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I'm inspired by the comments discussion on a (now deleted) question by XandarTheZenon. We're preconditioned to think in terms of a 2-sex lifeform, which is natural with diploid genetic material, and sexes that specialize with different reproductive strategies. This often leads to species that have sexual dimorphism. (At the very least, the sex organs are dimorphic.)

But that's not alien enough.

Why else would an animal-like species have different body plans within one species? In particular, it would not have the male/female division that Earth macrofauna life has. Reproduction could have a different number of sexes, or at least doesn't have the same kind of division where one partner invests most of the energy and sustenance and the other is non-essential to development, with different body plans and behavior evolving from that original source of difference.

How can an alien species have more than one body plan (and behavioral traits to match) yet individuals of all types are needed to make a family?

A human biologist would not find it immediately obvious that these are the same species at all. But there's a sound reason why there are different forms and furthermore they are all necessary as part of a family unit, rather than just being different lifestyles.

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    $\begingroup$ Do ants and bees count? You have the female queen who lays eggs, the short-lived males who fertilize the eggs, and sexless female workers. $\endgroup$ – Marion Apr 4 '16 at 3:05
  • $\begingroup$ I think the reason that most creatures have two sexes is because it is the best number of organisms to have genetic diversity with less risk of disorders. That or God wanted it that way. I personally prefer the latter. Ants and bees are different though. $\endgroup$ – Xandar The Zenon Apr 4 '16 at 3:36
  • $\begingroup$ I think 2 sexes is due to having diploid chromosomes. That goes back to primitive microbes, long before plants and animals (and fungi). I suppose 2 copies, allowing recessive genes and providing for diversity, is minimally "multi copy" and that turned out to be better overall than having 3 or more. $\endgroup$ – JDługosz Apr 4 '16 at 4:51
  • $\begingroup$ Rhink about disruptive phenotypes. Depending on which randomly selected chromosome you get you're either a white flower or a red flower. Also under some circumstances it's helpful to have this. For example big fish can fight good and small fish can hide well while middle sized fishes cant do any of it and thus die. It's still the same species but some are small and others are big because both have mating success while the midsized one cannot keep their genes inside the population becaude they do not have mating success and/or die too quick $\endgroup$ – BlueWizard Apr 4 '16 at 5:38
  • $\begingroup$ @JDługosz I think there are some plants who have 4 or 6 copies. They need to be divisible by 2 because otherwise the meiose wouldn't work. Or at least that's what I know. $\endgroup$ – BlueWizard Apr 4 '16 at 5:40
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The more social a species is, the more beneficial polymorphism becomes. A good example for extreme social polymorphism would be termites. Not only do they have the queen/male/worker division found in ants and bees, some species have many different kinds of specialized soldiers, ranging from melee fighters with big mandibles, guards with oversized heads that act as living doors, acid-spewing ranged attackers, and exploding chemical kamikaze bombers. Workers can include foragers, tower builders, diggers, nurses that take care of the children, grooms that tend to the king and queen, and farmers that cultivate fungi. Some species can have over 10 distinct castes!

A eusocial intelligent alien species might have even more specialists — a scientist caste, a military strategist caste, a communications caste, a data entry caste... Really, any specialized jobs we have as humans could potentially lead to a specialized caste in such a species.

Polymorphism can also allow different members of a less-social species to specialize in different lifestyles (consuming different kinds of foods, for instance) in order to minimize competition. However, this situation is often unstable in an evolutionary sense — unless it is the male and female morphs diverging into different lifestyles but coming back together to reproduce, it will often result in the morphs diverging into separate species.

Reproduction rarely requires more than two sexes. The main function of sexual reproduction is to recombine DNA, which you get just fine from two sexes. The minor benefit you get from adding another DNA source in the mix generally doesn't outweigh the complexity of having to arrange a threesome every time you want to reproduce. That doesn't mean it can't happen, but there's a good reason why on Earth at least, one to two sexes is the norm.

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    $\begingroup$ Diverging into different species: that's why casts are found in social insects: all the casts are produced by a queen, rather than among themselves. $\endgroup$ – JDługosz Apr 4 '16 at 13:45
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3 different members needed for reproduction

Perhaps not exactly 3 genders, but here on Earth we have many examples of 3 different organic entities needed for a successful reproduction: for example, the wasps of the genus Glyptapanteles, whose females inject their eggs into living caterpillars. There you need for the reproduction a male wasp, a female wasp, and a third entity that performs the incubation and feeds the babies (the caterpillar).

Yes, this is a dramatic example (the caterpillar dies), but it is not so bizarre to think of an alien species needing 3 elements to be involved in the reproduction process: two of them providing the ADN (or its equivalent) and the third one only providing the incubation (hopefully without being damaged in the process). In that case, you could have the same species but with 3 different specialized functions: the male, the female, and the carrier, and you would not be able to create a baby without any of them.

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    $\begingroup$ "this is a dramatic example (the caterpillar dies)" Not really. All sorts of species from octopi to spiders eat the male after the egg has been fertilized. I was just thinking about exactly an incubating but not genetic material contributing scenario this morning before reading your answer. $\endgroup$ – ohwilleke Aug 17 '18 at 23:22

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