A purr is a sound made by all species of felids. It varies between cats, 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, bears, badgers, hyaenas 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, Peters, in an exhaustive
review of the scientific literature, reached the conclusion that until then only ‘purring
cats’ and two species of genets, Genetta tigrina, and most likely also Genetta genetta,
had been documented to purr.
Mechanism 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,
comparing purring in a cheetah and a domestic cat found that the cheetah purred with an
average frequency of 20.87 Hz and 18.32 Hz, while the much smaller domestic cat purred
with an average frequency of 21.98 Hz and 23.24 Hz. Schötz & Eklund 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
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 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 compared purring
in adult, subadult and juvenile cheetahs and reported that while there was considerable
variation across most of the parameters analyzed – 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 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 ’ on the other, originally
goes back to Owen and was definitely introduced by Pocock, based on a difference in hyoid
anatomy. The ‘roaring cats’ 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, as the fifth
felid species with an incompletely ossified hyoid, purrs. All remaining species of the
family Felidae have a completely ossified hyoid which enables them to purr but not to
roar. However, Weissengruber et al. 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.
Reasons 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-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 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. See also
Kneading Trill consonant
References
External links Why do Cats Purr?
Cat Purr Generator Why do cats purr? Why do Cheshire cats grin?
The Felid Purr: A bio-mechanical healing mechanism Robert Eklund's Ingressive Speech Website
Robert Eklund's Wildlife Page Robert Eklund's Purring Page
For film clips of purring waveforms, see purring.org.