I had a fortress once (in the last version number) where I captured a whole bunch of elven and goblin prisoners and wanted to try setting them loose in an artificial cave system rather than use them in my arena as I had been. It took a long time as the cave was quite big, but eventually I got a large artificial cave with a small hole to drop prisoners into it. I dropped a bunch of goblins, elves, and the horses the elves brought into the cave and I don't remember how long it took, but eventually the elves, goblins, and horses alike all started having children. I know that in most cases the foals were instantly killed by all groups in the cave probably due to some strange thing with how allegiances are calculated. However I don't know if the goblin or elf children were killed as soon as they were born which is probably quite important to this idea. Another thing important to the idea though is invaders having children and I know they normally don't, and I don't know exactly what caused my invaders to have children, but I can make a few guesses. Of course the attack had ended by the time they were thrown in there because if it hadn't then the elves wouldn't have invaded after the goblins so that could be a part of it, the ambush or siege being officially over. Thinking about everything that's the only real thing I think might have changed the outcome from them not having any children to them having them, though if that doesn't work I could make a few more guesses that I really doubt would work.