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There are two humanoid races are in my fantasy story.

Race A is almost like humans, except they are in super human size, have super human strength, have a higher body temperature etc. Some sort of giant race I guess.

Race B is almost like elves. The only considerable different is that they are Oviparous (They lay eggs.)

What I need to know is that, is it possible that, A male from race A and a Female from race B have a child together, and vice versa? Simply, Can a mammal and Oviparous have child together if they have similar external body structure?

Both the races have similar external organs except the difference in color and size.

Both the races have similar mating methods.

Thanks in advance.

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    $\begingroup$ This seems to be up to the writer. If you are writing fantasy, then you are making up the rules of what can be. $\endgroup$
    – Escaped dental patient.
    Jul 31, 2019 at 15:41
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    $\begingroup$ The only answers to this will be opinion based. Even within mammals, species can't interbreed unless they're very close. Biology has a lot of checks and balances to prevent such a thing. A humanoid species also probably wouldn't be able to be oviparous as the birth canal would likely damage the egg (though leathery eggs like in reptiles might be a possibility). Humans are already extremely poorly suited to giving birth as it is. $\endgroup$
    – stix
    Jul 31, 2019 at 15:42
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    $\begingroup$ Yes. No. Maybe, possibly with some caveats -- for example a horse and a donkey can make a foal together, but the foal will most usually be sterile. Basically, it depends on the story which you want to tell. (Please note that an animal which looks mostly like a human but lays eggs is already highly implausible.) $\endgroup$
    – AlexP
    Jul 31, 2019 at 15:43
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    $\begingroup$ By biology No, monotremes are very far removed from placental primates. $\endgroup$
    – John
    Jul 31, 2019 at 15:55
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    $\begingroup$ In real biology, the number of genetic differences between two species can be used to define a measure of the "genetic distance" between them, and genetic distance is thought to be closely related to whether two populations are capable of interbreeding and producing viable offspring, see here. So even if an alien species evolved by convergent evolution to have a very similar bodily structure to humans, and even if they had placental reproduction, it's extremely unlikely they would be enough of a genetic match to interbreed. $\endgroup$
    – Hypnosifl
    Jul 31, 2019 at 16:58

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Peculiar Biologies Required

Normally one would think that the divide between oviparous and viviparous is very sharp. However, there is a report of a species that mixes and matches. Use that for inspiration.

Perhaps one or both of your two species may have a latent ability to switch between viviparous and oviparous mode, but due to the environmental parameters, only one mode has been active for hundreds of generations (the opposite modes in cases of the species-pair). And perhaps mating with the other closely related species would nudge the mechanism in the unusual direction (whichever one you need) for the mother. Or not nudge it, meaning the mother gives birth as usual for the mother-species, but is interfertile with the other species. Really, you can pick any of the variants if your creatures are (a) closely related and (b) not far from the oviparous-viviparous borderline.

Also, keep in mind that different species can sometimes still produce viable and reproduction-capable offspring. In fact there are species which procreate only through mating with a different species. So if you need two species to be close enough to produce fertile offspring, make them so; it's possible.

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No.

Or "rather unlikely" to be more precise, which might give you a way out for your story.

When two individuals can have fertile offspring, they are usually considered/defined as being in the same species. Individuals from closely related species may be able to produce infertile offspring, like mules from horses and donkeys.

The key to infertible-but-viable offspring is a close relationship between the species. Having a live-birth mammal and an oviparous mammal closely related sounds rather implausible, but there are oviparous mammals after all. So for your fantasy setting you could stipulate that a placentalia and a monotreme are close enough for infertile offspring.

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  • $\begingroup$ Excellent answer, exactly what I was going to write! +1. With some wand-waving explanations, you could explain other reasons for such a pairing. But biologically, this. $\endgroup$
    – DoctorWhom
    Aug 1, 2019 at 1:07
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If it is a literal "is there a scientific reason why they cant" then it's a solid no. Their DNA is simply not the same. Even if the sperm fertilized the egg the DNA combination would cause mayhem and kill the egg cell the moment it starts using the combined DNA.

Since this is fantasy I suspect you ignore the DNA part and are only interested in the development of the child.

Both birds and children in the womb move their bodies and as they grow they will get less movement range, so their development will be similar and there will be no joints and muscles that fail to develop due to lack of stimulation, which at first I thought would be a problem.

One of the biggest differences I can see is the way they are fed. A child in the womb receives nutrients from the mother which include immunization elements to protect the baby against diseases the mother is/has come into contact with. This increases the risks for the humanoid part of the being as it doesn't receive the immunization it is supposed to get. Otherwise there doesn't seem a lot different as long as the Elf eggs are at least as big as the full womb of the humanoids... which is probably the problem. The humanoids are BIG. their development will be a lot slower than the elf babies, and the maximum size they reach in the egg is likely going to be bigger than the egg that is build around elf-size children. In reverse it would be possible.

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  • $\begingroup$ What if the child in the egg have more similarities to it's elf mother other than giant father? $\endgroup$
    – Sangeetha
    Jul 31, 2019 at 19:41
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    $\begingroup$ @Sangeetha it depends on what size the giants are. The eggs have a very specific size and the food inside to grow its occupant, this effectively limits the maximum size to always perfectly fit the egg. So my initial idea that the giant would grow to break the egg was a bit stupid. However it now becomes a question of development. If the giant gets babies that are twice the size of the elves and the child takes after the elf it might be manageable, as the giant part of the child would be developed enough to survive. It would function like a premature birth and have higher death rates. $\endgroup$
    – Demigan
    Jul 31, 2019 at 22:37
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    $\begingroup$ It also depends on what kind of split of chromosomes and DNA and the like that we're talking about (if these species have them). For example, humans get 50% of our DNA from each of parents. If one of these species has a similar requirement, whereas the other does not, that could also prevent the baby to exist. You wouldn't even form a gametes. Also, keep in mind that Bonobos and humans are >96% similar in genetics, but we can't reproduce with each other. I think live-birth vs egg-birth are in themselves difficult limitations to conquer. $\endgroup$
    – arpanet101
    Aug 1, 2019 at 7:43
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Define ‘together’.

Eggs have an advantage over live births in that all they really need to gestate is a warm, safe place to be. The nutrients required for successful birth are already present in the egg.

If your oviparous species are females engaging in parthenogenesis (cloning themselves, without requiring a male to fertilise an egg), then Race A can be treated as literal incubators. Some hormonal changes may be required/desired in race A to allow for ingress of Race B’s eggs. Egg size matters. Fairy size eggs may not need anything special, baby size eggs may need quite a lot of work. Then ‘birth’ can either be of the egg just prior to hatching or of the hatched child and egg fragments. Either males or females of race A may be suitable candidates for this type of birth, depending on the... circumstances of the egg’s deposition. Sticky or hooked eggs can lodge in some truly unexpected places.

Needless to say some singularly squicky situations could arise here. Examples of this already exist in nature (Looking at you, spider wasps), though they generally result in the incubator being eaten alive by the children,

As for males of race B mating with race A, all I’ve got is that they’ve been preloaded with eggs (now sexually fertilised) much like a seahorse. In this case the B Male ovipositor is just used for depositing the eggs.

Obviously in this scenario the children are genetically only race B (and perhaps straight clones of their race B mother), but the member of race A may feel paternal/maternal instincts nonetheless.

If the babies of B/B couplings tend to kill their parents on birth/hatching and A’s are often sterile I can see this being a kind of social symbiosis evolving from a prior parasitic relationship. Sterile A’s get to have children, broody B’s get to not die in childbirth.

Win-win.

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No.

If there were some difference in the looks or body plan, you could expect a viable offspring (think of all those dog varieties). But with that huge difference affecting the reproductive system, there is no way they could be compatible: the human needs a placenta.

I would say even more: it is conceivable that genetic engineers would be able to modify human genome to produce something like eggs (after the placental stage), so that the offsprings were born in something like sacs (like dogs do), maybe even sacs with thick walls, and even maybe sacs that could survive some time detached from the parent (although this is highly unlikely because the living newborn would instantly need food and breath). But those changes would make that organism incompatible with humans. Because the changes involve the reproductive system and not something else.

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