Trigender Biology Inquiry

[Škjakto]

The other day, I was thinking about a hypothetical mammalian species that somehow ended up with 3 genders. Gender A bears the sperm and fertilizes the egg. Gender B bears the egg. Once it is fertilized by A, it is moved to gender C which nurtures the egg and gives birth. My question is, hypothetically speaking, what would their reproductive systems look like? I don't think Gender A deviates at all from typical male genitals.

[mèþru]

More likely is that either more than one gender gives birth or impregnates, as Gender C needs to pass on its genes so that there would be future generations of C. As some species are known for having dominant and submissive subgroups among males, it might no tbe hard to make the submisive group Gender C.
Transferred pregnancy seems like an unnecessary and risky feature. Perhaps Gender C takes care of the young in their early stages. The closest equivalent of carrying the pregnancy would be to carry the young in a pouch and feed them regurgitated food.

[Vijay]

Isn't it already possible to see humans as having three genders? There is such a thing as intersex people, after all (for example).

[mèþru]

Intersex people are rare and not really a biological sex as much as a group of various gender disorders.

[Chagen]

I hate to be that guy but you're actually talking about trisexual biology. Gender is a facet of a sentient being's personal viewpoint on their identity, and is a cultural construct.

As for your mammals, OP, I don't see why C would really be needed. But if such a system did exist, I'd assume that both A would have masculine genitals (i.e a phallic injecter of seminal fluid). Then B would have both feminine and masculine genitals: a vagina-like orifice for accepting sperm from A, and a phallic one for injecting the now-fertilized egg into C (which would be solely feminine).

[hwhatting]

I agree with methru and Chagen, a transferred pregnancy looks like an unnecessary risk to me. A scenario with a 3rd sex nurturing the young after birth looks similar to what happens with state-building insects like ants or bees. IIRC, the workers (= nurturers) of these insect species are sexually stunted females.

[Xephyr]

Iain M. Banks' novel The Player of Games has a humanoid alien race with three sexes. If I recall correctly, their equivalent of your B-gender had some kind of invertible genitalia that could function as both vagina and ovipositor.

Apropos of nothing, however, did you know that female spotted hyenas have super-sized clitorises called "pseudo-penises" which they use for mating, giving birth, and urinating, and which are nearly as large as regular penises on males, making the two sexes almost impossible to distinguish visually?

[mèþru]

Actually, you can distinguish them by size. A hyena clitoris is much bigger than a hyena penis.

[Xephyr]

"Much bigger"?

Female spotted hyenas have a clitoris 90 percent as long and the same diameter as a male penis (171 millimeters long and 22 millimeters in diameter)... author John C. Wingfield stated that "the resemblance to male genitalia is so close that sex can be determined with confidence only by palpation of the scrotum"

[mèþru]

Really? Guess I'm wrong.

[Salmoneus]

Iain M. Banks' novel The Player of Games has a humanoid alien race with three sexes. If I recall correctly, their equivalent of your B-gender had some kind of invertible genitalia that could function as both vagina and ovipositor.

Apropos of nothing, however, did you know that female spotted hyenas have super-sized clitorises called "pseudo-penises" which they use for mating, giving birth, and urinating, and which are nearly as large as regular penises on males, making the two sexes almost impossible to distinguish visually?

It's not only hyaenas, though they're the most famous example. Spider monkeys (iirc?) have similarly large penises. Iirc, however, the monkey clitorises are just that - whereas the hyaena organs are really pseudo-penises that just happen to have evolved from clitorises. Crucially, the vaginal canal travels through the "clitoris".

[This has consequences. Iirc something like a third of first-time hyaena mothers die in childbirth. It also means that sex is extremely awkward for both partners - very physically difficult for the male, and apparently very unpleasant for the female. It also means that rape is essentially impossible]


I once worked on an alien species that instead took inspiration from a third animal: the fossa. Fossas are interesting because the female has a pseudopenis, but only in adolescence. Iirc it's thought that the young females have evolved to purposefully mimic the appearance of young males, in order to improve cross-sex bonding and avoid rape and unwanted courtships.


------

Regarding the OP: I agree with the other commenters that this is a pretty unlikely scenario: what is gained by transfering the egg?

That said, I once worked on an alien species that did exactly this. In their case, the key point was that the C sex was immotile - essentially a living incubator that in some respects took the role of a hive queen. Males and females thus avoided pregnancy, retaining their mobility, with the immobile egg-bearer allowing young to grow much larger before birth, which has a lot of advantages (they're smarter, need less protection, and don't need special diets). It also has a sociological function: as the males and females don't know which young have hatched from their egg, they know that the young of the colony include their own progeny, but have to treat all the young as potentially theirs, not knowing (though I guess in some cases they can guess) which are which. The cost of all this, however, is an incubator (or a couple of them) that has to be protected by the whole of the colony, and that must be fed (voluminously).

[Vijay]

Intersex people are rare and not really a biological sex as much as a group of various gender disorders.

Why does it matter that they're rare, why are they "not really a biological sex," and what is a "gender disorder"?

[mèþru]

I am not sure how to say it in terms that are not offensive, but I don't mean to say that anything is wrong or unnatural about intersex people:
Intersex people tend to have some genetic disorder and/or hormone imbalance. Other then that, they have X chromosomes and sometimes Y chromosomes, just like females and males. Their rareness is completely irrelevant. I have no idea about why I said it, so I feel stupid.

[Pedant]

There is of course another possible option...at least genetically. Correct me if I'm wrong, here, but we rely for determination of male or female on the X and Y chromosomes, yes? Is it possible, therefore, to have a second set of chromosomes, the combinations of which might provide an extra variation? So, for example, if one were to have chromosome pair HT (hypothetical) as well, providing the possibility of either parthenogenesis or perhaps equal ability to pass or receive gametes, and have T as only being active when combined with an XX pair in the other set, this would give us:
HHXX--full female
HHXY--full male
HTXX--meel™ "third form" (originally mieler /mi:lǝɹ/, from 14C French mielieur /miɛliœr/ 'better [form]', derived from Latin melior)
HTXY--"hidden third form" (indistinguishable from male)
That said, we're all looking at a standardized third sex at the same stage of the life cycle. Is it possible, for example, to have an animal that is sexually active and capable of reproduction at one stage of its life cycle in a hermaphroditic manner, which then undergoes metamorphosis and becomes either male or female? The cultural implications (assuming that a species with this system achieved sapience) might be interesting...

*And yes, I'm copyrighting it; don't know if I can, but I'm trying.

[Fixsme]

Switching sex is actually pretty common for fishes. For instance, many grouper species, individuals are first female then male.

[Vijay]

I have no idea about why I said it, so I feel stupid.

I just wanted to say I'm sorry for making you feel that way. I'm not sure you did anything wrong by saying that; I guess I was just trying to understand your point.

[Astraios]

The rarity of non-XX/XY sexes is precisely the reason that intersex people are irrelevant in the discussion of mammalian species with more than two reproductive roles. If X0 individuals (for example) were anywhere near as common as XX or XY individuals, or played anywhere near as important a role in human reproduction as XX and XY do, then you could usefully say that the human species has the three sexes XX, XY, and X0. Since they are neither common nor essential for reproduction, intersex people are not a third sex, but a biological accident. Just because some deviations from the XX/XY system can occasionally (one in thousands of live births) produce viable (but generally infertile) individuals does not mean mentioning those individuals brings anything useful to a discussion of a mammalian species with three equally essential reproductive functions (insemination, oviposition, incubation).

Aberrations aside, many species have only two chromosomal sexes, but multiple ‘genders’, which is another thing the OP could consider, such as the ruff, which has three types of male: territorial, satellite, and ‘faeder’. The territorial and satellite males exhibit dimorphism (from females as well as from each other), but the faeder males look similar to females. All three male genders are inherited and permanent; there is no gender-bending or hermaphroditism, nor is there any significant variation in the behavior of different males besides their courting strategy (outside the lekking season, the female look-alikes associate with other males, not with females). And re what Pedant asked, male ruffs’ gender is an autosomal polymorphism; that is, it has nothing to do with the sex chromosomes (all three male types are ZZ), and is more similar to other types of intraspecies variation like melanism or blood types. So you can have multiple genetically determined gender roles with only two allosomally determined sexes, which obviates the need to mess around with multiple sets of chromosomes, which sounds messy.

If you really want actual multiple sex-determining chromosomes, you’d be better off looking at fish than at mammals, though be aware that multiple sex chromosomes doesn’t necessarily mean you end up with multiple reproductive roles, and there will be other behavioral or morphological consequences as well. Platyfish, for example, have three sex chromosomes, but only two sex roles (females can be XX, XW, or YW, while males can be XY or YY, but all five types perform only one of the same two reproductive roles). It seems that the X chromosome also determines bright coloration, while the W chromosome does not. This means that individuals with an X are brightly colored, which is disadvantageous in females, making them vulnerable to predation, but advantageous in males, as it wins them more mates. Females with an X are therefore less likely to reproduce, while males with an X are more likely to do so. This selective pressure for X in males ensures that the X chromosome doesn’t die out in females, and the selective pressure against X in females ensures that the Y chromosome doesn’t die out in males, so you’re left with about equal numbers both of the ‘ideal’ XY (bright, male) and YW (dull, female), and of the ‘incidental’ XX (bright, female), XW (bright, female), and YY (dull, male).

All of that said, the OP is essentially talking about a parasitic behavior, in which neither biological parent cares for their offspring, foisting the hard work onto some other individual. The only way this can be viable such that the non-parent gender would actually be a willing partner is if it has an incentive; either what Sal suggests (the parents care for the incubator), or the ants that hwhatting mentioned (the parents are so rare that the carers have no choice but to raise someone else’s young to ensure the colony’s viability), or some other strategy. Otherwise, it’s a lot of trouble on the incubator’s part (and a lot of risk on the parents’ in transferring the embryo to a third party) for no reward at all.


EDIT: I should also say, it would also be useful to have the specific details of this reproductive strategy in order to answer the OP’s question about the appearance of the species’ genitalia.

The inseminator would have a penis, yes, while the ovipositor and the incubator don’t necessarily need anything different to a boring old vagina. If you’re open to the idea, the oviposition could simply take place in a manner similar to marsupial birth, where the fetus crawls out of the mother on its own into the pouch (or in this case, into the incubator’s womb/pouch), where it continues developing. This strategy is probably the least alien, most “mammalian” (in the sense of Terran mammals); you essentially get one ‘male’ and two ‘females’, both of whom give live birth. It seems likely the ovipositor would cooperate with the incubator in this strategy, nurturing and protecting it, while the inseminator would behave like a typical mammalian male, just out to fertilize as many eggs as he can.

On the other hand, if you’re really set on having the zygote oviposited into an incubator, in a similar fashion to insemination, then the mother might need a specialized eversible organ like Xephyr mentioned, so she can first receive the sperm and then impregnate the incubator with the zygote. This sounds like it could take place in a similar manner to traumatic insemination, where for example male bedbugs pierce the female’s body cavity and forcibly inject sperm, or like parasitic wasps depositing their offspring inside other animals.

Alternatively, fertilization could be external, so the mother deposits an egg, the father fertilizes it, and the incubator picks it up. This strategy reminds me of some frogs, who leave all their offspring with a single carer until they develop from tadpoles to froglets. Or the unfertilized egg and sperm could both be delivered directly to the incubator by a similar organ in both parents, an ovipositor/penis, in which case you essentially have two ‘male’ sexes, both of whose genetic material is necessary to create viable offspring, and one infertile ‘female’, in whom the egg and sperm meet and become viable. This strategy also sounds like parasitism, though I can’t think of anything similar in the real world.


EDIT2:

That said, we're all looking at a standardized third sex at the same stage of the life cycle. Is it possible, for example, to have an animal that is sexually active and capable of reproduction at one stage of its life cycle in a hermaphroditic manner, which then undergoes metamorphosis and becomes either male or female? The cultural implications (assuming that a species with this system achieved sapience) might be interesting...

Autoparasitism! Fun. Such a drastic metamorphosis doesn’t sound likely in a mammalian species, but it could perhaps be that when a female reaches a certain age, she undergoes menopause, and she becomes a target for younger females looking for a surrogate womb for their fetus to crawl into. Or, to put it less violently, young females are incapable of hosting a fetus, and older females undergo a sort of second puberty, in which their womb develops fully and they seek out younger females to surrogate for. True hermaphroditism and sex changes just don’t really sound like an option for a mammalian species.

[mèþru]

This link provides nice, possibly inaccurate at times explanations of gender roles in various animals.

[2+3 clusivity]

Well, one easy way to do this would be to have a caste based biology as in many colonial insects.

[mèþru]

I thought that the OP meant chromosomal sexes rather than behavioral groups.

[2+3 clusivity]

The original post is ambiguous: the OP says gender rather than sex but notes a structure suggesting a difference in sex.

The other day, I was thinking about a hypothetical mammalian species that somehow ended up with 3 genders. Gender A bears the sperm and fertilizes the egg. Gender B bears the egg. Once it is fertilized by A, it is moved to gender C which nurtures the egg and gives birth. My question is, hypothetically speaking, what would their reproductive systems look like? I don't think Gender A deviates at all from typical male genitals.

Honestly, given this read out, a caste system still works.

[The male] bears the sperm and fertilizes the egg. [The queen/female] bears the egg. Once it is fertilized by A, it is moved to [the worker (perhaps an otherwise non-reproductive male or female)] which nurtures the egg and gives birth.

Not sure about the last part about birth ... maybe the workers have kangaroo pouches or some such plumbing.

[Xephyr]

The original post is ambiguous: the OP says gender rather than sex but notes a structure suggesting a difference in sex.

Good heavens, people, words can have more than one meaning. "Gender", in addition to more restricted definitions in linguistics and in sociology, also has a common meaning entirely synonymous with "sex". You know this. I know you know this. Everyone knows that everyone knows this. What is the point in pretending to not understand Škjakto's post?

[Salmoneus]

The original post is ambiguous: the OP says gender rather than sex but notes a structure suggesting a difference in sex.

Good heavens, people, words can have more than one meaning. "Gender", in addition to more restricted definitions in linguistics and in sociology, also has a common meaning entirely synonymous with "sex". You know this. I know you know this. Everyone knows that everyone knows this. What is the point in pretending to not understand Škjakto's post?

It can have that meaning, but I'm not sure it's clear that it does here. Or rather: I'm not sure that the 'common meaning' distinguishes between biological caste differences (in size, appearance, behaviour) and chromosomal (or other genetic) sex differences. I'm not even sure that 'sex' necessarily makes that distinction for most people, let alone 'gender'. I think caste-based 'pseudosexes' would fit the OP just as well as chromosomal sexes.


Astraios: thanks for that post!

I don't remember the details off-hand, but I think I was probably envisaging my conspecies transfering the egg to the incubator by means of an ovipositor. This could be an 'in-built' organ, or a repurposed one - I'm thinking of how some cephalopods have repurposed limbs to transfer sperm. So, a tail used as an ovipositor?

[Astraios]

I don't remember the details off-hand, but I think I was probably envisaging my conspecies transfering the egg to the incubator by means of an ovipositor. This could be an 'in-built' organ, or a repurposed one - I'm thinking of how some cephalopods have repurposed limbs to transfer sperm. So, a tail used as an ovipositor?

Or a clitoris.


EDIT: Now that I think about it, that system (where both parents transfer their gametes into an external incubator, wherein they become a viable zygote and are carried to term) reminds me of viral recombination. This can happen when a host cell is coinfected with two different strains of a virus simultaneously; the strains interact within the host and can generate novel progeny with genes from both parent strains. Obviously this isn’t very close to intraspecific reproduction in living organisms, because the viruses and their hosts are so different (and the host generally dies), but something vaguely similar does at least exist.

One could perhaps posit a non-viral version of coinfection in proto-multicellular lifeforms as a starting point for the evolution of the separate incubator system, if one wanted a whole phylum of conanimals reproducing via non-parent incubators. I don’t know how competitive this system would be against other forms of reproduction, but handwaving that wouldn’t exactly be a sin. Or, to avoid any flies in one’s primordial soup, the incubating conspecies could just be the odd ones out, like how primates (and a couple of others) are with our menstrual cycle.

In any case, the look and origin of the ovipositor would obviously depend on how this coinfection strategy evolved, and how the incubator’s receptacle works, if it even has one (as opposed to hypodermic impregnation). Probably also an important factor is whether a gamete or a zygote is transferred to the incubator. If both reproducing individuals are transferring gametes, then it would make sense for both ‘sperm’ and ‘egg’ cells to be cheap to produce and quite motile, or at least more motile than Terran mammals’ eggs, so that both can be transferred to the incubator with relative ease, and find each other quickly inside it. This could leave you with really similar organs for insemination and oviposition, if not the complete obsolescence of ‘female’ as a meaningfully distinct category from ‘male’. If they’re transferring already-viable zygotes, though, which are presumably rarer and more valuable than unfertilized gametes, then the female might have a particular specialized organ, or it could simply be an orifice whereoutfrom the fetus/egg is laid onto a repurposed tail (for example), and ‘manually’ transferred into the incubator.

[Salmoneus]

I don't remember the details off-hand, but I think I was probably envisaging my conspecies transfering the egg to the incubator by means of an ovipositor. This could be an 'in-built' organ, or a repurposed one - I'm thinking of how some cephalopods have repurposed limbs to transfer sperm. So, a tail used as an ovipositor?

Or a clitoris.


EDIT: Now that I think about it, that system (where both parents transfer their gametes into an external incubator, wherein they become a viable zygote and are carried to term) reminds me of viral recombination. This can happen when a host cell is coinfected with two different strains of a virus simultaneously; the strains interact within the host and can generate novel progeny with genes from both parent strains. Obviously this isn’t very close to intraspecific reproduction in living organisms, because the viruses and their hosts are so different (and the host generally dies), but something vaguely similar does at least exist.

One could perhaps posit a non-viral version of coinfection in proto-multicellular lifeforms as a starting point for the evolution of the separate incubator system, if one wanted a whole phylum of conanimals reproducing via non-parent incubators. I don’t know how competitive this system would be against other forms of reproduction, but handwaving that wouldn’t exactly be a sin. Or, to avoid any flies in one’s primordial soup, the incubating conspecies could just be the odd ones out, like how primates (and a couple of others) are with our menstrual cycle.

In any case, the look and origin of the ovipositor would obviously depend on how this coinfection strategy evolved, and how the incubator’s receptacle works, if it even has one (as opposed to hypodermic impregnation). Probably also an important factor is whether a gamete or a zygote is transferred to the incubator. If both reproducing individuals are transferring gametes, then it would make sense for both ‘sperm’ and ‘egg’ cells to be cheap to produce and quite motile, or at least more motile than Terran mammals’ eggs, so that both can be transferred to the incubator with relative ease, and find each other quickly inside it. This could leave you with really similar organs for insemination and oviposition, if not the complete obsolescence of ‘female’ as a meaningfully distinct category from ‘male’. If they’re transferring already-viable zygotes, though, which are presumably rarer and more valuable than unfertilized gametes, then the female might have a particular specialized organ, or it could simply be an orifice whereoutfrom the fetus/egg is laid onto a repurposed tail (for example), and ‘manually’ transferred into the incubator.

Yeah, I was thinking of the latter, I think. Though both the crawling-foetus and double-intromission options have things to recommend them.

I think this all sounds more realistic as an anomaly than as a whole branch of multicellular life - because it seems specifically connected to certain forms of society. It only really works with social species, and it's something I can sort of see evolving with the development of more social species (like a weaker form of a hive 'queen' system). The problem then is where the incubators come from - are they a novel anomaly (a gentic mutation that found a use)? Or are they repurposed from an already-existing caste- or sex-form? Not sure.

[Interesting idea about the clitoris, incidentally. Maybe I'd subconsciously discounted that because I'm not sure about the evolution of alien clitorises - are they really necessary? Though apparently at least some reptiles have them too, so it's not just a recent mammal thing.]



Speaking about genitalia, incidentally, and unrelated to the topic, I had a thought a while ago: what about something between internal and external fertilisation? The two parties create a sealed space between them (suckers, tentacles, etc), into which both parties eject gametes. The female then pulls the fertilised egg back into itself. Does anything like this occur in nature?
(Clever thought: this could evolve perhaps when a species with external fertilisation in water moved onto land, with the sealed chamber being filled with water to mimic the ancestral fertilisation environment?)