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In haplodiploid sex-determination system, female/diploid individuals are developed from fertilized eggs by means of sexual reproduction, while male/haploid individuals are developed from unfertilized eggs by means of parthenogenesis.

So assume there is a highly intelligent haplodiploid species. All female/diploid individuals of this species have the humanoid appearance (even though they do not necessarily look like human females), while all male/haploid individuals are mere egg-sized "balls" that possess neither human form nor sentience. The male/haploid individuals are produced periodically in the way similar to menstruation (or hens laying eggs) and have a very short lifespan respectively. As a result, female/diploid individuals of this species would exchange the male/haploid "eggs" (or "testes") that they produce with each other when they need to reproduce female/diploid offsprings of next generation. Since those male/haploid "eggs" (or "testes") exist outside of the bodies of female/diploid individuals and are disposable by nature, this species is technically not hermaphroditic.

So my question is whether such a radical sexual dimorphism between female/diploid and male/haploid individuals is plausible or not. It seems that in most haplodiploid species like Hymenoptera (ants, bees, and wasps) the physical morphology of male/haploid drone individuals is not radically different from female/diploid individuals despite the fact that they are technically also sperm pockets exchanged between female/diploid queen individuals of different colonies. One possible reason why this may be implausible is that sexual maturity always takes a long time (maybe one decade or even more so) to reach for a highly intelligent humanoid species even though all male/haploid individuals are just insentient "eggs". In other words, it is simply too costly for female/diploid individuals to produce disposable "testes" on a regular basis.

Or maybe the reproductive mechanism of this species can actually work?

Update 1: Another possible issue may be that the radical sexual dimorphism between female/diploid and male/haploid individuals diminishes the main evolutionary advantage of haplodiploidy. It has been proposed that haplodiploidy may evolve as a mechanism to get rid of defective genes that are much more visible in haploid individuals. But if those male/haploid individuals are mere balls, then it would be nearly impossible for female/diploid individuals to observe the existence of those harmful mutations. That is to say, if male/haploid individuals are mere balls, then the risk of accumulating recessive traits (or inbreeding) would be greater for them.

Update 2: Since @Willk points out that this species is functionally equivalent to simultaneous hermaphrodites, the post now can boil down to the question whether or not it is evolutionarily plausible for a human-like species to evolve a way of fertilization without the involvement of penile objects.

PS: I prefer using terms like "diploid" and "haploid" to terms like "female" and "male" to avoid some unfortunate implications.

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  • $\begingroup$ Welcome to the Worldbuilding Stack Exchange! Your highly interesting question has been written is a confusing manner for non-biologists, could you perhaps rephrase it to make the question more accessible for other readers? $\endgroup$ – A Lambent Eye May 11 at 14:19
  • $\begingroup$ @A Lambent Eye Thank you for your advice! I'll try to reduce my use of biological jargons in this post. $\endgroup$ – H. Hayashida May 11 at 14:30
  • $\begingroup$ Interesting question! While I probably would have upvoted it anyway, you get my +1 for sure for including a clear definition of your terms at the opening to the question. $\endgroup$ – Cyn May 12 at 0:35
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These are hermaphrodites, and what you are calling males are actually gametes.

Your "males" are haploid (like gametes), and are nonsentient (like gametes). They are produced in excess (like gametes) and their sole function is to mediate sexual reproduction (like gametes).

Your "females" can make these male gametes and also receive them, presumably to combine with some internal different sort of gametes. Because the "females' can make both types of gamete, they are hermaphrodites.


Answering the comment:

/the question becomes whether it is evolutionarily plausible for a humanoid, mammalian-alike species to evolve a non-penetrative way of reproduction other than parthenogenesis/

Sure. The gamete you describe (the "male") is an egg. You could call an egg an external womb though probably fairer to call a womb an internal egg, since eggs were first. The "male" gamete is big and presumably resource rich. It is fertilized by the other partner with a spray of gametes onto the egg (in fish, called milt). The embryo develops inside because that is how eggs work. There is no reason humanoids could not lay eggs.

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  • $\begingroup$ Thank you for your insightful answer! I think you are right that they are functionally equivalent to simultaneous hermaphrodites, so the question becomes whether it is evolutionarily plausible for a humanoid, mammalian-alike species to evolve a non-penetrative way of reproduction other than parthenogenesis? $\endgroup$ – H. Hayashida May 11 at 16:48
  • $\begingroup$ @H.Hayashida I sense the possibility of an "external womb" question, that (as far as I know) would be quite novel. Bravo for breaking new ground. $\endgroup$ – Don Qualm May 11 at 19:00
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There could definitely be such a difference, just look at other examples of sexual dimorphism. What you are thinking of is called sexual gigantism, and is predominant in spiders. While perhaps not originating in the exact way that you are describing, take a look at an example of this, and you will see that the females can be much bigger and more complex than the males. This is rampant throughout many other spiders, simply because the females have to raise the young while the males die soon after fertilization. This is why they are smaller and less complex, because when they are basically disposable, you don't want them to have to be so big. The males also die soon after fertilization. Concerning Hymenoptera, while there is not such a drastic dimorphism, it is still there.

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  • $\begingroup$ Thank you very much for your informative answer! I think the main difference between the golden orb-weaver spiders and the haplodiploid species of this post is that those male spiders are still functional individuals and have intrasexual (male-male) competition regarding mating despite the danger of being eaten, while the males/haploids of this species lack individuality completely like post-fusion male anglerfishes. $\endgroup$ – H. Hayashida May 11 at 15:50

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