The function(s) of a long-distance signal: the orangutan long call Brigitte Spillmann, Anthropological Institute and Museum University of Zurich Switzerland
Summary of the project
Long-distance vocal communication in widely dispersed, semi-solitary species often mediates individuals’ ranging behavior and social relationships (MacKinnon 1979, Galdikas 1985a, van Schaik 1999, Mitra Setia, Delgado et al. 2009, Singleton, Knott et al. 2009). Long calls are the most conspicuous long-distance vocalization in orangutans. They are emitted only by flanged males. Long calls are audible up to 1 kilometer for human observers on the ground and are therefore key regulators of encounters between dispersed individuals in a dense rainforest where visibility is limited (Galdikas 1983, Mitani 1985b). Two functions of long calls are generally considered: repulsion of male rivals and mate attraction (Mitani 1985a). Nevertheless, long call function is poorly understood, given that recent work in our group has shown that: (i) there is a third likely function, travel coordination with non-receptive females (Mitra Setia and van Schaik 2007); (ii) long calls can be divided into acoustically distinct types (Spillmann, Dunkel et al. 2010); and (iii) social systems, and in particular male-male competition and male-female relationships, differ between Sumatran and Bornean orangutans (Delgado and van Schaik 2000). In this project, we intend to examine all three possible functions of long calls and their information content on both islands, in order to elucidate the flexibility of vocal signaling in a geographically variable social system. We followed focal males to record their ranging, social and vocal behavior, but also established a network of 20 autonomous digital field recorders (autonomous acoustic monitoring AAM) throughout the area to get a complete overview of long call dynamics for 24 hours. The combination of traditional follows and novel auditory monitoring will create an accurate picture of flanged males’ presence in the area, and thus allow us to reconstruct a model of the communication network over time and space and the ranging responses of individual males and females to the long calls of other flanged males. In addition, playback experiments will answer questions about females’ and males’ reactions to long calls in a controlled way and thus to elucidating long call function. As this project is not yet finished results included here refer to the validation of AAM.
Methods and data collected This project is based on a fine-grained approach (simultaneous follows, recordings of long calls, playback experiments) and a coarse-grained approach (autonomous acoustic monitoring AAM) that will be merged together for data analyses. Field work took place at two field stations: Camp Tuanan (2°09’ S, 114°26’ E) in Borneo (Indonesia) from February 2012 until April 2013 and Camp Suaq Balimbing (03°02’N; 97°25’E) in
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