Determining whether a species' vocal communication system is graded or discrete requires definition of its vocal repertoire. In this context, research on domestic pig (Sus scrofa domesticus) vocalizations, for example, has led to significant advances in our understanding of communicative functions. Despite their close relation to domestic pigs, little is known about wild boar (Sus scrofa) vocalizations. The few existing studies, conducted in the 1970s, relied on visual inspections of spectrograms to quantify acoustic parameters and lacked statistical analysis. Here, we use objective signal processing techniques and advanced statistical approaches to classify 616 calls recorded from semi-free ranging animals. Based on four spectral and temporal acoustic parameters-quartile Q25, duration, spectral flux, and spectral flatness-extracted from a multivariate analysis, we refine and extend the conclusions drawn from previous work and present a statistically validated classification of the wild boar vocal repertoire into four call types: grunts, grunt-squeals, squeals, and trumpets. While the majority of calls could be sorted into these categories using objective criteria, we also found evidence supporting a graded interpretation of some wild boar vocalizations as acoustically continuous, with the extremes representing discrete call types. The use of objective criteria based on modern techniques and statistics in respect to acoustic continuity advances our understanding of vocal variation. Integrating our findings with recent studies on domestic pig vocal behavior and emotions, we emphasize the importance of grunt-squeals for acoustic approaches to animal welfare and underline the need of further research investigating the role of domestication on animal vocal communication.

Department of Cognitive Biology University of Vienna Vienna Austria

Department of Cognitive Biology University of Vienna Vienna Austria; Department of Psychotherapy and Biopsychosocial Health Danube University Krems Krems Austria

Réserve de la Haute Touche Muséum National d'Histoire Naturelle Obterre France; Equipe Interactions Cellulaires et Fertilité UMR0085 Physiologie de la Reproduction et des Comportements Institut National de la Recherche Agronomique Nouzilly France

Voice Research Lab Department of Biophysics Faculty of Science Palacký University Olomouc Czech Republic

Zobrazit více v PubMed

Algers, B. 1993: Nursing in pigs: communicating needs and distributing resources. J. Anim. Sci. 71, 2826—2831. PubMed

Arnold, K. & Zuberbühler, K. 2008: Meaningful call combinations in a non‐human primate. Curr. Biol. 18, R202—R203. PubMed

Baotic, A. , Stoeger, A. S. , Li, D. , Tang, C. & Charlton, B. D. 2014: The vocal repertoire of infant giant pandas (Ailuropoda melanoleuca). Bioacoustics 23, 15—28.

Boersma, P. & Weenink, D. 2014: Praat: doing phonetics by computer [Computer program]. Version 5.3.12 edn, University of Amsterdam, The Netherlands, Retrieved from http://www.praat.org/.

Boisseau, O. 2005: Quantifying the acoustic repertoire of a population: the vocalizations of free‐ranging bottlenose dolphins in Fiordland, New Zealand. J. Acoust. Soc. Am. 117, 2318. PubMed

von Borell, E. , Bunger, B. , Schmidt, T. & Horn, T. 2009: Vocal‐type classification as a tool to identify stress in piglets under on‐farm conditions. Anim Welf. 18, 407—416.

Bouchet, H. , Pellier, A. S. , Blois‐Heulin, C. & Lemasson, A. 2010: Sex differences in the vocal repertoire of adult red‐capped mangabeys (Cercocebus torquatus): a multi‐level acoustic analysis. Am. J. Primatol. 72, 360—375. PubMed

Bradbury, J. W. & Vehrencamp, S. L. 1998: Principles of Animal Communication. Sinauer Associates, MA, Sunderland.

Bradshaw, J. W. S. & Cameron‐Beaumont, C. L. 2000: The signalling repertoire of the domestic cat and its undomesticated relatives In: The Domestic Cat: The Biology of Its Behaviour (Turner D. C. & Bateson P. P. G., eds). Cambridge University Press, New York, NY, pp. 68—93.

Briefer, E. F. 2012: Vocal expression of emotions in mammals: mechanisms of production and evidence. J. Zool. 288, 1—20.

Byrne, R. W. 1982: Primate vocalisations: structural and functional approaches to understanding. Behaviour 80, 241—258.

Charlton, B. D. , Zhihe, Z. & Snyder, R. J. 2009: The information content of giant panda, Ailuropoda melanoleuca, bleats: acoustic cues to sex, age and size. Anim. Behav. 78, 893—898.

Charlton, B. D. , Ellis, W. A. , McKinnon, A. J. , Cowin, G. J. , Brumm, J. , Nilsson, K. & Fitch, W. T. 2011: Cues to body size in the formant spacing of male koala (Phascolarctos cinereus) bellows: honesty in an exaggerated trait. J. Exp. Biol. 214, 3414—3422. PubMed

Charlton, B. D. , Ellis, W. A. , Larkin, R. & Fitch, W. T. 2012: Perception of size‐related formant information in male koalas (Phascolarctos cinereus). Anim. Cogn. 15, 999—1006. PubMed

Cheney, D. L. & Seyfarth, R. M. 1990: How Monkeys See the World: Inside the Mind of Another Species. University of Chicago Press, IL: Chicago.

Düpjan, S. , Schön, P.‐C. , Puppe, B. , Tuchscherer, A. & Manteuffel, G. 2008: Differential vocal responses to physical and mental stressors in domestic pigs (Sus scrofa). Appl. Anim. Behav. Sci. 114, 105—115.

Fischer, J. & Hammerschmidt, K. 2002: An overview of the barbary macaque, Macaca sylvanus, vocal repertoire. Folia Primatol. (Basel) 73, 32—45. PubMed

Fitch, W. T. 1997: Vocal tract length and formant frequency dispersion correlate with body size in rhesus macaques. J. Acoust. Soc. Am. 102, 1213—1222. PubMed

Fitch, W. T. 2010: The Evolution of Language. Cambridge University Press, MA: Cambridge.

Fitch, W. T. & Fritz, J. B. 2006: Rhesus macaques spontaneously perceive formants in conspecific vocalizations. J. Acoust. Soc. Am. 120, 2132. PubMed

Garcia, M. , Charlton, B. D. , Wyman, M. T. , Fitch, W. T. & Reby, D. 2013: Do red deer stags (Cervus elaphus) use roar fundamental frequency (F0) to assess rivals? PLoS One 8, e83946. PubMed PMC

Gingras, B. & Fitch, W. T. 2013: A three‐parameter model for classifying anurans into four genera based on advertisement calls. J. Acoust. Soc. Am. 133, 547—559. PubMed

Hauser, M. D. 1997: The Evolution of Communication. MIT Press, MA, Cambridge.

Hillmann, E. , Mayer, C. , Schön, P.‐C. , Puppe, B. & Schrader, L. 2004: Vocalisation of domestic pigs (Sus scrofa domestica) as an indicator for their adaptation towards ambient temperatures. Appl. Anim. Behav. Sci. 89, 195—206.

Illmann, G. , Schrader, L. , Špinka, M. & Šustr, P. 2002: Acoustical mother‐offspring recognition in pigs (Sus Scrofa domestica). Behaviour 139, 487—505.

Illmann, G. , Neuhauserová, K. , Pokorná, Z. , Chaloupková, H. & Šimečková, M. 2008: Maternal responsiveness of sows towards piglet's screams during the first 24 h postpartum. Appl. Anim. Behav. Sci. 112, 248—259.

Keenan, S. , Lemasson, A. & Zuberbühler, K. 2013: Graded or discrete? A quantitative analysis of Campbell's monkey alarm calls. Anim. Behav. 85, 109—118.

Kiley, M. 1972: The vocalizations of ungulates, their causation and function. Z. Tierpsychol. 31, 171—222. PubMed

Klingholz, F. & Meynhardt, H. 1979: Lautinventare der Säugetiere — diskret oder kontinuierlich? Z. Tierpsychol. 50, 250—264.

Klingholz, F. , Siegert, C. & Meynhardt, H. 1979: Die akustische Kommunikation des Europäischen Wildschweines (Sus scrofa L.). Der Zoologische Garten N. F. 49, 277—303.

Lartillot, O. , Toiviainen, P. & Eerola, T. 2008: A matlab toolbox for music information retrieval In: Data Analysis, Machine Learning and Applications. (Preisach C., Burkhardt H., Schmidt‐Thieme L. & Decker R., eds). Springer, Berlin, Heidelberg, pp. 261—268.

Leidig, M. S. , Hertrampf, B. , Failing, K. , Schumann, A. & Reiner, G. 2009: Pain and discomfort in male piglets during surgical castration with and without local anaesthesia as determined by vocalisation and defence behaviour. Appl. Anim. Behav. Sci. 116, 174—178.

Lemasson, A. & Hausberger, M. 2011: Acoustic variability and social significance of calls in female Campbell's monkeys (Cercopithecus campbelli campbelli). J. Acoust. Soc. Am. 129, 3341—3352. PubMed

Leong, K. M. , Ortolani, A. , Burks, K. D. , Mellen, J. D. & Savage, A. 2003: Quantifying acoustic and temporal characteristics of vocalizations for a group of Captive African Elephants Loxodonta Africana . Bioacoustics 13, 213—231.

Manser, M. B. , Jansen, D. A. W. A. M. , Graw, B. , Hollén, L. I. , Bousquet, C. A. H. , Furrer, R. D. & le Roux, A. 2014: Chapter six ‐ vocal complexity in meerkats and other mongoose species In: Advances in the study of behavior. (Naguib M., Barrett H. L., Brockmann J., Healy S., Mitani J. C., Roper T. J. & Leigh W. S., eds). Academic Press, Oxford, pp. 281—310.

Manteuffel, G. , Puppe, B. & Schön, P. C. 2004: Vocalization of farm animals as a measure of welfare. Appl. Anim. Behav. Sci. 88, 163—182.

Marchant, J. N. , Whittaker, X. & Broom, D. M. 2001: Vocalisations of the adult female domestic pig during a standard human approach test and their relationships with behavioural and heart rate measures. Appl. Anim. Behav. Sci. 72, 23—39. PubMed

Marler, P. 1975: On the origin of speech from animal sounds In: The Role of Speech in Language. (Kavanagh J. F. & Cutting J., eds). MIT Press, Cambridge, MA, pp. 11—37.

Marler, P. 1976: Social organization, communication and graded signals: The chimpanzee and the gorilla In: Growing Points in Ethology. (Bateson P. P. G. & Hinde R. A., eds). Cambridge U Press, Oxford, England.

Marler, P. 1977: The evolution of communication In: How Animals Communicate (Sebeok T. A., ed.). Indiana Univ. Press, Bloomington, IN, pp. 45—70.

Marten, K. & Marler, P. 1977: Sound transmission and its significance for animal vocalization. Behav. Ecol. Sociobiol. 2, 271—290.

Marx, G. , Horn, T. , Thielebein, J. , Knubel, B. & von Borell, E. 2003: Analysis of pain‐related vocalization in young pigs. J. Sound Vib. 266, 687—698.

McGlone, J. J. 1985: A quantitative ethogram of aggressive and submissive behaviors in recently regrouped pigs. J. Anim. Sci. 61, 559—565. PubMed

Melisova, M. , Illmann, G. , Chaloupkova, H. & Bozdechova, B. 2014: Sow postural changes, responsiveness to piglet screams, and their impact on piglet mortality in pens and crates. J. Anim. Sci. 92, 3064—3072. PubMed

Meynhardt, H. 1990: Schwarzwild‐Report: Mein Leben Unter Wildschweinen. Neumann, Melsungen.

Morton, E. S. 1982: Grading, discreteness, redundancy, and motivation‐structural rules In: Acoustic Communication in Birds (Kroodsma D. E., Miller E.H. & Ouellet H., eds.). Academic Press, New York, NY, pp. 182—212.

Nair, S. , Balakrishnan, R. , Seelamantula, C. S. & Sukumar, R. 2009: Vocalizations of wild Asian elephants (Elephas maximus): structural classification and social context. J. Acoust. Soc. Am. 126, 2768—2778. PubMed

Oczak, M. , Ismayilova, G. , Costa, A. , Viazzi, S. , Sonoda, L. T. , Fels, M. , Bahr, C. , Hartung, J. , Guarino, M. , Berckmans, D. & Vranken, E. 2013: Analysis of aggressive behaviours of pigs by automatic video recordings. Comput. Electron. Agric. 99, 209—217.

Puppe, B. , Schön, P. C. , Tuchscherer, A. & Manteuffel, G. 2005: Castration‐induced vocalisation in domestic piglets, Sus scrofa: complex and specific alterations of the vocal quality. Appl. Anim. Behav. Sci. 95, 67—78.

Range, F. & Fischer, J. 2004: Vocal Repertoire of Sooty Mangabeys (Cercocebus torquatus atys) in the Taï National Park. Ethology 110, 301—321.

Reby, D. & McComb, K. 2003: Anatomical constraints generate honesty: acoustic cues to age and weight in the roars of red deer stags. Anim. Behav. 65, 519—530.

Reby, D. , McComb, K. , Cargnelutti, B. , Darwin, C. , Fitch, W. T. & Clutton‐Brock, T. 2005: Red deer stags use formants as assessment cues during intrasexual agonistic interactions. Proc. R. Soc. Lond. B 272, 941—947. PubMed PMC

Reby, D. , Andre'‐Obrecht, R. g. , Galinier, A. , Farinas, J. & Cargnelutti, B. 2006: Cepstral coefficients and hidden Markov models reveal idiosyncratic voice characteristics in red deer (Cervus elaphus) stags. J. Acoust. Soc. Am. 120, 4080. PubMed

Richards, D. G. & Wiley, R. H. 1980: Reverberations and Amplitude Fluctuations in the Propagation of Sound in a Forest: implications for Animal Communication. Am. Nat. 115, 381—399.

Rothschild, M. F. & Ruvinsky, A. & International, C. A. B. 2011: The Genetics of the Pig. CABI, Wallingford.

Rowell, T. E. & Hinde, R. A. 1962: Vocal communication by the rhesus monkey (Macaca mulatta). Proc. Zool. Soc. Lond. 138, 279—294.

Sanvito, S. , Galimberti, F. & Miller, E. H. 2007: Vocal signalling of male southern elephant seals is honest but imprecise. Anim. Behav. 73, 287—299.

Scandura, M. , Iacolina, L. , Crestanello, B. , Pecchioli, E. , Di Benedetto, M. F. , Russo, V. , Davoli, R. , Apollonio, M. & Bertorelle, G. 2008: Ancient vs. recent processes as factors shaping the genetic variation of the European wild boar: are the effects of the last glaciation still detectable? Mol. Ecol. 17, 1745—1762. PubMed

Scandura, M. , Iacolina, L. & Apollonio, M. 2011: Genetic diversity in the European wild boar Sus scrofa: phylogeography, population structure and wild x domestic hybridization. Mammal. Rev. 41, 125—137.

Schön, P.‐C. , Puppe, B. , Gromyko, T. & Manteuffel, G. 1999: Common features and individual differences in nurse grunting of domestic pigs (Sus scrofa): a multi‐parametric analysis. Behaviour 136, 49—66.

Schrader, L. & Todt, D. 1998: Vocal quality is correlated with levels of stress hormones in domestic pigs. Ethology 104, 859—876.

Slocombe, K. E. , Townsend, S. W. & Zuberbuhler, K. 2009: Wild chimpanzees (Pan troglodytes schweinfurthii) distinguish between different scream types: evidence from a playback study. Anim. Cogn. 12, 441—449. PubMed

Stoeger‐Horwath, A. S. , Stoeger, S. , Schwammer, H. M. & Kratochvil, H. 2007: Call repertoire of infant African elephants: first insights into the early vocal ontogeny. J. Acoust. Soc. Am. 121, 3922—3931. PubMed PMC

Szekely, G. J. & Rizzo, M. L. 2005: Hierarchical clustering via Joint between‐within distances: extending ward's minimum variance method. J. Classif. 22, 151—183.

Tallet, C. , Linhart, P. , Policht, R. , Hammerschmidt, K. , Simecek, P. , Kratinova, P. & Spinka, M. 2013: Encoding of situations in the vocal repertoire of piglets (Sus scrofa): a comparison of discrete and graded classifications. PLoS One 8, e71841. PubMed PMC

Taylor, A. A. , Weary, D. M. , Lessard, M. & Braithwaite, L. 2001: Behavioural responses of piglets to castration: the effect of piglet age. Appl. Anim. Behav. Sci. 73, 35—43. PubMed

Titus, K. , Mosher, J. A. & Byron, K. W. 1984: Chance‐corrected classification for use in discriminant analysis: ecological applications. Am. Midl. Nat. 111, 1—7.

Vannoni, E. & McElligott, A. G. 2007: Individual acoustic variation in fallow deer (Dama dama) Common and harsh groans: a source‐filter theory perspective. Ethology 113, 223—234.

Vannoni, E. & McElligott, A. G. 2008: Low frequency groans indicate larger and more dominant fallow deer (Dama dama) males. PLoS ONE 3, e3113. PubMed PMC

Volodina, E. V. 2000: Vocal repertoire of the cheetah Acinonyx jubatus (Carnivora, Felidae) in captivity: sound structures and their potential for estimating the state of adult animals. Zoologicheskii‐Zhurnal 79, 833—843.

Ward, J. H. Jr 1963: Hierarchical grouping to optimize an objective function. J. Am. Stat. Assoc. 58, 236—244.

Warrens, M. J. 2008: On the equivalence of Cohen's Kappa and the hubert‐arabie adjusted rand index. J. Classif. 25, 177—183.

Weary, D. M. , Braithwaite, L. A. & Fraser, D. 1998: Vocal response to pain in piglets. Appl. Anim. Behav. Sci. 56, 161—172.

Whittemore, C. T. & Kyriazakis, I. 2006: Whittemore's Science and Practice of Pig Production. Wiley‐Blackwell.

Wilson, D. E. & Mittermeier, R. A. 2011: Handbook of the Mammals of the World: Hoofed mammals. Lynx Edicions, ES: Barcelona.

Yin, S. 2002: A new perspective on barking in dogs (Canis familaris). J. Comp. Psychol. 116, 189—193. PubMed

Zuberbühler, K. 2000: Interspecies semantic communication in two forest primates. Proc. R. Soc. Lond. B 267, 713—718. PubMed PMC

Zuberbühler, K. , Noë, R. & Seyfarth, R. M. 1997: Diana monkey long‐distance calls: messages for conspecifics and predators. Anim. Behav. 53, 589—604.

Classification of pig calls produced from birth to slaughter according to their emotional valence and context of production

Observation of rescue behaviour in wild boar (Sus scrofa)

Individual stability in vocalization rates of preweaning piglets