Ranging Behavior of Proboscis Monkeys in a Riverine Forest with Special Reference to Ranging in Inland Forest

Abstract

We observed a unimale group (BE-Group) of proboscis monkeys (Nasalis larvatus) comprising an α-male, 6 adult females, and several immatures from May 2005 to May 2006. We followed the group for 2014 h along the Menanggul River, Sabah, Malaysia (118°30′E, 5°30′N). Observations focused mainly on ranging behavior. We determined availability and seasonal changes in plant species consumed by the members of the group by vegetation surveys in a 2.15-ha area along 200–500 m trails in the riverine forest. During the observation period, the group ranged ≤800 m from the riverbank, within a total range of 138.3 ha. The daily path length of the group ranged from 220 to 1734 m (mean, 799 m), and daily path length correlates negatively with fruit availability. The monkeys were apt to remain within a small range in fruit-abundant seasons. Because the monkeys preferred to feed on fruits of dominant plant species in the study area, their daily path length may decrease on days when they feed on fruits. The core areas of the group’s home range were along the river because the monkeys typically returned to riverside trees to sleep. The group most often used areas that were nearer the riverbank and where the availability of fruits was higher. The most frequently used grids were the ones where the group often had sleeping sites and crossed the river. Avoiding predation may be the main reason for river crossing and selecting particular sleeping sites; hence not only food availability but also the risk of predation appears to influence the ranging of the BE-Group.