Concerning the double-letter line and their demise. A story I was playing with a character named Ku'sar (Tribe name was Ya'shir, but that is a different story altogether) on Gilgamesh.. since the sept CAN be rather small, its easy for something like a roving band of X monster to take out the tribe. Or an encounter with a beastman raiding party would do the same. So while yes, a low male birthrate can kill a double-letter line, but there's a lot other things that can bring it down as well. And if there are two or three survivors after said band of Aman'jaa runs roughshod over the Ya sept, Well, that's where we have intriguing story to play out.
