Across the primates, larger body mass often necessitates below-branch locomotion (such as arm-swinging or brachiation). While colobines are typically considered arboreal quadrupeds, Pygathrix exhibits a high degree of arm-swinging behavior in captivity (ca. 50% of locomotion time). However, little is known about their behavior in the wild. We examined two questions about the locomotion of Pygathrix nemaeus in the wild: 1) What is the proportion of arm-swinging in the locomotor repertoire? 2) Given the difference in sizes between the sexes and age categories in this species, do adult males, adult females, subadults, and juveniles differ in arm-swinging frequency? Based on the hypothesis that increased body mass is typically associated with below-branch locomotion, we predict adult males will arm-swing the most, followed by adult females with a ventral infant, adult females without an infant, subadults, and then juveniles. We recorded data between November 2016 and April 2017 in Son Tra Nature Reserve, Da Nang, Vietnam. This time frame encompasses the wet and dry seasons. We video-recorded P. nemaeus between dawn and dusk, 5 days a week, and recorded behavioral data from the footage. We found that juveniles used arm-swinging the most (34.3%), followed by subadults (21.7%), adult females (17.6%), adult females carrying infants (15.5%), and then adult males (9.8%). These results do not support the hypothesis that larger bodied individuals engage more in below-branch locomotion. It is not uncommon for colobine juveniles to show higher proportions of arm-swinging than adults, but more research is needed to understand the relatively high proportions of arm-swinging in adult P. nemaeus.

中文翻译：

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红腿杜克（Pygathrix nemaeus）中的手臂摆动：体重的影响

在灵长类动物中，较大的体重通常需要在树枝下运动（例如手臂摆动或手臂摆动）。虽然 colobines 通常被认为是树栖四足动物，但 Pygathrix 在圈养中表现出高度的手臂摆动行为（大约 50% 的运动时间）。然而，人们对它们在野外的行为知之甚少。我们研究了关于 Pygathrix nemaeus 在野外运动的两个问题：1) 运动曲目中摆臂的比例是多少？2）鉴于该物种的性别和年龄类别之间的大小差异，成年雄性、成年雌性、亚成体和幼体的手臂摆动频率是否不同？基于体重增加通常与分支下运动相关的假设，我们预测成年男性将手臂摆动最多，其次是带有腹侧婴儿的成年雌性、没有婴儿的成年雌性、亚成体，然后是幼体。我们记录了 2016 年 11 月至 2017 年 4 月在越南岘港山茶自然保护区的数据。这个时间范围包括雨季和旱季。我们每周 5 天在黎明和黄昏之间对 P. nemaeus 进行视频录制，并从镜头中记录行为数据。我们发现青少年使用手臂摆动最多（34.3%），其次是亚成鱼（21.7%）、成年雌性（17.6%）、携带婴儿的成年雌性（15.5%），然后是成年雄性（9.8%）。这些结果不支持体型较大的个体更多地参与分支下运动的假设。Colobine 少年表现出比成年人更高比例的手臂摆动的情况并不少见，