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In my world there is a race of humans who are disproportionately female. They are closely derived from and very similar to real-world humans, but are unable to interbreed with the other races as they are too genetically distinct to produce fertile hybrids. They do not reproduce themselves asexually, but rather all the permitted females in a tribe mate with one or a few males, because very few males are even born.

The culture is built in no small part from the premise that the vast majority of its people are women, but I want to know just how few males this species can get away with, considering they are genetically isolated and every generation faces a serious potential bottleneck effect.

I guessed with gut instinct that one male per hundred females would work, but I'm afraid that's a bit too conservative; for a stronger narrative and more spectacle, I'd also like to know if 1/1,000 would work. For context, the species as a whole probably has a population of between 5-10 million, and are heavily maritime so all tribes have access to each other.

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    $\begingroup$ The main obstacle you face in making this species scientifically believable is not the number of males required to avoid genetic bottlenecks. Rather it is FIsher's Principle which explains why the sex ratio in almost all species is close to 1 between males and females. You will need an explanation for why this evolutionary argument is being violated. $\endgroup$ – Mike Nichols Sep 16 at 2:16
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    $\begingroup$ @MikeNichols I'm thinking it was a fairly recent in evolutionary terms, like a few thousand years ago their numbers dwindled to the point where it just so happened that the only surviving tribes had the male non-viability problem. I should probably portray it as a temporary state, like natural selection is taking over and more males are being born in recent years, pushing back in the direction of Fisher's ratio. $\endgroup$ – Maddock Emerson Sep 16 at 2:34
  • $\begingroup$ This doesn't address your question enough to warrant making it an answer, but there are examples in human populations where we have shifted dramatically away from the 1:1 ratio-- just not nearly toward your proposed ratio. E.g., some Arawak tribes in the Guiana Shield region were noted by anthropologists around 1900 to have a population of about 2:1 women to men. I can't recall how much that depended on the sex ratio at birth, but I think that was at least a significant factor. $\endgroup$ – wordsworth Sep 16 at 13:28
  • $\begingroup$ The ancient TV series called Sliders had an episode like this $\endgroup$ – cybernard Sep 16 at 16:30
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    $\begingroup$ If you go from natural examples like wild horses or lions, even 1/100 is rather extreme. Lion prides have about 1 male/4 females: lionalert.org/page/social-organization-and-behaviour Horses might have one male to a dozen or so females. And in both cases, there's also a population of batchelor males that compete to replace the herd/pride males. $\endgroup$ – jamesqf Sep 16 at 18:30
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You need 5,000 mobile males to make your population stable, at a minimum

This question has already been addressed from a different perspective in recent years, namely around interstellar generation ships and unlike previous research that looked at minimum viable populations in the hundreds, the new research tends to indicate that we need around 10,000 humans to maintain genetic integrity and avoid in-breeding on a new planet, away from others who could introduce genetic diversity.

Halving that figure tells us the number of males your society would absolutely need, but the real problem then is distribution. In your society, if the males stay where they are born, within a few generations everyone is (more or less) a half-sister, genetically speaking. So, given that your population is mobile and has access to all each other's settlements, in order for your society to function your males are best shipped off to random 'other' settlements upon birth and raised by their newly adopted community. This would allow the greatest randomisation and distribution of the gene pool given the minimal seeding opportunities that the females have in their environment.

If you have a population of 5 million which is the lower bound in your question, then a ratio of 1:1000 is possible, but it would be at the absolute minimum threshold of viability. Increasing the size of the population, but not the ratio, would lead to enough men to make the society viable and build in some safety protocols around genetic defects within males, sterility, accidents, and the like.

That said, such a ratio is going to bring other considerations. Just to maintain zero population growth, each male is going to have to father at least 1000 babies over the course of their 'career'. This is a lot. Put simply, if they father a single child once a week it will take them 20 years to complete their obligations. But, that assumes that every attempt is successful, and that you don't also want to account for infant mortality, disease, disaster, population growth and the like.

Let's say that the male is successful only 50% of the time. Let's further say he has to sire 2000 children. That's 40 years of engaging in relations twice a week, minimum. Finally, that's assuming that there aren't false starts; some men (or even women) who turn out to be infertile, and therefore increase the load on the remaining men.

On the one hand, men will be revered in this society for the service they provide (no pun intended, seriously) but on the other hand, their ability to pursue interests of their own are now greatly curtailed. Their full time job at this ration would effectively be impregnation and a whole industry / religion / whatever would soon build up around this function, including the necessary distribution of male infants through the broader community of settlements.

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  • $\begingroup$ A lot of the stuff in this answer is stuff I figured out in the past few days thinking over this scenario. I appreciate having a number for minimum viability, thanks $\endgroup$ – Maddock Emerson Sep 16 at 2:40
  • $\begingroup$ So, basically, just like eusocial insects... males are in the minority, and have an extremely short lifespan. They are basically born to breed and die immediately after. $\endgroup$ – vsz Sep 16 at 11:06
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    $\begingroup$ @vsz - That's not what this answer says, and it would be horribly inefficient that way given it takes 10-15 years after birth for a human male to become a sperm donor. Instead, a male born and raised to sexual maturity would be conserved for as long as viable, at least 40 years, in order (as Tim points out above) to do his job. $\endgroup$ – T.J. Crowder Sep 16 at 11:08
  • $\begingroup$ @MaddockEmerson - FWIW, you'll likely need artificial insemination, or to deal with STD subplots given the males would need to have 104 partners a year... :-) AI may lower the likelihood of conception, which could be an issue for the numbers. (Separately: I see rebellious male subplots, where the male has some unsanctioned "alone time" to reduce his sperm count and odds of his product being effective. But I can imagine the society being designed to keep close track of a male's effectiveness and when he's outlived his usefulness...urp.) $\endgroup$ – T.J. Crowder Sep 16 at 11:13
  • $\begingroup$ @T.J.Crowder : I know, the lifespan would be different, but the few males to many females does resemble eusocial animals. And I would guess in the OP's race they could, with high likelihood be evolving towards eusociality, with males reaching sexual maturity sooner and sooner, because their hunting and fighting skills (which in human prehistory selected for strong and smart men) would be irrelevant, and then in the OP's race males could devolve into weaker, less intelligent, but faster maturing "drones". $\endgroup$ – vsz Sep 16 at 11:27
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Dunbar's Number

I love going back to this because it just feels so real. Primitive societies were limited to about 100-150 people, even in modern society we have trouble keeping track of more than that. I'm connected to a thousand or people on social media. I doubt I could give specific details about more than 200 of them and I doubt that more than 200 could give details about me. The theory is our brains aren't wired for that.

Up until the steel age, maybe late bronze age, you would need one male for every 50-75 women. (Current breeder and future breeder)

At the annual gathering of the clans any sons born in the previous year would be feted and sold to another clan who lacks the future breeder.

With writing and transportation becoming more accessible you could have larger groups of women sharing a man. A young vigorous male could perhaps service 1200 women in a year(3-4 times a day), older men perhaps could only handle two clans.

Once space age technology is reached, assistive reproduction ICSI would allow one male to serve millions.

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    $\begingroup$ Servicing 3-4 females a day is very inefficient in terms of conceptions. A human male will have the best chance of producing a pregnancy after 48-72 hours since last ejaculation (sorry, can't give a reference, but I read it in a biologically oriented source concerning infertility). Even if more attempts doesn't linearly reduce the success rate, more than once a day is likely to be problematic by the time your donor is past age 18-20. $\endgroup$ – Zeiss Ikon Sep 16 at 17:54
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You need to have a think about like humans do you want them to be, socially and sexually?

Humans are very bad at getting pregnant. Studies done on American newly weds who were actively trying to have a child (lots of sex, no contraception), showed that it took on average 6 months for them to conceive. Now that's an average, so some of them would have got lucky on their wedding night. But others would have taken much longer than 6 months. So the probability of each mating resulting in a baby is very low if your males and females are physiologically and biochemically identical to humans.

However, there are plenty animals which are much better at conceiving. The females come into season, they mate a few times (perhaps over a day or two), and get pregnant.

In mammals, the ultimate success in getting pregnant is the mountain hare, Lepus timidus. The female can mate once and get pregnant twice! She stores some of the sperm from a spring mating in her reproductive tract, and can use it to conceive a late summer litter. There is also a species of lizard (Uta stansburiana) where a particular male fathered 3 clutches of eggs: once from mating, and then 2 more after he had died, because the female stored his sperm.

The ultimate version of this is of course ants, wasps and bees: the queen mates once, and stores the sperm to continually lay fertilised eggs for the rest of her life. These eggs are all female and are workers and new queens. She also lays unfertilised eggs which develop into drones (males). Look up Haplodiploidy for more information.

So if you make your humanoids very efficient at mating, and give the women the ability to store sperm, a one night stand with one male can produce an entire lifetime's worth of babies for your woman.

The quirk with the mammals and reptiles which do this, is that they may still need to mate for the 2nd and subsequent pregnancies to start. Mating causes the female to ovulate. This is called Reflex or Induced ovulation. The female hare will mate with any old male for her second litter, and will have both his sperm and the first male's sperm present in her reproductive tract. The first male's sperm has a 'head start' in getting to the eggs.

It's not the sperm, its the sex which kicks the process off. So if you want, your women could have female lovers to trigger the second pregnancy and the ones after that.

Alternatively, real world humans are spontaneous ovulators: we have a menstrual cycle, and release an egg once every 21 to 35 days. So your women could have sex once, store the sperm and use a bit of it every time they ovulate.

The genetic bottleneck may not be as bad as you think. There are plenty species where the males fight and/or display for the right to mate. For instance elephant seals, red deer, birds of paradise, peacocks. The winners are 5% of the male population and they father 95% of the offspring. 15% of the males father the other 5% of the offspring. 80% of the males never, ever mate.

Therefore you can have a small number of males keeping your population going. In the situation above, 80% of male genetic diversity is lost every generation, but there is no sign of deer and elephant seals suffering from inbreeding.

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The few males, or diversity in general, are not so much the problem per se. While there are many opinions and theories ("research") on that field, the simple truth is that nature doesn't care. Nature will take whatever it is given, it has always done that. Some combinations are more favorable than others, but in the end nature just works with what it has.

Will there be some individuals with defects? Well sure, they'll probably die. That's called selection (actually, this may be the very reason why there are so few males. Think Rett syndrome or Bloch-Sulzberger (or anything that's on the X-chromosome). Males who get the allel are never born, they just die in utero). But whatever, some will die, and some will live. Those that live will reproduce. Those with fewer defects will (usually) reproduce better. Nature doesn't care if a few die.

In the early days of man, tribes which certainly didn't have 10,000 people wandered to other regions driven by hunger, cold, or whatever reason. They reproduced just fine. Take Rapa Nui as an example if you will, or the American Indians Bering strait theory, whichever. Those tribes (and single families) surely didn't have appropriate numbers which modern research deems necessary, but nature just doesn't care. They reproduced just fine.

What's a problem with your distribution is the females, actually. Women tend to synchronize their menstrual cycle when they live together. Which means that all women are fertile at the same time, give or take a day. Which, uh... is a problem.
A man produces enough swimmers to be successful once, twice, maybe thrice per day. Everything beyond that is wishful thinking. I mean, sure, he can copulate more often, it's just that the chance of causing a pregnacy goes towards zero. So if you have 60 women on one man, that works out mathematically over the monthly average, but if they're all fertile on next wednesday, then that's truly not the very optimum.

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    $\begingroup$ Yes, but after a women is pregnant she has no need of him for at least 8 months so another women takes her place in the rotation. 60 pregnancies a month for 17 months is approx 1000 babies in 1.5 years. $\endgroup$ – cybernard Sep 16 at 16:42
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    $\begingroup$ @cybernard: But you see, that is the very problem. You get 2 pregnacies in a month, not 60. The man is good for two shots per day. If lucky, that is (might only score once, or not at all). If women are in sync, i.e. all fertile on one day within a 28 day period, that's at most two pregnacies per 28 days possible. Because on every other day, do what you will, no pregnacy. Well, maybe 4-5, if you assume they might be off by a day, and sperm may live for longer than a day. But 60 is totally unrealistic, that won't work. $\endgroup$ – Damon Sep 16 at 18:26
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    $\begingroup$ Yeah, I'd heard dissenting opinions about menstrual syncopy and I don't have the credentials to give an expert opinion on it, but a quick glance through Google results seems to show that the scientific consensus is that it doesn't really happen. I'm gonna assume it doesn't as I don't see any way it could or any reason it should. $\endgroup$ – Maddock Emerson Sep 16 at 19:14
  • $\begingroup$ So, quick math: If we assume that one "well-supplied" male mates with 3 females a week, who each have about a 50% chance to be implanted (they would be keeping track of their menstrual cycles to maximize the chances of conception, and sperm stays viable in the womb for several days) and of those who are fertile there is a 25% chance of viability (50% of implantation fails to come to term, and most male embryos in this case are nonviable) at 52 weeks per year, 3 females per week, 156 *.5 * .25 = as many as 18.75 babies per year, but (concluded in next comment) $\endgroup$ – Maddock Emerson Sep 16 at 19:26
  • $\begingroup$ (continued) it would realistically be less than that, because once a particular female is pregnant she doesn't need to go back to the male, and he might just shoot a blank or not be in the mood, etc. So let's assume it's more like 10 successful pregnancies a year; in that case, it would take a healthy man 10 years to seed the next generation, which is totally feasible. I hope I did my math right here. $\endgroup$ – Maddock Emerson Sep 16 at 19:30
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Here's the problems, assuming they're still mostly human:

  1. Human females don't advertise (generally speaking) when they're fertile, and you're only looking at conception being even possible for 5 or 6 days per month. So, about 80% of the time, no chance of pregnancy just based on the odds. If you're talking a ratio of 1 to 100, and as mentioned a mature male might be able to pull off twice a day, you're talking 50 days to "service" all the females once. So, just rounding out, say two months. That means the vast majority won't get fertilized based on chance alone.

  2. Failure to implant and miscarriages are ridiculously common in humans, with some estimates that up to 90% of fertilized eggs do not result in a baby being born, with most being ejected before the woman even knows she's been fertilized. Even with the lower accepted numbers like 75%, that still means of the few women out of 100 the male has sex with at the right time, at least 3/4s of them or more won't end up with a child.

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  • $\begingroup$ I remembered hearing somewhere that it was 50% of fertilized eggs don't come to term. But we can still work with that. Consider that even though it's not obvious when humans are ovulating, we are smart enough to predict when ovulation will occur, usually within a week's time, and sperm remains viable in the womb for several days. On top of that, this species might have induced ovulation or some other mechanisms to increase the chances of conception. I did some basic math on this in the comments to another answer, but let's plug in some numbers closer to what's in your answer... (continued...) $\endgroup$ – Maddock Emerson Sep 19 at 0:09
  • $\begingroup$ For simplicity's sake say that if a man mates with one woman every other day (need time for sperm count to refill) and the chance of fertilization is 50% each (accounting for period tracking, women not knowing they're already pregnant, and other complications) then the chance of a fertilized egg coming to term is 10%, we get 365 * 0.5 * 0.5 * 0.1, which is up to 9 births per year. Even if we halve that to account for the prospect that males are rare because male embryos consistently miscarry, it takes about 20 years of a male being able to breed, to break even for the next generation. $\endgroup$ – Maddock Emerson Sep 21 at 16:20
  • $\begingroup$ And actually if we take your numbers exactly then it's even easier. two women a day with a 20% chance of implantation and a 10% chance of successful birth is 365 * 2 * 0.2 * 0.1 = 14.6, half of which (assuming non-viable male embryos as opposed to majority X sperm count) is 7.3, so they'd have statistically between 7 and 14 births per year, meaning it takes ten years to seed the next generation. $\endgroup$ – Maddock Emerson Sep 21 at 16:29
  • $\begingroup$ So I guess 1,000 to one gender ratio isn't quite doable without advanced tech, but 100 to one seems just on the edge of plausibility. That's where I like to be. $\endgroup$ – Maddock Emerson Sep 21 at 16:29
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What you care about is something called effective population size. This is a term used by population biologists who want a way to quantify the genetic characteristics of some non-ideal population as compared to an ideal one. So a population that goes through bottlenecks or a population with highly variable reproductive success will have a lower effective population size than their actual size because these features of populations increase genetic drift. Genetic drift is what we really care about because it’s a measure of how genetic diversity is lost in small populations over time and this is what your population needs to avoid.

The effect of unequal sex ratios on effective population size is as follows:

$$N_e = \frac{4N_mN_f}{N_m+N_f}$$

This tells us that the effective population size $N_e$ can be calculated from the ratio of males $N_m$ to females $N_f$ and that the more imbalanced that ratio is the smaller the effective population size is. For an in-depth explanation of how this formula works see our friends at biology.se. So now to understand how many males the species can “get away with” we only need to know how large the actual population is and how large the effective population needs to be. If you play around with this formula you will see that as soon as the sex ratio changes significantly from 1 the effective population size drops significantly. This decrease approaches an asymptote of 4x the population of the less frequent sex. This tells us that you can have a hugely imbalanced sex ratio as long as there is some minimum number of the minor sex to achieve a viable effective population size.

Now, what is the “minimum viable population”? Frankly, estimates vary and depend on many things. If you look at just the genetics then the answer tends to be in the hundreds. The estimate of 10,000 cited in Tim B. II’s answer is on the high end because they are expecting "at least one severe population catastrophe over the 5-generation voyage." That probably isn’t an awful factor to consider in the survival of a population on a planet either, but it’s also not strictly about the genetics of the population either.

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