Understanding how human presence influences animal behavior, specifically the behavior of nonhuman primates, has important implications for methodology in cognitive and behavioral studies, for our understanding of human–animal relationships and for animal welfare in captive settings. Conducted in the San Diego Zoo, this study partially replicates an existing study on human–animal relationships, which focused on familiarity effects on the human-directed behavior of great apes residing in the Toronto Zoo. Such a replication in a new location and with different individuals is necessary to expand sample size and to better understand how ontogeny and living environment influence human-directed ape behavior. All-occurrence sampling of human-directed behavior was used to gather data on two gorilla groups and one orangutan group, all housed at the San Diego Zoo. The aim was to compare ape-initiated behaviors toward zoo staff with apeinitiated visitor-directed behavior, in accordance with predictions of the Human–Animal Relationship (HAR) model. Overall, human-directed behaviors were more frequent toward visitors than toward staff, mainly caused by increased visitor-directed visual behavior, specifically brief glances. Given that visual monitoring has been seen as fearful behavior, potentially related to predator monitoring, this would confirm the expectation that lack of familiarity will lead to increased visual vigilance. However, the visitor-directed visual behaviors may not be antagonistic or fearful. Considering the specific behaviors and exhibit designs, curiosity may be an equally likely explanation in this context. Aiming to also understand whether these groups exhibited species differences in human-directed behavior, it was found that orangutans engaged in more affiliative behavior overall than gorillas. Comparing our results with the previous study, we conclude that while effects of familiarity are present, those effects are modulated by both individual ontogeny and exhibit design. While the HAR model is partially supported, additional factors may need to be included to fully interpret the observed behavior. Additional research is needed to further understand the effects of species, sex, and ontogeny on the predictions of the HAR model.
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