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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. The
contentment hypothesis, however, clearly cannot be the only reason cats
(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" and "food meow". 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 to be obtained from purring to
one's 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, there would likely have to be
something very important (survival mechanism) about the purr. There is also
a very good reason for energy expenditure (in this case creation of the
purr), when one is physically stressed or ill. It would have to be somehow
involved in their 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%.
Bone and muscles/ligaments
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 is rare.
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.
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 documented.
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.
Unfortunately, there 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
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
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 equipment.
It 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, 1994).
In Summery: 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
We think that this research could help explain why cats purr, and here is
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 at 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 believe so.
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.
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.