Abstract
Torso-orthograde (TO)-positional behavior is a unifying characteristic of extant hominoids. Previous studies have highlighted the unique use of forelimb-suspensory dominated locomotion and posture among hylobatids, a tremendously successful radiation of small hominoid primates, often neglecting the importance of other TO-positional behaviors, causing hylobatid locomotion and posture to appear more stereotypic and less versatile, relative to the closely related large-bodied nonhuman hominids (great apes). However, early and recent studies of hylobatid positional behavior have lacked the categorical detail necessary to effectively analyze and compare their TO-diversity and contextual use of TO-positional behaviors to those of other primates and in particular the large-bodied nonhuman hominids. To address the deficit of knowledge of the complete positional repertoire of hylobatids, we observed in detail the positional behavior of a large sample of adult white-handed gibbons (Hylobates lar) at Khao Yai National Park, Thailand. We evaluated the TO-positional repertoire of 24 adult gibbons (11 females and 13 males), and the contextual use of their TO-positional repertoire. Our results indicate that lar gibbons possess and use a diverse TO-positional repertoire that is comparable to that of large-bodied hominids. A collective and flexible use of TO-locomotion and -posture allows lar gibbons to maximize their exploitation of the arboreal canopy. We argue that hylobatids unique suspensory locomotion and posture coupled with a previously undocumented TO-versatility more accurately reflect the pattern of positional behaviors responsible for the small apes’ successful radiation and subsequent diversification across all of South and Southeast Asia’s forest habitats.
Published Version
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