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I'm stewing on an idea about the people of Earth encountering a humanoid race exhibiting a pregender phase. After a period of time, this pregender develops into either male or female based on environment (attraction to mates, see sequential hermaphroditism). Commonly, most of the humanoids will have transitioned to one gender or the other by adulthood. After transitioning there is no further change without surgery.

For the purpose of this question, please use humankind as the starting point.

Question: based on terrestrial creatures that exhibit sequential hermaphroditism, what are the most likely biological traits my species would have that humankind would not?

The best answer will:

  • Explain the necessity for each change.

  • Have links and references to existing terrestrial creatures to justify the changes.

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  • $\begingroup$ @JBH No no, I mean a type of sequential hermaphroditism. I have looked into several species of coral reef fish and mollusks for prior research. I will edit the question then. sorry about the "third Gender" confusion. $\endgroup$ Commented Nov 25, 2020 at 8:58
  • $\begingroup$ Thanks for the clarification, I have retracted my close vote. I edited your question to improve formatting and clarity. If you don't like what I did, you can either edit again or roll the edit back to your last version (it won't hurt my feelings either way). I did remove the question about behavior. I recommend you ask that after this question is answered as the selected traits will be the basis for behavior. $\endgroup$
    – JBH
    Commented Nov 25, 2020 at 9:15
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    $\begingroup$ The question title asks for a sequential hermaphrodite, but that isn't actually what you're looking for. A sequential hermaphrodite, by the definition of the term, will be both male and female at different times over its lifespan. What you're describing is something that is born without a determined sex that chooses one sex at puberty for the rest of its life based on various factors; I'm not saying that can't be interesting, but a given individual will only ever be one sex, not both. Using the term "sequential hermaphrodite" for your humanoid is just a recipe for confusion. $\endgroup$
    – Palarran
    Commented Nov 25, 2020 at 10:37
  • $\begingroup$ as far as i'm aware, in nature you only see true hermaphroditism as either 1- the creature always had both sexual structures fully developed, or 2- had one set of reproductive structures and changed it's morphology based on enviromental conditions (and which is almost exclusive to fish and gastropods), such as a dominant male clownfish converting into the dominant female after the previous one died. Other than those, the closest instances of an "asexual" period i know only last the development period inside the womb/egg (and usually this is just an undeveloped version of the species' female). $\endgroup$ Commented Nov 25, 2020 at 20:02

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We have a "pregender" phase in humans: prepubescence. Barring an unusually early puberty, your average eight-year-old boy and girl are more-or-less the same, physically (genitalia aside). It's not until puberty starts flooding the body with hormones that secondary sex characteristics--body hair, breasts, beard, wider pelvis, etc.--start to develop.

Your third-sex could have basically neuter or intersex genitalia that develop in one direction or the other as they hit puberty and sexual maturity. Presumably their reproductive organs aren't quite as differentiated as they are in humans, though.

Author's Choice

This is, of course, non-human biology that we're discussing, so there could be some characteristic completely unlike anything humans demonstrate, which presumably diminish as they develop into one of the other sexes. Maybe third-sex individuals have especially long earlobes, or extra-bushy eyebrows. They could have tiger-stripe patterns all over their skin. Maybe they simply remain at a preadolescent stature until their attraction triggers a growth spurt as part of their sex-specific puberty. Get creative with it, especially if you really want to play up the non-human aspect of it.

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  • $\begingroup$ The question asks neutral->male or neutral->female, not male->female or female->male, so it's not exactly the same as sequential hermaphroditism. $\endgroup$
    – Salda007
    Commented Nov 25, 2020 at 16:33
  • $\begingroup$ That would be a good question to ask the OP. I was merely pointing out that a change had occurred that had the potential of impacting your answer. Cheers. $\endgroup$
    – JBH
    Commented Nov 25, 2020 at 16:39
  • $\begingroup$ Also see: [en.wikipedia.org/wiki/G%C3%BCevedoce ](Güevedoce) as very related (but not quite the same) $\endgroup$ Commented Nov 25, 2020 at 19:38
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The answer of Salda007 covers the time where the secondary sexual characteristics (breasts, beards, pubic hair) are indistinguishable between male and female humans. But going even further back in time there is a time when even the primary sexual organs (genitalia) are indistinguishable: the early embryonic phase.

Every human being starts out as one single fertilized cell. As this cell starts to divide and develop into an embryo, the first things to develop are important body structures like the heart, brain and skeletton. At that stage there is no visible difference between boy or girl. It takes a few weeks until the internal and external sexual organs start developing.

The same stem cells that build the ovaries in girls can build the testicles in boys. The cells that build the penis in boys will build a clitoris and vulva in girls. The way those cells develop is most prominently influenced by the hormones of the embryo, but a strong hormonal imbalance or chemicals that act like hormones can disturb the development, resulting in babies with XY chromosomes (boys) to be born with a vulva and often with testicles.

If your alien race doesn't start producing sex hormones (estrogen or testosterone) before puberty at all, they could maybe stop the embryonic development of sexual organs and keep dormant stem cells in their bodies. Then with the onset of hormone production, breasts or beards aren't the only things to grow, but their dormant stem cells are activated again and their genitalia take the specific form of their new sex.

The children of that alien race would have gonads somewhere in their pelvic region and mesonephric ducts as well as paramesonephric ducts. Their external genitalia would resemble a bigger version of those of embryos.

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In Ursula K. Le Guin's The Left Hand of Darkness there is an alien species mysteriously related to humankind that exhibits sequential hermaphroditism as you describe, but with one difference: they have a 26 days sex change cycle.

They have both genitals. They will be androgynous andsterile for 22 days at a time. Then based on environment they will pick up either gender's characteristics and become fertile for four days. If they don't have contact with others, or if the population in the reproductive stage of the cycle is balanced, their body picks a gender at random.

Some of them are stuck in the reproductive cycle forever after puberty and thus have either male or female secondary characteristics more developed and permanent; Some force themselves into this condition through hormone treatment. In both cases they are called "perverts" by their society (though it does not seem to be a demeaning term for them).


Your humanoid race would probably approximate Le Guin's people who are stuck in a gender, which would also be like a person with true human hermaphroditism that has had either masculinizing or feminizing medical interventions after puberty. They would not necessarily be androgynous before puberty; They could also be "boyish" or "girlish" in appearance at random, even changing from one gender's characteristics to another before settling.

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