Abstract
Vocal individuality is a prerequisite for individual recognition, especially when visual and chemical cues are not available or effective. Vocalizations encoding information of individual identity have been reported in many social animals and should be particularly adaptive for species living in large and complexly organized societies. Here, we examined the individuality in coo calls of adult male golden snub-nosed monkeys (Rhinopithecus roxellana) living in a large and multilevel society. Coo calls are one of the most frequently occurring call types in R. roxellana and likely serve as the signals for contact maintenance or advertisement in various contexts including group movement, foraging, and resting. From April to October 2016, April to July 2017, and September to October 2017, we recorded a total of 721 coo calls from six adult males in a provisioned, free-ranging group and one adult male in captivity in Shennongjia National Park, China. We selected 162 high-quality recordings to extract 14 acoustic parameters based on the source-filter theory. Results showed that each of all parameters significantly differed among individuals, while pairwise comparisons failed to detect any parameter that was different between all pairs. Furthermore, a discriminant function analysis indicated that the correct assignment rate was 80.2% (cross-validation: 67.3%), greater than expected by chance (14.3%). In conclusion, we found evidence that coo calls of adult male R. roxellana allowed the reliable accuracy of individual discrimination complementarily enhanced by multiple acoustic parameters. The results of our study point to the selective pressures acting on individual discrimination via vocal signals in a highly gregarious forest-living primate.
Highlights
Vocal cues play an important role in the instant communication of social animals, especially when visual and chemical signals are not available or effective (Kondo and WatanabeElectronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.2009; Rendall and Owren 2002)
Based on the source-filter theory, we investigated the individuality in coo calls of adult male R. roxellana
We found that the CVb value of each acoustic parameter was higher than the mean CVw value, and all potential for individual identity coding (PIC) values were greater than 1 (Table 1)
Summary
The acoustic structures of certain call types display graded within-type variation (Soltis et al 2005) This acoustic variation usually conveys important functionrelated information about callers, such as group membership (Delgado 2007; Fan et al 2011), age (Charlton et al 2009a; Fischer et al 2004), sex (Charlton et al 2009a; Ey et al 2007), body size (Pfefferle and Fischer 2006; Reby and McComb 2003), social rank (Bergman et al 2003; Fischer et al 2004), and estrus state (Charlton et al 2010). Individual distinctiveness in acoustic features of the same call type has been reported in many species of social animals (e.g., Spheniscus demersus: Favaro et al 2015; Pan troglodytes: Levréro and Mathevon 2013; Papio hamadryas ursinus: Rendall 2003; Dama dama: Vannoni and McElligott 2007). Vocal individuality is essential for kin selection such as in the context of parental investment (Callorhinus ursinus: Charrier et al 2003; Papio cynocephalus ursinus: Rendall et al 2000), and for sexual selection, which involves male–male competition and female choice (Cervus elaphus: McComb 1991)
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