Cat vocalisations have been categorised according to a range of characteristics.
Schötz categorised vocalizations according to 3 mouth actions: (1) sounds produced with the mouth closed (murmurs), including the purr, the trill and the chirrup, (2) sounds produced with the mouth open and gradually closing, comprising a large variety of miaows with similar vowel patterns, and (3) sounds produced with the mouth held tensely open in the same position, often uttered in aggressive situations (growls, yowls, snarls, hisses, spits and shrieks).
Brown et al. categorised vocal responses of cats according to the behavioural context: (1) during separation of kittens from mother cats, (2) during food deprivation, (3) during pain, (4) prior to or during threat or attack behavior, as in disputes over territory or food, (5) during a painful or acutely stressful experience, as in routine prophylactic injections and (6) during kitten deprivation. Less commonly recorded calls from mature cats included purring, conspecific greeting calls or murmurs, extended vocal dialogues between cats in separate cages, “frustration” calls during training or extinction of conditioned responses.
Miller classified vocalisations into 5 categories according to the sound produced: the purr, chirr, call, meow and growl/snarl/hiss.
Purr
The purr is a continuous, soft, vibrating sound made in the throat by most species of felines. Domestic cat kittens can purr as early as two days of age. This tonal rumbling can characterize different personalities in domestic cats. Purring is often believed to indicate a positive emotional state, but cats sometimes purr when they are ill, tense, or experiencing traumatic or painful moments.
The mechanism of how cats purr is elusive. This is partly because cats do not have a unique anatomical feature that is clearly responsible for the vocalization. One hypothesis, supported 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. Combined with the steady inhalation and exhalation as the cat breathes, a purring noise is produced with strong harmonics. Purring is sometimes accompanied by other sounds, though this varies between individuals. Some may only purr, while other cats include low level outbursts sometimes described as "lurps" or "yowps".
Domestic cats purr at varying frequencies. One study reported that domestic cats purr at average frequencies of 21.98 Hz in the egressive phase and 23.24 Hz in the ingressive phase with an overall mean of 22.6 Hz. Further research on purring in four domestic cats found that the fundamental frequency varied between 20.94 and 27.21 Hz for the egressive phase and between 23.0 and 26.09 Hz for the ingressive phase. There was considerable variation between the four cats in the relative amplitude, duration and frequency between egressive and ingressive phases, although this variation generally occurred within the normal range.
One study on a single cheetah (Acinonyx jubatus) showed it purred with an average frequency of 20.87 Hz (egressive phases) and 18.32 Hz (ingressive phases). A further study on four adult cheetahs found that mean frequencies were between 19.3 Hz and 20.5 Hz in ingressive phases, and between 21.9 Hz and 23.4 Hz in egressive phases. The egressive phases were longer than ingressive phases and moreover, the amplitude was greater in the egressive phases.
It was once believed that only the cats of the genus Felis could purr. However, felids of the genus Panthera (tigers, lions, jaguars and leopards) also produce sounds similar to purring, but only when exhaling. The subdivision of the Felidae into ‘purring cats’ on the one hand and ‘roaring cats ’ (i.e. non-purring) on the other, originally goes back to Owen (1834/1835) and was definitely introduced by Pocock (1916), based on a difference in hyoid anatomy. 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. On the other hand, the snow leopard (Uncia 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. However, Weissengruber et al. (2002) argued that the ability of a cat species to purr is not affected by the anatomy of its hyoid, i.e. whether it is fully ossified or has a ligamentous epihyoid, and that, 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, the latter rendered possible by an incompletely ossified hyoid.
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