Vocalizations may serve to locate group members, alert group members of predators, convey behavioural states, attract mates or defend territories. Previous studies on vocalizations have concentrated on a wide range of species including birds (Nottebohm, 1972; Shutler and Weatherhead, 1990; Temeles, 1990; Confer, 1992; Langmore et al., 1996; Langmore and Davies, 1997; Penteriani, 2003; Hall, 2004), frogs (Gerhardt, 1994), cetaceans (Janik and Slater, 1998) and primates (Zuberbuhler et al., 1997; Geissmann, 2000; Kitchen, 2004; Braune et al., 2005).
Amongst primates, vocalization studies are becoming more important for conservation management, particularly for rare species that are difficult to observe directly. Vocalization studies are particularly useful and important as they offer a non-invasive approach allowing for both estimating population densities, and determining taxonomic identification of un-habituated individuals (Nietsch, 1999; Zimmermann et al., 2000; Ross and Geissmann, 2007). Two important methodologies include using vocalizations to survey primates by estimating numbers of groups through triangulation (Estrada et al., 2003; Geissmann and Nijman, 2006; Aldrich et al., 2008) or numbers of individuals by their unique vocal signatures (Boinski and Mitchell, 1997; Steenbeek and Assink, 1998; Kojima et al., 2003). A third method uses vocalizations as a tool for distinguishing taxonomic variation between species or subspecies (Haimoff and Gittins, 1985; Konrad and Geissmann, 2006). The relative stability of the structural units in primate vocal patterns (Kummer, 1970), due to their resistance to alteration by their environment (Doyle, 1978), allows for direct comparison between individuals, populations and subspecies. This feature is essential when assessing phylogenetic relationships (Struhsaker, 1970; Hohmann, 1989). Before such studies are conducted, however, it is vital that the basic vocal parameters of a species are determined (Brockelman and Ali, 1987).
The purple-faced langur (Trachypithecus vetulus spp.) is a highly vocal primate endemic to Sri Lanka. Adult males are the most vocal of all age and sex classes, giving characteristic loud ‘whoop’ calls (Hohmann, 1990). Similar to male grey langurs (Ripley, 1967), Nilgiri langurs (Poirier, 1970), howler monkeys (Altmann, 1959; Chivers, 1969) and gibbons (Geissmann, 2002), male purple-faced langurs emit a daily morning loud ‘whoop’ call as well as additional whoop calls throughout the day (Hohmann, 1990). Morning whoop calls are usually given shortly after sunrise and function to alert neighbours of group presence (Ripley, 1967). In comparison, whoop calls given later in the day aid in territory defence and are often accompanied by intense locomotive displays (Manley, 1986; Hohmann, 1990).
Four subspecies of purple-faced langur (T. v. vetulus, T. v. philbricki, T. v. monticola, T. v. nestor) are currently recognised, with a fifth subspecies (T. v. harti) postulated based on museum specimens (Deraniyagala, 1955; Nekaris and de Silva, 2008). The Critically Endangered Western purple-faced langur (T. v. nestor) (Fig. 1) is not only the least studied of these taxa (Eudey et al., 2006), but is also listed as one of the ‘Top 25 Most Endangered Primates’ (Mittermeier et al., 2006).
Fig. 1. Sub-adult male Western purple-faced langur eating a jack fruit in a home garden in the Talangama Wetlands; an almost total loss of forest habitat has resulted in this taxon’s classification of Critically Endangered (photo by R. Moore).
With less than 3% of its original habitat remaining, T. v. nestor is found almost exclusively in home gardens, making traditional line transects an unfeasible method to track its dwindling populations (Rudran, 2007; Parker et al., 2008). Furthermore, the exact limits of its geographic distribution are unknown (Dela, 2007), yet comparative vocal studies may offer a systematic means to confirm and classify subspecies at their boundaries.
Also of significance to the Western purple-faced langurs are the affects of increased human-wildlife conflict, decreased home range availability and increased encounters with neighbouring troops (Dela, 2004; Eschmann, 2007). By comparing factors such as call times, reasons for calling and number of calls per day between troops ranging in areas of different degrees of forest fragmentation and human population levels, it may be possible to assess the impact and the potential threat that human activity is having on purple-faced langurs (Nijman, 2001).
Although descriptive data are available regarding the male vocalizations of T. v. philbriki and T. v. monticola (Manley, 1986; Hohmann, 1990), as well as some qualitative data on T. v. vetulus (Douglas, 2006), no data have been published on the calls of T. v. nestor. Here, we quantitatively describe the calls of these langurs for the first time based on a short but intensive study of six troops at Talangama Wetlands, Sri Lanka. We also address several questions. Can whoop calls be used to distinguish males? What environmental and anthropogenic variables influence calling patterns? Can call parameters be used to distinguish the different taxa of purple-faced langurs?