Passage: Although strong and powerful, the mountain gorillas are generally gentle and very shy. Severe aggression is rare in stable groups, but when two mountain gorilla groups meet, the two silverbacks can sometimes engage in a fight to the death, using their canines to cause deep, gaping injuries. For this reason, conflicts are most often resolved by displays and other threat behaviors that are intended to intimidate without becoming physical. The ritualized charge display is unique to gorillas. The entire sequence has nine steps: (1) progressively quickening hooting, (2) symbolic feeding, (3) rising bipedally, (4) throwing vegetation, (5) chest-beating with cupped hands, (6) one leg kick, (7) sideways running four-legged, (8) slapping and tearing vegetation, and (9) thumping the ground with palms . Jill Donisthorpe stated that a male charged at her twice and, in both cases, when she stood her ground the gorilla turned away.

Question: What types of primate is severe aggression rare in?
stable groups