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Different cats can sound somewhat different when purring.
Domestic cat purring mixed with pronounced meowing
This sound is made when the guinea pig is enjoying itself, such as when being petted or held. They may also make this sound when grooming, crawling around to investigate a new place, or when given food.
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A purr is a sound made by at least some species of felids. It 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 viverrids (civet, mongoose, genet), bears, badgers, hyaenas (et cetera) purr. Other animals that have been said to purr are rabbits, squirrels, guinea pigs, tapirs, ring-tailed lemurs, elephants, raccoons and gorillas while eating. However, using a strict definition of purring that continuous sound production must alternate between pulmonic egressive and ingressive airstream (and usually go on for minutes), Peters (2002), in an exhaustive review of the scientific literature, 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 unique “neural oscillator” in the cat’s brain.
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. 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, though this varies from cat to cat; in the audio samples that accompany this article, the first cat is only purring, while the vocal production of the second cat contains low level outbursts sometimes characterized as "lurps" or "yowps".
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. Eklund and Peters reported no major differences as a function of age.
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 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, 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. 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.
It is unknown for certain why cats purr, but the following reasons are speculated:
Cats 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; to wit: "feed me." However, this variety of purring seems to be found only in cats in a one-to-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 labour.
Scientists at the University of California, Davis hypothesised 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. Bone density loss and muscle atrophy is a serious concern for astronauts during extended periods at zero gravity. During these periods musculo-skeletal systems do not experience the normal stresses of physical activity, including routine standing or sitting, which requires strength for posture control. Exposing these astronauts to sound frequencies similar to those of a cat's purr could counteract the deteriorating effects of low gravity.
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