778 H. Trinh-Dinh et al.
TABLE 1 Survey results for the cao vit gibbon Nomascus nasutus population in 2007 and 2016. Year
No. of gibbon groups
2007 2016
17–18 20–22
No. of transitory individuals
0 3–4
Total no. of individuals
102–110 107–136
Country
China China
Survey dates
Viet Nam 07–19 Sep. 07–14 Sep.
Viet Nam 11–24 Sep. 11–16 Sep.
No. of surveyors
22 17 31 16
No. of listening posts
18 6
21 8
survey posts in Viet Nam and six in China (Supplementary Table 1), located on outcrop peaks and ridges, where we could record gibbon vocalizations from several nearby val- leys. The surveys posts covered all potential gibbon habitats (Fan et al., 2013) and all locations where gibbons had been recorded during previous surveys and routine patrols. In 2016 we used the same survey points from 2007 and set up three new posts in Viet Nam and two in China (Supplementary Table 1), to sample an area of regenerating habitat that was considered suitable for gibbons (Fig. 1). The survey dates and number of researchers involved in
the surveys are presented in Table 1. To ensure that the field teams used the same techniques for all surveys, all research- ers attended a 2-day training course prior to the field sur- veys, although most had already participated in previous gibbon surveys in this area and were familiar with the mor- phological characteristics and calls of cao vit gibbons. A previous study of three groups found cao vit gibbons
vocalized mostly (96.4% of recorded calls) in the mornings, from 30 minutes before to 4 hours after sunrise (Fei et al., 2010). Individual groups vocalized on a mean of 69.6% of all monitoring days and the mean number of singing bouts on days when they did vocalize was 1.24 per day (Fei et al., 2010). As the sun rises at c. 6.30 in September in our survey area, we monitored all survey points during 06.00– 11.00 for at least 4–6 successive days. We occasionally occu- pied the survey points until 17.00 to observe gibbons when they were foraging nearby. Once the gibbon songs were heard, researchers recorded the date, location of survey posts, direction and estimated distance between survey posts and gibbon groups, and the start and end time of each song bout. We recorded the call type and the number of individuals producing a particular type of call. Call types included duet song bouts, male solo song bouts, female solo song bouts, isolated great-calls, and alarm calls (Feng et al., 2013). Song bouts or isolated great-calls were defined as distinct vocalization events if they were separated from previous calls by an interval of at least 5 minutes. During the survey, we tried to locate and observe gibbon
groups if they were calling close to the survey points. Because gibbons are canopy dwellers and canopy height is low in our study area, researchers could observe gibbons from survey points using binoculars (Fan et al., 2011, 2012). Once we located a gibbon group, we recorded the
start and end time of the sighting, and the direction and dis- tance of the gibbons from the researchers. We also recorded the group size and composition (including adult male, adult female, juvenile and infant), to distinguish between neigh- bouring groups. Adult male cao vit gibbons were defined as fully grown and black in colour. Adult females are yellow with a white facial ring and a large black crest extending from head to the centre of their back. Juveniles are black, smaller than adult males, and fully independent from their mother. Infants are black and still dependent on their mother (Mootnick & Fan, 2011). In addition, we also recorded floating individuals; i.e. young adults that have emigrated from their natal group and, at least temporarily, live on their own. To avoid double counting groups that live along the
border and could be heard simultaneously by both teams, researchers from both countries exchanged and discussed survey results after the field surveys, in May 2008 and October 2016. We calculated mean group size based on groups that had been observed and counted, and then multi- plied mean group size by the number of groups to estimate total population size.
Results
Population size of cao vit gibbons The combination of listening and sighting information from the surveys indicated there were 17–18 groups in 2007 and 20–22 groups in 2016 (Table 1). In addition, we recorded 3–4 floating individuals in 2016 (none were recorded in 2007). During the survey we counted all members of 14 groups in 2007, and of 13 groups in 2016. The mean group size was 6.3 ± SD 2.02 (n = 14, range 3–9)in 2007, and 6.4 ± SD 1.50 (n = 13, range 4–8)in 2016 (Tables 2 & 3). Consequently, the estimated population size was 102–110 in 2007 and 107–136 in 2016 (Table 1).
Group structure of cao vit gibbons
Most groups had two adult females (11 of 14 fully observed groups in 2007, and 12 of 13 groups in 2016). In 2016,we recorded one group with three females. Two females both
Oryx, 2020, 54(6), 776–783 © 2019 Fauna & Flora International doi:10.1017/S0030605318001576
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