We demonstrate that although auditory sampling is a useful tool, this method alone will not provide a truly accurate indication of population size, density and distribution of gibbons in an area. If auditory sampling alone is employed, we show that data collection must take place over a sufficient period to account for variation in calling patterns across seasons. The population of Hylobates albibarbis in the Sabangau catchment, Central Kalimantan, Indonesia, was surveyed from July to December 2005 using methods established previously. In addition, auditory sampling was complemented by detailed behavioural data on six habituated groups within the study area. Here we compare results from this study to those of a 1-month study conducted in 2004. The total population of the Sabangau catchment is estimated to be about in the tens of thousands, though numbers, distribution and density for the different forest subtypes vary considerably. We propose that future density surveys of gibbons must include data from all forest subtypes where gibbons are found and that extrapolating from one forest subtype is likely to yield inaccurate density and population estimates. We also propose that auditory census be carried out by using at least three listening posts (LP) in order to increase the area sampled and the chances of hearing groups. Our results suggest that the Sabangau catchment contains one of the largest remaining contiguous populations of Bornean agile gibbon.

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Acknowledgements: This work was carried out within the OuTrop-CIMTROP multi-disciplinary research project in the northern Sabangau forest, Central Kalimantan, Indonesia. We gratefully acknowledge the invaluable contribution of all the researchers and staff that assisted with the project. We gratefully thank the Centre for the International Cooperation in Management of Tropical Peatlands (CIMTROP) for sponsoring our research and providing invaluable logistical support. We thank the Indonesian Ministry of Science and Technology (RISTEK) and Director General of Nature Conservation (PHKA) for permission to carry out research in Indonesia. We are very grateful to Warren Brockelman, Mark Leighton and an anonymous reviewer for comments on earlier drafts of this manuscript.

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