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For the purposes of this question:

  1. The species is multi cellular and from an animalia like kingdom.
  2. Sex is only physically possible when 3 individuals of all 3 sexes are present together. With only 2 members, sex is neither possible, nor enjoyable.
  3. Fertilization is only possible when at least one sex orgasms.
  4. An individual cannot spontaneously change its gender.

How would such a system work, a) physically, and b), at the cellular level? (we need three different types of cells: sperm, egg, and ???)

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  • $\begingroup$ Just add a third of each necessary thing, a third type of gamete, only a third of dna is contributed by each organism, etc $\endgroup$
    – Topcode
    Jun 14, 2022 at 2:36
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    $\begingroup$ @Topcode or you could go the tried and tested route several fantasy writers have already gone, the third contributes no DNA but provides enzymes or hormonal triggers without which conception can't occur, the third sex is often in some senses a drone or neuter in this approach, the offspring can be either of the three 'genders' in this situation / a third sex might also be the one with the 'womb' recieving the fertilised egg which also makes offspring impossible without it / a 3rd sex doesn't have to contribute DNA. $\endgroup$
    – Pelinore
    Jun 14, 2022 at 2:55
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    $\begingroup$ @Pelinore the problem with that is that the third doesn’t spread any dna, and therefore doenst have much of an evolutionary pathway to well, exist. If a mutation happens that causes them to not spread any dna but instead give other supporting materials, well that might work but it didn’t spread any dna so oh well doesn’t exist anymore. $\endgroup$
    – Topcode
    Jun 14, 2022 at 3:00
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    $\begingroup$ @Topcode the same is true of insect drones in ants, wasps, bees and many many others, they spread their genes by proxy and that pretty obviously is a perfectly adequate evolutionary driver for a neuter / drone gender to exist, because it pretty much does already in nature, in spades, I don't see them not existing any time soon just because they don't spread any DNA themselves, do you? 🤨 $\endgroup$
    – Pelinore
    Jun 14, 2022 at 3:04
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    $\begingroup$ @Pelinore the difference is that they are related to each other, all sisters so they all pass on their genes if for example in an ant colony it survives and alates are created by the queen. In order for that to work in that specific 3 sex reproduction method you would need at least 2 of them to be siblings, which I don’t know if op wants that. $\endgroup$
    – Topcode
    Jun 14, 2022 at 3:09

5 Answers 5

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Asimov described a similar system in his work "The God themselves"

The latter have three sexes with fixed roles for each sex:

  • Rationals (or "lefts") are the logical and scientific sex; identified with masculine pronouns and producing a form of sperm. They have limited ability to pass through other bodies.
  • Emotionals (or "mids") are the intuitive sex; identified with the feminine pronouns and provide the energy needed for reproduction. Emotionals can pass freely in and out of solid material, including rock.
  • Parentals (or "rights") bear and raise the offspring, and are identified with masculine pronouns. Parentals have almost no ability to blend their bodies with others, except when helped by one or both of the other sexes.

All three 'genders' are embedded in sexual and social norms of expected and acceptable behavior. All three live by photosynthesis; whereas sexual intercourse is accomplished by bodily collapse into a single pool (known as 'melting'). Rationals and Parentals can do this independently, but in the presence of an Emotional, the "melt" becomes total, which causes orgasm and also results in a period of unconsciousness and memory loss. Only during such a total "melt" can the Rational "impregnate" the Parental, with the Emotional providing the energy. Normally, the triad produces three children; a Rational, a Parental and Emotional (in that order), after which they "pass on" and disappear forever. In the past, some triads have repeated the cycle of births (thus ensuring population growth), but the declining amount of solar radiation no longer allows that. "Stone-rubbing" is a practice of partially melting with solid objects like rocks, possible for Emotionals, but the other genders are only capable of it in a very limited form. It is an analogue of human masturbation and generally frowned upon.

At cellular level you can think of it like two sexes providing the genetic material while the third one providing the cellular protoplasm where the genetic material can operate. Since the protoplasm also contains genetic material (mitochondrial) it ensures the passage of the genetic information of the third sex.

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The Culture series novel “Player of Games” describes such a species. In it, they describe three genders: male, female, and “apex”. The male and female are biologically just that, they provide the egg and sperm. What both male and female lack, however, is the womb to carry the growing new person. Sex works by implanting sperm and egg into the apex. All three are fully required.

Interestingly, in this society, both male and females are considered lesser than the apex gender, in a highly stratified and sexist system.

I will note that making sex unpleasurable without all three present is a tricky thing to consider… after all, we have two sexes and it’s still plenty pleasurable to simulate sex while alone, and homosexual sex is pleasurable to those who like it. Restricting pleasurable sex to all three would likely require a mental component, similar to how truly heterosexual humans simply do not like to engage in homosexual relations.

Alternatively, the pleasure in sex could involve something like an electrical feedback loop. Our pleasure from sex comes from stimulation of nerves, but you could imagine a species that actually requires nerve ending to touch with another gender’s nerve bundle to be pleasurable. You could also extend that to requiring the third gender as well for the nerve bundles to work, possibly involving an internal feedback loop of sorts, such that lacking any of the genders breaks the circuit.

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  • $\begingroup$ Regarding making sex pleasureable only when all three are present: What about a chemical reaction? Each of the three secretes a different pheromone in the early stage of arousal. The 3 pheromones react with each other and create a 4th pheromone, which is an intense aphrodisiac for all three genders. $\endgroup$
    – Philipp
    Jun 14, 2022 at 12:27
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Mendelian Genetics:

Your genders are based around a not-so-simple but still Mendelian inheritance of chromosomes. There is one trait, and each gender can be AA, aa, or Aa. 50% of offspring are Aa, and these are an intersex gender. The AA and aa together each contribute an (A) from the AA and an (a) from the aa. All The Aa contributes an (A) or (a) to each conception. Twins are born, genetically different. You can have a quadraploid conception, or you can have two conceptions with each pregnancy. With two separate conceptions, the Aa individual might potentially produce two different kinds of sex cells, one compatible with the AA gamete, one compatible with the aa gamete. All contribute genetically, and if there are no aa or AA individuals, conditions are considered too extreme to allow reproduction. Thus this three-gender system is a safety check on when it is safe to reproduce.

This is kind of complicated, with the accompanying disadvantages of complexity, but with the quadraploid conception, you may potentially have recombination occurring between all four sets of chromosomes. All offspring could thus potentially have genes from all three starting parents. Thus the possible number of diverse offspring is significantly increased over conventional reproduction.

The social roles for your system are up to you, but I would suggest the Aa's are the worker ants of society. They make up the largest part of the potential labor force. Without at least one worker available to support a child, it is unsafe to reproduce.

The mechanism of the biology has two main options. The 'host' option (already presented in another answer) has the Aa intersex individual functioning as the womb for the other two sexes. The 'catalyst' option (presented in Alien Nation) would have the intersex individual somehow needed to stimulate one or both of the other genders. The pleasure may come from the release of the catalyst, which only occurs when both other genders (AA and aa) are involved.

There is likely a strong social aspect to this sex, where an Aa must feel safe and (dare I say) loved.

Such a system suggests an environment where there are long periods of relative good reproductive conditions, followed by periods of intense selective pressure. The high diversity allows at least some individuals with especially advantageous combinations of genes to survive the harsh times, while the good conditions allow the more complex reproductive scheme to be less of a disadvantage.

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You could delegate the pheromones' or hormones which cause the male and female members of the species to want to reproduce to a third sex - so the men and woman would neither have the desire to reproduce without the third sex being around.

Two variants of this. Either the third sex puts off the pheromones constantly so they just need to be around to get the first two sexes in the mood or the third sex may only put off the pheromones at certain times of the year or when its found an (evolutionary-wise) safe place to rear the babies.

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I'd go with two types of males (sperm producers) and a female (egg producer). Give them physical characteristics to distinguish them. The two types of male are each only capable of producing half the DNA required to fertilize an egg. The only way reproduction can happen is if all three orgasm; the two males must ejaculate inside the female; and only once the chemical signals are present to indicate the possibility of pregnancy, then the female is capable of orgasm. If an egg is available for release, the orgasm will cause it to be released, so it may be fertilized by the two types of sperm.

Unlike humans or other mammals, the eggs do not drop on a time schedule or cycle. For evolutionary reasons, they are preserved in the egg producer.

But like humans, much can go wrong, intentionally or otherwise. Say the two types of sperm can both fertilize the egg, but if two of the same type of sperm fertilize the egg, nothing happens. So there is only a 50/50 chance of pregnancy. Intentionally, birth control may be possible; in any of the three genders. Say the chemicals that signal a type of sperm is present are not in the sperm itself, but in the ejaculate, the fluid that carries the sperm. And it is possible, by chemicals or surgery, to prevent any sperm from mixing with the ejaculate. Likewise, just like female birth control now, chemicals may prevent the egg from being released even if the female orgasms. And scientists may be able to synthesize the chemicals in the ejaculate as a cream or lubricant, so females could orgasm with the aid of any males.

The male psychology would evolve to welcome the other type of male in sex and pregnancy, it would seem natural, and the female psychology would likewise evolve to be primarily sexually excited only when both types of male present. Sex with only one type of male present would seem weird and incomplete, to both males and females.

I think you could build up a culture around this.

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