At Zoo Atlanta in December 1994 a silverback ♂ Western lowland gorilla Gorilla gorilla gorilla group leader died of cardiovascular disease. The behavioural responses of the remaining members of the group were studied and a variety of individual and social behaviours was recorded. For 3 months following the death of the ♂ there was a significant increase in aggressive display behaviours, such as lunge, quadrupedal stiff stance, displace and head divert, and the mother‐offspring relationship intensified. An understanding of the ‘control role’ of the silverback ♂ and the behaviour of group members following changes in social group structure can help zoo managers plan their management strategy when periods of social change are unavoidable.

Group membership was modified among 4 gorilla groups at Zoo Atlanta in an attempt to increase compatibility and sexual behavior. The modifications ultimately moved 7 adult females among the groups. The moves were made by individually introducing the new group members to the old members, resulting in 18 total introductions. Several individual and social behaviors differed in the two conditions. The most obvious change was a temporary increase in aggressive display behaviors, as well as an increase in contact and social examination, in the postmovement condition. Additionally, the individual behaviors of travelling and eating increased after the introductions, while resting decreased. Other self-maintenance behaviors did not change. Finally, there was an increase in time mothers and offspring spent closer to one another following the moves. Previous research and anecdotal evidence suggest that gorillas respond to changes in group membership with aggression; these data support and extend those data to different agehex groups, postmovement trends in behavior, and other aspects of gorilla behavior. 0 1996 Wiley-Liss, Inc.

Three 27-month-old infant gorillas living with their mothers and a silverbacked male were separated to a cage for 24 weeks. The infants initially showed threat responses and increased locomotion, characteristic of the protest stage of anaclitic depression in children. Within several days, these were replaced by dorso-ventral contact among the infants as well as self-holding and fetal positioning. Additionally, social and solitary play and object examination occurred at lower levels through separation than in the pre-separation condition. These changes were characteristic of the despair stage of separation. There was a substantial recovery of many infant nonsocial and social behaviors in the later months of the separation. Upon reunion, the infants did not immediately engage in attachment behaviors with their mothers, and spent more time in contact with each other than with their mothers for the first several days, indicating detachment. Following this, there was an increase in mother-infant attachment behaviors.

Group membership was modified among 4 gorilla groups at Zoo Atlanta in an attempt to increase compatibility and sexual behavior. The modifications ultimately moved 7 adult females among the groups. The moves were made by individually introducing the new group members to the old members, resulting in 18 total introductions. Several individual and social behaviors differed in the two conditions. The most obvious change was a temporary increase in aggressive display behaviors, as well as an increase in contact and social examination, in the postmovement condition. Additionally, the individual behaviors of travelling and eating increased after the introductions, while resting decreased. Other self‐maintenance behaviors did not change. Finally, there was an increase in time mothers and offspring spent closer to one another following the moves. Previous research and anecdotal evidence suggest that gorillas respond to changes in group membership with aggression; these data support and extend those data to different age/sex groups, postmovement trends in behavior, and other aspects of gorilla behavior. © 1996 Wiley‐Liss, Inc.