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Many species of primates use tools and manipulate objects. Environmental objects, such as sticks and branches, are used in locomotion, display, conflict, nesting, and foraging. This study presents observations regarding endangered male Hainan gibbons ( Nomascus hainanus ) selecting sticks and then throwing and catching them repeatedly. This act of Hainan gibbons was termed as “juggling” behavior. This study is the first record of branch use of this kind in free-living gibbons. While it is impossible to experiment on this only remaining population of Hainan gibbons, the deliberate acquisition and then throwing and catching of a stick raises myriad questions regarding their function. The study determined that the juggling behavior of Hainan gibbons, in the process of their brachiation, helps them accurately judge the distance and support strength of an object. It was also found that an adult individual’s proficiency in juggling behavior was much higher than that of a youth. Of all gibbon species, the juggling behavior of Hainan gibbon has a high degree of behavior refinement. Gibbons have the longest forearm than any other primates, which helps them in such performances—a unique mechanism that allows them to perform such unique activities, including juggling.

“Juggling” Behavior in Wild Hainan Gibbons, a New Finding in Nonhuman Primates