Densities, distribution and detectability of a small nocturnal primate (Javan slow loris Nycticebus javanicus) in a montane rainforest

Abstract

Nocturnal mammals can be challenging to survey and, especially for many species that live in dense forest habitats, limited information is available on densities and distributions. We surveyed the endemic Javan slow loris Nycticebus javanicus in the montane forests of Mount Gede Pangrango, West Java, Indonesia. Surveys were conducted on 23 transects (260 h covering some 93 km) walking at variable speeds between 200 and 800 m h �1 . Densities on individual tran- sects varied from 0 to 52 ind. km �2 , with an overall density of 15.6 ind. km �2 (95% CI 9.7 to 25.2 ind. km �2 ). Encounter rates per kilometre were strongly influenced by the speed at which transects were walked, with fewer lorises detected at higher speeds. This effect was absent when consider- ing encounter rates per hour. Detectability and behavior of Javan slow lorises were not affected by the amount of lunar light and, in contrast to studies of some of their congeners, we found no evi- dence of lunar phobia or lunar philia. Our study shows that slow lorises are not homogeneously distributed in their montane habitat and occur at intermediate densities. However, encounter rates did not differ between disturbed and primary forest. Analysis of data from multiple surveys of lor- isforms, including this one, reveals no statistically significant relationship between survey effort or the speed at which transects are walked and estimated densities, but speed is positively correlated with encounter rates.