Now that others have mentioned it, 2 star boars can give birth to no-star boars if bred with a no-star. When I introduced 2 2 star boars into my singular of boar, once I had 4 or 5 2 star I wiped out all the no star boar I had. The next day I had 2 no star boar in my small singular of boar, so obviously the 2 star boar had given birth to no star boars.