Arm-swinging and habitat use of the red-shanked douc langur (Pygathrix nemaeus) in the Son Tra Nature Reserve, Vietnam

Amongst the colobines (Colobinae subfamily), the douc langurs (Pygathrix) are known to use higher proportions of arm-swinging behavior in their locomotor repertoire. Previously, researchers have suggested the increased use of arm-swinging to be a result of habitat constraints. For example, suspensory locomotion allows animals to travel across unstable substrates, such as the terminal ends of branches, than would be possible in above branch locomotion. In this paper, we attempt to identify patterns of habitat use and arm-swinging amongst the doucs. Specifically, we predict arm-swinging will occur significantly more on horizontal substrates, in the main canopy, on branches (medium-sized substrates), and in old-growth forest. Data was recorded in the Son Tra Nature Reserve, Vietnam between November 2016 and April 2017. The monkeys were filmed and data was extracted continuously. Results indicate the red-shanked douc arm-swings more often when they are in the main part of the canopy compared to the top of the canopy, and possibly on substrates smaller than a bough. No differences were found in frequency of arm-swinging when individuals were in old versus new forest or on horizontal versus oblique substrates. The patterns of arm-swinging behavior follow patterns similar to that of spider monkeys, especially in substrate preferences. This information is critical for understanding and conserving the habitat of the critically-endangered red-shanked douc.