American Journal of Primatology 70:566–574 (2008) RESEARCH ARTICLE Geographic Variation in Thomas Langur (Presbytis thomasi) Loud Calls SERGE A. WICH 1,2Ã , ANNE MARIJKE SCHEL 3 , AND HAN DE VRIES 2 1 Great Ape Trust of Iowa, Des Moines, Iowa 2 Behavioural Biology, Utrecht University, Utrecht, The Netherlands 3 School of Psychology, University of St. Andrews, St. Andrews, Scotland Geographic variation in primate vocalizations has been described at two levels. First, at the level of acoustic variation within the same call type between populations and, second, at the level of presence or absence of certain call types in different populations. Acoustic variation is of interest because there are several factors that can explain this variation, such as gene flow, ecological factors and population density. Here we focus on the first level in a Southeast Asian primate, the Thomas langur. We recorded male loud calls in four populations that differed in their geographic distances from each other and had varying geographic barriers in between them, such as rivers and mountain ranges. The presence of these barriers leads to expectations of loud call variation under the gene flow model, which are examined here. We conducted a principal components analysis to condense the number of acoustic variables. With a subsequent discriminant function analysis on the six principal component scores, we found that the percentage of loud calls that were correctly assigned to a population was relatively high (50.0–76.2%) when three randomly selected loud calls from each male were used. Using the discriminant functions from this analysis to predict population membership of the remainder of the loud calls yielded lower, but still relatively high correct assignment percentages (26.2–66.7%). Analyses to examine the influence of barriers on similarities between populations confirm our expectations. We discuss that differences in loud calls are probably most parsimoniously explained by gene flow (or the lack thereof) between the populations and that future studies of genetic differences are crucial to test this hypothesis. Am. J. Primatol. 70:566–574, 2008.  c 2008 Wiley-Liss, Inc. Key words: long-distance vocalizations; gene flow; Indonesia; Sumatra; Leuser INTRODUCTION Acoustic variation in primate vocalizations has been demonstrated at several levels. At the indivi- dual level, it has been shown that individual calls show variation [e.g. Chapman & Weary, 1990; Marler & Hobbett, 1975; Snowdon & Cleveland, 1980; Waser, 1977; Wich et al., 2003a,b]. In addition, there is within individual variation related to various contexts [Clark & Wrangham, 1993; Crockford & Boesch, 2003; Digweed et al., 2005; Gouzoules et al., 1984; Norcross & Newman, 1993; Seyfarth et al., 1980; Vercauteren-Drubbel & Gautier, 1993; Waser, 1977; Wich et al., 2003a; Zuberbu ¨hler et al., 1997]. On a different level, it has been shown that there is geographic variation in the acoustic structure in calls of primates and non-primates [Bradbury & Vehren- camp, 1998; Crockford et al., 2004; Doutrelant et al., 1999; Green, 1975; Maeda & Masataka, 1987; Marshall et al., 1999; Mitani et al., 1992, 1999; Notman & Rendall, 2005; Peters et al., 2000]. The majority of primate studies have focused on chim- panzees [Crockford et al., 2004; Marshall et al., 1999; Mitani et al., 1992, 1999] and more recently orangutans [Delgado, 2003, 2007]. As a result, we know very little about how general the results of these studies are for other primate species. In addition, although these studies report geographic variation, the factors determining such variation are still poorly understood. The most commonly consid- ered factors of influence on call variation are anatomical differences that are often thought to be related to body size differences [Bowman, 1979; Fitch, 1997; Ryan & Brenowitz, 1985], differences in habitat acoustics [Bertelli & Tubaro, 2002; Waser & Waser, 1977; Wiley & Richards, 1978], the sound environment caused by local biota [e.g. Sorjonen, Published online 14 January 2008 in Wiley InterScience (www. interscience.wiley.com). DOI 10.1002/ajp.20527 Received 11 April 2007; revised 17 December 2007; revision accepted 27 December 2007 Contract grant sponsors: Netherlands Foundation for the Advancement of Tropical Research (WOTRO); The Treub Foundation; The Dobberke Foundation; The Lucie Burgers Foundation for Comparative Behaviour Research; European Commission; Government of Indonesia. Ã Correspondence to: Serge Wich, Great Ape Trust of Iowa, 4200 SE 44th Ave, Des Moines, IA 50320. E-mail: swich@greatapetrust.org r r 2008 Wiley-Liss, Inc.