A Cat's Purr...

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

One hypothesis, backed by electromyographic studies, is that cats produce the purring noise by using the vocal folds and/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] Despite competition from various contenders, the record for the loudest purring cat continues to be held by Smokey, owned by Ruth Adams (UK). On 25 March 2011, Smokey purred at 67.7 dB.^[4]

No cat can both purr and roar. The subdivision of the Felidae into "purring cats" on the one hand and "roaring cats" on the other, originally 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.

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,^[5][6] or eating. Some purring may be a signal to another animal that the "purrer" is not posing a threat.^[7]

Purring sometimes seems to be a way for cats to signal their caretaker 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.^[8] Cats often purr when distressed or in pain, such as in labour.^{[5][9][10][11]} This purring may trigger a cat's brain to release a hormone which helps it in relaxing and acts as a painkiller.^[12]