Possible antipredator behavior associated with river crossing by proboscis monkeys (Nasalis larvatus )

Proboscis monkeys' (Nasalis larvatus ) river crossing behavior was examined as a potential behavioral response to predation threat. N. larvatus' major predator at the Tanjung Puting National Park in Kalimantan Tengah, Indonesia, appeared to be the false gavial (Tomistoma schlegeli ), a crocodilian. An adolescent female proboscis monkey was captured and drowned by a false gavial during this study. The monkeys crossed the river at its more narrow points, with several individuals or groups crossing simultaneously. Increased vigilance prior to crossing and leaping as far across the river as possible were also observed. Proboscis monkey groups often associate at their sleeping sites at the river's edge. A potential function of these associations may be to allow groups to synchronize their movement with other groups during river crossings.