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

Abstract

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.

Behavioral changes following alterations in the composition of a captive bachelor group of western lowland gorillas (Gorilla gorilla gorilla)

“…Hoff et al () found a significant increase in non‐contact aggressive behaviors by all remaining members in a family group at Zoo Atlanta following the death of their dominant silverback. This is similar to the trends reported in one of the two family groups at the Pittsburgh Zoo group in which one of the non‐dominant silverbacks had died (Less et al, ). While group behaviors did not shift significantly at the Pittsburgh Zoo, researchers reported an increase in solitary behaviors and an increase in aggression between the two remaining silverbacks (Less et al, ).…”

“…Unlike we predicted, the gorillas showed very few significant behavioral shifts following the death of the dominant silverback. Decreases in feeding and sociality have been observed in both family and bachelor groups after the death of a silverback in other studies (Hoff, Hoff, & Maple, ; Less et al, ); this was not observed in our study. We expected to see shifts in agonistic behavior as the group hierarchy restructured following Juma's death, particularly in our older silverback individuals (Jontu and Little Joe), however, this was not observed.…”

“…In the case of the introduction transition, we predicted there would be an increase in affiliation and a decrease in aggression similar to that observed in the Bronx Zoo (McCann & Rothman, ) and Zoo Atlanta studies (Hoff et al, ). Based on previous reports of deaths in multi‐male groups, we predicted that the death of the dominant silverback at the Saint Louis Zoo would result in a temporary increase in aggression due to the immediate need for social restructuring similar to that found in the Cleveland Metroparks study (Less et al, ). Demographic transitions may also result in temporary increases in group or individual tension and/or abnormal behavior as an immediate response to stress.…”

“…At the Bronx Zoo, an increase in the amount of affiliation by the dominant silverback (determined by proximity) was observed when five infants were introduced to the group (McCann & Rothman, ). At the Cleveland Metroparks Zoo, there was an increase in aggressive behaviors and a general period of social restructuring following the death of a silverback in an all‐male group (Less, Lukas, Kuhar, & Stoinski, ). During a study at the Taronga Zoo on periods of social instability—including the removal of two young adult females—researchers found elevated glucocorticoids, aggressive displays, and fighting.…”

Human–animal relationships in zoo‐housed orangutans (P. abelii) and gorillas (G. g. gorilla): The effects of familiarity

“…Behaviors were classified, based on behavioral similarity or apparent behavioral ends, into four types: visual, aggressive, hiding, and affiliative (see Table II for full descriptions of HDBs). Additionally, affiliative behaviors, were designated as close (<3 m) or distant (>3 m), consistent with proximity definitions for studies of zoo orangutans and gorillas [Edwards & Snowdon, 1980;Less et al, 2010;Nakamichi & Kato, 2001;Poole, 1987;Stoinski et al, 2003Stoinski et al, , 2004Whilde & Marples, 2011;Zucker & Thibaut, 1995].…”