Estimation of White-Handed Gibbon Density and Population Size in Huai Kha Khaeng Wildlife Sanctuary, Western Thailand

All 20 gibbon species (Hylobatidae) are listed as Threatened or Endangered on the IUCN Red List. The four species of gibbons occurring in Thailand survive only in well-protected forests and need to be effectively monitored for conservation management. Because of their habit of duetting and living in small-group territories, gibbons are usually surveyed using acoustic methods employing point counts. We surveyed the white-handed gibbon (Hylobates lar) population in Huai Kha Khaeng Wildlife Sanctuary in western Thailand to determine the long-term viability of the population. We combined the listening-point method using arrays of listening posts with the point transect Distance method with prior random placement of sample points. We placed 39 sample arrays in dry (seasonal) evergreen forest (DEF), the optimal forest type for gibbons in the sanctuary, and 34 arrays in selected areas of mixed deciduous forest (MDF). During the 20 months of survey (2017–2021), we estimated an average density of 3.74 groups km⁻² for DEF and 1.10 groups km⁻² for MDF. The total number of gibbon groups in the habitats was estimated at 2568 (95% CI: 2156–3063) in DEF and 1482 (95% CI: 1172–1887) in MDF. A multiple covariate distance sampling analysis revealed that forest type had a major effect on gibbon density, while altitude and distance to poaching signs had little or no effect. The most serious threat to gibbon populations in the WEFCOM complex in the future is likely to be increased dryness and degradation of forests caused by the global anthropomorphic increases in temperature.