Five-domain-based evaluation of environmental enrichment in Japanese macaques (Macaca fuscata) to reduce aggressive...

Compared with wild troops, Japanese macaques (Macaca fuscata), which are commonly housed in cage-like enclosures in zoos and laboratories, face limited feeding opportunities, leading to intense competition and frequent aggression. Such aggression often results in injuries, highlighting the need for methods to reduce aggression in captive settings to improve welfare. This study evaluated the effects of three enrichment strategies-fire hoses with branches, external feeders, and plastic tubs with straw-designed to promote spatial usage and diversify feeding opportunities in a Japanese macaque group in Hirosaki City Yayoi Ikoi Park. The frequency of aggressive behavior and spatial utilization across vertical cage layers was recorded. Generalized linear models revealed that all the tested enrichments significantly reduced intense aggressive and displacement behaviors. Fire hoses increased the usage of the second layer, whereas external feeders and plastic tubs decreased the usage of the top layer, which was used most in the control condition, promoting group dispersion. These interventions improved the environmental opportunity and choice (environmental domain of the Five Domain model), decreased aggression-related injuries (physical health domain), encouraged to express natural behaviors such as climbing and foraging (behavioral domain), and reduced fear and discomfort due to the proximity of dominant individuals (mental domain), collectively supporting enhanced welfare.