In some fantasy works there are races, like some interpretations of harpies and mermaids, among others, that are all female, and need to seek out other races, like humans, to reproduce, the daughters always being members of their mother's race, and if they even can give birth to male children, they are always the father's race? What would provide a race that reproduces like this enough of an advantage to make it viable and continue its use?
closed as too broad by JBH, Pingcode, L.Dutch♦, Aify, Secespitus Jun 1 '18 at 8:00
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For most scenarios, this is a disadvantage.
If you look at most 'mate for life' species, the reason why they do it (if you take emotions out of the equation) is that their children take a large percentage of their efforts to raise during the time before they're adults themselves.
Birds are a classical example of this; raising chicks usually means someone guarding the nest and someone else fetching food. It's a two bird job, and that means a mating pair that not only conceive together, but raise together.
Not all mammals are mates for life, but many travel in herds or packs where (again) the raising of children and support for the group is shared among the group; some fetch food, others defend, others raise (or at least protect) the children.
Humans mate for life (not a perfect model I know but bear with me) because our children are being born at much lower levels of development than other creatures, and that leads to a much larger investment in their upbringing and protection before they're adults themselves.
If we take the human model however, as many will know it's not a perfect system and the effort required to maintain the pairing is often in conflict with the effort of raising children. I've met as many single parents who say raising their kids is actually easier without the other partner around as I have those who bemoan the fact that they've been 'stuck with the kids with no support'.
On the other hand, species whose reproduction is largely based on opportunistic encounters (largely solitary animals) are more likely to have children that take less effort to raise, and which a single parent can do with relative ease.
In the scenario described above, one has to assume 'opportunistic pairing', where the father has no further contact with the mother after conception. Both harpies and mermaids (for example) would live lives that are very confrontational to conventional humans, if not dangerous (men can't breathe in water for instance). So, the assumption has to be that the male's reproductive duties end at conception. For that reason as well, I'm going to further presume that the reproduction produces ONLY females of the species of the mother (y-chromosome sperm rejected through biological handwavium).
In most models as described above, that means that there is a lack of support for the mother that would assist with the raising of the child. As such, there can only really be 3 scenarios where this makes sense;
The all female species is a solitary species, who invest some of their time in raising their young only to abandon them or drive them off once they reach a certain age. They're unlikely to abandon them at birth unless they're proficient breeders due to the high mortality rate that would ensue, but clearly they invest a minimum of effort in their children before expecting them to support themselves.
Clan Based Model
The species may be social in nature but only if they adopt a clan or village based mutual support model. This would allow all members of the clan to assist with the raising of the children. Ideally, the raising of children would be done by the elderly; they possess superior experience and the willingness to pass this on to the next generation, but are inferior at hunting, gathering, or manual labour which would be done by the younger women who aren't actively pregnant or still nursing. This way, the bulk of the energy investment in raising the young is managed by those with the most experience and the least ability to assist in more material areas.
Somewhere in between these two is the concept that a child DOES take energy to raise and the mother is expected to do this by herself outside of a community support model. In such a case, it's likely that the mother would see anything other than the raising of the child as a distraction. There would be no further contact with the father as supporting his needs both as a husband and a parent would distract from the business of raising the child. Such a species would quickly develop a culture that venerated the women who created new members of their society. This would in turn lead to respect for the ascetic nature of taking care of children despite the adversity and sacrifice it entails.
In short, either the effort in raising children isn't needed, is done by someone else or is valued as a measure of status within the species. Outside of that, there's little reason to do it that I can see.