Quantitative Analysis of Tarsier Duet Calls from Field Surveys Reveals a New Acoustic Form in Gorontalo (Indonesia)

Islands are natural laboratories for the study of speciation mechanisms, such as allopatric diversification and adaptive radiation. Our study focuses on the long northern arm of Sulawesi, which is the home of three known tarsier species: Tarsius spectrumgurskyae, T. supriatnai, and T. wallacei. The precise location of the boundaries was unknown, but a biogeographic hypothesis for the island made predictions as to where they would be. We used acoustic surveys to locate tarsier faunal boundaries and contact zones along both coasts of the northern peninsula. We analyzed the duet parameters of 82 tarsier duet calls from 49 locations. Our results revealed four acoustic groups: Manado (T. spectrumgurskyae), Gorontalo (T. supriatnai), Tinombo (T. wallacei), and a previously unknown group between Manado and Gorontalo forms, which we call the Labanu form. Our results on the south coast revealed faunal boundaries associated with geographic barriers. Along the north coast, faunal boundaries were not associated with geographic barriers. Intensive survey efforts identified heterospecific groups in a single spectrogram. The study region has undergone significant deforestation, particularly the region where the Labanu form is found. We suspect this form to be a stable hybrid, formed by secondary contact between T. spectrumgurskyae and T. supriatnai. We estimate that the Labanu form would be Red Listed as Endangered should it be determined to be a new species. Follow-up genetic studies are urgent to validate the taxonomic status of the new acoustic form before it becomes extinct due to habitat loss.