I'm looking for a very grounded evolutionary justification for the existence of a hermaphroditic sapient and social species, preferably roughly mammalian but not mandatory, which is not as easy as it would appear to justify. In the real word most hermaphrodite species are simple species, and most live in isolated areas where finding another of their species is harder thus increasing the benefit of ensuring the ability to mate with any you find, and/or yourself if none are found in some species.

There is a reason more 'complex' species don't tend to be hermaphrodites (forgive my simplification of referring to species like mammals as more complex, I'm not implying evolutionary levels, just more extensive genetic code). In theory it doesn't cost much for a female to have 'male parts' and be a hermaphrodite. However, if you have a hermaphrodite species some will be naturally better at being a 'male', and these better males will tend to produce more offspring since strong males can produce more offspring then strong females due to lower investment per child. Over time the strong males will 'realize' they benefit more from being a strong male and will focus more and more at securing mates as a male, which usually leads to the males 'losing' the ability to mate as female to instead focus on being better at out-competing others as a male to make them better at producing lots of young. Eventually males split off as separate from hermaphrodite as the only way to compete with other males and then the non-males specializes as females only to be the best at producing children and because they can't compete with specialized males for mating rights.

That's oversimplified explanation above I know it. The point is I want to avoid this issue, to justify hermaphrodite sapient that make sense evolutionary and won't diverge into separate sexes. They must also live socially with no shortage in potential mates.

I would prefer simultaneous hermaphrodites, though I'll accept sequential hermaphrodites IF there is an equal number of both sexes and no cast system where only the strongest/biggest etc transition to the other sex.

  • $\begingroup$ The conditions under which the species lives must place greater selection pressure on being hermaphroditic. Vertebrates which are simultaneous hermaphrodites, such as certain fish, are those which are hard pressed to find mates. In order to ensure their genes are passed down they are ready to impregnate and be impregnated as soon as they find a mate. $\endgroup$ – Anonymous Oct 11 '16 at 20:16
  • $\begingroup$ @Anonymous "they are ready to impregnate and be impregnated as soon as they find a mate"? According to my 'research' in college the only thing I need to add to an environment to make that happen is lots of alcohol... $\endgroup$ – dsollen Oct 11 '16 at 21:03
  • $\begingroup$ "ensuring the ability to mate with any you find, and/or yourself" Just saying that for a species or individual to possess both these separately rare qualities is doubly rare. Even if you've got the equipment for male and female there's usually a problem getting the plug to reach the socket, so to speak. $\endgroup$ – Lostinfrance Oct 12 '16 at 18:15
  • $\begingroup$ @Lostinfrance actually it would be more accurate to say that evolution decided to prevent the socket from accepting the plug. Self-fertilization is often undesirable and many hermaphrodite species have evolved to intentionally avoid it. But some do practice it, of those that do it's sometimes a last resort option only if no mate can be found. $\endgroup$ – dsollen Oct 12 '16 at 18:19

You can avoid some of your hermaphrodites going off to become polygynous males, by having a selection pressure which requires both parents present to successfully raise the offspring. Basically, have some biological quirk which imposes monogamy and/or cooperative breeding.

For instance, most birds are monogamous, because the eggs have to be incubated by a warm parent sitting on them AND because both parents can feed the chicks. Most mammals are polygynous because only mum is pregnant and only mum can supply milk, so dad can run off to find another female to mate with.

If you don't want something like an egg, then a tough environment, makes it likely that both mammal parents will be required to work their butts off to keep their kids fed and protected.

Since your creatures are hermaphrodites, both of them can provide milk for the baby. Or crop milk, like pigeons do. Or shed their skin for baby to eat, like caecilians do. Or make honey/royal jelly like bees.

Also you can make having the baby physically draining, like in seahorses. Mum seahorse accumulates the resources to make the eggs, but then passes them over to dad to incubate, while she goes off to feed up and recuperate, ready to produce the next brood.

In hermaphrodites, perhaps the parents have to take turns to give birth, because it takes so long for their bodies to accumulate enough body fat for baby's brain, or phosphate for baby's bones, or calcium for the egg shell, or whatever. So a bit like the seahorses, except it would be:

  1. Herm A gives birth, passes baby to Herm B for initial few months of breast feeding while Herm A recovers from the resource drain of the pregnancy.
  2. Both Herm A & Herm B breast feed the baby from month 3 onwards until it is a toddler. Herm A's fertility is suppressed as it is still recovering its reserves after the pregnancy.
  3. Herm B gets pregnant and its milk dries up. Herm A continues to feed toddler for a few months then weans it just before Herm B gives birth.
  4. Herm B now gives birth... Rinse and repeat.

If you want to make pregnancy really tough, or milk production equally demanding as pregnancy (in many mammals lactation is more energy draining than pregnancy, which is why births are timed to coincide with seasonal food abundance), then perhaps you require three or four hermaphrodites to successfully raise a child to weaning age.

  • $\begingroup$ definately the best answer so far. I think I'll need some more thought on it, but the only one I liked as a viable justification. Hard part is simply figuring out what can strain the body enough as to make having another child so difficult on a parent that they can't try again for so long, but I think I could work something out of this. $\endgroup$ – dsollen Oct 17 '16 at 14:43
  • $\begingroup$ @dsollen It just needs to be something in the ancestral environment where they evolved. It doesn't have to still be true once they invent agriculture, hairdressing college and telephone sanitisers. For instance, in humans on a subsistence hunting diet in southern Africa, breast feeding suppresses fertility a lot, because in the dry season fatty meats are in short supply, so producing milk is a big effort. The effect is far less in westerners with a copious supply of dairy fats and all the deep fried mars bars they can eat. Phosphate for phospholipids to make the nervous system is a good bet. $\endgroup$ – DrBob Oct 17 '16 at 16:56

The species is simultaneously hermaphroditic (but not capable of self-fertilization) because their ancestors rarely found mates and had to ensure that they would reproduce every chance they got (Source). I suppose their ancestors were solitary tool users similar to cephalopods (Source). By the time they developed sapience and a social structure, their reproductive cycle was too entrenched to go away (similarly to how tetrapods, like us, cannot become hexapods despite the advantages).

In fact, humans are genetically hermaphrodites because males and females both have the genes for becoming male or female (Source). Sometimes these genes are expressed incongruously and an individual will display both male and female traits, but such individuals are generally sterile.


Sexual recombination is a huge evolutionary advantage and the primary driving factor behind the complexity of organisms in which it has evolved. Separation into two sexes that are both required for mating guarantees the inclusion of additional genetic information. As you mentioned, in environments where other mates may or may not be available, hermaphrodism makes use of the benefits of sexual recombination while allowing failsafe reproduction within a single organism. However, the option for hermaphroditic self-fertilization does restrict genetic diversity and therefore development.

If changing conditions resulted in a hermaphroditic species that now found mates readily available, then need for self-fertilization would disappear, but the ability for a single organism to reproduce with itself would continue to be a hindrance to the rate and extent of evolutionary advancement. An alternative solution to two diverging sexes could be periodic sex cycles, where each individual transitions back and forth between the two sexes. The species would gain access to the benefits of sexual recombination, such as the eventual development of social behavior and sapience, while maintaining a hermaphroditic sex system. In fact, this system could actually increase the genetic diversity over binary sex systems, because any two individuals could mate. Variation in each individual's cycle duration would allow any two individuals to reproduce in either configuration at some point or another (A is male and B is female at one point and vice versa at a later point in time).

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    $\begingroup$ What if there was a mechanism that prevented self-fertilization? This isn't necessarily possible in the strictest sense, but if the animal cannot self-fertilize by natural means (in other words, it would require in-vitro fertilization or something else that generally requires technology to accomplish), you might be able to avoid the problem of self-fertilization prior to the rise of civilization. At that point, given the example of humanity, you already effectively have the final form of this creature; selection pressures against hermaphroditism won't have time to act before the story opens. $\endgroup$ – Palarran Oct 12 '16 at 0:50
  • $\begingroup$ the majority of hermaphrodite species can not self fertilize if I recall correctly, definitely a fair number of them. However, sapience seems far less likely to evolve outside of a social situation, at the very least sapience that work well in social groups don't evolve without an extensive period of social organization. Thus I don't think you can jump too quickly from "isolated hermaphrodite" to "social sapient" stage and presume no significant change to the sexual reproduction had time to occur? $\endgroup$ – dsollen Oct 12 '16 at 14:26

I think there is a fair amount of evidence for unnecessary/sub optimal organs sticking around long after the reason they were selected for has passed.

I think all that is required for humanoid hermaphrodites to believably exist is for it to have been selected for at some point in their evolutionary history and never cause enough of an issue to be selected against.

There are enough theoretical variation in environmental and genetic factors given the diversity in the universe that it is fairly likely for complex hermaphroditic species to exist somewhere (given the assumption that complex life forms aren't extremely rare)

Remember earth is one data point and there are many examples of stagnated or sub optimal evolution even here. If you are worried about readers not finding what you come up with reasonable hang a lantern on it and point out that the unlikely often happens one in a million times.


Regular, massive climate changes.

Hermaphroditism would only be preserved in human-like vertibrates if there were regular occurrences that reduced population to the point where individuals found it difficult or impossible to find mates. In this scenario, the ability to reproduce without a sexual partner would allow individuals to repopulate after a population crash leaving only a small number of individuals, without needing to find a partner of the opposite sex. This would be especially beneficial if the environment changed from a very harsh one to a very forgiving one, in which child rearing was very easy, and benefits from having a partner to help raise children were minimal.

One way this could happen is if your creatures evolved on a planet with a long, eccentric orbit. This could produce global seasons far harsher and lasting far longer than the seasons we see on Earth. An intense summer or winter could drastically alter the climate, killing off a large portion of its inhabitants, after which a long spring and summer characterized by a near perfect climate for growing would see rapid blooms of life, fertilized by dead plants and animals killed by the previous winter.

Populations of hermaphrodites, in this scenario, could repopulate more rapidly than those comprised of single-sexed individuals, and would be more resilient to seasonal population crashes. All individuals, in good times, would be able to birth children, increasing the birth rate of their community.

  • $\begingroup$ (Edit: I misread the question and wrote a comment that doesn't apply.) $\endgroup$ – Lostinfrance Oct 12 '16 at 18:09
  • $\begingroup$ This is a very good explanation for maintaining hermaphroditism well into the terrestrial environment. $\endgroup$ – Anonymous Oct 12 '16 at 18:18

What if the species was hermaphrodite due to social evolution rather than natural evolution. Human hermaphrodites exist today, but are commonly surgically altered to be one sex or the other. What if hermaphrodites became popular and desirable for some reason and so became the majority sex.

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    $\begingroup$ There are no human hermaphrodites. There are humans who appear to have both sex organs, but they are only biologically one sex and only one of those organs 'works', if either. They can play the role as male or female, whichever sex they were born as, but not both. It's also pretty rare occurrence and usually not generic so no reason it would become common enough to be a majority. $\endgroup$ – dsollen Oct 11 '16 at 21:00
  • $\begingroup$ Yes good point. However my point is more about the social aspect of a mutation. Is a mutation rare because of natural evolution or society. What if a society encouraged a certain mutation? What if there was a society that bred for certain mutations? $\endgroup$ – jomki Oct 11 '16 at 23:36
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    $\begingroup$ @jomki You can't really breed for new mutations, especially something as wide in scope as hermaphroditism; you can only breed for existing traits. You can breed cows to produce more milk, for instance, but you can't breed them to suddenly have six legs. $\endgroup$ – Palarran Oct 12 '16 at 0:45
  • $\begingroup$ @Palarran I think jomki was implying since intersexed individuals exist they could be bread to spread that trait, though that wouldn't work. Intersexed individuals are not due to genetic mutation as I recall, but rather usually caused by other birth defects that would not 'breed true' into the next generation. Plus, koinophillia implies that there is no way a species as a whole (not just a smaller culture) would ever prefer to spread a rare mutation that causes significant physical change for esthetic reasons, it would feel wrong to them on an instinctual level to see. $\endgroup$ – dsollen Oct 12 '16 at 14:32

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