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Every species I'm aware of on earth has two sexes (with the possible exception of species which change their sexes to allow self fertilization).

Is it possible for a species to evolve which could have more than two sexes? How would such a race be competitive against species which only required two beings to meet and form a partnership?

I've edited this question to use the word sex rather than gender as I'm looking for biologically based answers and wasn't aware of the distinction some answers have clarified.

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    $\begingroup$ I'M guessing that your talking about their sex and not gender(I can't find a better word for it) A lot of people now believe that gender is a social identity more than a biological one. I think sex is more appropriate when it comes to biological differences. For example: some people are born female but want to be considered as males. Some will even go into surgery in order to make their gender and sex match. $\endgroup$ – Vincent Sep 29 '14 at 14:49
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    $\begingroup$ Not every species have two sexes. There are sexless species like bacteria and archaea en.wikipedia.org/wiki/Prokaryote#Reproduction $\endgroup$ – Envite Oct 3 '14 at 12:37
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    $\begingroup$ No-one seems to have mentioned mating types in fungi. They are (in some cases quite close) analogies to multiple sexes surely? Eg Paramecium bursaria. $\endgroup$ – Francis Davey Dec 14 '14 at 22:54
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    $\begingroup$ I know it's only a single cell, but I just wanted to throw this into the mix: newscientist.com/article/… $\endgroup$ – Joe Bloggs Sep 21 '15 at 10:02
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    $\begingroup$ An alien race has three sexes in the Isaac Asimov novel,The Gods Themselves. $\endgroup$ – CWallach Jan 4 '17 at 23:52

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Technically yes.

All that genders need to do (functionally) is to get 46 chromosomes into the egg to enable it to be a full creature. Likely aliens would have a different number of chromosomes than 46. 46 isn't divisible by three, or four, so it doesn't make as much sense. Likely there would be something like 138 chromosomes, with chromosomes forming in triplets.

However, the more creatures it takes to mate, the more difficult it would be for it to keep a leg up. Social scandals would be bigger, and more easily made known (three tongues to tell the tale, not two). They would also be less easily able to recover after an incident that removes much of the species. They would also reproduce more slowly especially if "monogamy" (more like "di" or "tri"-gamy) was still practiced. But this alone wouldn't doom the species to fail, other factors would probably contribute more.

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    $\begingroup$ "Technically aliens could have a different number of chromosomes" -- I'd really rather phrase that as technically they could have the same number of chromosomes; the likelihood that they would would be very small. $\endgroup$ – a CVn Sep 29 '14 at 11:11
  • $\begingroup$ @MichaelKjörling Is that better? $\endgroup$ – DonyorM Sep 29 '14 at 11:37
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    $\begingroup$ Seems likely that the social norms of having 3+ sexes would provide an entirely different experience than we humans are used to. An example would be a tribe mentioned in Freakonomics podcast on the economy of suicide. Suicide is such a completely foreign concept that when a man described his mother killing herself that they laughed. They found it completely and utterly ridiculous that anyone do something so stupid like take their own life. $\endgroup$ – Wayne Werner Sep 29 '14 at 21:51
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    $\begingroup$ "Likely aliens would have a different number of chromosomes than 46." There's nothing special about 46. Humans have 46 chromosomes, but Gorillas have 48 (which I'll note is a multiple of 3), most bears have 74, carp have 104, and Kangaroos have 16. There are even some species that have odd numbers. (The male swamp wallaby has 11.) $\endgroup$ – tobyink Oct 2 '14 at 11:25
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    $\begingroup$ Came here to post that, @tobyink! en.wikipedia.org/wiki/List_of_organisms_by_chromosome_count $\endgroup$ – evandentremont Nov 5 '14 at 23:55
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It is not only possible, apparently it actually exists on earth. The abstract of the linked article reads:

Two recently discovered cases of genetic caste determination in social insects might provide the first example of a major evolutionary transition from two to more than two sexes. I argue here that the system can be interpreted as comprising primarily individuals requiring gametes from three parental types and having four sexes from the perspective of demographic extinction. Additionally, I show how this mating system can be seen as a major evolutionary transition. For these populations, it is apparent that the mechanism for a three- or four-sex system does not lie within the myriad of possible arrangements of chromosomes within individuals, but at the next level of evolutionary complexity, with the arrangement of chromosomes among individuals within a social system.

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  • $\begingroup$ Nice catch, I was about to post something similar. To add to this answer for the sake of completeness, given the full question, there are also tribes with more than 2 genders (primitive, but they do have socially accepted genders that don't fit into male or female) and there's examples of this in fiction already. One trip to memory alpha gives plenty of examples with possibly more speculation. On why they'd be competitive - they wouldn't unless they could also shift between sexes or reproduce without partners, which introduces biological issues. $\endgroup$ – mechalynx Sep 30 '14 at 18:51
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    $\begingroup$ Pity that the article is not accessible $\endgroup$ – Envite Oct 3 '14 at 11:14
  • $\begingroup$ Out the top of my head I remember Eric Flint's gukuys in Mother of Demons (two sexes, four genders; most emotional relationships are between infertile females of the same sex, sex between a truemale and an infertile female being seen as perverted and obscene) and Varley's Titanides, that have two sexes, two genders (with, I think, three sexual organs each), and from one to four parents, fertilization being a fairly baroque undertaking which also becomes a plot device later in the Gea trilogy. $\endgroup$ – LSerni Sep 20 '15 at 22:49
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    $\begingroup$ If we want to get really technical, Flowers do this, the third "sex" is called Bees. $\endgroup$ – Ryan Feb 25 '16 at 18:22
  • $\begingroup$ While I can't read the article I suspect it's what I recently ran into on Through the Wormhole--"two" species of ants, breed with your own "species" and you get a princess, breed with the other "species" and you get a worker. Any given sex act only involves two but you need all four for either species to survive. $\endgroup$ – Loren Pechtel Sep 28 '16 at 0:49
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There are several ways on which species can reproduce sexually with more than two sexes. These are the simplest (and there are way others).

Option a: X/Y system with YY sex (two individuals)

Human system of sexual chromosomes X and Y giving XX and XY (which is similar for other species like birds, which use ZW and ZZ) can be extended to three sex determinations XX, XY and YY. In this system there would be three sexes, which all can engage in sexual intercourse with each other. I'll not describe sexual organs! This has two suboptions:

Option a1

Each individual can have sexual intercourse with every other individual, including same-sex. This implies there is no definite "female" sex that lays eggs or can be pregnant.

  • XX - XX gives only XX descendence
  • XX - XY gives 50% XX descendence and 50% XY descendence
  • XX - YY gives only XY descendence
  • XY - XY gives 25% XX descendence, 50% XY descendence, 25% YY descendence
  • XY - YY gives 50% XY descendence, 50% YY descendence
  • YY - YY gives only YY descendence

Idea can be that eggs with haploid configurations mix with each other in water like frog eggs do with frog sperm. Also something more elaborate with internal fecundation (which I again do not describe).

Option a2

Each individual can have sexual intercourse only with different sex individuals. This yields the same table as above, but with three options removed:

  • XX - XY gives 50% XX descendence and 50% XY descendence
  • XX - YY gives only XY descendence
  • XY - YY gives 50% XY descendence, 50% YY descendence

Ideas for fertilization can be more elaborate, especially for internal one, but still there is no "female" sex.

Option b: X/Y/Z system (two individuals)

Human system of X and Y giving XX and XY can be extended to a third chromosome type Z, giving (in principle) options XX, XY, XZ, YY, YZ, ZZ. Then these need to be assigned to three sexes, or rule that some of them are impossible (e.g. YY is impossible for humans). There are so many subtions that I will not explore them all. Some are:

Option b1

XX is female, XY and XZ are two different types of male. Females have sexual intercourse with males of the two types (I do not rule out homosexual relations, it is simply that they are not interesting for breeding). Each relation can give offspring of female or same male type of the father.

Option b2

XX, YY, and ZZ are three types of female, and XY, XZ and YZ are three types of male. All types of male can have sexual intercourse with all types of female. Offspring have the same chromosome as the mother and one of those from the father, which means that it may be different from them both (e.g. XX female and YZ male yield an XY male or XZ male).

Option c: Triploid system (three individuals)

In these systems the chromosomes come in triplets, not in pairs. This can happen with only one type of sexual chromosome, X, which can be present or not in the gametes, giving XXX for females, XX0 for "half-females" and X00 for males (000 being impossible). Or it can happen with two types of sexual chromosomes, X and Y, for which options XXX, XXY, XYY and YYY must be assigned to different sexes (or ruled impossible). Or even more complicated settings.

Please read this answer in conjuntion with that of Monty Wild. His gives an evolutionary view about how this could have happened.

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  • $\begingroup$ I have just realized a mistake. Editing. $\endgroup$ – Envite Feb 11 '15 at 21:45
  • $\begingroup$ Very late, but there are animals on earth that are similar to Option b1. $\endgroup$ – Michael Richardson Sep 24 '15 at 15:11
  • $\begingroup$ Details: I believe there is a lizard with 3 males and (I think) 2 female forms. I cannot find the page I previously read about them on. I did find a similar entry for a bird (Ruff : Philomachus pugnax) which has 3 types of males and 1 female. One male is territorial and gathers a harem. One male is a satellite, and just hangs around the edges, sneaking access to the harem. One male is a female mimic, which has free access to the harem. $\endgroup$ – Michael Richardson Sep 24 '15 at 15:17
  • $\begingroup$ you don't need to extend human sexual chromosome, there already exist 9 different combinations on humans, but mostly people only know about XX and XY.... $\endgroup$ – Charon Aug 9 '16 at 5:48
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    $\begingroup$ @渡し守シャロン 9 different viable, fertile combinations on humans? which ones? $\endgroup$ – Envite Aug 11 '16 at 7:05
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On Earth, most species that are capable of sexual reproduction have two complete copies of their genetic material (diploid), one set from each of their parents. There are exceptions (mainly in plants) where there are a larger, even number of copies (4 - quadruploid or 6 - hexaploid).

Some have three copies (triploid), however, since our form of sexual reproduction involves (at its most basic level) taking half of the copies of genetic material into each reproductive cell (which becomes haploid, containing one copy of the genetic material), the question as to how the process of meiosis deals with a choice of three copies rather than an even number of copies results in errors that make sexual reproduction non-viable.

However, in an alien system of sexual reproduction, individuals could be triploid or have higher ploidy, while each sex cell is haploid (or perhaps diploid or having higher ploidy but less than the normal ploidy by at least 2). In order to re-establish the correct ploidy, there may be a situation where an A-Egg and a B-Sperm combine to form an AB-Egg, which is then fertilized by a C-sperm, and becomes a viable embryo at that point. Such as system would require three types of gamete, and could result in three distinct genders, all three of which would be required to reproduce. there are possibilities involving higher numbers of genders, but the likelihood and additional advantage of that becomes steadily lower as the number of genders increases.

This could arise even though it is not necessarily the most efficient way of introducing genetic diversity through the expedient of adequacy. Evolution is not actually "survival of the fittest", but "survival of the adequate". If such a system evolved, it may require too great a step backwards to asexuality in order to change the number of genders. Since a 3-gendered system would be better than asexuality, if it evolved first, there is every chance that it would not change to a 2-gendered system unless 2-gendered competition arose and outperformed the 3-gendered species.

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  • $\begingroup$ +1 for correctness, and wouls have voted another +1 if possible for the references to ploidy, the main point here. $\endgroup$ – Envite Oct 3 '14 at 11:18
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It is possible, but it's also highly unlikely.

Sexual reproduction is an excellent mechanism for spreading successful mutations through the population and allowing it to adapt to changing conditions. The rapid shifts it allows helps massively in resisting parasites and diseases.

However all the benefits of sexual reproduction are immediately found as soon as you have two parents. Including more parents into the mix does not add any further advantages, however it does complicate the mating process. More effort is needed to bring three individuals in the right combination than is needed to bring two together.

It's noticeable that throughout nature you have everything from plants, to insects, to animals and two genders is what everything settles down to. The only exceptions are species which have non-breeding members (such as bees) but there the non-breeders contribute to the success of the overall colony and aren't involved in breeding at all.

You could very feasibly have a species with two genders, or with fluid gender, but there would need to be something very strange in the environment that gives an advantage before more got involved.

One possibility might be something like the Flouwen

There they don't have separate genders at all, instead they gather in groups and each split off a section of themselves. The split sections merge and then a new individual emerges already knowing some of what the parent Flouwen put into it (they can speak, swim, hunt, etc immediately) but needing to learn other things.

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    $\begingroup$ Neuter, such as with bees, is a viable third gender. Especially if they have some advantage that's useful for the species over the ones involved in reproduction. $\endgroup$ – Bobson Sep 29 '14 at 20:20
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Two sexes is the most likely pattern because math favors it. If two organisms are searching for each other, the most efficient pattern is to have one remain stationary and the other motile. If the mutual goal is reproduction, then it's obvious that the stationary partner can devote the conserved resources towards the creation of the future offspring while the motile one is better off using resources to produces a lot of seekers. The stationary partner becomes the egg and the female while the motile partner become sperm and male.

But there are a few plants on earth that have three sexes. It has to do with the details of their chromosome formation and they're all insect pollinated. I don't remember the details but they did evolve out original two or dual sex plants.

Still just two sexes is the simplest and therefore likely the first to evolve. You'd have to come up with a plausible selection pressure that shape a species into going through all the extra work of evolving three or more sexes.

The more powerful effect would not be the number of biological sexes but rather morphism, i.e. how different the sexes are. Humans are a rather strongly sexually dimorphic species. Human males and females are readily distinguishable by size and body shape. Dogs and cats by contrast are not. You can't be sure of the sex of a dog or cat without checking their genitals. Behavior and thus social interactions would follow morphism. Humans have always had a strong division of labor between males and females but in monomorphic species like wolves, there is little to none. All wolves hunt and male and females lead but more pack leaders are females.

It's likely that increasing human skull size made pregnancy increasingly difficult in turn causing women to devote more of their body to the process while having to sacrifice other functions. Certainly in the apes and in the human fossil record, division of labor increase with skull size.

So, even if you had a species with a dozen sexes, if they had little division of labor between sexes and therefore were monomorphic, then all the sexes would be interchangeable for anything not related to their reproductive function (as in dogs and and cats.) On the other hand, the greater the division of labor, then the dimorphism could be very extreme with lots of different social roles.

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I'm surprised no-one has already mentioned Iain M Banks's Player of Games which includes an alien species with three sexes:

The Azadians have three sexes: Males with testes and penis, an intermediate ("Apex") sex with a reversible vagina and ovum, and a female sex with uterus and a retrovirus that slightly modifies the implanted egg.

Quoted from http://tvtropes.org/pmwiki/pmwiki.php/Literature/ThePlayerOfGames

The mechanism here is biologically plausible though as others have pointed out whether such a species would actually evolve naturally is perhaps unlikely and it is possible the inherent complexity would cause such a species to die out before it reached an advanced stage.

In the book this is explored in somewhat more detail. He describes an alien society in which the middle sex (the apexes) are dominant with the females treated as possessions/slaves and males having limited status. See this Google Books search result for the passage which describes it from the perspective of an outside observer.

Of course he describes various exceptions to this during the narrative but that sums up the general status quo encountered by the main character.

The other thing to consider is that while such a species might not evolve naturally but a sufficiently advanced species might choose to introduce new sexes or have the ability to change between them. This might be done via genetic manipulation or via nano-technology.

Again Iain M. Bank's The Culture who are the main feature of many of his science fiction novels (including the aforementioned Player of Games) have only two sexes but the ability to change between them at will:

Citizens of the Culture refer to a normal human as "human-basic" and the vast majority opt for significant enhancements: severed limbs grow back, sexual physiology can be voluntarily changed from male to female and back (though the process itself takes time)

From http://en.wikipedia.org/wiki/The_Culture#Physiology

So in a sufficiently far future/advanced alien species setting you could explain the existent of additional sexes via technological means if you wished to do so.

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  • $\begingroup$ Nobody's mentioned it because fictional works does not demonstrate feasibility - it only indicates the creativity of the writer. $\endgroup$ – slebetman May 9 '16 at 7:11
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While I'm too late to be considered for the prize, I thought I'd provide an answer anyways.

In addition to the tendency to latch onto the human experience as the norm for discussion of multi-sex species, or even as the norm for discussion of sex in general (more on that anon), there seems to be an underlying assumption that, if a species has 3 sexes, all three have to contribute to produce a child, and the situation just gets worse as the number of sexes increases. Sorry, but no.

First, I recommend Olivia Judson's "Dr. Tatiana's Sex Advice to All Creation". It's an excellent overview of the evolutionary biology of sex.

Now. About sex. Sex is the mixing of genetic material for the purpose of reproduction. Recreational activities are not germane. The division of a species into two sexes has obvious advantages, as Tim B pointed out, but it has a big disadvantage: generally speaking, half the population is off-limits. There is no obvious reason why differentiation cannot occur to provide sexes A,B and C, where A can mate with either B or C, and B with C as well. In this case, each sex has an increased number of potential partners. It's sort of like Woody Allen's old joke that being bisexual doubles your chances of a date on Friday night. Judson suggests that the question should be "Why are there only 2 sexes?" with the underlying understanding that it seems improbable. Certainly the evolution of multiple sexes in a species with very low population density (such as specialized deep-sea scavengers) seems a possibility, since it would make finding an appropriate partner easier.

There exists a fungus, for instance, with 28,000 sexes (http://botit.botany.wisc.edu/toms_fungi/feb2000.html), and a slime mold with 700 sexes is known, along with a protozoan with 7 sexes (http://en.wikipedia.org/wiki/Tetrahymena#T._thermophila:_a_model_organism_in_experimental_biology).

But overall, yes, 2 seems to be the norm. Why that is, is not obvious, despite claims to the contrary. Apparently, as with so much of evolution, it's just what happens to work best.

Now. As to humans. Talking about sex in purely human terms is hopelessly parochial, not to mention speciesist. Even at a fairly basic functional level, we are hardly suitable as the gold standard. Among other things, we are specialized as k-selection strategy types (small number of offspring) with no mating season and high sociability. We are apparently mostly fairly monogamous, as evidenced by relative testicle size, and recently accumulating genetic evidence suggests that, among mammals and birds, monogamy is one of the rarest practices to be found. As a matter of fact, there are a number of species in which having multiple partners increases fertility in the female.

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This reminds me of this comic.

More seriously, the human race being only "two genders" is pretty big misconception. Here's the "genderbread" person shown there for reference:

Genderbread person

You have sex, gender identity, gender roles, sexual orientation, romantic orientation, sexual behaviour, etc - all of which are traits that people try to squeeze into that concept called gender, but it's really just a fuzzly box that sorta describes most people. But there's always exceptions - intersex, trans*, genderqueer, and neuter people being a few. And I think there's tons of story potential in tweaking any one of these in a fictional world.

More specifically on the topic of worldbuilding, here's a list of some stories made tweaking gender norms in worlds (having only males or females, everyone being both male and female, extreme segregation, etc.) that might help for inspiration.

Just remember, as a fantastic writer told me: "With any world it's usually best explained through the readers discovering the world through a character, because that's how we as humans think." It's admittedly kinda weird to do in some of these contexts though, but still interesting I think (though I'm trans* so I'm probably biased in seeing a lot of the pain that our current gender norms in society puts me and other trans* people through).

As some more ideas:

Imagine a society where equivalent "western" culture was more like some Native American tribes in being supportive of gender-variant individuals, and even seeing them as fulfilling an essential part of their society in some way (say as healears or special role players in the sun dance). That kind of thing could probably be carried over to a modern society in some way I suspect. Maybe some kind of government position, or probably more reasonably just no cultural barriers against LGBT issues and coming out or anything was never relevant because it was never seen as abnormal, or something like that?

Or imagine a society where most people are intersex, maybe even to the point where if someone was "too male" or "too female" (though those terms probably wouldn't be used there) they'd be considered as having a "disorder of sexual development" and require surgery to be more "normal" by becoming what we see as intersex, similar to often how intersex people are treated today.

Of course there's also the option of having a society where gender-lines are very very rigidly defined but I feel that would probably be less interesting because our society's been like that in the past in some ways, unless it was either a backdrop to something more significant that was unrelated, or so severe that it was shocking to us.

Personally I've always thought it would be fun to write some kind of story from the perspective of a member of a hive-mind society (think sorta bees but more extreme) where gender isn't really even relevant because being an individual isn't even relevant, and everyone is sorta attached to a whole. Or making a world where everyone sees gender more like that alien does: Simply something that's a part of someone, just like being a chef or scientist.

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    $\begingroup$ Very interesting and insightful, but not an answer. I'll not downvote. $\endgroup$ – Envite Oct 3 '14 at 11:25
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    $\begingroup$ I agree with @Envite that this is a good answer, but it's not a good answer for this question. There may be multiple genders among humans, but there's still biologically only two sexes - XX and XY (discounting genetic disorders which have multiple chromosomes). I know the question originally did use the word "gender", but from the rest of the question, it was pretty clear that the OP was discussing reproduction. Effectively, this is a really long comment. $\endgroup$ – Bobson Oct 3 '14 at 17:56
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    $\begingroup$ Saying "like Native Americans" reduces over 240 distinct and radically different cultures to a hollywood archetype. Some cultures had a concept of gender switching but most didn't. In most of the warrior cultures, cultural gender and sex grew sharply delineated to a higher degree than in Old world cultures. The Blackfoot Sioux would castrated and torture to death effeminate men. $\endgroup$ – TechZen Oct 10 '14 at 8:08
  • $\begingroup$ I'm very sorry TechZen! This Wiki Page seemed to give the impression that it was a very common practice, however I didn't realize that by saying that I was putting all native american cultures into a box. I edited the answer since the point of this post was to point out that boxes are harmful and that people are unique. $\endgroup$ – Phylliida Nov 5 '14 at 21:34
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Depends how you define things. For more than 2 parents contributing 3 different categories of genetic material to offspring? Even here, it's no.

But three sexual partners are needed.

In a recently discovered hybrid system within the harvester ant genus Pogonomyrmex, queens must mate with two types of males to produce both reproductive individuals and workers. These ants are the first species known which truly has more than two sexes—with colonies effectively having three parents— argues Joel Parker of the University of Lausanne, Switzerland.

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  • $\begingroup$ Welcome to the site han, excellent real world example. $\endgroup$ – James Feb 26 '16 at 3:56
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In Octavia Butler's brilliant Xenogenesis Trilogy, the alien species Oankali have three sexes, all necessary for reproduction: for the male and female sexes the intercourse is impossible without "the third" sex, the ooloi. Their culture prides itself on being "non-hierarchical", in contrast to humanity which they consider destructively backward due to its hierarchical model. Here is a brief review, giving a good idea of how Butler presents this culture. Spoiler: it's far from idyllic.

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Actually, human ADN comes from 3 sources:
- Haploid male cell (sperm)
- Haploid female cell (egg)
- Mitochondrion ADN

The mitochondria is usually seen as mother's material, but it's not. We twist the complex reality to fit the traditional thinking "1 mother + 1 father"

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    $\begingroup$ Excellent observation! The mitochondria is essentially our hidden third sex. $\endgroup$ – slebetman May 9 '16 at 7:12
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To extend from the multiple types of answers already given, you might consider dipping into Judith Butler's profoundly influential book Gender Trouble. In this book, she makes a remarkable argument about the now-common distinction between "gender" and "sex."

In short: the usual view (post-second-wave feminism) is that "gender" is a cultural or individual or whatever choice or interpretation or representation that is or is not directly correlated with an underlying biological substrate; and "sex" is the biological substrate, divided into two. Butler argues that, in a large number of ways, the claims and assumptions about the biological absolute duality of sex are problematic. As several answers and comments have already noted, there is extant evidence of sex not being necessarily dual. For example, if we grant that "male" includes several distinct expressions of chromosomal material, yet we accept that these are all "the same thing" because they all come out functionally "male" with penises and whatnot, then we're imposing a preconception of binarism on what we know about the biological substrate. Conversely, when we encounter examples in which material is expressed in a very unusual way, we label these "abnormal" and therefore insignificant -- thereby reaffirming our preconception that there are only two sexes. If that's the case, then the claimed absolute link between "sex" and objectively-existing biological substrate is ideological, in which case the difference between "sex" and "gender" is that "gender" is required culturally to bind itself to the absolute dominance of "sex" because of the power of the cultural presupposition of binary sex. Which is to say, "sex" and "gender" differ principally on the basis of who's got the power.

I am not saying that Butler is necessarily entirely right or wrong about any of this, but reading her work may jolt you into dramatically different ways of thinking about your question and the results you're trying to get in your worldbuilding project.

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    $\begingroup$ Butler's work, like a lot of the genres writing is premised on the idea that the human cultures are arbitrary and arise purely out of human interactions without regard to any interaction with either our own biology or the general environment and local conditions. Human cultures aren't about decoration or arbitrary choices, they're about survival. Different cultures arrive at different details because of their differing environments. It's easy to see why articulate intellectuals like Butler are drawn to the idea that culture is just an arbitrary story that can be rewritten at our whim. $\endgroup$ – TechZen Oct 10 '14 at 7:55
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    $\begingroup$ .... but that clearly not the case. $\endgroup$ – TechZen Oct 10 '14 at 8:03
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As a further wrench into the works, consider that in many species, the breeding unit is not the pair, but the pack/herd/swarm.

Wolves: When the pack is at carrying capacity (in balance with their food supply) then often only the alpha female and alpha male mate. Depending on the usual prey, the pack may include siblings, cousins, nephews, uncles of the breeding pair. The entire pack raises the pups.

If food is plentiful, more breeding takes place. If food is scarce, the female may not come into heat, or will produce smaller litters.

Bees. 1 queen, usually 1 drone, everone else helps. (Note: There are ways to trick domestic bees into having multple queens in the hive. This is economically useful for ramping up colony populations fast to meet peak blooming time, without having to make more hives.

Herds. Usually 1 dominant male, sometimes a few. Sometimes 2 tiers of breeders -- Dominant male covers most of the females, secondary males cover 1 or a few each.

In terms of a sentient species, you may get the biological equivalent of Greek Phaetry http://www.oxfordreference.com/view/10.1093/oi/authority.20110803100324887 http://www.oxfordreference.com/view/10.1093/oi/authority.20110803100324887

or of the warrior societies of North American aboriginals.

From the point of a selfish gene, if you don't breed, you better help the ones who share your genes to do so.

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I love the concept of the triploid sexual system. I first saw a third sex suggested in Alien Nation, though I wonder whether it was fully triploid, or if the third sex was merely a catalyst. A catalytic third sex might arise in the case of a species that is too reproductive, causing famine or war.

Triploid reproduction could evolve as a mutation, with the third sex being originally unnecessary. The third sex injects its DNA into a fertilized diploid egg. The resulting embryo has advantages and a chance to be of the third sex. In time, further mutations cause the egg to be infertile without both kinds of sperm. The lower reproductive rate might not be disastrous and might even allow for a more stable population as medicine extends the lifespan.

Triploid reproduction could even begin through genetic manipulation, as a method of birth control. The third gender might initially be rare and under government control. Broken genes inserted that can only survive with all three parts. Perhaps the third gender only graces the most productive, healthy and intelligent of the generation, making sure that they produce more heavily.

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I'm surprised no one has mentioned Larry Niven's Pierson's Puppeteers, which have three Genders involved in reproduction. I say gender as only two of the beings involved contribute genetic material, so strictly speaking this is straightforward two sexes reproduction. However, it does add an interesting twist to reproduction, where a third party must be involved.

The third gender is a non-sentient being who is parasitised by the puppeteers, with both sperm and an ovum deposited in its body. In puppeteer society, when a couple get permission to breed, they go and pick out a host together.

This method of reproduction was one of the Puppeteers closely guarded secrets, as they rightly guessed it would alienate other sentient races.

A narrative description of the process is described in the novel series Fleet of Worlds, Juggler of Worlds, Destroyer of Worlds and Betrayer of Worlds, although I'm afraid I don't recall which book the details feature in.

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The OP makes it clear that he's referring to sex, and not gender, so let's leave all discussion of gender to one side.
Sexual reproduction supercharges evolution. it allows the assortment of genetic materials from two individuals to produce the genome of a third, new, individual, thus ensuring that there are new genes (produced by mutation) and new assortments of genes, produced by meiosis, to face the environment in new ways.
But why TWO sexes? The answer is not culturally determined. The answer does not depend on sociology or (other forms of) fiction. It depends on the chemistry of DNA, which forms the double helix we all know and love, thereby making itself stronger and more resistant to random change during life. Let's represent a diploid strand as AA' in one individual, and BB' in a second. These separate into A - A' and B - B' at meiosis. When they combine, they can recombine as AB, A'B, AB', A'B': so where before there were 2 genotypes, there are now 4, and it may be that the new combinations are more advantageous for survival than the old. This is how evolution works.
As long as DNA chemistry works this way, this is the way things have to be. Two strands, 2 individuals to contribute their shares.
Nature is prolific and ingenious, so at higher levels of organization (and by higher I mean only higher in an organizational sense), all sorts of arrangements to bring this mechanism about are possible.
Some of these arrrangements are more likely in physical, chemical, biological or environmental terms than others, and it seems that the two-sex arrangement has particularly high survival value, for it is omnipresent. Let's refer to these as supra-sexual arrangements.
Among these survival arrangements is the universal presence of mitochondria in animal cells. Nitochondria are not a third sex. They are originally parasitic microorganisms that became symbiotic in animal cells, giving energy in exchange for protection and transport. This arrangment was obviously highly beneficial. Mitochondria carry their own genes.
Eukaryotes produce motile sperm cells and sessile ova. Sperm cells are basically stripped-down packages containg a haploid genome, and little else. Ova contain the genome, in a fully-functioning cell, which contains mitochondria. When this cell is fertilized,, each time thereafter that cell division occurs, each new cell contains about half of the mitochondria of the parent cell, and these mitochondria then reproduce asexually within the cell.
So in the adult organism, all the mitochondria are descendants of the originals in the ovum that gave rise to all those cells, which is why we can trace mitochondrial lineages through the mother, but not through the father.
In the case of the queen ant cited above, she no doubt requires fertilization by two individuals to produce two types of offspring because these two individuals have slightly different genomes. This is a neat demonstration of a supra-sexual arrangement.
Another such supra-sexual arrangement might be the rich bachelor uncle who sets up his nephew in a house of his own so he can get married. This has high survival value, which is why it is common in all human groups. In English we call this nepotism.

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Two sexes are familiar: individuals who each undergo meiotic cell division and combine their partial genomes. An individual might be male or female or switch back and forth but the essence is meiosis and combination of 2 individuals.

One individual might undergo meiotic cell division and combine two such of its own cells. This is still sex in that the meiosis is the key piece of sex - it still shuffles the genomic deck and offers the possibility for improvement over the parent. Many plants are hermaphrodites capable of self fertilization.

The last is asexual reproduction: no meiosis, no shuffle, just regular mitosis keeping genome intact and a clone of the parent. Good if your parent is good. Things use asexual reproduction to maximize the population of an organism that is doing well in a given environment - like that aquarium strain of the seaweed Caulerpa that has given up on sex and is one big clone.

Sex is helpful if circumstances are not ideal. Some organisms do mitosis and asexual reproduction when the living is easy and then fall back on sex in hard times.

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