Between a species that is hermaphroditic and a species that has males and females, then what would be the pros and cons of either one from a biological standpoint?

Anything not regarding the biology itself—culture, language, interspecies relations, etc—can be handled on my own. I just want to know the pros and cons of either method—and considering the conditions of their natural ecosystem in which they would have evolved—and determine whether one would be more likely for them to evolve than the other for practical evolutionary reasons.

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    A hermaphroditic animal species (or a monoecious plant species) most usually needs a mechanism to forbid (or at least, discourage) self-fertilization. (Because self-fertilization would negate all the advantages of sexual reproduction, and then why would the species expend energy on sexual reproduction instead of parthenogenesis?) "Whether one would be more likely for them to evolve than the other:" most animal species are not hermaphroditic. No higher animals are. – AlexP Dec 9 at 0:18
  • @AlexP, what's the difference between hermaphroditic reproduction and biological asexuality (if there is one)? I'm thinking the former is likely what the OP's imagining, but the later might be what he's expecting, if I'm remembering my biology correctly. – JBH Dec 9 at 6:41
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    @JBH: Hermaphroditic species work just like any other species which reproduces sexually, it's just that any individual can produce both male and female sex cells. Think flowers; most plants have "perfect" flowers, which have both stamens (that produce pollen) and carpels (that contain ovules). – AlexP Dec 9 at 9:00
  • Possible duplicate of Biology of hermaphrodite species – Sava Dec 9 at 11:16
  • @Dead Knight - I think that this question may be based on a false premise, i.e. that a hermaphroditic species is non-sexual. Could you rephrase it? Are you referring to asexual species? en.wikipedia.org/wiki/Asexual_reproduction – chasly from UK Dec 9 at 17:47

I think the single largest point comes down to specialization. Males and females are, as a general rule, not identical: there are physical differences beyond their reproductive organs. The best adaptations for giving birth, for instance, could weaken one's ability to run or to remain upright (see: the human pelvis, which in women is a compromise between giving birth to large babies and supporting bipedal movement). In a hermaphroditic species, that option of specialization is gone, since all individuals will (barring genetic disorders, deformities, etc.) follow the same body plan. Sure, you could hypothesize that there are different body plans with different strengths and weaknesses, where one is stronger than the others, where one is faster.....where one is better at taking on a particular reproductive role, and then you're on the road right back to separate male and female individuals. If you want to tilt this in favor of hermaphrodites, you're going to need a reason why biological specialization within a species is ill-suited to your world; that's a very tall order.

Another point is the added cost of supporting both sets of reproductive organs. That means more tissue, more mass, which necessarily involves a greater energy/food requirement. Nature doesn't have anything so convenient as the grocery store: if dividing a hermaphroditic species into a two-gender species reduces energy costs significantly (which it does), evolution will favor that outcome. However, hermaphrodites are able to reproduce whenever any two individuals come together: they thus have a wider choice of partners, because they don't have to wait for an individual of the correct gender. In a social species like humans, this would be a trivial advantage, but for a species that generally lives alone or in tiny groups that rarely meet, that broader choice could be vital in having the chance to reproduce at all. The probable civilization and psychology of such a species is an exercise for the reader to determine (and is out of scope in this question).

  • Specialization is for Insects - Heinlein. ;) – Sava Dec 9 at 11:17
  • If specialization was really such an advantage, why do nearly all the species on earth only have two sexes? Wouldn't three or four or five different body types (not even necessarily related to sex), like in some kinds of ants, have been better? – Peter Shor Dec 10 at 13:00
  • @PeterShor You have to contrast it against the problems entailed with multiple sexes. If any of 3 or more sexes can reproduce with any other and have children, evolution will generally weed them down to the most efficient child-bearer/egg-layer/"female" and the most efficient "male"; the other sexes will be eliminated over time by natural selection. If three or more sexes are required to reproduce at all, then you get the problem of having individuals of all the required sexes meeting together to reproduce, which is a significant problem (especially if the sexes are not evenly distributed). – Palarran Dec 10 at 13:22
  • Then clearly specialization is not such a great advantage. If specialization were such a great advantage, then when the third sex became rarer, the fact that it was specialized in a way the other two weren't would make it more competitive, and it wouldn't be eliminated. – Peter Shor Dec 10 at 13:24
  • And many fungi have two or more mating types, which correspond to sexes (some only have one), and as far as I can tell from the web, these correspond to no specialization whatsoever. While specialization may play a role, it seems to be a minor factor. – Peter Shor Dec 10 at 14:57

2-sex reproduction has evolved multiple times, strongly suggesting that there is a good reason for doing so. As for exactly what that reason is, there are a number of theories. Most likely, all of them play some part in the reason why this seems to be the trend among life on Earth.

The advantage of hermaphroditism is obvious - more potential mates for everyone. So I'm going to mainly focus on the advantages of a 2-sex system.

Self-fertilization is impossible

Self-fertilization reduces genetic diversity, which makes a species more vulnerable to predators and parasites. While many hermaphroditic species today have worked out alternative methods to avoid self-fertilization, like the complex reproductive organs of snails or alternating generations in plants, these mechanisms are complicated. The earliest aquatic life generally reproduced by simply releasing their gametes into the water, and splitting the sexes to prevent self-fertilization was one very common strategy.

Many fungi actually have a method of combining the advantages of the hermaphroditic system with the impossibility of self-fertilization by having tens to hundereds of different "sexes", (or mating types) that are functionally identical except that they can only breed with a type not their own. This increases the chances that any fungus they encounter will be compatible while ensuring they are not compatible with themselves.

Male seekers, female choosers

On the most basic and primitive level, the difference between males and females is that females expend a large amount of energy to gestate a child - whether constructing an egg or in a uterus - while males spend less. This simple distinction creates a number of trends with the benefit of maximizing the ability of good genes to spread.

The most basic of these is that the female does not have to expend much energy finding a mate - the males come to her and often compete for her attention. Instead, the female's role is to choose the most genetically fit male, a role that does not require a lot of energy expenditure. This energy can then go towards gestation and producing strong offspring. This also allows a highly fit male to produce a lot more offspring, while unfit males reproduce little if at all. This streamlines the evolutionary process.

Further specialization of roles

Once a basic male-female split exists, it is possible for males and females to specialize in other, more interesting ways. Among many animals where the male brings a "nupital gift", the female gets easy extra resources from her suitors. Among spiders, the female gets an easy extra meal of her suitors. Among mammals, the male often serves as a protector of the group and will have special adaptations for fighting, like horns or large size. Male birds often use their bright colors to lure predators away from the nest while the drab-colored female remains hidden and protects the eggs.

One interesting trend is that the more monogamous a species is, the smaller the difference between males and females becomes, and the more "typical" roles tend to blur - probably because if the male is spending less energy seeking new mates, he can spend more energy raising the young. Many birds share nest-watching and food-seeking duties, and emperor penguins reverse the typical roles by having the male watch the egg while the female finds food. While humans have some of the typical mammalian "male protector" traits like males tending to be larger and stronger, this difference is far less pronounced than it is in other apes.

This would depend if the hermaphrodites still paired off to reproduce, or they were capable of reproducing by themselves.

The big advantage of two distinct creatures combining to produce a third is diversity - more chances for the unique combination to produce a creature better adapted to where it lives, or able to live elsewhere.

It's why sexual creatures evolve so much faster than asexual creatures. Greater diversity in the reproduced DNA strand. Hermaphrodites who reproduced by themselves would be at a distinct disadvantage after several generations, due to the lack of diversity.

There is also some specialization in the gender, generally speaking. I'm on shaky PC ground here, so I'll be careful... My wife pointed out to me that women tend to be better at non verbal communications than men... determining what someone is thinking by their actions. The reason for this is: a lot of women spend part of their life dealing with humans that can't express themselves verbally... small children. This may also be why most elementary school teachers tend to be women... better, certainly more patient, at dealing with small children.

Life on earth displays more than the boring sex reproduction we know : 2 sexs, B only able to reproduce with A and vice versa. If B is as abundant as A, then it has only 50% chances to be able to reproduce with a random individual.

Some organisms however have more than 2 sex types. Let us assume you have 20 sex types equally distributed : A1, A2, A3, ... Each type can make sex and reproduce with all other types and not with its own. Therefore a random organism can reproduce with 95% of the population.

Since sexual reproduction enhances genetic traits mixing and combination it is excellent for adaptation to new environments but from an ecological point of view, it is limited by abundance of your compatible partners. By multiplying sex types I would say that this kind of reproduction is really advantageous.

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  • //I am writing a post-apocalyptic novel (french only)// : Ah, so you're french then? : it might be you just need different (English) words for what you're saying then, sexuality (in the English meaning of the word at least) can't possibly have any effect on genetic traits, I think you may have meant sexual reproduction? – Pelinore Dec 10 at 0:58
  • I beg tyour pardon, you are right, I was thinking of the ecological concept, sexual reproduction indeed – Méridional Dec 10 at 14:54
  • Could you cite a species with more than two sex types? Links to a wikipedia article or scientific name would be preferable, to avoid the language barrier. – hszmv Dec 10 at 19:08
  • Sure, here is a fungal example : en.wikipedia.org/wiki/Schizophyllum_commune And here is a scientific book (look at pages 270-271) : b-ok.cc/ireader/2653930 – Méridional Dec 12 at 9:21

Biology has two types of hermaphrodite species:

  1. Simultaneous hermaphrodites. They are both male and female in one body. For instance slugs and snails.
  2. Sequential hermaphrodites. They start out one sex, then turn into the other. Wrasse (a type of fish) are this type of hermaphrodite.

I'm going to assume you are mainly interested in the simultaneous hermaphrodite type. Both types mix up the genes when mating, just like regular non-hermaphrodite species do. It is sexual reproduction not asexual reproduction.

Hermaphrodite Pros

  • Everyone you meet is a potential mate, not the mere 50% of the population as for 'normal' species. This is particularly valuable if others of your species are rarely encountered. Or if you live in a 'boom or bust' ecology, where you need to reproduce fast when the winter/dry season/famine ends and the good times come.
  • When resources are scarce or patchy, you can conserve your resources and be male when you mate (producing sperm is cheap and fast). When the going is good you can spend lots of your resources and be female when you mate (producing eggs and babies is expensive and slow). This works best if food resources are unevenly distributed across the population (i.e. there are skinny hermaphrodites and fat hermaphrodites at the same time), rather than if the whole area is in the grip of a famine.
  • If everyone has the same body form, society is more egalitarian. Of course, mature, experienced hermaphrodites may still lord it over clueless adolescents or beat up infirm, old hermaphrodites. But if there is no sexual dimorphism, then there is no male/female dominance hierarchy which lead to things like lions bullying lionesses away from their kill. It all starts out as a level playing field. Anyone can potentially become the leader of the pack. (This is of course, all about biology - if you add in culture then being the offspring of the King means you get to be the next king by inheriting his title and wealth).

Hermaphrodite Cons

  • Being 'female', producing eggs and/or babies takes a great deal more time and effort and resources (food you need to eat to manufacture the egg/baby) than being 'male' and producing sperm. The courtship of various mollusc hermaphrodite species is therefore a fight to determine who gets to be male and do the impregnating and who gets to be female and be left holding the baby after they mate. In some sea slugs, they try to bite each other's penis off! (It does grow back later).
  • In resource terms, a mated pair of hermaphrodites who are both pregnant will require more food, territory, etc to support the pregnancy and the offspring than a male-female pair where only the female is pregnant. (The male-female exception would be a species like deer, where the hind is growing a fawn, placenta and milk while the stag is growing new antlers, so both sexes need lots of food to produce several kilograms of new flesh and bone).

Male and Female Pros

  • In monogamous species (most types of birds, for instance), two parents can dedicate their time and resources to raising one batch of offspring. They can split tasks between them - one guarding the eggs or chicks, one fetching food. Two hermaphrodites can also do this, but both have to have smaller clutches of eggs or they will be working harder than the bird couple to feed them. Mum bird lays 6 eggs, each hermaphrodite lays 3 eggs each.

Male and Female Cons

  • In the majority of species where the males are much bigger and stronger than the females, it is for the specific purpose of males beating up other males to prevent them mating. 5% of the males are responsible for 95% of the offspring. 15% of the males are responsible for the other 5% of the offspring. So 80% of the males never win a fight, or never win enough fights and die as virgins. All the females get to mate. Being male is a high-risk strategy. Most mammals have the above sort of mating system: it's called polygyny. (In species where the females are bigger and stronger it sometimes for the above reason, but can also be for a variety of other reasons, such as females being cannibalistic).

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