^Disincentivized to exist. Whatever social constructs they build on that to make it seem like being a zhavey is respectable or cool doesn't take away from the fact that the zhen (regardless of species) does not transmit information to the next generation, and hence is less likely to have ever developed.
The other three sexes parasitize the zhen, and unlike a human female, and no matter what the zhen gets out of it psychologically and materially, she receives no payment evolutionarily speaking.
"Avatar, Book 2" describes a zhavey as the "biologically and socially closest mother".
Either the author uses the term "biological closeness" incorrectly or she is contradicting the assertion that the zhen does not transmit genetic material. A surrogate mother, even if she wound up keeping it, would not be biologically
closer to the resultant child! That's effectively what zhens are--surrogate mothers. It takes as much investment to produce a surrogate mother as it does to just produce a fertile child. The benefits the surrogate mother can provide don't accrue to whoever's making it.
Or look at it another way:
A zhen serves about the same function as a bird's egg does--keeping the developing fetus safe and fed. Eggs come from the tissue of the mother and aren obviously not accorded recognition as an organism separate from both the mother and the child; nor do they traditionally grow legs, arms, and brains.
While I suppose an egg with legs, arms, and brains is going to be a very effective egg, it's a hell of a tremendous investment by an organism to make such an egg that it intends to deliver to unrelated organisms who will then use that egg to gestate their
offspring. Why not just develop a fertile
offspring instead, who can transmit your
genes instead of those of strangers? It's the exact same amount of work.
If an Andorian quartet (trio, really) manages to get one child out of a union before one dies, and that child's a zhen, they have already reproductively failed, and might as well not have tried at all. Meanwhile, the four-sexers who are all
fertile are totally outbreeding them.
From an individual thaan/chan/shen organism's point of view, I can see the appeal of the zhen--you can offload reproductive duties onto this chick, and go find another, and another. But what would give rise to that? In a parasitic system, at least the parasitor does not actively create its own parasitee--it free rides
on another. Building a zhen just to parasitize it is not free riding at all. Indeed, the thaan/chan/shens that came before them are reproductively penalized
every time they give life to a zhen, and hence that trait is less and less likely to be carried forward.
The behavior of the parasitoid wasp sums up poignantly what thaan/chan/shens are doing to the poor zhen, and what she should expect to get out of the transaction:
Warning: gross as hell--but educational!*
The questions that bear on the existence of the three sex + walking egg system are: what happens to that species of butterfly if every
caterpillar wound up like that poor dope; and what would happen to that species of wasp
if they had to grow that caterpillar first before letting other
wasps parasitize it?
*If anyone's confused by the unexplained mention of the "wasp virus," like I was, this is helpful: http://en.wikipedia.org/wiki/Polydnavirus