be a ceremony. He turns female, and with this her intelligence and capacity for magic increases.

A queen might opt to have no other females - although unless the males can turn female on their own without her help this would be a risky proposition for her line should she die. She might transition females to produce offspring because the numbers of her House have been depleted by war, or because she needs a few of the intelligent females for strategic long term endeavors, magic or otherwise.

• is Mortal Affairs your creation or a reference to something I'm not familiar with? I'm familiar with D&D Drow somewhat. May 10 '18 at 3:27
• @J.J Hartly - I think Mortal Affairs is a sort of province in the larger D&D world - maybe a longstanding campaign or world. "Drow" are owned by D&D so to use that term they have to be some sort of D&D. May 10 '18 at 11:01

While most answers either use a simple proportion or shift-off to alternate biology, I'll focus on lifespan problem. So, my answer is not specific to elves, it's about longer-living (and probably genetically engineered) people.

The problem with the proportion is that you would assume the phases of life to take the same share of one's life, but longer since the whole life is longer. Meaning that if it takes 18 years to get mature now, it would take like 40 years, if we would live for, like, 200 years. (Folks in their teens were regarded adults in earlier history.) One real reason (beyond proportion) for such tendency would be a prolonged education. You need to learn until you are 40 to be a proper and productive member of that society.

The current tendency with human populace is less prolonged adolescence (even despite my above reference to the Dark Ages), but more the prolonged geriatrics. While adolescence and active adult life (such as working age up to retirement) stay relatively the same (see the enhancement of the retirement age as the recent trend in various counties though), the time left after retirement enhanced vastly. Like, people previously retired with 60 and were sort-of-expected to die at 65-75. Now they retire at 65 and can well live beyond 90.

So my point is that when scaling up the life expectancy to elvish, you are better off when you don't blindly scale up things, but set two ages, the maturity and the retirement, and then play accordingly.

Let's take the life expectancy of 200 years, more than doubling the human life span. It seems reasonable to extend the adolescence to 25 years at least. (Notice that if your elvish (other biology!) populace has not gone though industrial revolution, antibiotica boom, and progress in public healthcare, that would severely extend the life span from the natural figures – basically, if they have always lived for 200 years, then the physiology of an elf is also different. This was the long way to say that they don't hit the puberty at 13-14. Make it 18-20.)

Now, while in humans the typical retirement age is somewhat above becoming grand parents (it's possible to become a granny with 40, while 50-60 is more reasonable), we don't know how it works in elves. The linear scaling of a proportion would have saved us from this, providing for longer generations in elves. If we assume somewhat human behaviour, even is stretched-up a bit, it seems reasonable for our elves to typically have kids at 40-50-60. (Of course, they can get pregnant at 20, but so can human teenagers.)

This means that we have elvish grandparents at 100 and grand-grandparents at 150. 3/4 of the life span (150 in elves) seems like a lot (75 in 100-year-lifespan humans), but it's 60 in 80-year-lifespan humans. Around this seems to be a perfect age for retirement. Making adjustments for ageing populace and what not (similar to what governments of developed countries tell us, when they bump retirement age), let's set the retirement age at 160. This leaves you with enough time to travel the world. Like, to visit the elvish Paris and snip an elvish pic of them in front of elvish Eiffel tower.

Provided system has a few major consequences.

It was typical for longer in human society that grandparents provide help to parents, especially if the latter a young and working. Elvish grandparents would have not retired yet! They are in their primes! So, either grand-grandparents need to jump in, or the grandparents need to take some kind of a sabbatical. Other systems might be in place, it is a given to hire grannies, or elvish children are not that care-intensive (remember, different biology!). It might be that elvish grand-grandparents don't give a damn about their grandchildren, preferring to travel the elvish world instead.

Next, you have four living generations at a time instead of three in humans. This is huge. For once, more family hierarchy. It does not matter what the pops said, as grand-pops can override him, and so on.

Even worse, the elves multiply faster than they die out. The are on an expansion tour. They need more and more resources. They fight colonial wars for the elvish oil and elvish rare metals, that are required to fabricate evlish Li-ion batteries in elvish smartphones.

If elves are not united (which they probably are not), there is a lot of friend-foe mentality. If they are already (after a huge bloodshed, probably), they need some kind of a frontier where to push a horde of youngsters that have no purpose and place in life. A frontier like space. So, expect your space-faring elves to fight wars with humans or space-orcs.

Next, high population density and big re-population potential leads to a diminishing perception of elf life. (And, by extension, of life of any sentient being, including humans.) So, the elves can kill for nothing. They are used to fight in never-ending wars and to die. Probably, the duel culture is thriving. For a reference mind-set, take Japanese samurai from the Fighting Provinces era.

Ooops, that's much more D&D drows than Tolkien high elves.

An alternative, when keeping the life span and even above life span proportions, is to change the biology more. For example, an elf can get pregnant only from her eternal love, and it takes, like, 90 years to find. Then we have elvish children at around 90 and grandchildren at around 180, which is a) well in the retirement age, and b) a bit over-extended human situation, hence only three generations at once. Problem solved!

The consequence is having a huge "bachelor gap" in the society. Elves now mature at 25 and are left for their devices until around 90 before founding a family. Interim families, long world-travel (even before retirement, wow!), yuppie career spree, going on an adventure (who said "asari from Mass Effect?"), or other things might ensue.