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Inspired by this question: Sexual reproduction without biological sex

And, to a lesser extent, this question: Would a society of simultaneous hermaphrodites have gender roles?

I'm not particularly well-versed in biology at all, let alone the wide variance of sexual characteristics that exists in the world, so I apologize if I've misunderstood/misused any terminology here. (Or if my questions are somehow just beyond stupid in a way that I failed to realize.)

For my purposes, I'm imagining a (preferably as close to human as possible) species that has exactly one biological sex. Every individual of this biological sex has all the necessary anatomical requirements to be the father or mother in a reproductive exchange. In case it's somehow relevant, their anatomy is also arranged in such a way that one couple can be simultaneously participating in two exchanges. This means that both members of one couple could be simultaneously pregnant with a child fathered by the other member of the couple.

So the main question here: If you have two children of the same two parents, but with swapped parental roles in their conception (the father of one is the mother of the other and vice versa), is there any inherently meaningful difference between the two siblings beyond two siblings who have the same parents as each other, both of whom are in the same parental roles?

I realize that a society comprised entirely of such a species could easily create social constructs such as children taking the surname of their biological mother or labeling such cross-siblings as something indicative of a relationship closer than a half-sibling but further than "actual" siblings. I'm less concerned with purely social constructs that might arise than I am with actual, meaningful differences that would affect them in the future more than "actual siblings" would be expected to experience.

If a social construct were to arise from such differences (as I expect they would), then I'm definitely interested in those sorts of downstream effects. In fact, the main reason I'm asking the question is so that I can eventually come up with exactly those kinds of downstream effects. But I'd like to set the foundation of my social constructs on something more solid than just other social constructs.

For instance, as AlexP brought up in comments, the intra-uterine environment can be very important because of genomic imprinting and/or metabolic imprinting. Aside from potential disorders that might happen at the statistical fringe, what noticeably-common differences might we see between cross-siblings? Since both parents could nurse the child, would some differences be mitigated by having a child's father nurse them instead of the mother? Would differences in mitochondrial DNA cause significant differences? Are any noticeably-visible or significant characteristics only heritable from one parent? These are all sub-questions of which I'm unsure of their relevance to the larger discussion.

I wish I could be more specific in what I'm looking for, but I'm struggling to find the words to describe it at the moment. Hopefully, I can think of a better way to word all of this fairly soon and edit my question accordingly.

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    $\begingroup$ If this strange species uses a genetic framework similar to humans, then there will indeed be a little more difference between them than between two ordinary siblings. The small extra difference comes from their development in different environments; for example, different genomic imprinting or metabolic imprinting. In humans (and other mammals, of course), the intra-uterine environment is of great importance. $\endgroup$ – AlexP Jan 25 '18 at 2:03
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    $\begingroup$ @AlexP - imprinting is the right answer. Why don't you flesh it out? $\endgroup$ – Willk Jan 25 '18 at 2:21
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    $\begingroup$ The best use of Wikipedia articles is to provide an entry point. Now that you know what to look for (epigenetics, evo-devo, genomic and metabolic imprinting) you can start doing the research. Don't forget that siblings who are not identical twins can be quite different; the little extra difference due to different intra-uterine environments will most likely be much smaller. @Will, let's wait and see if somebody whose knowledge of biology is not decades out of date will answer. $\endgroup$ – AlexP Jan 25 '18 at 3:20
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    $\begingroup$ The biology I've learned is confused by the phrase 'simultaneous hermaphrodite". Hermaphrodites are just hermaphrodites. Two hermaphrodites copulating together can impregnate each other. Please don't try to impose a version of human reproductive gender roles on hermaphrodite species. They will have their own gender identities & be comfortable with them. Parents both Mum & Dad. Easy! Happens all the time. Please remember gender identities are social & cultural constructs. Don't worry they'll cope in their own way. Please drop the "simultaneous" nonsense, it's only confusing. $\endgroup$ – a4android Jan 25 '18 at 11:24
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    $\begingroup$ @a4android Dropping the "simultaneous" would drastically alter the scope of the question. Hermaphrodites are not "just hermaphrodites". Some species behave as one sex for part of their lifecycle, and transition to the other later--in some cases possibly even switching back and forth--but never have both reproductive systems active at once. Those are sequential hermaphrodites. Simultaneous hermaphrodites have both sets active simultaneously, and can impregnate each other. They are both types of hermaphrodites, but they have drastically different reproductive strategies. $\endgroup$ – Logan R. Kearsley Jan 25 '18 at 21:29
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That depends.

Do they use isogametes, or sperm and ova?

If they use isogametes, do they lay eggs, or use internal gestation / live birth?

If they use isogametes and lay eggs, the potential differences will be utterly negligible--limited to things like the effects of diseases or nutritional deficiencies in the mother that might compromise egg production.

If they use isogametes but have internal gestation, then there will still be no genetic differences, but the developmental environment has more opportunity to produce differences. I would not, however, expect them to be large enough or common enough to be of any social consequence. (Doesn't mean a society of such creatures couldn't still decide that the facts of parentage were important, though, purely as a social construct, maybe for purposes of inheritance or whatever.)

If they use dimorphic sperm and eggs, do their cells include any organelles with independent genetic material, like mitochondria or chloroplasts?

If their DNA (or equivalent) is strictly nuclear (or equivalent), then can we also assume "normal" reproduction of individuals with diploid genetic structure via haploid gametes that each carry half of the parents genetic information, despite their physical dimorphism? If so, there will be no genetic differences, and this is the same as the isogamete case--only the effects of the developmental environment may be relevant.

If they do have organelles with independent genetic material, then there will be potentially significant differences between children with different mothers in the same pair of parents, because organelles will come only from the mother. In humans, this is why certain metabolic diseases are passed strictly through the maternal line.

Additionally, if the assumption of "normal" haploid-diploid recombination of nuclear genetic material with equal contributions from each parent is wrong, and they have some other more complex reproductive system which introduces different proportions of genetic material from each parent independent of the organelles, then you could easily get very pronounced and obvious differences between children of different mothers.

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    $\begingroup$ This is exactly the kind of explainer I was hoping to get. Thank you! I'm holding off on marking this as the accepted answer for the moment because this is all so far out of my wheelhouse that I need to look up a bunch of terms to fully understand what you're saying and I may edit the question a little bit after doing a deeper dive on all the info here. I also need to decide whether I want to minimize or maximize differences between cross-siblings, but I'm still only at the very earliest stages of thinking this through at the moment. Any thoughts you might have would be greatly appreciated. $\endgroup$ – Aporia Jan 25 '18 at 22:44
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Logan already gave a great answer to the biological differences of the reproduction itself. I want to address the evolutionary psychology and how it would in term affect cultural differences, as I believe there will be some significant ones.

Cheaters sometimes Prosper

For the record whenever I say male or female please interpret that as meaning 'the herm who played the role of male (providing sperm) or female (birthed the child) in a given mating', it makes the answer much cleaner to not have to constantly add that qualifier

One issue that has shaped human culture and laws since their onset unfortunately still plagues our poor herms, and that is the possibility of infidelity. The male can never know with 100% confidence that he was not cheated on by his partner. Thus one would generally trust the child that came out of their womb as being theirs more then they trust a child birthed by another herm who could have theoretically mated with a different male and have birthed that male's child instead of yours.

This effect would start as one of evolutionary psychology. A hem will instinctually prefer investing resource on the child that definitely shares their genetics over investing resources on a child that potentially may not share their genetics.

How significant this psychological inclination to prefer children you birthed is depends on how assured the male is that he is the father of a child. If the animals were polyandrous or polygynandrous (ie the herm playing the role of female regularly mates with multiple males) then a herm would dedicate most of their resources to the children they birthed since there is a very real possibility they didn't father the child of a herm they mated with. If a species instead split off in such a way that a mongomous couple was isolated from all other herms during mating time so that it would be difficult for a partner to find someone to cheat with then the male is far more likely to treat children equally since he is relatively confident of the paternity of the child.

Just because a species is 'monogamous' does not mean this doesn't matter, because every monogamous species every studied still had regular cases of extra-pair copulation (ie cheating), simply put cheating is too effective an evolutionary strategy for it to never occur in a species. For humans the frequency of non-paternity events (ie the guy who thinks he is the father isn't) seems to be somewhere around 2-4% in modern history. I would presume that number would be higher in prehistory, where there was less technological or cultural means of assuring paternity, but we don't really know. That matters because the frequency of non-paternity events in prehistory, when our instincts for handling the possibility of non-paternity would have evolved, would decide how much effort were willing to dedicate to raising our partner's children.

If we assume your herms had a human-like mating strategy then I'd say a 3-6% frequency of non-paternity would be common in prehistory, which means that on an instinctual level a herm is willing to dedicate 3-6% more effort into raising a child it birthed over a child it's partner birthed.

Arguably in the 'traditional' nuclear family, where two herms raise a pack of baby herms, the issue with infidelity is small enough to likely only play a small role in differences in how children are raised. However, there are situations where paternity is less certain where that lack of confidence would lead to more significant preference for children the herm birthed itself.

Studies show that non-paternity events are more common in couples that aren't married, couples with less education, couples of lower socioeconomic level, couples that regularly fight or have marital/relationship problems, and couples where travel or work keeps the two individuals apart for large lengths of time. In these couples the higher chance of a non-paternity event would translate to an instinctual preference for the child one birthed being much higher. As an extreme example if a herm has a documented history of cheating then it's quite likely that their partner is going to favor the child that they birthed because of the very real difficulty of being certain of the paternity of the other child. The point being that there will be certain situations where you may expect to see a marked preference for one child over another due to paternity concerns.

Socially I imagine this would lead to a likely cases of inheritance putting preference on children birthed by a herm over those fathered by the herm. Of course without separate sexes passing inheritance down the male line no longer makes sense, so in most cases, where a couple is married and have intermingled their assets, it wouldn't really come up since neither of the adult herms would have a higher claim to those commingled assets then the other herm does. Still in cases of royalty or other situations where one herm clearly has a greater claim to assets or some genetic lineage then the other herm likely children birthed by that herm would get priority for inheritance and similar policies.

Milking pregnancy for all it's worth

(I really need to come up with a better pun)

Assuming that your herms are roughly similar to mammals then that would mean they need to provide milk to their young. While it's quite possible that both herms would be able to produce milk and nurse a young it's possible that the one that birthed the young, and went through all the hormonal cues of pregnancy, would either be the sole provider of milk or the primary one. This actually factors in to the above point, since a herm is more willing to provide resources to a child they know shares their DNA the female would have more incentive to provide milk to the child then the male would, since she knows she is feeding her own child.

If the female either is the only provider of milk, or the primary provider, then this would also render her the primary caregiver, since until very recently it was impossible for the non-lactating male to feed a nursing child. If this happens the 1-2 years of nursing and child-rearing would likely impart an increased bond between mother and child over that of the father, and likely the mother would traditionally be the primary care giver even after the child stopped nursing, as she would have already grown to know the child's needs and personality better while nursing the child. The fact that the mother already is more invested in the care of the child she birthed, if only by a small amount, would further encourage the mother to continue as child-rearing everything else being equal.

In a modern society where there are plenty of means for a non-lactating individual to care for a nursing child these sort of restrictions need not apply, but cultural and social norms would already have been built up over generations during which only the lactating herm could care for the child. These norms would still be taught and passed down to children, even if they don't necessarily make sense in a modern society, and thus would likely still be impacting gender/parenting roles even in a more modern society.

Of course this is one area you get plenty of say in, it's easy enough to create herms where either both partner's lactate equally or children don't nurse, in which case this entire section becomes moot.

It's allot harder to be a dead beat mom

One could expect that issues with a male not sticking around to care for the child will still happen in a herm society just like it does in our current society. Thus single mothers raising children will happen far more then single fathers still, and thus the legal system will likely have protections in place for herms who birthed a child if her partner isn't assisting in providing for it. In fact if a herm is raising a child by itself it will likely be presumed to have birthed the child for this reason.

Gender disparity will still exist, because you can be a dick even if you also have a vagina

I need to add a very important caveat to this entire section, that I'm speaking purely about evolutionary psychology. The below is a true and known aspect of evolution that can be demonstrated. That does not necessarily mean I believe that it should be used to dictate anything about how modern society should be structured or believe it makes a significant difference between sexes in modern society. In fact I'll go so far as to say someone trying to use this as claims of superiority of one sex over the other or to justify treating sexes differently in modern society is butchering evolution and science and is all around a terrible person

It's a known fact that herms have a preference when it comes to mating, they would prefer to play the role of a male. Males dedicate far fewer resources into producing young then the female does, allowing the male to dedicate those extra resources in potentially mating with other herms and producing more offspring. This preference is more significant with a polygamous mating system then a monogamous one, as the male is free to mate with other herms immediately after impregnating a female in a polygamous mating system. However, even in a monogamous mating system this preference will still exist as there is always sexual conflict with the two sexes trying to offload as much work as possible on their partner's even in monogamous systems.

Thus the evolutionary psychology of the herms will be two fold, first a preference for being the male, and secondly a belief that whichever herm played the role of the male is likely 'stronger' or more evolutionarily fit then the one playing the role of the female, as he was able to somehow convince/force the female to accept the less preferable role.

This bit of evolutionary psychology is going to have a significant affect throughout all of your society and culture, which will generalize favor the ideal of herm-as-male. Your see it in everything from royalty and other rich/important figures always insisting on mating as the male to society treating unwed females as a bigger problem then unwed males to Axe body spray commercials promising that your get all the herms begging you to be their male because all the herms are culturally encouraged to be males over females on some subtle level so playing the role of male is just 'sexier'.

Unfortunately we have also seen consistently that those with power will do everything they can to consolidate power, in fact males in human history did quite a bit of power consolidating by exploiting a small physical size advantage into things such as laws and policies that gave females less legal rights as males to policies design to refuse women education or the ability to get a job (so they had to stay dependent on their male partners) or spreading of misinformation about women that denigrated them to make males seem more powerful by comparison (like claims that women are less intelligent or that they were too 'emotional' to hold a job or that they were more comfortable being subservient).

Please note this is not a situation limited to males. Females have been shown to be just as willing to consolidate power whenever they were in a position of power, it just so happened that differences of physical size and childbearing costs generally put males in a better position to do so in the past.

In a herm society I could see somewhat similar tendencies occurring. While any herm could mate in any position the ones in power, be it financially, politically, or physically, will both consolidate power, to put themselves in a better position in the future, and likely insist on mating in the male role. As a side effect they may also create systems that make it harder for a herm who has already mated in the female role to gain 'power', in any format, in order to cut back on competition and ensure that once they have mated as a male they will get to keep that position.

As an example I could see herms making claims that females who are pregnant or nursing shouldn't be allowed to attend school as they need to dedicate all their energy to their child and/or would be too distracted/distracting to get her education. While there would be all kind of arguments why this just 'made sense' one of the unspoken reasons would be that of consolidating power. If two herms mated at a young age the one that mated as a male would be able to continue going to college while his partner couldn't, thus he would have the education that allowed him to get a better job and thus be better suited as the 'bread-winner' then the female who was refused an education. When the time comes around for another child to be had he can thus argue he is too busy bringing in the money to be pregnant and thus insist the other herm again be the female. Basically taking one opportunity of being the male in the first mating as an excuse to justify his gaining the skills necessary to always get to maintain the role of male.

All the above is talking in very large sweeping terms across all of society. It's quite possible that many herms would regularly swap the male and female roles and that some herms would have a preference for female role for instance. Likewise it's quite possible many of the herms who created or propagated systems which helped males consolidate power may not fully understand how those systems were in fact assisting in the consolidation of power. In short, I can anticipate patterns likely to show up across the society as a whole, but that doesn't mean you should presume every individual in that society share's that belief or motivation.

Regardless the net result is that a sort of gender role where herms mating as males were preferentially favored, or herms that mated as females were somehow subjugated, is not at all impossible. If such a case existed that would then have a very significant effect on how a child would treat each of their parents. For instance such a system would likely encourage forcing female mated herms to be responsible for child rearing, which in term would affect the parental bond with the child as in my previous point. For that matter the child may grow up to treat each parent differently based off how society treated the two types of mating. For instances your likely to hear "my dad can beat up your dad" sort of arguments which cite the herm that mated as a male as the one to beat up because society expects male mated herm's to be 'stronger' then female mating herms.

Put more generally once society creates gender roles based off of method of mating those gender roles are going to affect all aspects of life, including how the children and parents interact with each other. A whole separate question could be dedicated to just how many difference such roles could create, so I'm not going to waste time explaining each one in depth.

Down with gender roles

If you don't want these sort of gender roles to be as prominent in your world there is, luckily, a simple evolutionary justification for decreasing these roles. If monogamous mating are the norm then it's possible that herms regularly alternated between the male and female mating role every other mating.

From an evolutionary stand point this would make sense because, as I stated before, it takes more energy for the female to birth a child then for the male to father it. If the goal is to pop out children quickly then waiting for the female to regain the lost calories/resources she expanded in birthing a child could slow the rate at which pregnancies could happen. By alternating who plays the role of male it would be possible to better spread out the resource cost of multiple mating's, thus potentially allowing more total mating for a monogamous pair.

If such a system had evolved before modern society and culture evolved then gender roles would be far less drastic, since it would be the evolutionary norm for partners to alternate matings and thus every herm would be expected to occasionally mate as a female. This wouldn't entirely remove the problem though, as evolution would still favor the male role of mating and that would still have some affect on their evolutionary psychology. In particulate there is a possibility that modern societies, where medicine and technology made child mortality lower and thus couples spent more time on raising existing children rather then birthing new ones to replace deceased kids, that a preference for the male role may grow in 'vogue' as it no longer became necessary to alternate roles in mating just to keep up with the demands of constant pregnancies. Still, this would at least significantly decrease the degree of preference for specific gender roles.

There is another catch with this possibility, the fact that it only makes sense when your goal is to constantly produce kids. Putting most of your effort into producing lots of kids, rather then in raising existing kids, is more of an R-type strategy by definition, and generally sapience is primarily a k-type mating strategy. In layman's terms the really smart animals likely would have a large gap between pregnancies while the took the time to raise and teach their newborn smart child how to use his smarts to survive, and if that happened then a herm has enough time to catch up on the calories lost during the birth in the years between pregnancies spent raising the existing child.

The solution to such a problem would be to put the herms in a harsh environment where infant mortality is extremely high. I'd cite penguins as the perfect example of this. The male and females trade off time spent caring for their unborn child (in this case an egg) by passing the egg back and forth because it's the only way the two of them can manage to raise an egg in such a harsh environment. While alternating pregnancies is a bit different then trading eggs there are quite a few analogs that one can draw. still I'm getting a bit off topic now so I'm going to stop here by saying if you wanted to ensure herms traded off mating roles it's probably worth asking a separate question so we can go into more detail about the reasons.

Who's your daddy?

Finally there could be quite a significant difference in parentage based off of mating role because it's entirely possible that 'realistic' herms would never know their father. Generally hermaphroditism only persists in a world where it's hard to find mates due to individuals of the species being highly isolated. I already went into quite a bit of detail as to why that was at the beginning of this answer, so rather then repeat myself I'll encourage you to check out that answer.

The point being it's quite possible that herms only met up long enough to mate and then went their separate ways, meaning a young herm would only be raised by it's mother and thus there would be a significant difference in how a herm was raised depending on which herm was their mother.

This point gets a little complicated if you actually want intelligent herms with their own culture, because the degree of isolation necessary for hermaphrodites to stay an evolutionary good idea isn't exactly supportive of evolving sapience or culture. In fact I have a separate question just for trying to justify the existence of sapient herms.

However, I can say that it's quite possible that herms at least started as a mostly isolated spaces and thus their instincts are less in favor of monogamy and/or put much higher emphasis on the maternal bond over the paternal since in their distant ancestry the paternal bond didn't really exist, how much that is true depends on how you justified your sapient herms existing in the first place.

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