The loud calls of adult male Western purple-faced langurs are similar in structure, function and distribution throughout the day to those of other colobines (Ripley, 1967; Poirier, 1970; Marler, 1972; Vogel, 1973; Horwich, 1976; Oates and Trocco, 1983; Herzog and Hohmann, 1984; Hohmann, 1989). The calls comprised characteristic acoustic units. Analyses of the organisation of tonal units revealed a common syntax structure between individuals, as well as individual differences. These calling patterns are comparable to those documented for the subspecies T. v. philbriki (Hohmann, 1990), T. v. monticola (Manley, 1986; Hohmann, 1990) and T. v. vetulus (Douglas, 2006).
The frequencies of Western purple-faced langur calls are well-adapted for optimal propagation over long distances. Calls with frequencies ranging from 0.10-1.00 kHz propagate well, while calls with frequencies near 0.2 kHz have low attenuation rates (Waser and Waser, 1977; Wiley and Richards, 1978; Wiley and Richards, 1982; Waser and Brown, 1986). The frequency range of T. v. nestor in Talangama Wetlands was 0.13-5.38 kHz. Although the maximum observed frequency is much higher than the previously reported optimal frequency range, these measurements were taken from the harsh barks within a call. In addition, because of the repetitive nature of the whoop units that make up the bulk of purple-faced langur loud calls, encoded information is thought to degrade slowly, thus allowing intact messages to travel further distances before losing their content (Wiley and Richards, 1978; Wiley and Richards, 1982; Waser and Brown, 1986). The average formant frequency of T. v. nestor calls was 0.36 kHz. This frequency, which has been observed in a variety of primate species (Struhsaker, 1970; Oates and Trocoo, 1983), is believed to interact minimally with reverberation and ambient noise (Waser and Brown, 1986). Given the large differences observed between the frequencies of the harsh barks and the whoop units, it is possible that the harsh bark functions to transmit information to individuals relatively close by, while the whoop units transmit information to individuals further away.
Loud calls are useful in distinguishing individual purple-faced langurs. Significant differences between individuals A, B, G, L and P were found in the number of phrases per call, the number of residuals per call, the formant frequency and the fundamental frequency. Based on these findings, it is possible to deduct that information regarding the caller’s identity, location and behavioural state is contained within these call parameters (Busnel, 1977; Cowlishaw, 1996). Distinctiveness of calls can also facilitate future survey efforts, allowing researchers to detect groups based on calls alone (Geissmann et al., 2005). In addition loud calls of other primates are known to contain specific information regarding the physical fitness of the caller, the size of the troop and the presence or absence of young (Wilson et al., 2001; Kitchen, 2004). It is possible that the differences observed in the calls of the males in this study were the result of individually distinctive messages regarding troop composition and fitness encoded into the loud calls.
Although troops were followed from dawn till dusk, there was a marked difference in the numbers of calls heard throughout the day, with the fewest calls occurring in the evening. The majority of calls were observed in the morning and correlated with increased activity levels during that period (Eschmann, 2007). This is consistent with findings for the other subspecies (Hohmann, 1990; Douglas, 2006) as wells as with other colobines (Ripley, 1967; Poirier, 1970; Chivers, 1973; Vogel, 1973; MacKinnon, 1974; Horwich, 1976; Herzog and Hohmann, 1984; Hohmann, 1989), cebids (Altmann, 1959; Horwich and Gebhard, 1983), titi monkeys (Aldrich et al., 2008), and lesser apes (Chivers, 1973; MacKinnon, 1974; Geissmann, 2002; Geissmann and Nijman, 2006; Konrad and Geissmann, 2006). Increased calling in the morning hours may be the result of optimal calling conditions. A combination of warmer air above the tree canopy and cooler air below it acts as a reflector of sound, increasing the intensity of the call at further distances (Waser and Waser, 1977; Wiley and Richards, 1978).
Although environmental characteristics certainly impact calling times, anthropogenic disturbance also seems to play a role. The earliest morning call of T. v. nestor was heard at 05:27 hrs, which was half an hour before sunrise, later than reported by Ripley (1967) for the species as a whole. The latest morning call occurred at 06:25 hrs. The majority of calls were heard prior to the technical sunrise, in conjunction with the first detectable signs of the sun. Daily human activities did not appear to affect the timing of morning calls, as calls were not dependent on the levels of noise from neighbouring homes (Eschmann, 2007). In comparison, the majority of T. v. vetulus morning calls occurred post-dawn when the sun had fully penetrated to the forest floor (Douglas, 2006). The fact that T. v. nestor calls before sunrise may be related to the meagre canopy continuity in the Talangama Wetlands. Most trees have been cut down to make room for housing developments; the few that remain occur in fragmented belts throughout the villages (Eschmann, 2007). Because of this, much of the monkeys’ home ranges are exposed to direct sunlight.
Loud calls of colobines are thought to function in the maintenance of a troop’s territory (Eisenberg et al., 1972). Similar to findings regarding T. v. philbriki and T. v. monticola (Hohmann, 1990), the majority of the loud calls of T. v. nestor were induced by sunrise and neighbouring troops of monkeys. Morning calls are believed to space troops, while daytime calls are thought to aid in the defence of home ranges (Ripley, 1967; Eisenberg et al., 1972; Manley, 1986; Hohmann, 1990). As purple-faced langurs are extremely territorial (Ripley, 1967; Manley, 1978; Manley, 1986; Nekaris and de Silva Wijeyeratne, 2008), these results are not surprising. As observed in other primates (e.g. Kitchen, 2004), two or more neighbouring alpha males commonly responded directly to the loud calls of each other with their own loud call. This calling pattern is ideal for triangulation studies (Estrada et al., 2003), providing yet another non-invasive method to estimate much-needed population densities of these monkeys in a highly fragmented urban landscape (Rudran, 2007).
Human activity seemed to have little effect on the calling behaviours of langurs in the Talangama Wetlands. Although people often came into contact with troops of monkeys, the monkeys rarely responded. In fact, monkeys were so desensitized to humans that they often refused to move from feeding trees even when they were being verbally threatened or when rocks were thrown at them. All human-induced loud calls were the direct result of interactions with the unfamiliar researchers and not the locals. Airplanes and vehicles, which have been previously reported to induce loud calls (Hohmann, 1990; Dela, 2007), elicited no response from Western purple-faced langurs. The consequence of living in such a highly urbanized environment has desensitised the langurs to the effects of most human activity.
Although loud calls were recorded under a variety of weather conditions, the majority of calls were observed when the weather was either cloudy or overcast. Horwich (1976) has reported a significant preference of Nilgiri langurs (T. johnii) for calling when the weather is cloudy and misty. The data regarding weather and calling patterns of langurs in this study do not necessarily reflect a preference for vocalizing under cloudy conditions, but more likely reflect that the study took place during the monsoon season. In order to depict more accurately how weather influences call patterns and distribution throughout the day, further research should be conducted during non-monsoon periods. Decreased occurrences of calls during the rain may stem from the fact that rain decreases propagation and increases attenuation of calls (Wiley and Richards, 1978), suggesting that vocal studies must not exclude days affected by monsoon rains when assessing the impact of weather on the vocal repertoire of purple-faced langurs. Most studies that use primate calls to estimate densities exclude rainy days (Brockelman and Ali, 1987; Buckley et al., 2006); such methods should be reconsidered when estimating small and fragmented populations of purple-faced langurs.
In this study we have provided evidence that purple-faced leaf monkey calls are distinct amongst individual males; this combined with their ritualised morning occurrence makes them ideal for future survey work of this Critically Endangered subspecies. Quantitative differences between taxa are also evident, and future work should not only aim to compare the calls of all four recognized subspecies (T. v. nestor, T. v. vetulus, T. v. philbriki and T. v. monticola), but also the calls of individuals who are suspected of being T. v. harti. This type of comprehensive study would ultimately validate the existence of the fifth postulated subspecies, while also providing a means for determining the boundaries of each subspecies. As human encroachment on their habitat continues, langurs are being forced into smaller ranges. Long-term studies of T. v. nestor should address the impact of habitat reduction on the vocal behaviour of this highly territorial taxon.