A purr is a tonal fluttering sound made by some species of felids and two species of genets. It varies in loudness and tone among species and in the same animal.

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.

Mechanism [ edit ]

The mechanism by which cats purr is an object of speculation. There is a unique "neural oscillator" in the cat’s brain of uncertain significance.[1]

Vocal folds/laryngeal muscles [ edit ]

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.[2] Combined with the steady inhalation and exhalation of air as the cat breathes, a purring noise is produced with strong harmonics.[3]

Degree of hyoid ossification [ edit ]

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 [ edit ]

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.

) and a domestic cat ( ) 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.

Purpose [ edit ]

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,[4][5] or eating. Some purring may be a signal to another animal that the purring cat does not pose a threat.[6]

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.[7]

Cats often purr when distressed or in pain, such as in labour.[4][8][9][10] This purring may trigger a cat's brain to release a hormone which helps it in relaxing and acts as a painkiller.[11] 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.[12][13]

See also [ edit ]

Notes [ edit ]