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.

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 .

Most colobine monkeys primarily move through their arboreal environment quadrupedally. Douc langurs (Pygathrix spp.), however, are regularly observed to use suspensory behaviors at the Endangered Primate Rescue Center (EPRC) in Northern Vietnam. Previous work has linked variation in scapular morphology to different modes of primate arboreal locomotion. Here we investigate whether the shape of the Pygathrix scapula resembles obligate brachiators (gibbons) or obligate arboreal quadrupeds (other cercopithecoids). Using a MicroScribe G2X 3D digitizer, the positions of 17 landmarks were recorded on 15 different species of nonhuman primates (n = 100) from three categories of locomotor behavior: brachiator, arboreal quadruped, and unknown (Pygathrix). All analyses were conducted in the R package geomorph. A Procrustes analysis uniformly scaled the shape data and placed specimens into the same morphospace. A Principal Component Analysis was used to examine scapular shape and a Procrustes ANOVA was conducted to test for shape difference in the scapulae. A pairwise analysis was used to compare the means of the locomotor categories and identify any statistically significant differences. A phylogenetically controlled Procrustes ANOVA was also conducted using a phylogeny from 10kTrees. Results show Pygathrix scapular morphology is significantly different from both arboreal colobine quadrupeds (p < 0.01) and hylobatid brachiators (p < 0.01). It does, however, share some features with each including a long vertebral border, like other cercopithecoids, and a more laterally projecting acromion process, like the hylobatids. The principal difference segregating Pygathrix from both the arboreal quadrupeds and the brachiators is the more medially placed superior angle. These nuanced morphological characteristics associated with suspensory behaviors may be useful for inferring suspensory locomotion in the primate fossil record.

The genus Pygathrix consists of three species, the red-shanked douc langur (Pygathrix neameus), the grey-shanked douc langur (P. cinerea) and the black-shanked douc langur (P. nigripes). These primates are found in Southeast Asia in Laos PDR, Cambodia, and Vietnam, where they occur only east of the Mekong River. The Endangered Primate Rescue Center (EPRC) in Northern Vietnam has lent itself to comparative locomotor studies of the red-shanked and the grey-shanked douc langurs where both have been observed to regularly engage in brachiation and forelimb suspension. The grey-shanked douc langur was observed engaging in forelimb suspension approximately 56% of all locomotor bouts whereas the red-shanked douc langur used forelimb suspension about 46% of all locomotor bouts. Historical literature placed the doucs into the semibrachiator locomotor category but also suggested arm-swinging was infrequent. Scapular shape plays a central role in the ability for a primate to engage in forelimb suspension and brachiation-type behaviors. We examined the scapular diversity within the genus Pygathrix to document morphological variation. We observe that the douc langurs share a similar scapular morphology and therefore by extension share a locomotor repertoire that includes suspensory behaviors. We used 17 3D landmarks to gather shape data on the scapula of the genus Pygathrix. Our results show no significant difference between the scapular shape of the three species (P = 0.354, P = 0.344). These results suggest Pygathrix is monomorphic for scapular shape.