Do cultural evolutionary processes shape the long-distance vocalisations of wild chimpanzees (Pan troglodytes)?

Cultural evolutionary theory has gained increasing levels of support over past decades (Boesch and Tomasello, 1998; Noad et al., 2000; Whiten, 2000, 2017a; Boesch, 2003; Whiten et al., 2017; Aplin, 2019), with many citing cultural evolution an additional layer to genetic evolution for many species (Mesoudi, Whiten and Laland, 2004; Whiten, 2017a). Cultural evolutionary theory predicts that when individuals innovate new behaviours within a population, these may then spread to local conspecifics through social learning (Whiten, 2000; Whiten and van de Waal, 2018). If these behaviours offer significant fitness benefits, they may persist over time, despite having no genetic basis (Boesch and Tomasello, 1998; Laland and Janik, 2006; Whiten et al., 2017). This process of cultural evolution is theorised to be able to produce population-level differences in the common behavioural practices of animals (Boesch, 1996; Whiten, 2005; Mesoudi, 2016, 2017; Aplin, 2019). Empirical evidence largely supports the theory that chimpanzee (Pan troglodytes) feeding, self-maintenance and social behaviours are shaped by cultural evolution (Boesch and Tomasello, 1998; Biro et al., 2003; Luncz, Mundry and Boesch, 2012; Gruber et al., 2015; Mesoudi and Thornton, 2018). Little attention has been given to the possibility that chimpanzee vocalizations may also show culturally driven variation. Vocal learning pathways have been identified in generating culturally distinct calls of passerine birds (Jenkins, 1978; Lynch, 1996; Aplin, 2019) and cetaceans (Deecke, Ford and Spong, 2000; Noad et al., 2000; Garland Ellen C. et al., 2017; Mahaffy et al., 2018). Evidence from captive chimpanzee populations suggests that the acoustic structure of pant-hoot calls (Marshall, Wrangham and Arcadi, 1999; Crockford et al., 2004) and food grunt calls (Watson et al., 2015) may also be shaped by vocal learning from conspecifics. However, evidence for vocal learning pathways within primates remains controversial, with publications such as Watson et al. 2015 being criticised for omitting key environmental and psychological variables within their analysis (Fischer, Wheeler and Higham, 2015). Given the well documented capacity for social learning in chimpanzees (Biro et al., 2003; Hobaiter et al., 2014; Whiten, 2017b; Whiten and van de Waal, 2018) - including reported evidence of vocal learning (Marshall, Wrangham and Arcadi, 1999; Watson et al., 2015) - and the prevalence of cultural variation in feeding, grooming, and social behaviours (Boesch and Tomasello, 1998; Biro et al. 2003; Luncz, Mundry and Boesch, 2012; Gruber et al. 2015) I propose to investigate the existence of population-level differences in chimpanzee vocalisations which may be attributed to cultural evolutionary processes. The long-distance 'pant-hoot' display offers a novel opportunity for cultural comparison. Pant-hoot calls are multi-purpose longdistance calls which are ubiquitous within wild chimpanzee populations, and therefore can be sampled from a wide range of study sites (Fedurek et al. 2013, 2016; Fedurek, Donnellan and Slocombe, 2014). Moreover, the extended durations of pant-hoot displays, as well as their complex vocal structures, provide many spectral, temporal and behavioural elements that can be exploited in inter-population comparisons (Fedurek, Zuberbuhler and Semple, 2017). For these reasons, it is proposed that panthoot calls are used as a case study to assess for cultural evolutionary processes shaping chimpanzee vocalisations.