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Hm. Maybe there's merit to the theory that they only have one child apiece. I'll have to work on the math for that -- but a one-to-one ratio is usually decently stable, methinks. That'd really be the only way to do it, and would also account for their strict policy that no one is allowed to leave the island.

Well, I presume that if they only live 1000 years or so (as my theory postulates), they would look that old towards the end of their lives regardless of how Kurama looks now. Demons can have different lifespans as far as I know, just like different species of animals. As for them having one child each, one person responding to this thread mentioned it out of hand and I didn't remember it from the show -- but I haven't read enough of the manga to know for sure anyway, and it is a better way to account for a small, stable population without the factors of disease and violence.

As for how Yukina ditched them . . . I don't think the show ever says. It'd make a good fanfic, actually, although I suppose it's probably been done before.

Darn Togashi and his plot bunnies! ...Grr. This will bug me until I die.