Acoustic properties of long calls given by flanged male orang-utans (Pongo pygmaeus wurmbii) reflect both individual identity and context

Long calls by flanged male Bornean orang-utans (Pongo pygmaeus wurmbii) serve as a long-distance communication signal in this semi-solitary species and allow individuals to adjust their ranging behavior. Long calls can be heard up to circa 1 km in dense rainforest. Only flanged males emit them, in various contexts: spontaneously (where no disturbances from the environment are perceived by human observers), when highly aroused by another male's long call or a falling tree nearby, or right after having pushed over a dead tree themselves. In this study, acoustic analyses of orang-utan long calls at Tuanan in Central Kalimantan not only confirm the discrimination of individual males by their long calls but also demonstrate the discrimination of context based on the long calls' acoustic structure, which is further supported by the females' ranging responses according to long call contexts. Females with dependent offspring move away from spontaneous long call sources but appear to ignore long calls elicited by disturbance. Hence, Bornean orang-utan females perceive measurable differences in acoustic characteristics of long calls given in different contexts. These findings concur with vocal discrimination of contexts in other non-human primates.