The Felid Purr:
A bio-mechanical healing mechanism
Has been published and peer reviewed in the proceedings from the
12th International Conference
on Low Frequency Noise and Vibration and its Control
held in Bristol, UK, 18th to 20th September 2006, and first
presented in 1998 and made public then.
- Presented at the 2001, 142nd
annual Acoustical Society of America, American Institute of Physics,
- Layman's version published in
Cat Watch Newsletter from the Cornell University College of Veterinary Medicine
Cat Purr Analysis
Partial reference list
readers. This paper is not designed to give veterinary or medical
advice. In addition it is copyrighted with patent pending**
All smaller felids, including the domestic
cat, caracal, serval, puma, ocelot, and even some large cats such as lions and
cheetah purr. Since the 1970's no one has pursued research into the 3000 year
old question, "Why do cats purr?" Perhaps it is because, one, we didn't have the
knowledge we have now, and two, it was simply easier to assume that cats purr
when they are content, which cannot be argued-they do purr when they are
content. However they also purr when frightened, severly injured, giving birth
and even while dying. Because of this, the contentment hypothesis clearly cannot
be the only reason cats purr:.
(1) A vocalization is used to display a particular emotion or physiological
state. This enables an individual in society or pack to be able to express
themselves. As any cat owner knows well, there are different "meows" for
different emotions. A cat owner knows the difference between their cat's
"fearful hiss", "food meow", and "let me oooouuutttt!!!" This cannot be
applied to the purr however. Cats purr even when they give birth and when
severely injured in a barren cage at the veterinarian's. There are cases of cats
purring when they are in grave physiological or psychological stress, as well as
when they sit on your lap. Therefore, purring really cannot be considered a
vocalization, as the purr is produced under differing emotions or physiological
states. As an example, a cat hissing when he/she was happy and when he/she was
scared, would confuse the rest of the cat's companions and probably would lead
to him/her being ostracized.
(2) Natural selection insures that a particular trait be advantageous to an
animal. Admittedly, there is some benefit obtained from purring to ones'
self or to kittens, (a sort of kitty lullaby if you wish). Yet, there does
not appear to be a strong 'survival' advantage to this behavior, unless, of
course, you wish to constantly display submission. For the purr to exist in
different cat species over time, geographical isolation etc. there would likely
have to be something very important (survival mechanism) about the purr. There
is also would have to be a very good reason for energy expenditure (in this case
creation of the purr), when one is physically stressed or ill. The vibration of
the cat's diaphragm, which with the larynx, creates the purr, requires energy.
If an animal is injured they would not use this energy unless it was beneficial
to their survival. When was the last time you heard someone singing, or
humming to themselves (before pain drugs, and they weren't on any when they came
in) when they were in the emergency room with a broken leg? The purr has
to be somehow involved with survival.
Old wives' tales usually have a grain of truth behind them, and most people have
heard of a cat's "nine lives." There is also an old veterinary adage still
repeated in veterinary schools which states, "If you put a cat and a bunch of
broken bones in the same room, the bones will heal." Any veterinary
orthopedic surgeon will tell you how relatively easy it is to mend broken cat
bones compared with dog bones which take much more effort to fix, and take
longer to heal. There is excellent documentation of the cats' quick recovery
from such things as high-rise syndrome. First mentioned by Dr.
Gordon Robinson in 1976, high-rise syndrome was later studied by Whitney, W.,
and Mehlhaff, C., (1987) the Journal of the American Veterinary Medical
Association. They documented 132 cases of cats plummeting many stories
from high rise apartments, (average 5.5 stories) some suffering severe
injuries. Interestingly, 90% of these cats survived. The record for survival
from heights is 45 stories, however most cats suffer from falls of 7 stories or
more and manage to live.
There has been some
research which that suggests that domestic cats are in general less prone to
postoperative complications following elective surgeries. Using computer
records, Pollari and Bennet, (1996) state that complications following surgery
for dogs undergoing castration to be averaged at 9.8%. The same surgery for cats
lists the rate of complications to be 1.2%. Dogs undergoing overiohysterectomies
(OHE) had complications 17.4% of the time and cats 8.4%. In another study by the
same authors comparing paper records with computerized documentation, dogs
undergoing castration complications varied from 2.4% to 22%, in cats 0.0% to
6.3%. With OHE complications varied from 6.5% to 17.7% in dogs and 3.6% to 16.%
in cats. Lund et al. (1999) the records of 31,484 dogs and 15,226 cats at 52
veterinary practices to determine the most common disorders. Arthritis in dogs
was listed as 2.4% of the population and was not listed as being reported in the
cat. The prevalence of lameness in dogs occurred 3.1% of the time, in cats it is
not mentioned as being reported. Healthy dogs were listed as 6.8% of the dog
population, healthy cats 9.5%.
Although it is
impossible to standardize the healing time for dogs and cats in clinically
occurring fractures, due to the type of fracture, amount of trauma to soft
tissues, the type of treatment, the standard evaluation time or the after care,
some general statements can be made, (Johnson, 2001). Cats do not have near the
prevalence of orthopedic disease or ligament and muscle traumas as dogs do.
Additionally, Toombs et al. (1985) suggests that non-union of fractures in cats
Osteo diseases that
are rarely found in cats but can be found in all breeds and sexes of dogs
include; Osteochondritis dissecans of the proximal humerous, scapulohumeral
joint luxations, hip dysplasia. Osteo diseases in which cats are completely
unaffected include fragmented coronoid process, ununited anconeal preoceese,
traumatic elbow luxation, elbow subluxation, and legg-perenes. Osteosarcoma
occurs much less frequently in the cat then in the dog. Johnson, 1999.
Osteoarthritis and CPPd have only been found in large cats that were raised in
zoological parks. The frequency of effected cats in the wild is apparently so
low, that they are infrequently effected by these diseases in the wild.
(Rothschild et al., 1998)
Myeloma is a tumor of
plasma cells originating in the bone marrow. Only eight cats with multiple
myeloma have been reported to have osteolytic bone lesions. 56% of all dogs
reported with this condition involve bone. The metastatic behavioral differences
between dogs and cats is that tumors in the dog involve the whole body, whereas
in the cat it involves the distal ends of the extremities. In Lameness
With regard to the
prevalence of ligament and muscle injuries and disease, those that are seen
regularly in dogs but not in cats include, cranial crutiate ligament ruptures,
meniscal injuries (torn ligaments), muscle contusions and strains, muscle
contracture and fibrose, quadricepts contractor and inialsinatus, bicipital
tenosynovitis, medial patellar luxation, lateral patellar luxation,
osteochronditis dissecans of the stifle, and ligamentous injury of the tarsus.
One explanation for
the lack of trauma or disease found in cat bone and muscle/ligaments is that
cats are more sedentary then dogs, however this is a supposition and is not
There have been
studies that indicate that purring can aid in dyspnea as Cook in 1972 suggests.
Kidd et al. in 2000 found in a study with 11 cats and 17 dogs with acute and
subacute myocardial necrosis, none of the cats in the study had dyspnea,
although all the dogs did. The overall incidence of primary lung tumors in the
dog is 1.24%, and in the cat, .38% (Miles, 1988)
Free skin grafting is
often used for the treatment of large skin defects on the distal limbs of dogs
and cats. However while using this technique in dogs, the overlapped skin edges
of the graft usually become necrotic by 3 days postoperatively, and need to be
debrided. In cats, the grafts are usually viable even after six days.
has been no research that has attempted to explain the extraordinary
ability cats have for healing themselves.
Just two years ago, Dr. Clinton Rubin and his associates made a fantastic
discovery. They found that exposure to frequencies between 20-50 Hz (at low dB)
creates the robust striations of increased bone density, Clinton Rubin, (1999),
Strain mediated augmentation of bone mass and morphology: Is it possible to
harness the anabolic potential of mechanical stimuli without necessarily
requiring exercise?, Wellcome Trust. In one study chickens were placed
on a vibrating plate every day for 20 minutes, and grew stronger bone,
National Geographic, January 2001, p. 11. This discovery of anabolic
frequencies between 20- 50 Hz (at low dB), is a tremendous breakthrough.
Astronauts in space loose bone density in zero gravity, and this method could
help them maintain healthy bones. Dr. Rubin's group has begun research trials
with humans, designed to test whether this non-invasive method halts
osteoporosis and perhaps even renews bone growth in post-metapausal women; J.
Zhi, and M. Hadjrargyrou, (1999) The expression of a novel and a known gene,
unregulated by disuse is down regulated by anabolic mechanical stimulation,
American Society of Bone and Mineral Research. This method is not yet FDA
approved, although it is hoped it will be soon. Additionally, Chen et.al
(1994) The effects of frequency of mechanical vibration on experimental
fracture healing, Zhonghua Wai Ke Za Zhi, in his work with rabbits, found
that frequencies of 25 and 50 hertz promote bone strength by 20%, and stimulate
both the healing of fractures, and the speed at which the fractures heal.
There is also documentation that low frequencies, at low dB are helpful with
regard to pain relief, and the healing of tendons and muscles. Vibrational
stimulation between 50-150 Hz has been found to relieve suffering in 82% of
persons suffering from acute and chronic pain (Lundeberg, 1983). In 1999,
M. Falempin and S.F. In-Albon discovered that mechanical vibration at 120 Hz
counteracted atrophy in tendons after hind-limb muscle loading.
Biomechanical stimulation which uses mechanical vibration of standardized
frequencies from 18 - 35 Hz is used in Russian sports medicine. This
technique improves the relaxation of strained muscle structures and increases
the stretching ability of capsules and tendons. Lake in 1992, found that
biomechanical stimulation prevents a decrease in muscle strength and muscle mass
and the oxidative capacity of thigh muscles, following knee immobilization after
sports injuries. The use of low frequency therapy also applies to tendon
healing. It can increase the mobility of upper ankle joints by 16- 19 %,
Klysczt et. al, 1997, Biomechanical stimulation therapy as physical treatment
of arthrogenic venous insufficiency, Hautarzt. Exposure to frequencies
between 2-100 Hz results in in the reduction of muscle spasms and more
pronounced reduction of the spasms occurs the longer the treatment is applied,
(D. Ardic, A. Buljina, 2000). After ten days of short periods of biomechanical
stimulation, upper mobility of ankle joints improved by 16 and 19 degrees and
was accompanied by the healing of venous ulcerations after skin flap
transplantation, (Klysch, T. et al., 1997). It is interesting to note that
Biomechanical stimulation is also used in public gyms and work-out centers to
increase muscle mass. A web search will bring up many manufactures of such
has also been found that in- phase chest wall vibration at 100 Hz, is known to
decrease dysponea in patients with chronic obstructive pulmonary disease while
at rest (Cristiano and Schwartzstein 1997; Nakayama, et al., 1998; Sibuya,
Vibrations between 20-140 Hz are therapeutic for bone growth/fracture healing,
pain relief/swelling reduction, wound healing, muscle growth and repair/tendon
repair, mobility of joints and the relief of dyspnea.
We think that this
research could help explain why cats purr, and here is why:
Fauna Communications has recorded many cats' purrs, at a non-profit
facility and the
Cincinnati Zoo , including the cheetah, puma, serval, ocelot and the
domestic house cat. After analysis of the data, we discovered that cat purrs
create frequencies that fall directly in the range that is anabolic for bone
The dominant and fundamental frequency for three
species of cats' purrs is exactly 25 Hz, or 50 Hz the best frequencies for
bone growth and fracture healing. All of the cats purrs all fall well
within the 20 - 50 Hz anabolic range, and extend up to 140 Hz.. All the
cats, except the cheetah have a dominant or strong harmonic at 50 Hz.
The harmonics of three cat species fall
exactly on or within 2 points of 120 Hz which has been found to repair tendons.
One species within 3 Hz and one within 7 Hz.
Eighteen to thirty-five Hz is used in therapeutic
biomechanical stimulation for joint mobility. Considering the small size of many
of these cats, especially the domestic cats, it is interesting to note that that
all of the individual cats, have dominant frequencies within this range.
In fact, some of the cats, have 2-3 harmonics in this range.
The frequencies for therapeutic pain relief are from
50-150 Hz. All of the individual cats have al least 5 sets of strong harmonics
in this range.
Therapeutic frequencies for the generation of muscle
strength lie between 2-100 Hz. All of the individual cats have al least 4 sets
of strong harmonics in this range.
Therapy for COPD uses 100 Hz, all of the individual
cats have a dominant frequerncy of exactly 100 Hz.
There is another clue found in a study performed by Dr. T. F. Cook, (1973)
The relief of dyspnoea in cats by purring, New Zealand Veterinary Journal. A
dying cat who could not breath (they were considering euthanasia), was
found to breath normally once it began purring. The purring opened up the cat's
airway, and improvement was "remarkable and the next day commenced to eat...."
Three species of cats have a strong harmonic at exactly 100 Hz, the vibrational
frequency found to relieve dyspnea. One species within 2 Hz and one species
within 7 Hz of 100 Hz. It could be that the cat's purr decreases the
breathlessness by vibratory stimulation.
Is it possible that
evolution has provided the felines of this world with a natural healing
mechanism for bones and other organs? Researchers at Fauna Communications
Being able to produce
frequencies that have been proven to improve healing time, strength and mobility
could explain the purr's natural selection. In the wild when food is plentiful,
the felids are relatively sedentary. They will spend a large portion of the day
and night lounging in trees or on the ground. Consistent exercise is one of the
greatest contributors to bone, (Karlsson et al, 2001), and muscle (Roth et al,
2000; Tracy et al 1999), and tendon and ligament strength (Simoson et al, 1995;
Tipton et al 1975). If a cats exercise is sporadic it would be advantageous for
them to stimulate bone growth while at rest. As well, following injury,
immediate exercise can rebreak one and re-tear healing muscle and tendon
(Montgomery, 1989). Inactivity decreases the strength of muscles (Tipton et al,
1975). Therefore, having an internal vibrational therapeutic system to stimulate
healing would be advantageous, and would also reduce edema and provide a measure
of pain relief during the healing process.
question...What about other cats that don't purr???
Unfortunately there is no easy way to test this hypothesis. Strangely, after
speaking with several of the foremost specialists on animal bones, it was
discovered that there has apparently never been a study on any small cat
bones, not serval, caracal, puma, ocelot, or domestic. Only cheetah and tiger
bones have been studied, and tigers do not purr. Cheetahs do purr, but they are
one of the most unique and specialized forms of the felid family. The cheetah's
bones were found to have dense remodeling (growth), which apparently is found in
carnivores and in humans.
Purring-cat physiology would have to be compared to non-purring cat
physiology to test this theory. The study would have to be entirely
There are inherent difficulties in discovering whether purring aids in healing,
as purring-cat physiology would have to be compared to non-purring cat
physiology. The dilemma is that most all cats purr, even under duress. They are
even capable of producing a purr following a laryngectomy (Hardie et al, 1981),
due to vibration of the diaphragm (Stogdale and Delack, 1985). A naturally
occurring, non-purring cat is very rare, and this effect is usually associated
with a physical problem. Cats that have physical problems related to purring
cannot be admitted to the study because of the possible variables presented by
the physical disability. Therefore, any research would have to be non-invasive
and observation based.
Given the data on
anabolic frequencies, fracture and healing research, the exact match of the
frequencies and amplitudes of the cat's purrs to vibrational therapy research,
time proven adages, biomechanical therapy, studies on tendon and muscle repair
and Dr. Cook's study, it is certainly not a leap of faith to speculate that the
cat's purr is a healing mechanism. Having a natural way to increase strength,
and decrease healing time, would indeed be very advantageous and would explain
the purr's development.
It is suggested that purring be stimulated as much as possible
when cats are ill or under duress. If purring is a healing mechanism, it
may just help them to recover faster, and perhaps could even save their life.
We are currently
gathering veterinarian case studies and beginning a study to test the cats'
purr-healing theory. No cats will, or have been harmed in this study. All of
Fauna Communication's studies are non-invasive. We need your help for this
research. We thank you for your support!
Please send your
tax-deductible donations to:
Communications Research Institute
P.O. Box 1126,
Hillsborough, N.C. 27278
before submitting your e-mail to our site we e-mail you and request your
permission. We will not mention your name or e-mail address,
unless you request it. Also before you e-mail
us on this topic, we are not
attempting to disprove the contentment theory, we are merely supplying an
additional hypothesis as to what the purr may do. Additionally,
this research is not designed to determine how the purr is
For more information:
E-MAIL or United States (919) 732-1322
The scientific version of
this paper has been submitted for review.
Many thanks to Shelley
Adams, Dr. John Currey, Dr. Clinton Rubin,
Dr. Terry Cook, Dr.
Margerie Lindeke, Jacqui Roddick,
Cincinnati Zoo, and
all the other professionals and helpful people
we contacted about this
Copyright (2001) Fauna
Communications Research Institute
Cat's Purr Scientific Abstract
Author: Elizabeth von Muggenthaler
Location: Fauna Commun. Res. Inst., P.O. Box 1126, Hillsborough, NC 27278,
A current hypothesis suggests the purr indicates contentment, however, cats purr
when they are severely injured or frightened. Forty-four felids were recorded
including cheetahs, ocelots, pumas, domestic cats, and servals. A Sony TCD-D8
Digital Audio Recorder (DAT) and Statham Radio microphones recorded the purrs.
FFTs and spectrographs were performed using National Instrument's Polynesia. An
accelerometer was also used to measure domestic cat purrs. Every felid in the
study generated strong frequencies between 25 and 150 Hz. Purr frequencies
correspond to vibrational/electrical frequencies used in treatment for bone
growth/fractures, pain, edema, muscle growth/strain, joint flexibility, dyspnea,
and wounds. Domestic cats, servals, ocelots, and pumas produce fundamental,
dominant, or strong frequencies at exactly 25 Hz and 50 Hz, the two low
frequencies that best promote bone growth/fracture healing [Chen et al., Zhong.
Wai Ke Za Zhi. 32, 217--219 (1994)]. These four species have a strong harmonic
exactly at, or within 2 Hz of 100 Hz, a frequency used therapeutically for pain,
edema, wounds, and dyspnea. An internal healing mechanism would be advantageous,
increasing recovery time and keeping muscles and bone strong when sedentary.
[Published with permission from the New Zealand Veterinary Journal; work
supported by Endevco.]
1.) Domestic cat
purr at 150 Hertz Fourier Transform
2. Domestic cat purr at 100 Hertz
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