Human predation and Kloss's gibbon ( Hylobates klossii ) sleeping trees in Siberut Island, Indonesia

Kloss's gibbons ( Hylobates klossii ) and Mentawai langurs ( Presbytis potenziani ) on Siberut Island, Indonesia, both sleep in emergent trees 34–55m tall, situated on crests and upper slopes of hills. They differ in that 91% of gibbon sleeping trees examined were free of lianas, whereas 89% of langur sleeping trees were draped with thick, woody lianas hanging from the branches to the ground. Because indigenous hunters climb lianas to shoot primates in treetops, gibbons are less susceptible than langurs to nocturnal human predation. Hence, the gibbons' preference for—and apparent control of—the limited supply of vineless trees gives them an advantage over langurs.