I'm trying to create alien species that is hermaphroditic, with strong sexual conflict. In essence each being tries to trick or force (by mechanical or hormonal means) its partner to take role of female during copulation, as to avoid costs of bearing the young. Their behavior is in this case is similar to snails or flatworms.

The question is: what kind of environmental pressures favor hermaphroditism over regular sexual reproduction.

Can very unstable environment that requires fast breeding or makes meeting other members of the species be such pressure? Any others?

Update: after reading answers I think I am going to change my Herms.

They evolved from hermaphroditic ancestors. In fact hermaphroditism is now dominant on their homeworld by some evolutionary accident. That removes the problem with establishing how they became hermaphroditic in the first place.

Each Herm has both female and male gonads. During copulation they exchange complex cocktail of hormones trying to suppress sperm production and force ovulation in each other. The winner forces its partner to became pregnant and goes looking for another occasion. It creates evolutionary arms race. Winners produce lots and lots of children.

There was once isolation in time between tribes. Herms' homeworld has short good seasons. Winters and summers are harsh and most lifeforms spent them in cryptobiosis. Now, when climate was tamed this ancestral ability helps Herms in interplanetary travels.

Bidirectional hermaphroditism is the basis of social development. Some subtype of hormonal cocktail may be used to control subservient members of the tribe. Later in industrial age some chemical tyrants may order production and dispersal of such hormones on mass scale. They are called husbands of a nation in symbolic terms. Their tyranny lasts until some of their subjects acquire resistance. Herms' warfare is rather slow, sneaky and eugenic.

There are also other types of social arrangements. Rare parthenogenetic clans specialize in science, philosophy and religion. There is also quite new development - participatory relationships where partners use modern medicines to ensure equal exchange of genetic materiel.

Thanks everybody for help.

Update 2: I forgot to add. Herms are not humanoid. Their bodyplan is roughly similiar to them: https://en.wikipedia.org/wiki/Triops, more vertical, with longer legs. This may change.

  • $\begingroup$ Welcome to Worldbuilding.SE! We're glad you could join us! When you have a moment, please click here to learn more about our culture and take our tour. This question has the potential to be too broad and primarily opinion-based. Are we assuming an Earth-like planet (e.g., Earth, with hermaphroditic humans)? Have you studied sexual dimorphism and its evolution? $\endgroup$
    – JBH
    May 15, 2019 at 16:44
  • $\begingroup$ Understanding how we got to where we are would help you refine your question. $\endgroup$
    – JBH
    May 15, 2019 at 16:44
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    $\begingroup$ Note that sentient beings tend to have very small effective populations, so that the selective pressures need to be very high in order to drive evolution. Remember that evolution is driven simultaneously by natural selection, sexual selection and genetic drift; which of the three forces is the most important depends on population size and the magnitude of the selective pressures. $\endgroup$
    – AlexP
    May 15, 2019 at 16:58
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    $\begingroup$ Honestly, I think that a more difficult question to answer is: what kind of environment favors species whose existence is dependent on the coupling of two distinct, codependent sexes? What advantage do we gain by cutting our mating pool in half? What advantage is mating over splitting like amoeba anyway? It's easy to imagine how a species in our own environment, but which doesn't have our sexual limitations, would flourish better, and maybe even be happier for it. $\endgroup$ May 15, 2019 at 22:27
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    $\begingroup$ @boxcartenant I agree that the "objectively beneficial" observation is inherently faulty, but it's hard to argue with success. If evolution really is pure randomness, then I'd vote that every "save the planet!" style environmental effort isn't just wrong, it's evil, because that assessment makes humanity the single most precious commodity in the universe. A commodity that has little chance of forming anywhere else or in any way. A commodity so rare, that any price paid to prosper it is worth paying. $\endgroup$
    – JBH
    May 17, 2019 at 2:22

6 Answers 6


The primary reason I know for a species to favor hermaphrodism is isolation. In the case of flatworms, the worms do not form social structures, thus favor hermaphrodism because it allows them to reproduce any time that two worms happen to meet while burrowing through the earth.

That said, in animals with a defined social structure, evolution seems to favor a bisexual structure with defined gender roles over a hermaphroditic structure, and creatures with a social structure over creatures without one, as complexity of the creature increases. Humans, for instance, being the most complex life-form on earth, also have the most complex social structure.

You want a world which requires inhabitants to rarely make contact with each other. Perhaps resources are scarce, but the resource spots are slow but steady, allowing only one inhabitant, and possibly a growing offspring to live off it. The world will also be older, because less interactions means slower reproduction, smaller population, and all advances (evolutionary, technological, etc.) would be slower.

The psychology would be unique and possibly aberrant and distasteful to humans. The inhabitants would be able to cope with prolonged isolation, decades on end, and would be inherently distrustful of all other members of the species. If reproduction is built on deception, each member of the species would be lucky to have more than one or two friends, and all friendships would be incredibly platonic in nature. There would be a strong sex drive, stronger than humans possess to ensure reproduction would happen. Lastly, it's also probable that children mature psychologically faster than human children, and do not require protection after a handful of years of development.

EDIT: Also, I recall some an old Sci-Fi which had a race deliberately turn themselves hermaphroditic, but that was so they could reproduce with themselves. If you could come up with a reason why a species would turn themselves hermaphroditic, that could also work. Book in question is Foundation & Earth, but to make sense of what's happening in it, you'll need to read the Foundation trilogy and Foundation's Edge first, and reading The Naked Sun and I, Robot would both be useful

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    $\begingroup$ Kind of makes intelligent hermaphroditism seem unlikely since many of the factors that make intellence a valuable trait also make hermaphroditism loose its value and vice versa. $\endgroup$
    – Nosajimiki
    May 15, 2019 at 18:26
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    $\begingroup$ Absolutely. On Earth, anyway. That why I suggested the environmental and psychological changes. $\endgroup$
    – Halfthawed
    May 15, 2019 at 18:27
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    $\begingroup$ By what basis do you consider humans the most complex creatures on earth? We do have the most complex social structure, and we believe that we are the smartest... by our own terms. It's quite anthropocentric. There are many creatures with many more complex functions and organs than ours. And even more efficient social structures. Just because we can kill each other in the fanciest ways ever known does not make us more "complex" in general. $\endgroup$
    – Oxy
    May 16, 2019 at 6:10
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    $\begingroup$ @Oxy We also have the more complex (sets of) behaviours and build the most complex objects and structures. Intelligence is a difficult concept to define, not to say quantify, but an individual bee is nothing but a simple robot with less programs than a dishwasher, and dolphins have not build any temples. The fact the digestive system of a ruminant is a chemical plant of unbelievable complexity compared to that of humans does not do much for the intelligence of cows. $\endgroup$
    – Rekesoft
    May 16, 2019 at 9:18
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    $\begingroup$ @Rekesoft I already said that our social structure is the most complex. And by some measures, our brain is the most intelligent. Indeed, I do think the human brain is the most complex BRAIN. But we are not a brain. We are much more than a brain, and the parts that are not brain are quite average if not mediocre. Since we are talking about reproduction, more complex and sophisticated reproductive organs, dna (or equivalent) processing and mixing systems, etc, could very well play a much bigger role in the social results of hermaphroditism than brain size itself. $\endgroup$
    – Oxy
    May 16, 2019 at 9:32

So good news, there is a biological term for this, and is found in vertebrates. The actual term is called Sequential Hermaphrodites and many examples have a complex social structures.

Within this classification, there are three catagories:

  • Protandry, in which all members are born male and a dominant member will turn female. Clownfish are the most prominate members of this type, where all fry are male, and the largest adult fish will turn into a female, and will mate with exactly one male fish with a harem of non-breeding males. Should any member of the adult school die, all subescquent members rank up, with the breeding male becoming the new breeding female and the highest rank non-breeding male becoming the breeding male. The female will not reverse.

  • Protogyny, sort of the reverse and more common than Protandry, this is where the species is born female and the dominant member becomes male. The example species from Wikipedia throws a monkey wrench into it, by having two types of males: Initial Phase Male (iMale) and Terminal Phase Male (tMale). All members of this species are born either female or iMale (which looks and behaves as a if it was female, but is incapable of breeding) and the tMale is the dominant breeding fish. It's best to think of both males as a effeminate male and a dude-bro jock male rather than either being a true third gender. Upon the death of the tMale, the next dominant fish will emerge from the most dominant female or iMale in the role.

    • Bidirictional: Very common, the wikipedia example here gives this as a fish that establishes a heirarhcy system where the more dominant members will be male and subordinant will be female. Genders can change as an individual rises or falls in social standing and will occur over time as a new community is formed.

In all cases, the dominant gender is usually also the largest gender, which suggest that, yes, size matters to these species for some evolutionary stand point. The bigger you are, the harder it is to fall to predation, disease, and dissention in the ranks. The behavioral nature of this shift and biological advantages is poorly understood because many specimens demonstrating Sequential Hermphrodism are noted to exhibit a quality of behavior commonly classified as "hating an audience while they fuck" as the scientific jargon refers to it. To be fair, a giant in a scuba suit isn't something most humans fined turned on by, but it does make understanding the relationships difficult to say the least.


From a purely evolutionary perspective, hermaphroditism is only helpful when you live insolation. Supporting both sex organs means you invest more resources into your reproductive system and have more organs that could possibly malfunction causing your death; so, there tends to be a slight evolutionary pressure towards splitting up the genders unless encounters with your own species are rare.

But rare encounters are bad for an intelligent species. A big part of intelligence is being able to coordinate efforts as a group to do more than an individual could do just trying to survive.

But... occasionally we see a species that values a trait so much that evolution pushes it to the point that it is detrimental to the species like over-sized antlers or a peacock's tail. I think the most likely case is that you have a very social, intelligent, two gendered species much like humans, but without our conflictive nature. Instead of viewing sextual and genetic differances as a point of conflict, this species sees differences as a good thing that they are genetically predisposed to embrace so when a random mutation does create a hermaphrodite, as it sometimes does in people, this person is prized by potential mates as being better for embodying both genders. They find it easy to join with the best mates and have many children creating more hermaphrodites who also find the best mates and have many children.

After many many generations, single gendered children would become rare compared to the hermaphrodite gene. Once hermaphroditism becomes the norm, strong sexual conflict might emerge as a cultural norm, but once this begins to take hold, the environmental pressure will again be in the direction sexual separation and their society would lose a lot of its cohesion; so, I would not expect this to be a very sustainable situation for this species.


There's no clear understanding of what environmental issues may favour hermaphroditism over normal sexual differentiation; for instance, if you consider the fish that demonstrate it, there's no obvious differences in environment or behaviour compared to other fish that live around it. The blue-banded goby (Lythrypnus dalli), for instance, isn't much different from other small reef-dwelling fish that would explain why it is a bidirectional sequential hermaphrodite and they aren't. It may simply be that it is just because it is.


Sibling groups.

  1. Intelligence is an outgrowth of sociality. Your species is intelligent because it lives in large family groups of a mother and siblings; these are all littermates from the same gestation. The siblings (led by the mother, and then together if it dies) work cooperatively to forage / hunt etc. Siblings are not potential mates with each other. Cooperativity is selected because the gene is the fundamental unit of fitness and these siblings are closely related, all with the same mother and father.

  2. The cost of being a mother is that it must forgo the help of its siblings and strike out as a new group with the new offspring once they reach reproductive age. Nephews / nieces are potential mates but that is less than ideal and so some distance is evolutionarily advantageous.

  3. Individuals will go on what amounts to raids as males, attempting to impregnate an individual in a different group. Reproductive age individuals will usually be in the company of their sibling group and so will be defended. It is very possible the tables might be turned on a would-be male who will return to its group impregnated. Siblings will help each other in these raids; a successful sortie as a male means success for the siblings too.


Do you want this species to come from a planet where sexual dimorphism exists in some other species? If that's not necessary to you, you can just posit that sexual dimorphism never occurred as a variation on this world. Natural selection only comes into play once a variation exists.

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    $\begingroup$ Sexual dimorphism is so valuable from an evolutionary standpoint that hermaphroditism is an unstable equilibrium in almost all environments. Considering how easy it is to get a single-gender individual, "never occurred" isn't a believable explanation. $\endgroup$
    – Mark
    May 15, 2019 at 20:03

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