Effects of river width on the selection of sleeping-site by Proboscis monkeys (Nasalis larvatus) in Sabah, Malaysia

Previous studies have suggested that Proboscis monkeys sleep in trees located by riverbanks and cross rivers at their narrower points to effectively avoid land-based and aquatic-based predators. In this study, we provide some evidence in support of the low predation pressure theory from land-based predators of Proboscis monkeys at the Padas Damit Forest Reserve in the west of Sabah, Malaysia. We compared the mean width of the Garama River adjacent to where Proboscis monkey sleeping trees were located (mean=31.0m; SD: ± 11.3; Range: 10-50m; n = 88) with the mean width of the entire length of this river surveyed for Proboscis monkey sleeping trees (mean=33.3m; SD: ± 13.4; Range: 13-68m; n = 31) , i.e. approximately 8.4 km long . We found that Proboscis monkeys choice of sleeping trees was not dependent on where the narrowest sections of the river are (two-sample independent t-test: t = -0.912; d.f. = 117; P= 0.36). We suggest that this maybe an indication of low land-based predation pressure. We also argue that the reluctance of Proboscis monkeys to cross large rivers mayindicate a high aquatic-based predation pressure.