The Function of Adult Male Long Calls in Wild Orangutans (Pongo pygmaeus) (Indonesia).

Delgado, Roberto Antonio, Jr., 2003, The Function of Adult Male Long Calls in Wild Orangutans (Pongo pygmaeus) (Indonesia). Ph.D. thesis (Primatology), Duke University.

This study investigated the signal content and function of adult male long calls in wild orangutans (Pongo pygmaeus). In addition to using existing long term comparative data, orangutans were studied for a combined 18 months at several field sites throughout Indonesia. Over 3,000 hours of observational data were collected from the two primary study populations at the Ketambe Research Station in northern Sumatra and the Cabang Panti Research Station in West Kalimantan. In all, the study populations comprised 31 habituated animals. Long calls produced by males were recorded in the field and later analyzed for their degree of variance in selected acoustic features using a digital signal-processing program designed for the study of animal vocalizations. Field playback experiments were conducted with adult subjects to test hypotheses regarding the possible signal content and function of adult male long calls. Immediate and delayed responses including ranging data were examined in relation to the listener's familiarity with the signaler, the speed and duration of the call, and, for females, their inferred reproductive status based on relative infant size and the behavior of associated males.

Acoustic analyses of adult male orangutan long calls revealed sufficient variation in signal properties to allow for individual discrimination. Differential behavioral responses to the playbacks of long calls indicated that experimental subjects distinguished between classes of familiar and unfamiliar flanged males. Moreover, it appears that long calls can act as a coordinating signal between dispersed parties. Signaling males effectively indicate their direction of subsequent travel and females in the vicinity may seek, if necessary, vocalizing males for reproductive benefits including preferred mating access or protection against sexual coercion such as harassment.

Orangutan long calls are a good means of distinguishing among individual males. While structural acoustic features may not necessarily reveal the inherent quality of a signaling male, the speed and duration of long calls could still reflect a vocalizer's current condition or motivational state. The functions of adult male long calls in wild orangutans are largely receiver-dependent and the adaptive value of these long distance vocalizations is probably maintained by sexual selection (Winzeler database online).