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A hypothetical species reproduces by females impregnating males a la seahorses. However, their reproductive system appears woefully ill-suited to this task. Their genitalia are outwardly similar to and compatible with placental mammalian genitalia. The mechanics of copulation are outwardly identical: penetration, friction, and ejaculation on the male's part. How does the ovum get fertilized and delivered to the father's prostate/uterus?

UPDATE: It seems the most efficient transfer of genetic material is in one direction only. The feasible alternative to ejaculation or oviposition would be siphoning.

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    $\begingroup$ The thing which makes me noise is: how do you classify male and female in that species? $\endgroup$ – ESL Dec 16 '16 at 22:29
  • $\begingroup$ @ESL: by the type of gametes they produce. That's why seahorses have male pregnancy: the ones that get pregnant produce sperm. $\endgroup$ – Anonymous Dec 16 '16 at 23:09
  • $\begingroup$ Look at fish that are related to seahorses but not so advanced in this adaptation. Pipefish simply stick the egg sack around the male’s tail (on the outside). $\endgroup$ – JDługosz Dec 17 '16 at 9:17
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TL;DR

If you want males to give birth, they need genitals that can give birth. That's why male seahorses don't have external penises (and why they have vagina-like openings).

If you want two transfers of genetic information, do it safely. You ask for two, but seahorses only have one.

The detailed version

Seahorses have a one-way transfer - females inseminate the males. Going back and forth twice (male->female->male) as you suggest increases the risk of losing the egg, so in nature, there is usually just one transfer.

Conversely, if the male gives birth, he will have an opening large enough to do so. This is why male seahorses have vagina-like openings and not external penises - if you're taking the "male birth" part it is important to take the genitals that allow male birth.

I'm going to try to find a balance between "identical" genitals and males that give birth, but obviously you cannot pass a baby through a penis. These facts are hard to change, so "identical" genitals must be stretched.

Safe transfer of fertilized eggs from female to male

This is possible, but even seahorses don't risk it; it's easiest to transfer genetic material once. Consider one of the following ways to make the transfer safely.

  • Males could "suction" the resulting mixture with the penis.
    • If you want human-like birth rates (on average, one child) then this is not safe; it's too risky to lose the only egg there is.
    • If you opt for more than one offspring each time (seahorses have thousands of offspring, for instance) the risk may be negated; why does it matter if you lose a few eggs when most of them made it?
  • The female could gather the fluid containing the egg, and condense it (or drain it into a nice pod), then eject the egg in this substance / object back into the male through her own process (orgasm? round 2? who knows.).
    • This has the added bonus that you can complicate births further. Males can receive the eggs of other males.
  • You could ditch eggs during this transfer and bring them in at the last step. Males could ejaculate something similar to sperm into the female, who would edit her own DNA into as many sperm as possible. This final fluid would be siphoned or ejected into the male, and the first one to reach the lining of the male's uterus would begin to develop - similarly to the "first sperm wins the race" idea.

Safe male birth

There are three main ways to do this - to have a vaginal opening, to lay eggs, or to be severely mutilated (die?) in the process of childbirth.

  • The "vaginal opening" idea sounds most promising. You describe males with uteruses (uteri? what is the plural of uterus?) so it seems like this would be the case
    • You can choose to have the penis expand, just as mammalian vaginas contract and grow larger, to give birth, although I'm guessing it would be extremely uncomfortable. It may render the penis useless.
    • Alternatively create a new opening adjacent to the uterus; this is safe, comfortable, and can be out of the way.
  • Egg-laying via the penis (oviposition) will allow you to keep genitals as they are
    • This would defy the mammalian characteristic of giving live birth, but it would require the least physical change
  • Mutilation during childbirth (no organs suited to birth, so the offspring crawl out) is possible, and may even occur in nature. This may be too gruesome of a path.

Note: I don't know if anything I've said here is realistic from an evolutionary standpoint. The idea of male to female to male transfer is already stretching it past what we've seen on Earth.

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  • $\begingroup$ Thanks for the explanation. I figured that the penis/vagina or ovipositor/brood pouch dynamic would remain the most efficient method. Hyenas give birth through a phallic clitoris, but this is quite risky. $\endgroup$ – Anonymous Dec 17 '16 at 19:24
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    $\begingroup$ @Anonymous Birth through the pseudo-phallus (female hyena penis) is also an option and relates to the idea I provided about the penis dilating/contracting - but note that 60% of hyena cubs die because of this $\endgroup$ – Zxyrra Dec 17 '16 at 19:47
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Everything works like you would expect to see for any typical mammal species, but then there is one extra step:

After the eggs are fertilized the father switches from blow to suck and vacuums/pumps the eggs into his uterus like pouch. As for what happens when the eggs are ready to hatch or the fetuses are ready to be born... the dad is likely going to be in for a very painful experience.

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    $\begingroup$ all I can think is "It's not a tumor!" $\endgroup$ – PipperChip Dec 16 '16 at 23:53
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Swap which ones produce the sperm and the egg

The definition of male and female in biology will clarify this. The female is simply the member that makes the more costly/expensive reproductive cell. So You can just alter the current system so that the former-male produces the egg and inserts it into the former-female who now produces the sperm.

This is pretty much what we see in seahorses. For all intents and purposes the male seahorse is really a mother seahorse. The male seahorse just happens to produce sperm so they are classified as "male".

To use modern day humans as a similar example of swapping sex. Our current day male and female can keep pretty much keep the exact same bodies, their sex is just swapped with each other via who produces the more complex gamete.

Now you can use the same reproductive apparatus that we have today, but now by definition the girls are boys and the boys are girls.

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