How can my sapient sea-star centaurs have four different sexes?

Question

The Cochlequestrians (from the Latin for “snail horse”). are a sapient species much older than humans. They have much more advanced technology than other sapients. They resemble Pierson’s puppeteers to a degree:

Image owned by Wayne Barlow

Imagine a horse with the head of a brachiosaurus, the skin of Mewtwo, no hooves or tail, and elongated humanoid arms.

One very interesting classification of theirs is that they have four sexes: child (kru), man (hin), woman (ulhin), and senior (zudra). Children don’t have reproductive organs until they reach their equivalent of puberty, where these organs grow in. In old age, the reproductive organs atrophy.

Is the situation echoed in Earth biology?

Edit: This system is genetic, not by choice. All kru have an even chance of being hin or ulhin, but can’t change their sex.

Edit 2: Kru have NO genitalia. Hin and ulhin do. Zudra still do, but they do nothing and often get surgically removed for convenience. Cochlequestrians have separate excretory organs and reproductive organs. And this system is about biological sex, not gender identity. And Cochlequestrians cannot change their biological sex without surgery or drugs.

Answer 1

So first things first, gender is a product of language, not of biology. The biological term for this is "Sex" as in "All Peafowl with large tail feathers are members of the male sex of the species."

While it's certainly possible that an animal can be born without developed genatalia and will develop as they mature, and many species are capable of changing their sex due to environmental factors, none of these qualify as a third sex (true gender shifting rarely is reversed (i.e. a male becomes a female and later becomes a male) though it's not unknown. You also have cases where some males develop characteristics typical of sexual dimorphism within their species while others develop more feminine characteristics (but retain male genitalia). This often occurs in species where males compete to breed with females in social structures. The more effeminate males develop this way to get close to females and impregnate them without having to compete against other males that have an overwhelming edge.

That all said, no biologist would describe this as a "third" or "fourth" sex, but rather a metamorphosis. After all, menopause occurs in human females at a certain age, but nobody claims that a women who goes through this process is "no longer a woman" (Or if they do, they likely quickly learned why this is a bad idea and never do it again... providing they survive the first time). Similarly, many insects have larval stages where they look nothing like their adult forms. While I will not look for information on whether you can determine a caterpillar's sex or not, (feel free to do that yourself. I'd like to not have anything related to "Caterpillar genitalia" in my search history, thank you very much.) I do have suspicions that, given the radical transformation from juvenile to adult forms, it's safe to assume that there are a lot of differences between a caterpillar's genataila and a butterfly's. Almost every animal will have a life cycle period where they are unable to reproduce for a period of time following their birth, yet no species is considered to have a "third sex" during this period.

As a rule, evolution favors efficiency (we are not Pokemon. There is not a "final" evolution of animals, there is just evolution). Sharks developed well before the Dinosaurs and didn't change all that much. Turns out, they outlasted a lot of sea life and thus had little reason to evolve. As such, sex is not the most efficient way to reproduce (that would be mitosis) but it has advantages over the other forms of reproduction in ensuring genetic diversity. Most of it's inefficiency comes from the fact that, well, it takes two to tango. So the more partners required, the more in-efficient the process becomes, and that inefficiency does not translate to any new benefits over a simpler "two player mode" while "multi-player" reproduction is way more fun than single player mode.

Another reason classifying these stages as biological sexes is not ideal is the that the juvenile and the elder do not take part in procreation. That is only possible by the adult male and the adult female. And in biology, the truest test of Male or Female is to look at the chromosomes, as all sex in every species are determined by at least one pair of chromosomes (Not all XX/XY, that's a mammal thing. Many birds, reptiles, fish and insects use a ZZ/ZW system with the chief difference being the ZZ is a male and ZW is female.). And the Duckbilled Platypus has it's sex determined by 10 chromosome pairs (because of course it does) that are still looked to all XX pairings or all XY pairings. (I'll admit that I do not know if species that change gender some how change their chromosomal pairs, but in all likelihood, that isn't happening as the changes are normally triggered by environmental factors).

In short, the species is possible on a sex level (Similar to not having three or more sexes, having two or more heads is not efficient in an evolutionary sense which is why it rarely happens, and when it does so, it rarely survives for long in the wild, but that's out of scope of this question.). But it would not be classified as a creature with four biological sexes. That said, as gender is a construct of language, not sex, if the creature is self-aware OR named by an intelligent species that recognize more than two genders (there are some societies where this happens) than they may speak of life stages in terms of gender... but it's not biological or scientifically creating sexes beyond the two.

Answer 2

Primitive Gonads

Look at a foetus young enough and you cannot tell if it's a boy or a girl. It does not have a penis or vagina. It does not have testicles or ovaries. It has a pair of so-called gonads. When the foetus gets old enough, hormonal messages tell the gonads whether to turn into testicles or ovaries, and to build the rest of the genitals accordingly.

Your species develops the same way, except the gonads remain unspecialised until puberty. (It still needs somewhere to do peepee though. Do your sea stars do peepee through their genitals? Do they do peepee at all?) Only then do hormonal changes make the gonads develop into their equivalent of testes or ovaries.

Hormones also alter the general body shape, so hin and ulhin can be told apart from kru at a glance. This is also realistic. It happens in humans. That's why all the boys and girls from Peanuts look the same:

But Johnny Bravo

looks nothing like Meg from Hercules:

If you were to check the chromosomes (or equivalent) of a kru you could tell which of the two adult sexes they will become when their gonads descend fully. But they are not there yet.

The zudra happen later in life and are the equivalent of the menopause or andropause in humans. The different hormones gradually modify the adult body to remove any sexual dimorphism. This is perfectly realistic. Check out Hormone Replacement Therapy in humans. Over several years it can change your face and body shape to look like an entirely different person.

Answer 3

So, after extracting the distracting politics d'jour,³ what you have to begin with is a pre-pubescent juvenile, a fertile male, a fertile female, and a post-fertile adult. A great many terrestrial species are like that. Humans are like that.

Take it a step further, and probably more along the lines you're looking for: a sexless¹ pre-pubescent juvenile and a sexless² post-fertile adult.

The first exists: Cetomimidae. The juvenile tapefish is sexless until it reaches puberty and morphs into either a bignose or a whalefish.

The second, on the other hand... I know of no terrestrial creature, great or small, that has an adult phase of sexual development that become sexless after fertile sexuality. All instances I could find depended on the creature being asexual to begin with (hermaphroditic or parthenogenetic). When such a creature reaches the stage in their life that they are no longer fertile, they are sexless.

But that's not what you appear to be asking for. You appear to be asking for a creature who was once a fertile male or a fertile female becoming sexless. Unless by "atrophy" you only mean "shrink from disuse" rather than actually disappear. The former, IMO, may exist through suspension of disbelief. The latter is much harder for me to blindly accept — but not impossible.

Conclusion

I believe there is enough terrestrial evidence to fully rationalize the juvenile and adult life stages of your creatures. I don't believe there's enough to fully rationalize the third and final adult life stage.

However, just as the Pierson’s Puppeteers you use as your reference are, from the perspective of terrestrial life, wholly implausible and yet capable of suspension of disbelief (I loved the books...), I think your creature idea is just as capable of suspension of disbelief.

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_{¹ I am not using the word "sexless" in the way biologists do. Nor am I using it in the way modern sociopolitical thought seeks to. I am using it only to reflect the idea of "undeveloped sex," meaning that the stage of development precedes fertile adulthood and no sexual organs have developed. Additionally, there is no way to know which sexual organs will develop until pubescence when the body, based in this case on unstated rules of the OP's world, elects one sex over the other (and we'll ignore the imperfection of life for the purposes of this answer).}

_{² I'me diverging even further with my use of "sexless" for post-fertile adults. A sexless post-fertile adult could be nothing more than an adult that no longer engages in sexual behavior. At the extreme, a sexless post-fertile adult might be one where the sexual organs are re-absorbed into the body such that the result strongly resembles (if not mirrors) the sexless juvenile. The OP's use of the word "atrophy" doesn't make the end result entirely clear, but at least acknowledges that the post-fertile adult life stage is one where participation in sexual behavior no longer occurs even if the appreciation for the "opposite sex" (as would be seen in the fertile adults) might.}

_{³ That line refers to the original form of the question. It was edited after my answer. Cheers.}

Answer 4

Sequential Hermaphrodites

Not all animals that perform sexual reproduction have multiple genetic genders (sexes). The first form of sexual reproduction was hermaphroditic, meaning that every individual is born with all of thier specie's genetic material and reproductive organs. They are not male or female, but male and female at the same time.

In contrast, humans are gonochoric organisms meaning we are born with only part of the genetics of our species. With the exception of a few rare mutations, males and females are created from insemination to be two genetically distinct organisms; so, even before we develop any sexual characteristics invitro, we are already male or female, and no matter what surgeries or chemical manipulations you perform, you can never genetically be the other gender, because those genes are absent from your chromosomes.

However, between gonochoric organisms like ourselves and true hermaphrodites are sequential hermaphrodites. These organisms carry all of the genetics of thier species in them when they are born, but only express certain sexual organs at specific points in thier life cycle. We often call these organisms male and female at different parts of thier life cycles based on what sex organs they present, but genetically speaking, they are never male nor female, but hermaphrodites. They are actually born with both ovaries and testes, but only have 1 or sometimes both sets of gonads active at different points in thier lifecycle.

The most plausible explanation for your aliens is that they would be born with both ovaries and testes (or thier alien equivalents there-of), but both are dormant at birth and at old age, but in mid-life 1 and only 1 set of gonads becomes active... but they are aliens; so, if you want them to grow sex organs mid-life instead of invitro, there is no hard rule saying you can't, that's just not how life evolved here on Earth.

Answer 5

Possible even have all genders on one specimen. There are lots of species who are undistingishable at child then take one gender and change it to opposite basing on enviroment pressure. Old age will be little more problematic but possible.