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If a hermaphrodite animal (like slug, snail, etc) finds a partner they can mate immediately.

If another animal with "normal" reproduction (lets say a mouse) finds a partner they can only mate if they are opposite genders.

So it seems logical that the hermaphrodite way of reproduction is more successful than the "normal" way.

But it is not, as far as I know all higher developed species are using, the standard way of reproduction (male and female).

Why? What are the disadvantages of hermaphroditism?

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    $\begingroup$ The group of fish known as hamlets en.wikipedia.org/wiki/Hamlet_(fish) are simultaneous haermaphrodites, like snails. Many other fish are sequential hermaphrodites, but that is probably outside the scope of your question. $\endgroup$
    – Alan Boyd
    Nov 7, 2012 at 9:27
  • $\begingroup$ in brief - its quality not quantity. sort of like asking 'why do we bother getting married and buying houses when we could just hook up at a bar every weekend?' hermaphrodites can reproduce more easily, but other organisms invest more in their offspring and also often have a harder time getting around to mating. no judgments here, just trying to make it pretty plain. $\endgroup$
    – shigeta
    Nov 7, 2012 at 20:15

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Firstly I'll clarify that you are talking about simultaneous hermaphrodites rather than sequential hermaphrodites (1st one sex, then the other e.g. the limpet Patella vulgata).

It is perhaps easiest to address the question by countering it and asking why dioecy (2 sex systems/2 gonochoric types e.g. male and female) is better? As you have pointed out there are obvious advantages to being a hermaphroditic species such as more chance of mating - more likely to provide an advantage at very low population densities where interactions are infrequent.

There are two key disadvantages of hermaphroditism which I will briefly cover but have been discussed in this paper and probably other costs.

The first is energy costs. Maintaining the capacity to produce male and female gametes will be more costly than maintaining one. This gives the hermaphrodite a fitness disadvantage because energy is rarely an infinite resource. Therefore at higher population densities, when mating opportunities are not rare, the gonochoric individuals will have a higher fitness because they have more energy. Monogamy is also analogous to rare encounters but true monogamy is rare (1 partner for life).

The second disadvantage of hermaphroditism is self fertilisation. This will cause an increase in homozygosity and lead to inbreeding depression (reduced fitness).

So you are right to some extent...

the hermaphrodite way of reproduction is more successful than the "normal" way.

...but the conditions which give rise to an hermaphroditic advantage are restricting. Overall, the above costs, combined with the obvious complexity of evolving the ability to produce male and female gametes, the ability to both fertilise and be fertilised, pregnancy and birth, and mating systems, mean that it is often more beneficial to be a dioecious species. Thus dioecy evolves.


EDIT: Question Raised by @Single_Digit

I have been pondering this question for a while and I get what RG255 is saying. I'm just not sure I entirely buy it. Take earthworms, for example. They are simultaneous true hermaphrodites (as far as I understand). The anatomy doesn't have to be that complex*. They simply have two genital openings (one for eggs and one for sperm) and they line up in a "69" (excuse the vulgarity) position. This should, in theory, minimize the inbreeding depression. However, it doesn't eliminate the maintenance of two sets of reproductive systems. But most organisms are not internally fertilized mammals with wildly complicated systems of internalized embryonic care. Most species lay a pile of eggs that a male squirts sperm on or squirt eggs while the male squirt sperm and then they hope for the best.

I would think the advantages of simultaneous parenting (after all, many MANY species' males don't provide much in the way of child care) and its fitness advantage would vastly outweigh the burden of a second set of reproductive organs.

With that said, I don't have a better explanation, but I find the question a very interesting one. The linked article is pay-walled, aside from the abstract, but I still disagree with some of its tenets. To me true hermaphroditism should be very common (I realize it isn't) in species that don't need two parents to raise offspring, but do benefit from some (as in one parent's) parental care. I recognize that it would do little to help species that merely dump gametes and leave because specialization of one reproductive system would likely do the job better and both genders equally contribute under that type of system.

So, RG255 convince me! Clearly there are good reasons, since gender (or asexual repr) is the norm, but I need more/better evidence.

    • Yes I realize they would need separate internal anatomies for each type of gamete, but still...

My response:

You have presented one example of hermaphroditism and used that as evidence that all species should be hermaphrodites. Earthworms are small slow creatures living in soil, I don't imagine they have high rates of encounter, and therefore low rates of encountering the opposite sex, therefore hermaphroditism would be favoured as discussed above.

Further, you say most species are external fertilisers (do you have a reference for this?) and therefore it is not costly be a hermaphrodite. I don't see your logic there, the cost is not necessarily to do with the cost of bearing child, producing & maintaining the gonads and gametes is also a costly process. I would argue that this is extremely complex. This is not just on a morphological level but also physiological: in non-hermaphroditic species the sexes have very different, and often, conflicting gene expression and hormone production patterns. Hermaphrodites would not be able to optimize to the fulfilling both the male and female roles.

Finally, you pointed out that the worms do not inbreed. Inbreeding avoidance does not have to be the cause of the hermaphroditism persisting, if the environment/other factors favour hermaphroditism. I never said that both were simultaneously necessary.

I hope this clarifies it for you, if not please expand as to why, I am on here because I want to help people understand biology properly!

Further response from @single_digit:

Well fair enough about my external fertilizers comment. I don't have a reference, but I was thinking all multicellular life and I'd have to imagine that when you factor in plants, that external fertilization is relatively the norm (as is hermaphrodism (dioecy) for the plants). As to earthworms, I disagree about your description of them. Their densities are actually pretty high, so I'd wager they encounter each other frequently, so I'm not sure where that leaves them in terms of pressures for hermaphroditism. Your point on the physiologic/hormonal issues of maintaining the systems is one I haven't previously considered. I honestly don't have any clue as to how daunting (or simple) that is, but I'd imagine that the sophistication of the systems would play a pretty key role. Makes me wonder how much this has been researched in true hermaprhodites. I suppose the main thing I keep coming back to is the overwhelming disadvantage gender has in terms of potential to create offspring. Males in many (most?) species essentially act as little more than sperm donors, thus half the individuals have effectively zero fitness. That just seems like an overwhelming advantage for hermaphrodites.

My Response

Why do you consider half of the individuals to have zero fitness? Fitness is widely accepted as the number of offspring a parent produces because this is directly related to number of copies of their genes passed to the next generation. Sperm donor type males achieve increased fitness by mating as do females - with out the male they would never be fertilized. The key disadvantage of dioecy is the halved (assuming equal sex ratio) frequency of potential encounters that could lead to mating. The general disadvantages of hermaphroditism are inbreeding depression and high cost & complexity.

Single_digit:

Zero fitness isn't exactly correct, but if we look at parental care as conveying a survival advantage for K selected species, and huge numbers of offspring conveying an advantage for r-selected species (obviously the type of env affects this) does a deadbeat dad really optimize for either of these? Passing on genes is fine, but if offspring survival is low, does it matter? Does it simply boil down to the maintenance of two repr systems plus decreased fitness from inbreeding vs the increased reproductive success from extra child care? Or is there more?

My response:

r/K selection theory has generally been disregarded in the evolutionary biology community due to the substantial evidence against it so it is unhelpful to think of selection in this way. As long as the 2 sexes strategy is more successful at passing on genes than a hermaphroditic strategy it will (should) prevail. Dioecy will be more successful if the hermaphroditism introduces to much cost through production and maintenance of sexual organs/gamete and inbreeding whilst not attaining substantial gains from higher potential mating frequency.

@Single_Digit

Interesting about r/K selection. I hadn't heard that. Do you have any links? I'd be curious to learn more there. I incorrectly earlier made a comment about dioecy where I meant monecy. But this seems to beg the question, why is monecy/hermaphroditism so much more prevalent in plants? Obviously there are different survival pressures, but I'd think the same basic principles would apply as in animals, but the condition seems to be far more common than in animals.

My Response

I seem to remeber there being a reason in plants, don't have time to look it up right now. The work about r/K selection was Reznick/Stearns/Charlesworth. Reznicks is the most recent and more overview type paper - best place to start: http://www2.hawaii.edu/~taylor/z652/Reznicketal.pdf

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  • $\begingroup$ also want to add that hermaprodites are not found in larger animals because the reproductive system becomes more complicated and difficult to maintain. sexual dimorphism is a much better advantage when you have live birth with amniotic sacks, umbilical cords etc. $\endgroup$
    – shigeta
    Nov 7, 2012 at 18:34
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    $\begingroup$ @rg255 Thank you very much, this is a great answer! Sorry if my question does not contain many hard-to-spell words but I'm not a biologist and my first language isn't english, I'm just curious about everything :) And yes you're right, I was talking about simultaneous hermaphrodites. :) $\endgroup$
    – Marc
    Nov 8, 2012 at 0:46
  • $\begingroup$ That's fine, it was easy to understand! great question, really got me thinking & I study sexual selection! $\endgroup$
    – rg255
    Nov 8, 2012 at 9:05
  • $\begingroup$ belated reply, but I think that sexual dimorphism is the bigger motive. It's easy to evolve ways to avoid accidental self fertilization, and production of male gambits for a female is a relatively trivial addition to her energy investment in eggs, so even a minor mating chance as a 'male' would warrant the cost to hermaphroditeism over pure female. The big distinction is that males could evolve to be even better males, better compete for rights to make with multiple females etc. A strong hermaphrodite will always choose to mate as male over female, so why not adapt to be better at that role $\endgroup$
    – dsollen
    Jul 21, 2015 at 17:17
  • $\begingroup$ The book "The selfish gene" gives an answer for you to get ideas from to make your answer. It's obvious that for almost any species that reproduces sexually, it wouldn't be any less advantageous to produce a kid the same way but asexually. For each sexually reproducing species, if it evolved to reproduce asexually, it would be an advantage to do away with males if the species had separate genders and do away with the male parts the species is hermaphrodite. I think it would be better for you to explain that and then answer why not all sexually reproducing species are hermaphrodytes. $\endgroup$
    – Timothy
    Jun 20, 2016 at 2:02
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I'm not at a biologist, but I have a pet theory I'd like to throw into the ring: hermaphroditism tends to be unstable in a similar way to strong sex skews. Imagine species A where 90% of offspring are female; at moderate population density, the 10% male population will be more than sufficient to maintain the females, and having 90% of the population able to breed will out-compete another species with a 50/50 distribution. However, a mutation which tends to produce more males will tend to become more common, because the males of species A reproduce much more often than the females. Eventually, this stabilizes at roughly equal frequencies for the sexes.

Back to the hermaphroditic species C, imagine that a mutation led to a variant which had a flawed female reproductive system, meaning it could not lay as many eggs when fertilized, but it had some other advantage which made it better at mating: perhaps by diverting energy from the female systems, it was slightly larger and stronger and could intimidate others, or perhaps it put this energy into a stronger immune system. Eventually, the population evolving in this direction could lose its ability to lay eggs or birth young entirely, and instead focus entirely on being better at finding mates to impregnate.

Looking at it from the other side, if some individuals de-prioritize that side of things and instead concentrate on being the best egg-layers and care-providers possible, that could be a stronger evolutionary position than trying to be equally good at both roles. Eventually, this split would produce recognizably male and female subgroups.

If this idea has already been proposed and either taken seriously or shot down, I'd appreciate any pointers to further reading.

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I have been pondering this question for a while and I get what RG255 is saying. I'm just not sure I entirely buy it. Take earthworms, for example. They are simultaneous true hermaphrodites (as far as I understand). The anatomy doesn't have to be that complex*. They simply have two genital openings (one for eggs and one for sperm) and they line up in a "69" (excuse the vulgarity) position. This should, in theory, minimize the inbreeding depression. However, it doesn't eliminate the maintenance of two sets of reproductive systems. But most organisms are not internally fertilized mammals with wildly complicated systems of internalized embryonic care. Most species lay a pile of eggs that a male squirts sperm on or squirt eggs while the male squirt sperm and then they hope for the best.

I would think the advantages of simultaneous parenting (after all, many MANY species' males don't provide much in the way of child care) and its fitness advantage would vastly outweigh the burden of a second set of reproductive organs.

With that said, I don't have a better explanation, but I find the question a very interesting one. The linked article is pay-walled, aside from the abstract, but I still disagree with some of its tenets. To me true hermaphroditism should be very common (I realize it isn't) in species that don't need two parents to raise offspring, but do benefit from some (as in one parent's) parental care. I recognize that it would do little to help species that merely dump gametes and leave because specialization of one reproductive system would likely do the job better and both genders equally contribute under that type of system.

So, RG255 convince me! Clearly there are good reasons, since gender (or asexual repr) is the norm, but I need more/better evidence.

* - Yes I realize they would need separate internal anatomies for each type of gamete, but still...


Well fair enough about my external fertilizers comment. I don't have a reference, but I was thinking all multicellular life and I'd have to imagine that when you factor in plants, that external fertilization is relatively the norm (as is hermaphrodism (dioecy) for the plants). As to earthworms, I disagree about your description of them. Their densities are actually pretty high, so I'd wager they encounter each other frequently, so I'm not sure where that leaves them in terms of pressures for hermaphroditism.

Your point on the physiologic/hormonal issues of maintaining the systems is one I haven't previously considered. I honestly don't have any clue as to how daunting (or simple) that is, but I'd imagine that the sophistication of the systems would play a pretty key role. Makes me wonder how much this has been researched in true hermaprhodites. I suppose the main thing I keep coming back to is the overwhelming disadvantage gender has in terms of potential to create offspring. Males in many (most?) species essentially act as little more than sperm donors, thus half the individuals have effectively zero fitness. That just seems like an overwhelming advantage for hermaphrodites.

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  • $\begingroup$ You have presented one example of hermaphroditism and used that as evidence that all species should be hermaphrodites. Earthworms are small slow creatures living in soil, I don't imagine they have high rates of encounter, and therefore low rates of encountering the opposite sex, therefore hermaphroditism would be favoured as discussed above. $\endgroup$
    – rg255
    Nov 26, 2012 at 21:00
  • $\begingroup$ Further, you say most species are external fertilisers (do you have a reference for this?) and therefore it is not costly be a hermaphrodite. I don't see your logic there, the cost is not necessarily to do with the cost of bearing child, producing & maintaining the gonads and gametes is also a costly process. $\endgroup$
    – rg255
    Nov 26, 2012 at 21:01
  • $\begingroup$ I would argue that this is extremely complex. This is not just on a morphological level but also physiological: in non-hermaphroditic species the sexes have very different, and often, conflicting gene expression and hormone production patterns. Hermaphrodites would not be able to optimize to the fulfilling both the male and female roles. $\endgroup$
    – rg255
    Nov 26, 2012 at 21:01
  • $\begingroup$ Finally, you pointed out that the worms do not inbreed. Inbreeding avoidance does not have to be the cause of the hermaphroditism persisting, if the environment/other factors favour hermaphroditism. I never said that both were simultaneously necessary. $\endgroup$
    – rg255
    Nov 26, 2012 at 21:02
  • $\begingroup$ I had to break that up in to small sections because there is a character limit. I hope this clarifies it for you, if not please expand as to why, I am on here because I want to help people understand biology properly! $\endgroup$
    – rg255
    Nov 26, 2012 at 21:04
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It's actually quite simple if you consider the following two points: 1.Males only have to mate with TWO MORE individuals in each reproductive cycle to outcompete a hermaphrodite. 2.mobility is important and this gives male-only animals a big advantage.

I can't see anyone clearing expressing this already but apologies if someone has. Simple animals generally produce a lot of eggs to pass on their genes to as many offspring as possible. However this slows them down, when looking for a mate, in spending time laying their eggs, and in expended energy producing the eggs. In such a hermaphroditic population, eventually a male will randomly evolve that doesn't bother to produce eggs. Providing that this gives it an advantage over its hermaphroditic counterparts, such that it can mate with at least two more egg-laying individuals in the time taken for the hermaphroditic version to produce and lay its eggs, then this male-only version will produce more offspring sharing its genes than the hermaphrodite would. This means that more of the next generation will also be male only.

This ensures that males always exist (at least at some percentage of the population), as chances are they will mate with many more individuals than their slower hermaphroditic competitors and pass on their genes more successfully. Simple.

(This is probably also the reason that most plants are hermaphroditic, as mobility is not a factor.)

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    $\begingroup$ Welcome to BiologySE. We always appreciate references to accompany answers as a way to add legitimacy as well as allow other users less familiar with the subject to read for more information. $\endgroup$ Apr 1, 2017 at 12:44
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This is a bit expressed in an unclear way but I think it solves the problem.

I think the reason is simply that if you have all the hermaphrodites being difficult about their mates they will evolve fast because the best mates will mate more and the worst won't mate a lot. But if the hermaphrodite individual is hard to get, it also won't be motivated to pursue sex and it will be hard for the 'best' individuals to mate more cause they will be too hard to get. So the requirement to impress while also being hard to get are contradictory in non differentiated individuals. In most animals, the difference in libido of the female versus that of the male can solve this paradox. One has to show they have good genetic material (the male) and the other just have to be hard to get. This is pretty shitty for females, but nature often is. I think the fitness of this difference almost disappears in monogamous species where both have an advantage if they are hard to get. So we can hope hermaphroditism is not an advantage only for an intermediary stage of evolution but can become one again in higher species. This would alleviate some of the sexual conflict.

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The non-hermaphroditic sex systems I'm most familiar with arise from hermaphrodite populations. Suppose a male-only mutant arises, it can cheat the hermaphrodite system by putting its reproductive energy into the "cheap" sperm and fertilising many eggs, instead of sharing the energy burden of expensive eggs and cheap sperm. the male-only out-competes the hermaphrodites and so over generations this male-only trait spreads. the male + hermaphrodite population has an excess of males, selecting for any female-only mutant to do well. Suppose a mutant female-only arises. She Put all her energy into egg production, so there are more and/oor better eggs. A female-only outc-ompetes a hermaphodite if there is an EXCESS OF MALES. A female-only does less well than a hermaphrodite if there is a male/female balance

So, in hermaphrodite species, we see avenues to the formation of a male-only and female-only subpopulation

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    $\begingroup$ Hi Stephanie! Welcome to Bio.SE! Please update your answer by citing reputable source(s) of support for your claims. Otherwise, your answer is likely to be downvoted or deleted. Thanks! $\endgroup$ Dec 31, 2018 at 3:45
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The female & male organs compete against each other. A stud hermaphrodite has little incentive to have female organs. A human stud who can have 1000s of kids would be slowed down considerably if he\it had to spend 9 months\year being pregnant & raising infants & it's costly to maintain female organs.

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