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I want to know if my interspecific interbreeding is realistic or a total fiction.

Most animal hybrids are sterile (mostly the heterogametic sex according to Haldane's rule). Some famous examples include mules and ligers. Some famous exceptions include most hybrids from the Canis genus (dogs, wolves, and coyotes) and grolar bears.

In my world, there are sixteen living human species (mammals from the Homo genus): anatomically modern humans, merfolk, angels, demons, ogres, dwarfs, halflings, giants, vampires, therianthropes, goblins, trolls, orcs, elves, gnomes, and wizards. They can all interbreed with each other, but female human hybrids are more likely to suffer from endometriosis and PCOS, and male human hybrids are more likely to suffer from oligospermia and teratospermia. That said, most hybrids, both female and male, are still fertile (it is more correct to call them sub-fertile).

So, I wonder if the fertility problems are realistic, because my species have a radically different appearance (merfolk, ogres, and giants have a seal-like blubber, vampires have fangs, therianthropes are covered with a gorilla-like fur, angels have huge hands like a bat's wings, demons have horns, etc.).

They also have different behaviour (demons are eusocial, therianthropes are as solitary as bears, giants, ogres, and merfolk are as solitary as orangutans, etc.).

Naturally, they are of different size (merfolk are as massive as belugas, giants are as massive as polar bears, ogres are as tall as the average anatomically modern human NBA player and as heavy as adult male gorillas, angels are as big as wandering albatrosses, in a colony of demons, at a metric tonne, the queen is always the largest individual, gnomes are as small as domestic cats, orcs are as heavy as adult male American black bears, etc.).

Finally, they have a different diet (ogres, merfolk, trolls, halflings, and wizards are all obligate omnivores like brown rats, demons, vampires, therianthropes, dwarfs, and orcs are all omnivores with carnivorous tendencies like raccoons, angels, giants, elves, goblins, and gnomes are all omnivores with herbivorous tendencies like gorillas).

Note: since my story is a satire against racism and speciesism, my species can be of any morality (I hate both the Exclusively Evil and the Always Good tropes).

Note 2: when I say my story is a satire against racism, there have been persecution, wars, and slavery between anatomically modern humans and other humans, there have been some genocides, but many survivors, and some females apparently became sexual slaves. Naturally, anatomically modern humans considered themselves the "perfect human", and the discrimination mostly concerns other human species. But, fortunately, there have been some consensual marriages between all human species, sometimes, because some non-anatomically modern humans were considered "honorary anatomically modern humans", this is comparable to "honorary Whites" in Apartheid South Africa.

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    $\begingroup$ Funny idea to make a fantasy satire about racism, upvoted. You'd like to know the plausibility of cross-breeding.. but for breeding you'd need contacts. You could make a diagram for that: how plausible would each combination be: orc-human, or wizard-elve, or giant-dwarf (=0) Question about the story: Is there any hostility to start with ? Or troubles in the past ? For wide scale racism to develop, you could need things like oppression, or war.. intelligence differences, culture differences.. suppose the "humans" on your planet would regard themselves as superior, why would that be, etc. $\endgroup$
    – Goodies
    Feb 5 at 10:42
  • $\begingroup$ What is a "therianthropes"? $\endgroup$
    – Daron
    Feb 5 at 13:35
  • $\begingroup$ A therianthrope is the generic name for werewolves, werecats, etc. $\endgroup$ Feb 5 at 13:37
  • $\begingroup$ I thought I knew what you mean by "as solitary as orangutans" which is one of your favourite descriptions. But now you also say "as solitary as bears" and I am confused as to the difference. It might be time to drop these expressions and just say directly what the important social features are. $\endgroup$
    – Daron
    Feb 5 at 13:38
  • $\begingroup$ Both animals are mostly solitary in the wild, except when they have children. But they can both live in small groups in a zoo without fighting each other. One difference is orangutans have a more complex social structure due to being smarter and having more facial expressions. So if something is smart as an orangutan (or smarter as I assume are all of these human species) then what does it mean to be as solitary as a bear??? Please help. $\endgroup$
    – Daron
    Feb 5 at 13:40

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Anatomical Incompatibility.

The problems are not genetic. They are anatomical.

If you take a healthy ovum from any subspecies and inject it with a healthy sperm from any species then the zygote will develop with similar success rate to a normal pure human fertilization.

The difficulty is with the sperms reaching the ovum in the first place. One species has a slightly thinner or longer Fallopian tube. Another has a slightly larger or smaller sperms. Another has a different Ph in the vagina. Another has a slightly curved uterus that makes it difficult for fertilization to occur unless the male has suitable specialized genitalia.

Let's throw some numbers at the wall. Suppose an average human sperm is 2.84 μm wide, with the vast majority being in the 2.6 - 3.1 range. An average egg can easily accept a sperm up 3.1 μm wide. On the other hand Ogres have average sperm of 3.05 μm. The average Orge and Average human are still compatible. But it is much more likely for an Ogre to hit 3.15 μm than a human and this makes them incompatible.

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