Calling patterns of Western purple-faced langurs (Mammalia: Primates: Cercopithecidea: Trachypithecus vetulus nestor) in a degraded human landscape in Sri Lanka

The study of calling patterns is a useful non-invasive method for determining population densities and the taxonomic relationships of rare or cryptic animal species. The Western purple-faced langur Trachypithecus vetulus nestor, endemic to Sri Lanka’s lowland rainforests, is severely impacted by forest fragmentation, with most remaining populations living almost completely in home gardens. Due to their shy nature, little is known about the behaviour of this subspecies; analysing the regular loud calls emitted by these langurs could allow for improvement of census techniques, clarification of their taxonomy, and an understanding of the impact of forest destruction on their behaviour. In 2007, we recorded the calling patterns of five male T. v. nestor at Talangama Wetlands. Time, duration, weather conditions, and stimulant of 253 calls were noted. Loud calls comprised three structural units: harsh barks, whoops and residuals. The average call contained 4 phrases and 3.8 residuals, was 38 seconds in length, had an average maximum frequency of 3.5 kHz, a formant frequency of 0.36 kHz, and a fundamental frequency of 0.2 kHz. Significant differences were found between individuals for the number of phrases and residuals within a call, two different phrase lengths, the formant frequency and the fundamental frequency. The earliest call occurred at 05:27 hrs, while the latest was made at 17:57 hrs. The greatest percentage of calls (73.5%) was heard in the morning (05:00-09:59 hrs), mostly stimulated by territorial battles with neighbouring troops. These results show that vocalisations can be used to distinguish individual males; as langurs are more often heard than seen, and most troops contain only a single adult male, vocalisations may be used to determine the number of troops in an area. Calls of this taxon also differed from the other subspecies, suggesting that they may be used to distinguish subspecies and their boundaries. Finally, calling behaviour differed from other subspecies. Deforestation may be a direct cause of different calling patterns. These baseline data form a valuable starting point for further studies of this Critically Endangered primate.