Predator guild does not influence orangutan alarm call rates and combinations
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Monkey alarm calls have shown that in the primate clade, combinatorial rules in acoustic communication are not exclusive to humans. A recent hypothesis suggests that the number of different call combinations in monkeys increases with increased number of predator species. However, the existence of combinatorial rules in great ape alarm calls remains largely unstudied, despite its obvious relevance to ideas about the evolution of human speech. In this paper, we examine the potential use of combinatorial rules in the alarm calls of the only Asian great ape: the orangutan. Alarm calls in orangutans are composed of syllables (with either one or two distinct elements), which in turn are organized into sequences. Tigers and clouded leopards are predators for Sumatran orangutans, but in Borneo, tigers are extinct. Thus, orangutans make a suitable great ape model to assess alarm call composition in relation to the size of the predator guild. We exposed orangutans on both islands to a tiger and control model. Response compositionality was analyzed at two levels (i.e., syllable and syllable sequences) between models and populations. Results were corroborated using information theory algorithms. We made specific, directed predictions for the variation expected if orangutans used combinatorial rules. None of these predictions were met, indicating that monkey alarm call combinatorial rules do not have direct homologues in orangutans. If these results are replicated in other great apes, this indicates that predation did not drive selection towards ever more combinatorial rules in the human lineage.
KeywordsCall combinations Alarm calls Predation Primates Orangutans
This study was financially supported by Fundação para a Ciência e Tecnologia (SFRH/BD/44437/2008), Wenner-Gren Foundation for Anthropological Research, Dr. J.L. Dobberke voor Vergelijkende Psychologie, Lucie Burgers Foundation for Comparative Behaviour Research, Arnhem, The Netherlands, Schure-Beijerinck-Popping Fonds, Ruggles-Gates Fund for Anthropological Scholarship of the Royal Anthropological Institute of Great Britain and Ireland, Netherlands Organization for Scientific Research (NWO), Primate Conservation, Inc., and Fundação Calouste Gulbenkian. We thank RISTEK, PHKA, TNGL, and BPKEL for the research permission in Indonesia and UNAS, BOS, and SOCP for supporting the project. We also thank Natasha Nieuwenhuis and the field assistants involved in the experimental work.
This research complied with the current Indonesian law.
Conflict of interest
The authors have no financial relationship with the organizations that sponsored the research. The authors declare that they have no conflict of interest.
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