Unexpected locomotor behaviour:: brachiation by an Old World monkey (Pygathrix nemaeus) from Vietnam

More than 70 h of positional behaviour data were collected on the red-shanked douc langur Pygathrix nemaeus, Delacour's langur Trachypithecus delacouri, Hatinh langur Trachypithecus laotum, and the white-cheeked crested gibbon Hylobates leucogenys in January and February of 2001 at the Endangered Primate Rescue Center of Cue Phuong National Park, Ninh Binh Province, Vietnam. Equal amounts of instantaneous and continuous bout data were collected for each of the four species. All animals were housed in enclosures of 10 x 5 x 3.5 m with similar substrates, and lived in small, species-appropriate social groups. The positional behaviour of white-cheeked crested gibbons was as expected with > 80% of locomotor bouts and 60% of postures being suspensory in nature. The positional behaviour of the Delacour's and Hatinh langurs was similar to that reported in the literature for other Asian colobines with > 90% of the locomotor bouts being quadrupedal walking, running, and bounding on tops of arboreal supports. Postures were dominated by sitting in and sitting out with forelimb suspension associated with < 10% of these behaviours. The positional behaviour of the red-shanked douc langur is intermediate in nature between the Delacour's and Hatinh langur and the white-cheeked crested gibbon. Locomotion is nearly evenly divided between arm-swinging (46%) and quadrupedal walking and running bouts (54%). Forelimb suspensory postures were three times more frequent for the red-shanked douc langur than they were for the Delacour's langur.