Why hermaphrodites?

What advantage does a worm gain by being a hermaphrodite?

It doesn’t need to go through the pain of dating to get laid.

Just a WAG, but doesn’t it double the chances of reproduction?

(S)he can have it both ways?

He/she can marry shimself.

Hermaphroditic organisms increase the likelihood that they will pass on their genetic material by making it unimportant what sex their contacts are. If you rarely run into another of your species that can make a difference.

It doubles its chance of a date on Saturday night.

And if you tell one to go @#$% themselves, they CAN!

The real question is why more animals aren’t hermaphrodites. After all, most plants are. Having separate sexes, one of which isn’t able to bear young, automatically cuts the reproductive rate for a species as a whole. (How much it is actually cut depends on the sex ratio.) It would seem to be a very large advantage for a species to be hermaphroditic, like worms and some fish.

Why sex itself exists is pretty well understood: the shuffling of genetic combinations produced by sexual reproduction enables species to adjust to changing conditions. While there are some obligate parthenogenetic animals (e.g. some all-female species of lizards) these do not seem to persist very long in evolutionary time compared to their sexually reproducing relatives. (Some species, such as aphids, while frequently reproducing parthenogenetically, have period sexual generations which enable them to get in the required genetic mixing.) But sexual reproduction can be accomplished fine if all individuals are hermaphrodites.

In contrast, why so many animals should indulge in the luxury of separate males, which do not contribute to the overall reproductive rate of the species (unless males assist females in help raising their own young) remains something of a mystery.

If the OP meant earthworms, they still need to mate with another worm. the male and female parts are far enough apart to prevent self-mating. The worms align, in opposite directions, and form a mucus-like sleeve around both worms. They exchange sperms and eggs, then they depart, without so much as a “Can I call you?”

Right. Very few hermaphroditic animals if any are able to self-fertilize. This would pretty much negate any point of undertaking sexual reproduction in the first place. You might as well just be parthenogenetic and save the trouble.

Although self-fertilization is more common among plants, many species are obligately out-crossing, with various mechanisms to ensure that a plant does not fertilize itself.

I have the idea that monosexuality evolved first in organisms where there wasn’t much difference in investement between producing the different types of gametes. When you simply shed gametes into the water column, it doesn’t much matter if you’re male or female. But if you shed both types you run the risk of self-fertillization. And so monosexuality evolved in multiple lineages as a mechanism to avoid that.

It’s only later that the sexes evolved different reproductive roles…eggs becoming a much larger investment than sperm, parental care, internal retention of eggs, internal fertillization, and so forth.

But it is really weird that so MANY animal species are monosexual.

Actually, that’s a good question: in how many animal lineages has male/female sexuality evolved? Is it a base animal characteristic? (I assume animal sexuality is separate from plant sexuality.)

I’m no evolutionologist, but here’s how I see it:

Start with a hermaphroditic species, where the bearing parent has to invest some time in gestating/nurturing/raising its young. Each individual will therefore only have a limited number of young - no more than 2 * reproductive lifespan / (gestation + nurturing period) - assuming it only fertilizes the other parent at the same time it is fertilized like worms do.

Now a male mutant comes along. It has no such restriction - it can fertilize lots of hermaphrodites if it gets to them early enough. Assuming its mutation breeds true - you get lots of mutant offspring. Continue the trend - what was originally a hermaphroditic population becomes a male + hermaphrodite species.

Somewhere along the line, a female mutation occurs as well - an individual is born which capable of bearing young, but not fertilizing other hermaphrodites. Assume that the mutation makes the female somewhat more likely to thrive than a hermaphrodite - perhaps not having to expend energy creating/maintaining the fertilizing equipment as well as the bearing equipment makes it live longer. Over time, the female mutants will crowd out the hermaphrodites, and you’re left with a male/female species.

Wow! That makes so much sense. The fact that earthworms live underground means that they will be able to mate with any other earthworm they bump into.

But, why are most plants hermaphroditic?

This is unlikely. The female “role” is generally much more expensive energetically, due to producing eggs instead of sperm. You wouldn’t save enough on not producing sperm-producing equipment to make up the difference.

Simultaneously hermaphroditic fish species, which take part in mating bouts in which they alternately play the male and the female roles, in turn shedding eggs and sperm, will attempt to “cheat” by playing the male role more often than their partner does. They will fertilize the others eggs, and then won’t reciprocate by shedding their own eggs.

While some algae have undifferentiated gametes, that differ only in mating strain (that is, there is no distinction between sperm and eggs), I am not aware offhand of such cases in animals. Even very simple multicellular animals such as sponges have differentiated eggs and sperm.

As I said above, basal animal lineages (e.g. sponges, jellyfish) are generally hermaphroditic. Male/female sexuality has almost certainly evolved a number of times, being present in lineages as distantly related as the vertebrates and arthropods.

Here’s an article which discusses some of the advantages of not having separate sexes. (warning: pdf)

I don’t mean to suggest that eggs and sperm will each cost the same amount, just that the hermaphroditic species will invest equal amounts in both sperm and eggs…if eggs are 10 times larger than sperm you make 10 times as many sperm.

Although viewing obligate males as “cheaters” might make more sense. You get enough male cheaters fertillizing hermaphrodites and it doesn’t make sense for the hermaphrodites to continue to produce sperm given that many mating partners won’t produce eggs for them. When you get a species with two sexes like this, the sex ratio will stabilize at some number…if you have too many males it means females have more offspring on average than males, if you have too many females it means males have more offspring on average than females.

So hermaphroditism is vulnerable to cheaters, whereas two sexes has a sex ratio equilibrium where there’s no reproductive advantage to being either male or female.

There is a lot of research being done on sexual allocation, and which in sex role animals should invest the most energy. But in the case you describe, males still have an advantage even if the energy investment in each role is equal, since a male can potentially have 10 times the number of offspring.

Here’s another article discussing some of the issues involved: Sex and the Single Hermaphrodite.