The mother bears are not specific PH's for Ursalua. I've killed rounds with 1 or 2 mother bears, and not had Ursa pop. I've killed rounds with no mother bears, and Ursa pops. I have had her pop after rounds with mother bears, but I zoned in once when there were 2 motehr bears, killed both, waited on repops, no Ursalua. I think any mesa bear can be the PH, best bet is to just kill every one of them.