Again with the "born pregnant" thing. If you actually watch and listen to the scene where Dr. McCoy makes that remark, I think it's pretty clear he was joking and didn't mean it literally.And imagine if those squirrels/chipmunks/mice/what-have-you were born pregnant and didn't leave until there was nothing left to eat in your home.
It's a factoid arising out of a misunderstanding, like Vulcans having sex only once every seven years.
Well, I'm not the only one who thinks that Bones was serious. Even Wikipedia states 'McCoy concludes that tribbles use over fifty percent of their metabolism for reproduction and that they are born pregnant.'