Recent work on the tarsiers of Sulawesi indicates that the degree of species differentiation is greater than has hitherto been thought. This is indicated by the acoustic differences between the vocal duets of mated pairs of Tarsius spectrum, Tarsius dianae and tarsiers from the Togian Islands (Tomini Bay, Sulawesi). This paper presents further evidence of taxonomic differentiation, based on the vocal behaviour shown during playback experiments. Six adult T. spectrum, kept in captivity, were stimulated by the playback of duets of their own species, those of T. dianae and those of Togian tarsiers. The results show that T. spectrum discriminates between the vocalizations of conspecifics and heterospecifics. The behavioural responses of the test subjects to playback of the various duets suggest that T. spectrum is unlikely to interbreed with either T. dianae or with the Togian tarsiers. This not only confirms the taxonomic differentiation between T. spectrum and T. dianae, but also supports the specific status of the Togian tarsiers.
Geographic populations of Sulawesi tarsiers differ conspicuously in the acoustic structure of the loud duet calls produced by mated pairs. There is evidence that the differences in call features relate to species differentiation. Duet calls can thus be used as a tool to identify cryptic species and for their monitoring in the field. We recorded duets from various geographically isolated populations of Tarsius tarsier in south and southeast Sulawesi to survey the species identity of the tarsiers in these regions. We compared their calls with the duet calls of the Manado tarsier (= Tarsius spectrum), Tarsius dianae, and the Togian Island tarsier. In addition, we recorded and analyzed the duet call of the tarsier from Peleng Island, Tarsius pelengensis. Qualitative characteristics of duet songs of both sexes, analysis of frequency and temporal parameters, and multivariate statistics show 4 distinct vocal forms that predict the existence of 4 new species. These are found on Selayar Island, on the south mainland (Bantimurung), on the southeast mainland (Puwato), and on the Islands of Kabaena and Buton. Our analysis of the call of the Peleng tarsier revealed striking similarities with the call of Tarsius dianae. Further morphological analyses are needed to support the proposed new species in south and southeast Sulawesi and to clarify the taxonomic affinity between the central Sulawesi tarsier species Tarsius pelengensis and T. dianae.
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