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I'm trying to design human like race where all the children are born as girls, but later in life some of them switch to male, but no sooner then 15. Usually only the most dominant women switch, so in normal times women are majority of the population, but in exceptional situations like war or overpopulation gender ratio might be reversed.

As soon as transformation starts woman is infertile until the transformation is finished. I have chosen this in order to avoid self impregnation. Transformation is irreversible, man can't transform back into woman. The woman must start transformation before certain age, otherwise she will stay woman for good. I haven't decided on the exact age until I know how long should the transformation last.

How many years should the transformation from woman to man take?

I don't know much about biology but I'm expecting some plausible approximation rooted on examples in either humans or animals, like Frostfyre's links about genetic disorder and mice hormone experiments.

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    $\begingroup$ 7 - because any arbitary number is as good as the next; Welcome to WB Lulawezi, although your question shows that you have thought about what you are asking and took time to post a good question it still is, what we call, opinion-based. This means it is very hard to answer your question with facts.. Please try to reword it in order to avoid this opinion-based-ness $\endgroup$ – dot_Sp0T Nov 10 '16 at 19:50
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    $\begingroup$ @Azuaron To support Koume: on female to male and on male to female should be a good start. $\endgroup$ – Frostfyre Nov 10 '16 at 20:10
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    $\begingroup$ @Azuaron There's a community in the Dominican Republic that has girls turn into boys at puberty. $\endgroup$ – Frostfyre Nov 10 '16 at 20:12
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    $\begingroup$ Given that this is a real condition, with real data to back it up (see @Frostfyre's comment), we should reopen this since it's not opinion based. $\endgroup$ – Azuaron Nov 10 '16 at 21:33
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    $\begingroup$ @Frostfyre: As far as I can tell, those aren't sex changes. There's some tissue that initially looks female, but they aren't actually female. The children are male with under-developed testes and not-yet-developed penises. Then puberty kicks in and everything goes (mostly) to normal. Still, I'm sure a species could be engineered to go fully female to fully male in a similar time. $\endgroup$ – MichaelS Nov 11 '16 at 9:30
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clownfish take about a month, hormone therapy takes about 2 years to show strong effects, so assuming they stay active approx six months to a 1 year a decent number for a natural process in a human size animal. if they go into a immobile cocoon metamorphoses stage like insects you could get away with a month, not including the binge eating build up. there really is not that much difference between male and female, growing the new nerves and organs is what takes the longest. basically the faster it happens the less other things they are going to be able to do at the same time.

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  • $\begingroup$ Also, at some point, there needs to be some mental component while the brain does rewiring, otherwise you get gender dysphoria. Most teenagers are thought to sleep a lot during identity formation and the result process would probably do the same, again. $\endgroup$ – aphenine Feb 3 '17 at 20:30
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About six months

As we live and grow, our bodies shape and reshape theirselves, in very young children at a rate of approximately their body weight in new growth per year, tapering off as we edge towards 25 years of age. While reforming skin and nerves already laid is complex in humans, it needent be in this putative breed, and we can use the base regrowth rate.

For (human) children around ten years of age; something on the order of a lost fingertip can be completely regrown in a month, so for something more complex and sizable, but not so sizable, about six months at the base growth rate would likely do. (We are after all talking about reformation of existing structures, and nothing so dramatic as a lost limb.)

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How about half an answer?

We have an example with humans. Normally we have males whose body responds normally to testosterone and they're male. We also have "males" whose body does not respond properly to testosterone, they appear female but test as XY and often exhibit failures of the reproductive system.

However, testosterone actually comes in two versions. There is a very potent form released in-utero that causes boys to be boys. It's possible to have a failure to respond to this form of testosterone but still respond normally to normal testosterone. The result is an apparently female baby that turns male when puberty hits.

https://en.wikipedia.org/wiki/5-alpha-reductase_deficiency

Unfortunately, I haven't encountered anything that says how long it takes but it never even occurred to me to wonder until this question came along. I'll leave tracking down this number as an exercise for the reader.

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However long you want it to take.

When the females of your species already have a fully developed penis and working testes, these could activate immediately.

When the females have these organs fully formed but dormant within their bodies and they first need to migrate outside and become active, it could be a matter of days or weeks. Human sperm cells have a gestation period of about 3 months, so when the testes were completely inactive before the transition, it would take at least that long until the specimen produces fertile sperm.

When these organs need to form from scratch, it might take month to years.

In either case, it might take longer for secondary sexual characteristics to change than for becomming fertile.

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    $\begingroup$ In human physiology, the penis develops from the same embryonic tissue as does the vagina, so both being present in the same person isn't really an option. $\endgroup$ – jwenting Nov 11 '16 at 6:56
  • $\begingroup$ @jwenting We aren't talking about humans here, though. $\endgroup$ – Philipp Nov 11 '16 at 7:15
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    $\begingroup$ There are intersex/hermaphroditus humans in the real world. The idea of a fantasy world with humans with fully functional (but one dormant until selectively activated by hormones) male and female reproductive organs is not too far fetched. $\endgroup$ – Achilles Nov 11 '16 at 16:49

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