Visitor effects on the behavior of captive Western lowland gorillas: the importance of individual differences in examining welfare.

Given the conflicting and somewhat limited findings available on the effect of zoo visitors on primate behavior, the primary purpose of this study was to provide additional data on gorillas' response to variations in crowd size and to look at what other factors-both intrinsic (e.g. personality, sex, and rearing history) and extrinsic (e.g. group)-might influence such responses. Subjects included four groups (three mixed-sex and one bachelor) of captive western lowland gorillas living at Zoo Atlanta. Overall activity budgets and rates of social behaviors in high and low crowd conditions were compared. Behaviors suggestive of general welfare did not vary as a function of crowd size alone, although we did observe one significant interaction and several trends. Specifically, two groups spent more time engaged in undesirable behaviors in the high crowd condition, whereas the other two groups showed the opposite response pattern. Additionally, males, but not females, showed a trend toward greater rates of contact aggression in the high crowd condition, although wounding rates did not vary with crowd size. We also found trends towards variation in response to crowd size as a function of individual personality ratings. These results suggest that although the visitor effect may constitute an impediment to optimal animal welfare, this may only apply to some individuals or groups. We emphasize the importance of continuing to explore individual differences and the limitations of a one-size-fits-all approach when describing influences on animal welfare.