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The Humerns are quite an odd species when it comes to mating. They have nine distinct sexes (not genders, they’re biologically distinct) all of which exhibit varying levels of dimorphism and all of which can interbreed with any sex but their own. Don’t even ask about their social structure or mating rituals...

Any given coupling has a distinct set of probabilities associated with it for the sex of the offspring (similar to these guys). Some couples cannot have children of a certain sex, others are near guaranteed to have children of a certain sex. The complex social and physiological harmonics this sets up are an evolutionary response to a harsh and ever changing planetary ecology, and ensures that their species can adapt to rapidly changing climates and survival requirements without actually changing too much.

Each pair will have exactly two children, as breeding is done by both parents interlocking and flooding a temporarily shared ‘womb’ with their gametes before separation. The gametes combine, embed in the lining of the parent’s womb, establish a small, simple placenta, mature into a larval/embryonic stage then engage in a frenzied bout of siblicide that ensures only one child is brought to full term per parent.

The question is this:

By what method can many (potentially hundreds) of these embryos be reduced to exactly one while minimising the risk of death of the survivor and also minimising risk to the parent?

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    $\begingroup$ "Some couples cannot have children of a certain sex, others are near guaranteed to have children of a certain sex." Does it work like bloodtypes? I.e. you get a random piece from either parent. Not every combination is possible (e.g. you won't have a B is neither of your parents does), but the specific combination of possible outcomes is randomly assigned? (e.g. for an OA and an OB parent, you have a 25% chance to end up as OO, OA, OB, AB) $\endgroup$ – Flater Oct 26 '17 at 13:14
  • $\begingroup$ @Flater: Yes, just far more complex. $\endgroup$ – Joe Bloggs Oct 26 '17 at 13:36
  • $\begingroup$ They need to average more than 1 kid each to make up for the ones that don't make it to breeding age. Currently that's about 2.1x per woman for humans. Used to be between 4 and 5 per woman due to childhood mortality. $\endgroup$ – Sherwood Botsford Oct 27 '17 at 20:46
  • $\begingroup$ With 9 sexes you have 9*8 = 72 possible sex pairing. Do all the animals on this planet work like this? How do you keep track in your story? Even with just 4, you would have 12 possibilities. $\endgroup$ – Sherwood Botsford Oct 27 '17 at 20:48
  • $\begingroup$ @SherwoodBotsford : Not all, but certainly the most successful species do. For the vast majority of pairings the there’s not really any need to keep track of anything.. $\endgroup$ – Joe Bloggs Oct 27 '17 at 20:58
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Once the embryos are formed, they rely on the energy supply provided by the egg. But that's not enough to make them grow until birth: they need to attach themselves to the womb wall and implant a placenta to start exchanging with the bearing organism.

The womb wall will establish a bond with only one embryo, and this lucky one will be the one capable of providing the strongest hormonal stimulus in a given time frame.

So, dear embryo, if you want to be fed, be sure you are quick to reach the wall, and be sure your hormone shot is the strongest one. Else you are going to starve.

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    $\begingroup$ It’s not who is strongest: it’s who shouts loudest. Elegant. $\endgroup$ – Joe Bloggs Oct 25 '17 at 6:39
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I ... hate to do this. But tiger sharks may point the way here: https://www.livescience.com/29198-shark-embryos-cannibalize-others.html

Relevant quotes:

Shark embryos cannibalize their littermates in the womb, with the largest embryo eating all but one of its siblings. [...]

That finding suggests the cannibalism seen in these embryos is a competitive strategy by which males try to ensure their paternity. [...]

Full-grown sand tiger sharks are approximately 8.2 feet (2.5 meters) long, and mothers typically give birth to two baby sharks, each about 3.3 feet (1 m) long.

So I guess we can do this, it should work ... but ... when Humern science advances enough to figure out what's going on, the Humerns are going to be absolutely shattered, emotionally.

Update: Comments asked if they'd really react badly. Some thoughts... I imagine there will be a time when they don't know what's going on (their pre-modern era). Depending on what their society is like before they find out, it may come as a shock. Don't know enough to say if their eventual attitude will be grief ("We are all born murderers, oh the anguish!") or satisfaction ("My sons are strong, rawr!"). Pure speculation: eventually they'll put the best possible spin on it, as people are good at finding ways to live with themselves

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  • $\begingroup$ Why would you hate to do that? Nature getting there first is awesome! $\endgroup$ – Joe Bloggs Oct 26 '17 at 5:37
  • $\begingroup$ @JoeBloggs I'm worried that the Humerns will be horrified when they discover what's going on. $\endgroup$ – akaioi Oct 26 '17 at 6:08
  • $\begingroup$ You assume they have the same outlook on life as us. If they’re of the mindset that life begins at birth then a bit of in-utero rivalry isn’t going to bother them that much. $\endgroup$ – Joe Bloggs Oct 26 '17 at 6:22
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    $\begingroup$ @Burki: Cancer is "natural" to human biology, but that doesn't mean we're okay with its existence. Similarly, murder comes natural to some humans but is abhorrent to others. The same applies to eating meat. Some relish it, some are neutral about it, others can't accept it as a way of life. People can be shocked by the nature of their own species, even if they're not shocked by their own (individual) nature. $\endgroup$ – Flater Oct 26 '17 at 13:47
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    $\begingroup$ @JoeBloggs I imagine there will be a time when they don't know what's going on (their pre-modern era). Depending on what their society is like before they find out, it may come as a shock. Don't know enough to say if their eventual attitude will be grief ("We are all born murderers, oh the anguish!") or satisfaction ("My sons are strong, rawr!"). Pure speculation: eventually they'll put the best possible spin on it, as people are good at finding ways to live with themselves. $\endgroup$ – akaioi Oct 26 '17 at 15:21
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I would take a parallel with young birds in a nest. Those that develop rapidly due to good placement in the womb, genetic superiority, or predestination grow stronger and more able to defend each other. You might end up with two or more equally strong sibling though, to which you could find an answer in stating that the method of attack could be some acidic secretion or ingestion of their siblings, making each kill strengthen the killer.

If two siblings still remain against all odds, and they are equally matched, some external influence could decide, or you could decide that twins simply are possible, just very rare. Exceptions make the rule. You could also state that it causes complications and either both children die in utero, or their father/mother dies due to complications at childbirth.

Note: Before the 1900s, many more women died due to pregnancy, and many more children were stillborn. If your society is not in the industrial age, perhaps a margin of error is even beneficial to both your world and its realism.

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At one point within the first couple of (days/weeks/months) each of the larvae will develop a poisonous substance, e.g. enzyme, that will kill each of its siblinngs within seconds but be immune to himself. The one that got randomly

(birds have mechanisms to make this non-random, but it doesn't seem to be the case with your species - or if you would follow the same strategy (eggs laying over a couple of days), you would just waste a lot of good food)

the most nutrition and is just overall the healthiest will develop that ability first and kill off all its siblings within seconds. Theoretically 2 could "mature" at the same time, but the chances of this happening are very, very slim because of the short time to kill off everyone and the long time it takes to mature.

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The embryos might engage in a survival of the fittest/there can be only one type of action, with weaker embryos being absorbed by stronger embryos, providing a significant portion or of the nourishment needed to grow, thus diminishing the burden of this nourishment from the parent.

This would ensure the strongest, fittest possible offspring. Twins would be bad luck, as they would have absorbed only half of the nourishment needed to come out strong and rearing to survive, and would therefore be weak and more likely to die young.

In addition, the parents would bulk up before the mating event, as the production of the many embryos would be very energy and resource intensive on the body. After the mating event, parents would be at near-normal levels of health, meaning that their own ability to survive would not suffer greatly from the need to nourish a growing youngling.

You might want to consider parents' bodies hardening the walls of the womb into an egg and expelling it from their bodies in some way, as being pregnant (bulky, unwieldy and hormonal to name a few issues) would be a serious detriment to survival in a harsh environment.

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The human system generally produces one viable embryo that combines the genomes of both parents exactly as desired. Why not just base it on the human system? Judging by the name you want some similarity, anyway. And you could use literature about humans as a reference, which would be a significant benefit. While the exotic factor would go down, it would also make the humerns more relatable.

So both parents would have wombs and would produce an egg. The periods would be synced via pheromones, so that both "mothers" would have a chance to get pregnant simultaneously. Humans already do this, so details can be copy pasted. Although if having both parents get pregnant at the same time makes sense, it kind of implies that there is a time pressure, so an outright mating season might be better.

Both parents would also produce sperm. This would then be ejected into the womb of the other, or if exotic is desired mixed between both wombs. The eggs in any case would reject sperm of the same sex, so it works the same. It might be nice to give well fed and lonely parents a chance of self-fertilizing. This kind of set up kind of implies that finding mating partners has been a major issue at some point of evolutionary history, so it would make sense.

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  • $\begingroup$ The reason I chose to use many gametes was to ensure that in any mating event it was a certainty both parents got pregnant. Having only one viable egg is certainly a was to ensure one child, but it’s much less reliable. $\endgroup$ – Joe Bloggs Oct 26 '17 at 5:44
  • $\begingroup$ @JoeBloggs Not really, you are still cutting it down to one at some point and the time you do the cutting at does not really affect reliability. What it does affect is the cost, which will be lower the earlier you cut down to one. In that sense doing the cut down to one early would probably be more reliable in real life as the one egg could be invested more resources into. $\endgroup$ – Ville Niemi Oct 26 '17 at 8:20
  • $\begingroup$ Rabbits mate immediately after birth. Embryos are fertilized, then stored just as fertile eggs until conditions are ripe for them to develop. $\endgroup$ – Sherwood Botsford Oct 27 '17 at 20:43

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