Behavioral response of captive western lowland gorillas (Gorilla gorilla gorilla) to the death of silverbacks in multi-male groups

In both free-ranging and captive western lowland gorillas, a silverback provides protection and leadership, mediating conflict within a group. In the wild, when a dominant silverback dies the group will disperse or transfer to a solitary male, unless a subsequent male is present to inherit the group. In captivity, studies have focused on groups containing one male and therefore it is unclear how gorillas respond to the death or removal of a silverback in multi-male groups. This study examined the behavior of a bachelor group (Cleveland Metroparks Zoo) and a multi-male, mixed-sex group (Pittsburgh Zoo and PPG Aquarium) following the death of two older silverbacks in 2005. Both of the younger but dominant males maintained their same level of dominance after the death. We predicted that agonism would increase after the death as groups struggled for social stablility. We did observe an increase in both agonism and displacements among the bachelor group, but only observed an increase in displacements among the mixed-sex group. Although we predicted that there would be no change in solitary behavior, both groups decreased feeding and the mixed-sex group increased self-directed behavior post-death. In the bachelor group, self-directed behavior decreased and undesirable behavior increased. We also observed a difference in spatial distance after the death with members of the mixed-sex group becoming more dispersed and members of the bachelor group more converged. This study demonstrates that there is a period of transition for multi-male groups after the death or removal of the oldest silverback. Future research could integrate physiological measures with behavioral analyses before and after the death or removal of a prominent member of the group. Zoo Biol 29:16–29, 2010. © 2009 Wiley-Liss, Inc.