Although true purring is exclusive to felids and viverrids, other animals such as raccoons produce purr-like vocalization. Animals that produce purr-like sounds include mongoose, bears, badgers, foxes, hyaenas, rabbits, squirrels, guinea pigs, tapirs, ring-tailed lemurs, and gorillas while eating.
Vocal folds/laryngeal muscles
One hypothesis, backed by electromyographic studies, is that cats produce the purring noise by using the vocal folds or the muscles of the larynx to alternately dilate and constrict the glottis rapidly, causing air vibrations during inhalation and exhalation. Combined with the steady inhalation and exhalation of air as the cat breathes, a purring noise is produced with strong harmonics.
Degree of hyoid ossification
No cat can both purr and roar. The subdivision of the Felidae into "purring cats" on one hand and "roaring cats" on the other goes back to Owen (1834/1835) and was definitively introduced by Pocock (1916), based on whether the hyoid bone of the larynx is incompletely ("roarers") or completely ("purrers") ossified. However, Weissengruber et al. (2002) argued that the ability of a cat species to purr is not affected by the anatomy of its hyoid.
The "roaring cats" (lion, Panthera leo; tiger, P. tigris; jaguar, P. onca; leopard, P. pardus) have an incompletely ossified hyoid, which according to this theory, enables them to roar but not to purr. However, the snow leopard (Uncia uncia, or P. uncia), as the fifth felid species with an incompletely ossified hyoid, purrs (Hemmer, 1972).
All remaining species of the family Felidae (‘purring cats’) have a completely ossified hyoid, which enables them to purr but not to roar. Based on a technical acoustic definition of roaring, the presence of this vocalization type depends on specific characteristics of the vocal folds and an elongated vocal tract, which is rendered possible by an incompletely ossified hyoid.
Frequency, amplitude, and respiratory variation
- Domestic cats purr at a frequency of 20 to 30 vibrations per second.
- Eklund, Peters & Duthie (2010), comparing purring in a cheetah (Acinonyx jubatus) and a domestic cat (Felis catus) found that the cheetah purred with an average frequency of 20.87 Hz (egressive phases) and 18.32 Hz (ingressive phases), while the much smaller domestic cat purred with an average frequency of 21.98 Hz (egressive phases) and 23.24 Hz (ingressive phases). Schötz & Eklund (2011) studied purring in four domestic cats and found that the fundamental frequency varied between 20.94 and 27.21 Hz for egressive phases and between 23.0 and 26.09 Hz for ingressive phases.
- Schötz & Eklund (2011) also observed considerable variation between the four cats as regards relative amplitude, duration and frequency between egressive and ingressive phases, but that this variation occurred within the same general range.
- In a follow-up study of purring in four adult cheetahs, Eklund, Peters, Weise & Munro (2012) found that egressive phases were longer than ingressive phases in four cheetahs. Likewise, ingressive phases had a lower frequency than egressive phases in all four cheetahs. Mean frequency were between 19.3 Hz and 20.5 Hz in ingressive phases, and between 21.9 Hz and 23.4 Hz in egressive phases. Moreover, the amplitude was louder in the egressive phases in four cheetahs.
- Eklund & Peters (2013) compared purring in adult, subadult and juvenile cheetahs and reported that while there was considerable variation across most of the parameters analyzed (amplitude, phase duration, cycles per phase and fundamental frequency) – mainly attributable to degree of relaxation/agitation in the animals resting or playing– previously reported observations that ingressive phases tend to be lower in frequency were largely confirmed. There were no major differences in these parameters as a function of age.
Purring may have developed as an evolutionary advantage as a signalling mechanism of reassurance between mother cats and nursing kittens. Post-nursing cats often purr as a sign of contentment: when being petted, becoming relaxed, or eating. Some purring may be a signal to another animal that the purring cat does not pose a threat.
Purring sometimes seems to be a way for cats to signal their caretakers for food. This purring has a high-frequency component not present in other purrs. This variety of purring seems to be found only in cats in a one-to-one relationship with a caretaker.
Cats often purr when distressed or in pain, such as in labour.  This purring may trigger a cat's brain to release a hormone which helps it in relaxing and acts as a painkiller. Purring may also be a healing mechanism to offset long periods of rest and sleep that would otherwise contribute to a loss of bone density. The vibrations and contractions of a purr show a consistent pattern and frequency around 25 Hz; these frequencies have been shown to improve bone density and promote healing in animal models and humans.
- "Why and how do cats purr?". Library of Congress. Retrieved 2011-04-10.
- K.M. Dyce, W.O. Sack and C.J.G. Wensing in Textbook of Veterinary Anatomy 3rd Ed. 2002, Saunders, Philadelphia; p156
- "How A Puma Purrs". Archived from the original on 2013-01-22.
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- Paul Leyhausen in Cat Behavior: The Predatory and Social Behavior of Domestic and Wild Cats, translated by Barbara A. Tonkin. New York: Garland STPM Press, c1979.
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- von Muggenthaler, Elizabeth (29 October 2001). "The felid purr: A healing mechanism?". New Zealand Veterinary Journal. doi:10.1121/1.4777098. Archived from the original on 2008-08-03.
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- Eklund, Robert & Gustav Peters. 2013. A comparative acoustic analysis of purring in juvenile, subadult and adult cheetahs. In: Robert Eklund (ed.), Proceedings of Fonetik 2013, the XXVIth Swedish Phonetics Conference, Studies in Language and Culture, no. 21, ISBN 978-91-7519-582-7, ISBN 978-91-7519-579-7, ISSN 1403-2570, pp. 25–28.
- Eklund, Robert, Gustav Peters, Florian Weise & Stuart Munro. 2012. A comparative acoustic analysis of purring in four cheetahs, Proceedings of Fonetik 2012, 30 May–1 June 2012, Gothenburg University, Gothenburg, Sweden, pp. 41–44. Download PDF from http://roberteklund.info here: . The paper can also be downloaded from http://purring.org
- Eklund, Robert, Gustav Peters & Elizabeth D. Duthie. 2010. An acoustic analysis of purring in the cheetah (Acinonyx jubatus) and in the domestic cat (Felis catus), Proceedings of Fonetik 2010, 2–4 June 2010, Lund University, Lund, Sweden, pp. 17–22. Download PDF from http://roberteklund.info here: . The paper can also be downloaded from http://purring.org
- Hemmer, Helmut. 1972. Uncia uncia. Mammalian Species, no. 20, pp. 1–5.
- Owen, Richard. 1834/1835. On the Anatomy of the Cheetah, Felis jubata, Schreb. Transactions of the Zoological Society of London, vol. 1, pp. 129–137.
- Peters, Gustav. Purring and similar vocalizations in mammals. Mammal Review, vol. 32, no. 4, pp. 245–271.
- Pocock, R. I. 1916. On the Hyoidean Apparatus of the Lion (F. leo) and Related Species of Felidæ. The Annals and Magazine of Natural History, Including Zoology, Botany, and Geology, vol. 28, series 8, pp. 222–229.
- Schötz, Susanne & Robert Eklund. 2011. A comparative acoustic analysis of purring in four cats. Proceedings of Fonetik 2011, 8–10 June 2011, Royal Institute of Technology, Stockholm, Sweden, pp. 9–12. Download PDF from http://roberteklund.info here: . The paper can also be downloaded from http://purring.org
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- Weissengruber, G. E., G. Forstenpointner, G. Peters, A. Kübber-Heiss & W. T. Fitch. 2002. Hyoid apparatus and pharynx in the lion (Panthera leo), jaguar (Panthera onca), tiger (Panthera tigris), cheetah (Acinonyx jubatus) and domestic cat (Felis silvestris f. catus). Journal of Anatomy, vol. 201, pp. 195–209.
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