Orang-utan

Orang-utans are the least vocal of the great apes, but they do have a system of auditory communication. Their most dramatic vocalization is the long call, given only by adult males. The call has been likened to the sound produced by large volumes of water roaring through steel pipes. The precise function of the long call is not known, but it probably serves to space out adult males in territories. It likely also serves other functions, including a display to attract sexually receptive females, and a signal to inform the community of the location of the dominant male. Males tend to call in bad weather, when another male is visible or calling, when close to a sexually receptive female, or as an element of copulatory behavior. Calls are audible up to 1.2 mi (2 km) from the source.

Other vocalizations include a variety of grunts, squeaks, moans, barks, and screams. Alarmed and agitated individuals produce "kissing" and "gluck-gluck gluck" sounds that seem to indicate their level of annoyance. These noises are often accompanied by aggressive physical displays, such as shaking and breaking branches. Males use their huge size and other secondary sexual characteristics during intimidation displays. They inflate their throat sac and elevate the hair on their shoulders and arms to make themselves look larger. When threatening, individuals open their mouths wide to show their teeth, and when fearful, they extend their prehensile lips.