Interesting reading that they thought Gugas had very poor fertility, he seems to bear out what my years in human fertility treatment suggested, if one half has poor fertility a fertile other half can overcome it.
I thought one of their problems was that he took a long time to fully dominate the (older) females. That wouldn't affect his fertility of course but I don't know what his mating record was like. Bikira had to leave the group as there was no breeding from her. Yet now he has young with three different females!