Arm-Swinging in the Red-Shanked Douc (Pygathrix nemaeus): Implications of Body Mass

Across the primates, larger body mass often necessitates below-branch locomotion (such as arm-swinging or brachiation). While colobines are typically considered arboreal quadrupeds,Pygathrixexhibits 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 ofPygathrix nemaeusin 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-recordedP. nemaeusbetween 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 adultP. nemaeus.