A Comparison of Hylobatid Survey Methods Using Triangulation on Müller’s Gibbon (Hylobates muelleri) in Sungai Wain Protection Forest, East Kalimantan, Indonesia

Abstract

Density estimates are a common tool for assessing potential changes in primate populations over time and for evaluating important habitat characteristics such as preferred food sources. There are several different methods for estimating the density and population of wild primates, though the accuracy of these methods across different habitats and species is difficult to assess. We calculated the density of the population of Müller’s gibbon (Hylobates muelleri) in the pristine and regenerating forest in Sungai Wain Protection Forest in East Kalimantan, Indonesia from May to July 2012. We collected data on the location of bonded pairs and compared the results of two different density estimate methods: triangulation and point transect sampling using Distance software. The triangulation method yielded population estimates of 486.9 ± SD 132.6 individuals in the pristine forest and 274.3 ± SD 179.0 in the regenerating forest. Distance analysis produced population estimates of 580.5 ± CV 20.6 and 388.4 ± CV 23.4 individuals for the pristine and regenerating forest, respectively. The difference in the density estimates between methods was not significant. We hypothesize that point transect sampling overestimated group density based on the unusually high estimate, but further investigation into the accuracy of point transect analysis using Distance with respect to gibbons is needed. We conclude that triangulation remains an important tool for hylobatid surveys because of its efficacy in locating gibbon groups using acoustic detection.