purr by Gemma Correll

A 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 and is often believed to indicate a positive emotional state, however, felines sometimes purr when they are ill, tense, or experiencing traumatic or painful moments. Purring varies between cats (for example by loudness and tone), and from species to species, but can be characterized as a tonal buzzing.

The term ‘purring’ has been used liberally in literature, and it has been claimed that many non-cats including viverrids (civet, mongoose, genet), bears, badgers, and hyaenas purr. Others are reported to purr only at specific times, for example rabbits, squirrels, guinea pigs, tapirs, ring-tailed lemurs, elephants, raccoons and gorillas are claimed to purr while eating. However, using a strict definition of purring that continuous sound production must alternate between pulmonic egressive and ingressive airstream (breathing in and out) and usually go on for minutes, a 2002 study reached the conclusion that until then only ‘purring cats’ (Felidae) and two species of genets, Genetta tigrina, and most likely also Genetta genetta, had been documented to purr.

The mechanism by which cats purr is ambiguous. This is partly because the cat has no unique anatomical feature that is clearly responsible for the sound, except for a ‘neural oscillator,’ a part of a cat’s brain responsible for the movement of the laryngeal muscles. 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 (voice box) to alternately dilate and constrict the glottis (vocal folds) 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. Purring is sometimes accompanied by other sounds such as low level outbursts sometimes characterized as ‘lurps’ or ‘yowps’ (though this varies from cat to cat).

Domestic cats purr at a frequency of 20 to 30 vibrations per second. Larger cats purr at lower frequencies, for example cheetahs purr with an average frequency of 20.87 Hz (egressive) and 18.32 Hz (ingressive). A study of purring in four domestic cats 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. There was also considerable variation in relative amplitude, duration and frequency between egressive and ingressive phases. A follow-up study of purring in four adult cheetahs in 2012 found that egressive phases were longer and louder than ingressive ones. A study comparing adult, subadult, and juvenile cheetahs reported that there was considerable variation across most of the parameters analyzed (amplitude, phase duration, cycles per phase, and fundamental frequency) but that disparities were mainly attributable to degree of relaxation/agitation in the animals and whether they were resting or playing, not age differences.

It was, until recent times, believed that only the cats of the genus Felis could purr. However, felids of the Panthera genus (tiger, lion, jaguar and leopard) 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 19th century English biologist Richard Owen and was definitely introduced by British zoologist Reginald Innes Pocock in 1916, based on differences in the shape of the feline hyoid (a bone below the chin that aids in tongue movement). The ‘roaring cats’ (lion, tiger, jaguar, leopard) have an incompletely ossified (semi-rigid) hyoid, which according to this theory, enables them to roar but not to purr. On the other hand, the snow leopard, as the fifth felid species with an incompletely ossified hyoid, purrs. All remaining species of the family Felidae (‘purring cats’) have a completely ossified, rigid hyoid which enables them to purr but not to roar. However, a study in 2002 argued that ossification of the hyoid is not a major factor in roaring, claiming it was better explained by specific characteristics of the vocal folds and an elongated vocal tract.

It is unknown for certain why cats purr, but they often purr when being petted, becoming relaxed, or when eating. Female cats are known to sometimes purr while giving birth. Purring may have developed as a signalling mechanism between mother cats and nursing kittens. German ethologist and cat behaviorist Paul Leyhausen interprets it as a signal that the animal is not posing a threat. Scientists at the University of Sussex showed in 2009 that purring, or some purring, seems to be a way for domesticated cats to signal their owners for food. According to Dr. Karen McComb and her team, purring in the ‘about to be fed’ context has a high-frequency component not ordinarily present. Humans report feeling an urgency to investigate and satisfy the cat’s needs. However, this variety of purring seems to be found only in cats in a one-on-one relationship with their caretakers. This ‘soliciting purr’ is different from a cat’s normal purring. Another theory states that purring triggers a cat’s brain to release a hormone which helps it in relaxing and acts as a pain killer. This may be a reason why cats purr when distressed or in labor.

Scientists at the University of California, Davis hypothesized that a cat’s purr can be used as 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 work during both inhalation and exhalation 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. Dr. Lyons, one of the scientists in this study, suggests that this finding may be applicable to astronauts during extended periods in zero gravity.

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