 Studies by McCracken and Bradbury (1977) have shown that unrelated females in the colony often form stable factions.Recent studies have shown, that even though females do live in stable co-operative groups or "tribes" with non-kin, and will protect unrelated pups of their own group, they attack and even kill pups from other groups living in the same cave.McCracken and Bradbury considered many possible reasons for group formation in "P. hastatus".