There, that got your attention,didn't it? (It often gets mine, anyway...) In our world, the usual reproductive strategy (among the 'higher animals' anyway) is two-sexed. Mammal, reptile, amphibian, fish, insect -- the details of how reproduction is accomplished vary greatly, but almost always involves a male depositing genetic material to fertilize the female egg. We have terrestrial examples of one-sexed reproduction, and technology now offers cloning, which is perhaps a variant on three-sexed reproduction: male and female to create a being, and then machines to reproduce it ad infinitum.
But having three distinct sexes is not a strategy that (to my knowledge, anyway) terrestrial evolution has seen fit to take. If that's so, then using the mandate "There Has To Be A Reason" tells us that there's no need for three sexes on Earth, or we would have them.
But for this world I was creating, three sexes were needed. Why? I mused on that myself for a long time. I did a lot of what I call 'mining' -- I scanned all the science periodicals and read anything concerned with sex, sexuality, reproduction, or evolution. Anything interesting I tore out (hey, I bought the magazines, okay?) and stuck in a folder called "Sparks" -- actually, whenever I read something that seems to resonate with a potential story idea, I tear it out or copy it, and toss it in that folder. It's my fodder, my compost pile. Periodically, I go through it and throw away stuff that no longer has the same impact, and occasionally, I'll find myself needing an article or reference in there for a story.) I picked up a few books on the subject that looked interesting and read them. One -- EXTINCTION by David M. Raup -- was especially filled with sparks.
I also had wonderful conversations on the subject with Denise, and with Becca Levin and Guy Allen, two good friends. Becca was especially helpful, as she's a veterinarian, and happily corrected me whenever my strictly amateur biology background betrayed me.
I eventually came up with three possible reasons for a third sex to arise:
THE BIOLOGICAL CONDOM:
If the whole intent of reproduction is the imperative "pass on your genes!" then let's look at mimicry in the animal kingdom. Suppose a third sex could make itself attractive to both the males and females of a species. They could first entice the male to mate. Then this 'false female,' having held and destroyed the male semen, could mate with the females, impregnating her with its own sperm. Perhaps this hermaphroditic creature could even change form or coloration, appearing female for the males, and male for the females
THE BIOLOGICAL ADDITIVE:
Take a male with 'low survival' sperm (maybe just a low sperm count, or perhaps sperm with low motility). Or perhaps a female with a particularly convoluted or perhaps even sperm-hostile environment. Or perhaps the male holds only a potion of the necessary DNA/RNA. Whatever (as my daughter Megen might say...) Then a third sex might arise who adds vital ingredients to the male semen, and passes along the 're-energized' sperm to the female.
THE BIOLOGICAL FILTER:
Mutation is a natural and expected part of reproduction. Without it, none of the wonderful variety of life on the world would have existed. But most mutations result in either non-viable, sterile, or damaged offspring. Too much or too rapid mutation will kill a species, not help it. Given an environment where mutation is the norm rather than the exception, a third species could be useful as a 'filter,' taking in the male sperm, culling the damaged sperm and passing along to the female only the 'healthy' ones. Of course, that takes care of only the male side of the equation and does nothing about mutated eggs, but if you use the usual mammalian model, with the testes hanging outside the body, well, the male genital equipment is the most exposed to environmental hazards.
Yes, I'm certain there are other and possibly better 'excuses' for three sexes -- and if you have one, you're perfectly free to go and write your own story about it. But the last one, especially, resonated with that first scene. Part of world-building is logic, but part of it is inspiration. This is going to sound corny and artsy-fartsy, but it's the truth... I suddenly saw the first large section of the book: a doctor performing an autopsy on the bog body, and I knew that she shared some of the characteristics of that body. I knew that reproduction was the problem for these humans as well, and because THERE HAS TO BE A REASON, that:
a) these humans were stranded where they were (or otherwise they just leave, and there's no story...)
b) for some reason yet to be explained, they also were out of touch with Earth (or they call for help...)
c) heavy and fast mutation was part of the problem for the humans (because the plot elements need to work together, and genetic stability seemed to be the best reason for a third sex to arise in the aliens...)
d) they were lower tech than our current level (or they could probably fix the fertility problem) -- and yes, there had to be a reason for that, too
e) the alien race was extinct (or our valiant colonists could just get the answer to their dilemma from them...)
You can see how all sorts of Neat Stuff comes cascading out of that simple little phrase... Of course, all this had to do with the human population, but I'll deal with that a little further along.
In addition, one of the 'spark' passages I'd collected had mentioned that one source of mutation-causing androgens was in water -- and every peat bog was after all once a lake...
Synchronicity like that can't be ignored.
I also knew that the alien 'midmales' (as I dubbed them), were mostly Filter, along with a touch of Additive, because rapid mutation and low fertility were the problems faced by our stranded colony.
So now I had a sense of the biology of the aliens -- and at that point, I had enough of an overview of the plot that I could even begin writing. I started on the autopsy scene, and I started thinking more about these aliens. After all, physiology is all well and good, but what really interests me is culture. And that's what I wanted to know more about now.
Who were these people, and what were they like?
TO MOVE ON, CLICK ON THE "ALIENS" STONE BELOW.