Geographical variation in orangutan long calls

Information on geographic variation in vocal signalling is important because it complements data used to infer phylogenetic relationships, has the potential to help understand call development, and may provide insights into social organization. A quantitative acoustic analysis of orangutan long calls was undertaken to compare males from six distinct sites across Borneo and Sumatra and revealed consistent differences among populations and between islands. Several acoustic parameters proved reliable for distinguishing among individuals, among sites, and between islands; populations differed significantly in the number of pulses per call, call speed, call duration, pulse duration and dominant frequency. The chapter discusses these findings in relation to proposed hypotheses that include the influence of ecological, genetic, and social factors. The findings suggest that the patterns of observed differences among orangutan populations are probably best explained by differences in either genetic characteristics and/or forest structure, but these hypotheses remain to be tested more rigorously.