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Housing different animal groups in close-by facilities is common in wildlife centers. However, the impact on animal welfare is insufficiently studied in the literature. In this study, we analyzed the behavior of two adjacently housed chimpanzee groups to investigate how intergroup interactions may affect their behavior and, thus, their welfare. We recorded occurrences of abnormal and self-directed behaviors, two well-known indicators of stress in chimpanzees. Furthermore, we explored the social responses to said intergroup interactions by recording all inter- and intragroup affiliative and agonistic behaviors. Finally, we measured the number of vigilance occurrences that individuals directed towards other chimpanzees as an indicator of interest. Generalized Linear Mixed Models (GLMMs) were used to assess whether and how social interactions between neighboring groups might influence their behaviors, taking gender, age, group, and intergroup participation into account. Our results suggest that intergroup interactions promoted the occurrence of affiliative behaviors between group members. However, intergroup interactions caused the chimpanzees to exhibit a higher number of abnormal and self-directed behaviors and increased vigilance towards their group members when agonistic intergroup interactions occurred. Thus, adjacent housing does impact the chimpanzees’ behavior and welfare and should be continuously monitored and assessed to promote and maximize welfare.

Wild chimpanzees live in large and complex social communities, but their complexity is determined by the number of potential social partners as well as the frequent changes in group composition due to fission–fusion dynamics. Alternatively, captive housed chimpanzee groups are usually much smaller and less complex. However, studies have shown that groups can be housed in adjacent habitats, potentially increasing the number of social partners, i.e., granting additional relationship opportunities and enhanced social complexity. While most social network studies analyze social groups as closed systems, this study assessed the impact of social interactions between two neighboring groups of chimpanzees, based on two social network indices (Vertex strength centrality and Deviation from edge weight disparity). Furthermore, Linear Mixed Models (LMMs) were employed to assess whether gender, age, and intra-/intergroup directionality influenced these indices. We recorded occurrences of social behaviors, social proximity and whether chimpanzees observed individuals in the other group as a measure of interest. Most social behaviors were directed towards group members; however, 13% were directed towards the neighboring group. Although enclosure barriers constrained the chimpanzee’s capacity to physically interact, it does not necessarily impede social interactions with the outside world. Hence, the presence of neighboring chimpanzees potentially provides additional relationship opportunities, increasing social stimulation and complexity leading to an enriching social environment.