In the real world all humans start off as females in the womb and then develop male organs before being born.

I'd like to expand that concept further to the humans in my world.

In my world, humans are all born females, turning into males later in life.

So all younger people would be females and all older people males, with sex being an indicative of age.

The result would be that all the people will become males once they get old enough.

In this scenario, is it viable for a species in which all organisms are born females but then turn into males after a certain age is reached to survive?

Assume that the sex change happens in adulthood around the age of 30-40 years.

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    $\begingroup$ This exists and strangely how a prostate medication was created en.wikipedia.org/wiki/Güevedoce $\endgroup$
    – user69935
    Commented May 31, 2020 at 12:53
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    $\begingroup$ I think that this question falls into the "too story-based" category. It is not about building a world, it is about figuring out what would happen in the world you already built. But why do you think that it would result in extinction? If you would explain your reason, then this would be an on-topic question, because we could then look for a change in your world which would make it viable. $\endgroup$
    – Philipp
    Commented May 31, 2020 at 13:23
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    $\begingroup$ Sequential hermaphroditism is a real thing in many real life species. They survive just fine. $\endgroup$
    – AlexP
    Commented May 31, 2020 at 13:42
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    $\begingroup$ @kyu, you are incorrect, protogyny does exist in animals, Indo-Pacific cleaner wrasse being a classic example. Also in the Labridae, Pomacanthidae, and Serranidae $\endgroup$
    – John
    Commented May 31, 2020 at 14:46
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    $\begingroup$ @user535733: Why do you think age differences are in any way repugnant? A 10-20 year age difference is by no means uncommon in western society, e.g. my late neighbor (over 100 when he died, wife in late 70s), the current US President and his spouse, the UK PM and his partner... $\endgroup$
    – jamesqf
    Commented May 31, 2020 at 16:12

5 Answers 5


In this scenario, is it viable for a species in which all organisms are born females but turn into males after a certain age is reached to survive?

Yes. It's very viable, given certain constraints, and there are even certain, although mild, advantages. First, the female stage needs to be for a sufficiently long amount of time, especially if the species is intelligent. Intelligent species tend to wait until later in life to have children - here 'later in life' means sexual maturity after at least a few years of development. Humans in particular take an astonishing long time, which makes sense because of the way intelligence vs instincts works - intelligence is always better in the long term, and the longer you can allow for intelligence to develop, the better the species is. But I digress.

You'll need a sufficiently long time of sexual maturity, I'm thinking at least 25 years, possibly longer. (Again, this isn't strictly necessary for an unintelligent species, but it's better for an intelligent species to have a longer span of time.) After that, you'll want a transition phase where the individual isn't sexually mature while they transition and then reassume sexual maturity as an adult. Funnily enough, this requires less time within sexual maturity because at this point the species is a mature adult, though obviously, the more time, the better.

The lifespan looks like this. 0-15 - development, 15-40 - female stage, 40-45 - transition stage, 45-death - male stage. There are a few advantages to this, namely, that since everyone can perform 'double duty' when it comes to reproduction, they have twice the womb capacity as a fixed race such as ours.

That said, I'm not certain female->male is the way to go. True, it has benefits. On the one side, you get much faster access to wombs, which is very important for a developing society. In primitive societies, wombs are really important because those are what you use to get the best of all resources - new offspring. And while you're restricted by the number of wombs, all it takes is a handful of males to fill those wombs, meaning it's better to lose men than women. That's why males are the disposable gender and why they're the warriors. (Among other reasons.) Having the species develop first as women so they can give birth to offspring and then become the disposable men seems like a good thing.

On the flip side, men are also pretty important as the warrior caste, responsible for the hard work and fighting and all that good stuff. If they're busy giving birth to children until 45 (okay, you can play around with the ages, but the thought still stands), then you run the risk of having a male society which isn't really that strong or fit - and that's pretty bad! In evolution, specialization tends to beat generalization, so if you had a tribe of these male/female humans vs a tribe of male humans + female humans, I'd take the latter over the former.

That said, there's no inherit problems in this structure. I means, society is going to be very interesting because every marriage is going to contain a 30 year age gap and you'll have to do some nimble footwork to avoid mother/father-daughter incest issues, but it can work. Theoretically.

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    $\begingroup$ I would think female-->male is the best. Since everyone will become male eventually there is no reason to make a sex disposable. The male traits that make them good at fighting or hunting wouldnt exist, and it is far more important to have young female bodies capable of childbearing without complications than "disposable" male bodies. It also makes sense to make women who survived into males. Their body might not be able to support childbirth without complications anymore and the surviving women obviously have genes worth spreading as they got that far, so male it is! $\endgroup$
    – Demigan
    Commented Jun 1, 2020 at 7:36
  • $\begingroup$ No need to have such a long period of being female. Every female needs to reproduce once or twice to keep the race going and growing at a slow rate. Say 5 times if there is a huge child or your women mortality. That makes a 5 to 10 years as 'breeding age' women, before turning into male. You may even make that change triggered by the person, so after a given age, women can choose to change, with an automatic change after a next given age. $\endgroup$
    – Willeke
    Commented Jun 1, 2020 at 7:50
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    $\begingroup$ @Willeke 5 babies in 5 years is quite an ordeal. Even if we're talking of infant mortality, which we likely aren't, and if the breeding starts as soon as possible, meaning you need a husband ready and willing. Also if the older gender has more life expectancy than the younger gender, you might want to figure out in advance what should happen when the male/female breeding pair turns into a male/male non-breeding pair. With traditional marriages, half of the wombs go unused, with planned divorces you get one aging man taking care of five adolescents. $\endgroup$ Commented Jun 1, 2020 at 9:41
  • $\begingroup$ 5 years from starting to breed when going for one or two offspring. 10 years for up to 5 kids. That is starting to count at full maturity, comparable to 20 years in Earth humans. (And what makes you think the males get more than a few years as male? Life may well keep the dangers for the males and Earth Humans used to have a much lower average age than they have now, 30'ish or 40'ish depending on whether there were wars or epidemics. $\endgroup$
    – Willeke
    Commented Jun 1, 2020 at 10:13
  • $\begingroup$ And why go for long term pairs? Stay together to get pregnant or at most stay together till the kids are beyond infant stage. $\endgroup$
    – Willeke
    Commented Jun 1, 2020 at 10:14

It already exists:

About 2% of fish species already do something like this, and it is perfectly plausible. (Source)

  • $\begingroup$ Very interesting article, just wondering whether you could summarize part of the information here in the answer as well, for future reference $\endgroup$
    – Enthu5ed
    Commented Jun 3, 2020 at 21:24

The species will survive, but not in the same way we do.

You have set up a situation in which, unless the population is rapidly contracting with sub-replacement birth rates for a long time, there will always be many more females than males.

First, any given age cohort will decrease in size as it ages, due to accident and disease. So under exact replacement birth rates, when all males are older than all females, the male-age cohorts will all be smaller than the female-age cohorts.

Furthermore, if the population is growing, younger cohorts will have a higher base size than older cohorts--which further increases the number of later-born women over the number of earlier-born men.

One way to fix this is to simply accept than some people will never reproduce as females. That may work in a modern industrialized civilization, where not everybody needs or wants to have kids, and populations stabilize at replacement levels. But "wasting wombs" is just not a viable strategy for a pre-industrial culture.

As a result, polygamy will be the universal default state for this altered humanity. In situations where population growth is slow, so the female:male ratio is less than 2:1, not every mature male will have to be a polygamist, but the most well-off--those who best able to support multiple wives and more children--will be. And when the population is growing rapidly, or when the male population has been depleted--say, after a war or famine or pandemic, all of which reduce population generally but affect males and the elderly more than females and the young--it would be seen as every male's duty to take multiple wives. In this instance, polygamy is not a reward for powerful old men at the expense of other males, who are then left discontented and serve as a source of violence--rather, it is the normal arrangement required for social stability, and a duty which some men may find themselves relieved from in times of unusual long-term prosperity.


Yes, provided that the women develop a means to safely and effectively suppress testosterone production in men once they are no longer needed for reproduction.


In this scenario, is it viable for a species in which all organisms are born females but then turn into males after a certain age is reached to survive?

Yes, but

  1. What if you suffer mass loss of life, you need to have the transition bought forward. The change may not be purely aged based, maybe pushed forward or back depending on environmental conditions. (like the frogs from Jurassic Park)

  2. If life expediency is about 30-40 they you don't have enough males. They need to both change to male healthy, and spend time health as a male.

  3. It might be advantageous for good female genes to be spread far and wide via male reproduction. Ie genes for a good female is capped with one child every 9 moths; the genes for the men are well dependent on the number of women. I feel like that genes for a good male/female would dominate, like a reward for making it that far.

    1. What about the comparative advantage of the female, that turns male early? What stops that gene setting up younger and younger males? Is their fighting? are they too weak to mate?

Modern Society: Need a pretty hard re-write. its pretty significant.

  1. Gender divide now makes no sense. You can't exclude women from careers etc, because they will become men; and all the men used to be women (some had children). This alone would have all sort of impacts that change society.

  2. It cuts away partners for life for breeding, as your partners all age into males. Do you have children as the man or women? Would this prevent the nuclear family? A guess communal child rearing still works.

    1. Now women may delay children, until they are men. If your left with any disadvantages around this (stay at home disrupt a career) none would would want children as the women.

    2. The window of female child birth is very limited in a modern society that needs to delay children (or child care) for additional education, it pushes back childbirth.


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