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Peters, G., & Tembrock, G. (1998). Subharmonics, biphonation, and deterministic chaos in mammal vocalizations. Bioacoustics, 9.
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Pérez-Barbería, F. J., Shultz, S., & Dunbar, R. I. (2007). Evidence for coevolution of sociality and relative brain size in three orders of mammals. Evolution, 61.
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Pérez-Barbería, F. J., & Gordon, I. J. (2005). Gregariousness increases brain size in ungulates. Oecologia, 145.
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Passilongo, D., Marchetto, M., & Apollonio, M. (2017). Singing in a wolf chorus: structure and complexity of a multicomponent acoustic behaviour. Hysterix, 28(2), 180–185.
Abstract: Wolf choruses ( Canis lupus ) are complex, multicomponent signals, composed by a series of different vocalizations emitted by a pack. Although howls, the main component, have been highly studied, poor attention has been drawn upon the other vocalizations of the chorus. In this study, we investigate the structure of the chorus by means of the analysis and the quantification of the different components, taking advantage both of the digital sound recording and analysis, and of the modern statistical methodologies. We provide for the first time a detailed, objective description of the types of call emitted during the wolf howlings, combining spectrographic examinations, spectral analyses and automated classifications, with the aim to identify different types of call. Our results show that wolf choruses have a rich, complex structure, that reveals six other types of call, to be added to those howls already described in literature. Wolf choruses are typically composed by other three different types of calls: the bark, i.e. relatively long calls characterized by low frequencies and the presence of harsh components (deterministic chaos); the whimper, characterized by a harmonic structure and a very short duration; and the growl, a call with a noisy structure, low frequencies but relative long duration. Although further investigations are necessary to understand the meaning of the different calls, this research provides a basis for those studies that aim to compare wolves and other canids vocal behaviour.
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Passilongo, D., Mattioli, L., Bassi, E., Szabó, L., & Apollonio, M. (2015). Visualizing sound: counting wolves by using a spectral view of the chorus howling. Front. Zool., 12(1), 22.
Abstract: Monitoring large carnivores is a central issue in conservation biology. The wolf (Canis lupus) is the most studied large carnivore in the world. After a massive decline and several local extinctions, mostly due to direct persecutions, wolves are now recolonizing many areas of their historical natural range. One of the main monitoring techniques is the howling survey, which is based on the wolves' tendency to use vocalisations to mark territory ownership in response to howls of unknown individuals. In most cases wolf howling sessions are useful for the localisation of the pack, but they provide only an aural estimation of the chorus size.
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Passilongo, D., Dessi-Fulgheri, F., Gazzola, A., Zaccaroni, M., & Apollonio, M. (2012). Wolf counting and individual acoustic discrimination by spectrographic analysis [Abstract]. Bioacoustics, 21.
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Palacios, V., Font, E., & Marquez, R. (2007). Iberian wolf howls: acoustic structure, individual variation, and a comparison with North American populations. J Mammal, 88.
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O'Brien, P. H. (1988). Feral goat social organization: a review and comparative analysis. Appl Anim Behav Sci, 21.
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Nowak, S., Jedrzejewski, W., Schmidt, K., Theuerkauf, J., Myslajek, R. W., & Jedrzejewska, B. (2006). Howling activity of free-ranging wolves (Canis lupus) in the Bialowieza Primeval Forest and the Western Beskidy Mountains (Poland). J Ethol, 25.
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