Effects of crowd size on exhibit use by and behavior of chimpanzees (Pan troglodytes) and Western lowland gorillas (Gorilla gorilla) at a zoo

Several factors are known to affect the overall well-being of captive apes, including facility design and the presence of and interaction with both familiar and unfamiliar humans. Several studies have examined the effects crowds of varying sizes have on the behavior of a wide taxonomic range of zoo-housed animals, including apes. However, previous investigations of visitor effects on ape behavior vary in scope and methodology, leading to inconsistent results across groups and institutions. Using data gathered through two long-term observation protocols, we aimed to determine how 6 chimpanzees (Pan troglodytes) and 5 Western lowland gorillas (Gorilla gorilla) housed at Lincoln Park Zoo responded to visitor crowds of various sizes. The dataset, gathered over 12 consecutive months, included both behavioral data collected via focal animal follows, and exhibit use data collected via whole group scans. As with previous studies, we investigated if and how the behavior of the apes varied as the number of visitors near their exhibits increased across several levels of visitor density. In addition, we looked at whether the apes in this study use the indoor areas of their exhibits that are nearest the public (within one meter) at a rate proportional to the size of that area, and whether their use of this space changed as crowd size varied. As crowd size increased, neither chimpanzees (P = 0.97) nor gorillas (P = 0.18) were observed to alter their use of the areas of their exhibit closest to zoo visitors. In addition, both chimpanzees and gorillas were observed in this area at a rate equal to or greater than expected by random movements at all three levels of crowd size analyzed. Crowd size had no effect on the frequency of most behaviors analyzed including abnormal, agonism, and grooming (P > 0.05). We observed a slight effect of crowd size on self-directed behaviors in both species though it was not the case that the largest crowds caused the highest rates of self-directed behavior. Although these data were collected at a single institution, our findings support the argument that well-designed zoo exhibits can be effective in removing the potentially negative effects of large crowds.