Human visitors affect the behavior of captive animals, which is the so-called visitor effect. The number and behavior of visitors may influence stress-related behaviors in captive animals, such as self-scratching, yawning, and visitor-directed vigilance. A captive social group setting that resembles the setting in the wild can be applied to alleviate such negative visitor effects and facilitate social behavior and interactions between individuals. In this study, we examined how the number and

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... havior of visitors are related to stress-related behaviors of a captive mixed-species gibbon pair comprising a yellow-cheek gibbon (Nomascus gabriellae) and a white-handed gibbon (Hylobates lar). The two gibbons were put through a process of separation during the study period, and we examined whether the social isolation stimulated the visitor effect. The frequencies of stress-related behaviors of the gibbons increased and social playing between them decreased with increasing visitors, particularly after social isolation. In the indoor enclosure, the gibbons increased their visitor-directed vigilance when visitors shouted or struck the glass partition. Our findings indicate that the number and behavior of visitors negatively affect captive gibbons and that a mixed-species social setting can help gibbons reduce visitor-induced stress.