Why Don’t More Animals Change Their Sex?

That's the title of an article from Yale University:

Most animals, like humans, have separate sexes — they are born, live out their lives and reproduce as one sex or the other. However, some animals live as one sex in part of their lifetime and then switch to the other sex, a phenomenon called sequential hermaphroditism. What remains a puzzle, according to Yale scientists, is why the phenomenon is so rare, since their analysis shows the biological “costs” of changing sexes rarely outweigh the advantages.

A report by Yale scientists in the March issue of The American Naturalist says that while this process is evolutionarily favored, its rarity cannot be explained by an analysis of the biological costs vs benefits.
...
“We were surprised to see that a hermaphrodite could spend 30 percent of its lifetime in the process of change sex, and still persist in a population,” said Kazancioglu. “This suggests that only huge costs can disfavor sex change.”

So, why is sex change so rare? And, why does one species of fish reproduce strictly as separate sexes, while another very closely related species flexibly changes sex?

The capability is there, buried in our genes. Well, buried in my genes, anyway. It appears that my body attempted to do this, but of course the change was very incomplete. Rather from being from a fully functional male to fully functional female, it was from somewhat dysfunctional and infertile pseudo-male to a functional but sterile pseudo-female. I'm speaking biologically here, not psychologically. My mind has always been female, but that's an irrelevancy. So I could be considered a transitional variety, not 100% successful as either sex (biologically speaking), yet functional enough, barely, just, not merely to survive but to reproduce. That's good enough for Evolution, a C- is as good as an A+ here. My work has also helped save tens of thousands of lives, so even if I didn't have children, having someone with my talents being thrown up occasionally would be good for the species as a whole. Objectively, I'm an asset to humanity, no matter how many Torr the situation would suck on a personal level.

The costs in an intelligent species whose emotional capacities are strongly sexually dimorphic are large. But while humans may have sexually differentiated brains, the differentiation is, compared with many mammalian species, not as great as most. And the differentiation in the body as a whole is relatively minor. Furthermore, it appears that perhaps 1 in 3 of us could perform well, or at least adequately, in either gender role. If that wasn't the case, transsexuality would be far more common.

I survived 47 years after all, and it would have only been when my reproductive years were behind me that the discomfort would have been too great to bear any longer. I'm less TS than most who transition, closer to being BiGendered without actually being so. The discomfort was intense, and the relief now things are right is beyond description.

It would be interesting to see what could possibly happen to us in the next few thousand years. Perhaps one in three might opt for serial or concurrent hermaphroditism. Barring cryonic neurosuspension though, I won't be around to see it. And I wouldn't avail myself of the opportunity anyway: if the possibility was there, I'd opt for 100% female. It's more my style. Because although I've tried to be objective, speaking of myself as a "biological pseudo-female", that's not who I am. Subjectively, emotionally, even spiritually, there's nothing "pseudo" about it and never was. While I may know that, others can't though. If they could, I wouldn't have done the boy act for so long. I wouldn't have had to.