Introduction

A central goal of animal communication research is to understand the function of vocalizations within a species (Bradbury & Vehrencamp 1998) and/or between species (e.g., Zuberbühler 2000). In this context, whether a communication system is better characterized as graded or discrete is a fundamental question (Marler 1975; Morton 1982; Cheney & Seyfarth 1990; Fitch 2010). While graded systems show continuous variation in acoustic structure, lacking strict boundaries between call types, discrete systems show acoustically distinct call types generally lacking structurally intermediate forms (Marler 1975, 1977; Keenan et al. 2013). To investigate this issue, a key step is to define a representative acoustic repertoire of the study species (Hauser 1997). Many studies, especially on primates, have defined vocal repertoires and explored this topic, documenting both graded (Rowell & Hinde 1962; Marler 1976; Fischer & Hammerschmidt 2002) and discrete (Zuberbühler et al. 1997; Arnold & Zuberbühler 2008) repertoires. Some mixed cases have also been reported and showed varying levels of gradation depending on call types and/or sex (Bouchet et al. 2010; Lemasson & Hausberger 2011; Keenan et al. 2013).

Acoustic continuity has also been investigated in non‐primate mammalian species vocalizations (Volodina 2000; Boisseau 2005; Stoeger‐Horwath et al. 2007; Nair et al. 2009). For example, the first description of the domestic pig (Sus scrofa domesticus) vocal repertoire by Kiley (1972), identified a degree of acoustic gradation in their definition of ‘grunt‐squeals’: an intermediate vocalization between ‘grunts’ and ‘squeals’. More recent work revisited the pig repertoire (Tallet et al. 2013), similarly concluding that gradation between the acoustic categories is prominent. However, most of the work carried out on domestic pig has focused on a single call type and specific conditions, thus neglecting much of the potential acoustic variability in this species. One reason is that the domestic pig is among the most intensively farmed species on the planet, and studies often focus on specific circumstances (i.e., those particularly relevant to welfare; Whittemore & Kyriazakis 2006). Examples of such focused studies include castration (Puppe et al. 2005), mother‐offspring recognition (Illmann et al. 2002, 2008), nursing (Algers 1993), experimentally‐induced stress (Marchant et al. 2001), or discomfort (Hillmann et al. 2004). This approach has been productive in that it has improved our understanding of how vocalizations reflect the physiological and emotional status of pigs (Schrader & Todt 1998; Düpjan et al. 2008) as well as their interindividual interactions (Kiley 1972; Schön et al. 1999; Melisova et al. 2014). It has also stimulated better assessment practices for housing conditions and overall treatment (Weary et al. 1998; Manteuffel et al. 2004; Puppe et al. 2005; Leidig et al. 2009).

Much less is known about the acoustic signals of the domestic pig's close relative and presumed wild forebear, the wild boar (Sus scrofa), from which domesticus traces its ancestry (Rothschild & Ruvinsky 2011). Wild and domestic forms remain closely related: hybridization events occur under natural conditions (Scandura et al. 2011), and a given pair of domestic and wild animals will not necessarily be more genetically divergent than two wild animals drawn from geographically distinct populations (Scandura et al. 2008).

Given this close relationship, the investigation of vocal communication in wild boars is needed to establish a comparative foundation for understanding the evolutionary origins and the potential effect of domestication on domestic pig vocalizations. Nonetheless, the vocal repertoire of wild boars has received limited attention, with the only existing publications dating from the 1970s (Klingholz & Meynhardt 1979; Klingholz et al. 1979). These studies were based on data gathered over several years in the wild by ethologist Heinz Meynhardt and colleagues. They classified boar vocalizations into ten call types split between three main groups: voiced sounds or grunts (including contact), unvoiced sounds or squeals/screams (including fear, complaint, defense, fight, isolation, and hunger calls), and intermediate sounds (including alert, alarm, and advertisement calls) (Klingholz et al. 1979). Even though this classification implies discrete call types, in a follow‐up paper these authors discussed acoustic gradation between call types, concluding that although many aspects of wild boar vocalization are continuous, there are also acoustic invariants that may identify calls as discrete entities (Klingholz & Meynhardt 1979). Despite the impressive expertise and understanding these authors developed, their conclusions are potentially limited by a number of factors. First, they relied on visual inspection of spectrograms to define acoustic parameters, which may not provide an objective and consistent measurement method. Second, they performed no statistical analyses on their data. And third, because some of these call types were defined by a single utterance and/or were produced by a single individual, generalizations to the population level may be questionable.

The aim of the current study was to revisit the wild boar vocal repertoire based on the work of Klingholz et al. (1979), taking advantage of advances in digital sound recording and analysis, as well as modern statistical methodologies used in classification designs (Boisseau 2005; Bouchet et al. 2010; Gingras & Fitch 2013; Tallet et al. 2013; Baotic et al. 2014). Our goal was to provide an objective description of the wild boar vocal repertoire and call types, with consideration of acoustical intermediates, as suggested by research on domestic pig vocalization (Kiley 1972). We also applied a more cautious approach to the call type evaluation, which avoids assumptions about the potential meaning of calls. Thus, following the lead of some recent work with domestic pigs (Tallet et al. 2013), we did not assign call types based on behavioral contexts. Rather, we took an acoustic/analytical approach, using multinomial logistic regression modeling and hierarchical cluster analysis of objectively defined acoustic parameters to evaluate a perceptual classification based on the one developed by Klingholz et al. (1979). With this work, we thus establish an acoustically based classification of wild boar vocalizations and provide a basis for further research comparing wild boar and pig vocal behavior. We examine our results in the context of acoustic continuity in animal vocalizations and discuss potential contributions of this study to animal welfare and our understanding of the relationship between domestication processes and vocal communication.