Twenty-five distinct vocalizations are recognized within the gorillas, many of which are used primarily for group communication within dense vegetation. Sounds classified as grunts and barks are heard most frequently while traveling, and indicate the whereabouts of individual group members. They may also be used during social interactions when discipline is required. Screams and roars signal alarm or warning, and are produced most often by silverbacks. Deep, rumbling belches suggest contentment and are heard frequently during feeding and resting periods. They are the most common form of intragroup communication. 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.
The entire sequence has nine steps:
(1) Progressively quickening hooting,
(2) Symbolic feeding,
(3) Rising biped ally,
(4) Throwing vegetation,
(5) chest-beating with cupped hands,
(6) One leg kick,
(7) Sideways running, two-legged to four-legged,
(8) Slapping and tearing vegetation,
(9) Thumping the ground with palms to end display