While fleshing out the details of my species, I have some concerns regarding reproduction. This species has several constraints which make it difficult to get pregnant and reproduce. These include:

Major issue:

  • To reproduce, a connection between the two needs to be made which is both physical and mental, and difficult* to establish succesfully

Other issues:

  • An individual can only have one partner in its lifetime. So reproduction can only occur with this other individual (this constraint is when the connection mentioned above is made properly: if the connection is made, they are partners for life. If not, they can break up and look for someone else)
  • Couples usually have two children (the mode, not the average)
  • The total population is small (<1000), so finding the right partner might be difficult

There are some positive notes to consider as well:

  • Health care is very good, so little disease and death except from old age
  • If above conditions are met, conception is usually successful
  • Species is intelligent, so careful decisions can be made

I am concerned that this species has too much of a difficulty to reproduce to make it viable or at least believable. You would think that such a species would probably have gone extinct. I would like the species to be sustainable.

My main question is: how can I make this species viable? What can I add to their culture / world / other to make their existence believable? In other words: are there any prerequisites I need to fulfil for it to work?

My first idea is to embed the 'find your soul mate' more into their culture, making it a major point on everyone's agenda. I'm not sure yet if this fits the world as it is in my mind, and would love some more input. I'm okay with the idea that the difficult connection as mentioned in the major issue above is more of a recent thing, and that in the past it was a lot easier. But that does not prevent them from going extinct in the (near) future.

*Difficult meaning that if the partner is not right, the connection can not be made completely and reproduction is not possible. This goes, of course, both ways. Whether a partner is 'right' depends more on the mental aspect and trust. It can be build between two individuals, but it takes time.

Addition: the size of the population and its viability, also regarding number of children or reproductive rates is out of scope for this question. That is already covered in an earlier question: How small can a population be with regard to genetic diversity?

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    $\begingroup$ Those limits seems to make the species quite difficult to recover from the inevitable catastrophes that befall struggling young species. Each member of the species must be quite tough - able to resist fire, flood, tsunami, severe weather, crop failure, disease, parasites, pests, etc. The juveniles must be invulnerable to predators and unlikely to succumb to periodic malnutrition or dehydration that occur before food storage becomes available. $\endgroup$
    – user535733
    Feb 15, 2018 at 20:33
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    $\begingroup$ "An individual can only have one partner in its lifetime." "Couples usually have two children (the mode, not the average)" - this is a path to extinction. You need to give your species some leeway here. $\endgroup$
    – Alexander
    Feb 15, 2018 at 20:57
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    $\begingroup$ Before partnering, if the individuals can't tell whether they're "right" for each other, then there will be "partnered for life" couples that can't reproduce. If correctly-partnered couples generally have 2 children, then the species will dwindle and go extinct. $\endgroup$ Feb 15, 2018 at 21:02
  • $\begingroup$ I am also afraid that I these constraints are too much, but I was hoping for some direction I had not thought of yet. @BrettFromLA: I will update my question, being partnered for life is when the connection is made properly. So you can't end up permanently with a partner that isn't right if you don't want to, you can go look for another one. $\endgroup$
    – Century
    Feb 15, 2018 at 21:07
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    $\begingroup$ @Alexander: worrying about sustain of the population regarding low number of children is not in scope of this question, but I understand your concerns. If the average is 2.25, with a reproductive rate of 90% the species should be sustainable. I did not add this information to this question as I wanted to focus on the other issues. (reproduction rate partly covered in a previous question: worldbuilding.stackexchange.com/questions/98551/…) $\endgroup$
    – Century
    Feb 15, 2018 at 21:12

6 Answers 6


In a somewhat evolutionary perspective, this type of reproduction might occur at a more advanced stage of a species progression. Having this type of connection implies a species that has consciously moved beyond their impulsive animalistic instinctual behavior.

To make this believable, other aspects of their culture/world should adhere to this kind of conscience based decision making. Here are some ideas:

  • You've mentioned healthcare, and death occurs from old age. Perhaps they've found a way to extend lifespan? Do these people live past 100 years of age? 200? How advanced are they? Do they have artificial organs to replace old ones?
  • Less than 1000 is extremely small for a population. Perhaps they live in a bubble, or some kind of enclosed environment This could keep them safe from outside calamities, other unfriendly species, or diseases.
  • Their small population size may imply they are more tribal, or communal. This would imply there is quite a bit of sharing. They may even share parenting duties.
  • There could be a more open view on sex. Through this openness it would be easier to find one's soulmate through trial and error without any feelings of guilt or shame. Love would be their focus and their attention, and priority. Fear is not something they cultivate. As much as finding their soulmate is important to them, jealousy and envy do not seem to mix well with a society like this.
  • Interbreeding with other species could also solve problems of extinction. Perhaps there are exceptions as the soul knows no bounds, right?

[side note: Forgive me for thinking of the movie "You Don't Mess with the Zohan (2008)." In it, the main character Zohan (Adam Sandler) realizes who his true love is by way of discovering he cannot get an erection for any other woman.]

  • $\begingroup$ I've accepted your answer because it delved deeper into the characteristics of their culture/world that I can take into account to make the species more believable. Though the other posts make valid points too. $\endgroup$
    – Century
    Feb 20, 2018 at 23:08

how can I make this species... existence believable? In other words: [what are the] prerequisites I need to fulfil for it to work?

With me, you'd have to overcome the following items which haven't been mentioned yet:

  1. From a genetic perspective, the most troubling item I see is the total population. A total population of 1,000 is a pretty small gene pool for a high order creature.

With two generations alive at the same time, and mating lasting a lifetime, you have 500 couples... 250 grandparents (infertile), 250 parents (fertile).
Even if the first thousand walked off a space ship unrelated to one another, it isn't many generations before you are likely/required to marry cousins.

  1. Unless you introduce a biological compulsion to reproduce (think of a dog in heat) you will have a lot of 'people' that don't like anyone that is left.
    With your mental connection requirement that means that even if they pair up as a result of social pressure, they won't produce offspring.

A biological compulsion is rare/non-existent in high order beings. That 'Vulcan thing' (pon-farr?) is the only one I can think of - and that one still strikes me as a horrible plot device (other Trekkies will disagree).

To solve #1 you could say that the limited population is a recent thing. Like in a lot of first world countries of today where the population is dropping.

Not sure how to fix #2. Maybe you can change Couples usually have two children (the mode, not the average) to three children. But even then, I'd say you need a population of thousands (maybe 16k would work).

Hope that helps

  • $\begingroup$ Thank you for your insight. Regarding one, that one is already covered in an earlier question of mine: worldbuilding.stackexchange.com/questions/98551/…. I will add it to the question to avoid confusion :) Still, would three children just be enough to solve all the constraints? $\endgroup$
    – Century
    Feb 15, 2018 at 22:16
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    $\begingroup$ Sorry, I missed that in the comments. It is still relevant though because you asked about believability - and it strikes me as not believable. I see two paths for you. You can either require the reader to accept it / suspend belief (how many fantasy stories have one dragon left?) or have a recent (in number of generations) catastrophe that indicates this isn't a sustainable number of creatures. $\endgroup$ Feb 15, 2018 at 22:21
  • $\begingroup$ I was hoping for not having to rely on entire suspension of disbelief, but some is still necessary I suppose. $\endgroup$
    – Century
    Feb 17, 2018 at 10:21
  • $\begingroup$ I was wondering where your estimate/guess/calculation of 16K comes from? To take into consideration. $\endgroup$
    – Century
    Feb 20, 2018 at 23:00
  • $\begingroup$ Well, it was sort of a back-of-the-envelope calculation. I started with your number of 1,000 which I didn't find believable, and started doubling it until it seemed believable. I couldn't decide if 8,000 was believable so I went to 16k. You can reference giant pandas, whose total living population is estimated to be (just under) 2,000; 200 of which are in captivity. I realize that a big part of the panda breeding problem is that they don't breed in captivity - but you have constraints on breeding also so it seems a good fit. $\endgroup$ Feb 21, 2018 at 15:47

The answer is always to abuse the daylights out of the requirements.

First off, you need to make it so that this physical+mental match results in children that are extraordinarily well adapted to the environment. In particular, you're going to need to use this match to do something to counter any inbreeding issues that occur with that small population.

Then, create a highly inequitable environment. The alpha male and female reproduce like rabbits, paving the way for the next generation. The remaining couples only reproduce to have two kids. That way, the mode remains 2, but the mean is stabilized by the reproductive rate of the alpha couple. The harder it is to attain the physical+mental match, the fewer couples reproduce, and the more children the alphas have to have to compensate.


Almost all of that already occurs in nature with one exception, the only 2 offspring is impossible as the wild condition, any disease, predators, or natural disaster would drive the species extinct in short order. Your species could never evolve in the first place IF this cannot change.

They could be like humans and experience a drop in number of offspring under very high standard of living (extremely low infant/child mortality) but have much higher birth rates under lower survival conditions. humans only have around 1.9 offspring in very high standard of living countries, since the offspring are all but guaranteed to survive they instead they instinctively invest much more energy and resources in each individual child. in worse condition humans have around 5-6 offspring, but only 2 on average survive to reproduce. when regions experience a rise in standard of living the population often spikes as it takes a little time for the number of offspring to change.

1000 individuals and advanced medicine are mutually exclusive. It is hard to see how they could have any form of technology or even much of a civilization with only a 1000 individuals. 1000 individuals is barely enough to support a single general practitioner much less a medical specialist. Your species will have a hard time supporting bronze age technology much less anything modern.

if they use DNA this number in of itself is a problem since they have little variability, 1000 individuals in not a healthy population of a sexual species.

Otherwise this is just exclusive monogamy which already exists in animals, the only trick is, unless one dies after mating, we know of no species with 100% mate fidelity, some are remarkably high however. Notably some will cheat but still stay socially bonded until death, prairie voles are good example. So you could have your species form perminate monogamous relationships and still have infidelity.

  • $\begingroup$ The number of children is not a physical limitation, they can have more. I can add to the world that if numbers are dwindling for whatever reason, they have more children, and if everything stabilizes, they 'return' to an average of 2.25. $\endgroup$
    – Century
    Feb 17, 2018 at 10:23
  • $\begingroup$ @Century with population this small it won't help. After earthquake, rocks falls, everybody dies you will not have enough people to restart your species. Unless number in your question is one after such incident. $\endgroup$
    – Mołot
    Feb 18, 2018 at 9:58
  • $\begingroup$ John, your edit is another question entirely, but indeed something I need to look into. Your last sentence was covered in the other question I mentioned in my question. @Mołot I will take that into consideration, good point. $\endgroup$
    – Century
    Feb 20, 2018 at 23:04

One of the proto-species of what was to evolve into humans had a rather unique adaptation in their females, which was a subtle re-positioning of the vagina so that babies could be birthed in a different position to other primates. The energy saving was minor, but it meant that the average number of children that the species had went from 2 to 3. That was enough to ensure their success. The Walking with Cavemen documentary series explains this in much better detail than I can in this answer if you want to revisit it outside of anthropological literature.

The point of this? Well, you're really dealing with 2 limitations here that are normally counter-balanced.

1) difficulty in reproducing
2) limited numbers

Take fish and insects; they have massive numbers of offspring because of their 'difficulty' in reproducing. In their case, the difficulty isn't in conception, it's in post-natal attrition. The massive numbers offset the difficulty in their species living to term through natural predation.

In your case, the difficulty is finding the right partner and conception.

Let's assume that only half the population of your species manage to find a partner and reproduce. The ZPGRC (Zero Population Growth Reproduction Constant) will be 4. That means, every female who DOES have children needs to produce an average number of 4 children just to maintain existing numbers. It's a myth that ZPG is maintained by banning new children; China is coming into a period of runaway ageing population because of their 1 child policy, for instance.

So; let's look at possible scenarios. Man and Woman love each other very much and reproduce. Man and Woman love each other very much, but one dies before reproduction (mating for life rules assume here no re-marriage). Man and Woman don't meet, or never find their perfect partner.

Then there's the kids. What's their rate of survival to reproductive stage? Especially if one partner dies after reproduction takes place. Humans have a system of welfare (in most countries at least) that is designed to take care of single parents and orphans and the like. Whether that is abused by some people or not is out of scope of this question and correspondence won't be entered into, but the relevant part of this is whether or not you have a system that has a vested interest in preserving the young and how effective it is at doing so.

All this adds up to give you a probability factor of every child reproducing, and THAT gives you the average number of children you need for population maintenance, not population growth. Sample scenario;

1) Children reach breeding age - 90%
2) People find the right partner - 80%
3) Partnerships are 'fruitful' - 80%

You start out with 1000 population. Working on a basis of 2 children (on average) per couple (assuming 100% mating pairs) you'll get 900 in the next generation.

But, only 720 of those will find pair up. Of those, only 576 will have kids, meaning 288 breeding pairs. That means for the next generation to produce a population of 1000, the average child count PER PAIR is just over 3.47 at which point the attrition rate starts over again.

Obviously, this is not as segregated as these figures describe; kids will be being born all the time and the generations blend with each other in the community. But, the real point here is that for your species to be viable, you have to do one of two things;

1) You increase the probability and / or convenience of reproducing
2) You increase the average number of children per breeding pair

The rest (as they say), is math.

  • $\begingroup$ I was afraid it would come down to these two factors. I was hoping to be able to increase the probability and get some input on how to achieve this, but I guess I have to rephrase my question / ask another if I want an answer to that. ;) $\endgroup$
    – Century
    Feb 17, 2018 at 10:27

Promiscuity until reproduction occurs would be helpful. After that, the couple would be partnered for life.

However, there are still issues of natural disasters, diseases killing one member of a couple before the second child is conceived, etc. Therefore, the 2-children-per-couple limitation should be social and intentional. Then if the group of <1000 gets too low, couples would have more than 2 children for a generation or two, to get back to their ideal community number.

  • $\begingroup$ It is indeed a social/cultural thing, not physical. Couples can have more children. Indeed, average needs to be 2.25 with a reproductive rate of 90%, so some couples should have more than two children. I am still concerned however that all these constraints drives this species on the edge of extinction, even if they have more children. $\endgroup$
    – Century
    Feb 15, 2018 at 21:16

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