CAT COMMUNICATION - THE ENIGMATIC PURR
The cat’s purr is an important means of communication and, depending on the cat's situation, it can convey contentment, pleasure or be placatory behaviour (i.e. "I am not a threat to you"). As well as purring when happy, cats also purr when severely injured, frightened or giving birth. A cat may even purr when close to death.
Purring is also found in the cheetah, puma and in most small cats such as the serval and ocelot. The pantherine big cats such as lions, tigers, jaguars and leopards can’t purr because their throats are built for roaring; the structures surrounding their voice box (larynx) aren’t stiff enough to produce a purr. Conversely, the small cats, puma and cheetah screech or yowl instead of roaring. Although there are a few reports of purring-type sounds - a breathy groaning sound rather than an in-and-out purr - from lions and tigers, it seems that a cat can either purr or roar, but not both. The only other animals that truly purr are civets, genets and mongooses.
Natural selection favour the retention of advantageous traits. For the purr to have been retained in the different cat species over time, and retained after the species became geographically isolated, purring must be linked to something advantageous to the species. Personally, I like the quip that purring “is the sound of a cat manufacturing cuteness!”
HOW DO CATS PURR?
It’s only recently that science has worked out the mechanism of purring. This is because there is no unique anatomical feature that is clearly responsible for the sound. Along the way, there have been a number of theories.
Early theories can be generalised as “air turbulence theories”. At first, the larynx was believed to be the source of the purr. However, cats that had had a tracheotomy could still purr even though the air bypassed the larynx.
The Diaphragm-Larynx Theory suggested that the sound was due to positive pressure in the lungs causing the glottis at the back of the throat to rapidly open and close which caused the vibrations audible to owners as purring. Rapid, rhythmic contractions of the muscles of the larynx and diaphragm meant purring could also be felt as a vibration in both the throat and stomach, which is where those muscles are located. Alternatively, purring was a type of snoring, where the cat’s air stream was interrupted by a structure in the larynx. However, cats are capable of producing a purr following removal of the larynx (Hardie et al, 1981), due to vibration of the diaphragm (Stogdale and Delack, 1985).
A similar theory suggested that vibrations of the false vocal cords, located just behind the true vocal cords, in the larynx caused the sound. The Soft-Palate Theory suggested that cats could voluntarily "flutter" the soft palate to produce the purring sound.
Having dismissed the larynx as the source, researchers attributed the purr to vascular (blood vessel) sources. They noticed that cats most often purred while being petted and, at the same time, the cats tended to arch their back. They reasoned that the arching of the back bent the aorta (major artery in the chest), forcing blood to eddied around a sharp bend. This caused turbulence that was audible as a purr. To investigate this, they gave a local anaesthetic in the abdomen and made an incision so they could manually palpate (examine) the aorta for fremitus (vibration). Another group of investigators suggested the source of the purr was due to turbulence in the vena cava, rather than the aorta, when the cat’s back was arched. These theories hinged on the cat arching its back and did not explain how cats could purr when lying down.
It was believed that it was the completely ossified hyoid that enabled domestic cats, and other small cats, to purr but not to roar. However, Weissengruber et al. (2002) argued that purring ability is not affected by the anatomy of its hyoid (whether it is fully ossified or has a ligamentous epihyoid), but depends on specific characteristics of the vocal folds and an elongated vocal tract, the latter being made possible by an incompletely ossified hyoid.
Electromyographic studies of laryngeal muscles revealed regular patterns associated with purring. As some of the laryngeal muscles contracted when the mouth is closed, the glottis closed partially, resulting in a build-up of pressure behind or inside the glottis (the space between the vocal chords). The resulting air turbulence as it passed through the narrowed opening caused the purr when the cat was inhaling and exhaling and in postures where the back isn’t arched. This produces the familiar in-out rhythmic purr that can continue over long periods of time. Researchers call the audible result of this airflow a "tonal buzz".
Purring is, therefore, a non-vocal sound caused by vibration of structures in the throat. However, a truly ecstatic cat sometimes vocalises (uses its vocal cords) while purring, resulting in a shrill noise.
THE “PURR CENTER”
Studies have shown, that the movement of the laryngeal muscles is signalled from a unique “neural oscillator” (Frazer-Sisson, Rice, and Peters, 1991 & Remmers and Gautier, 1972) in the cat’s brain. This "purr centre" was located in the infundibular area of the brain, connected to the hypothalamus. Among other things, the hypothalamus interprets emotions and decides whether sensory stimuli are pleasant or painful.
With pleasant stimuli, the hypothalamus releases endorphins (natural opiates) that stimulate the "purr centre" causing the cat to purr. Pain also stimulates the hypothalamus to release endorphins, this time to help block the pain, which is why cats also purr when they are in pain.
TYPES OF PURR
Purring is one of the lowest pitch sounds made by cats. The average frequency of the purr is approximately 27 Hz. Schötz & Eklund (2011) found considerable variation between cats regarding relative amplitude, duration and frequency between egressive (exhaling) and ingressive (inhaling) phases of purring, but the variation generally occurred within the same general range. Once a cat establishes its purring frequency, this remains relatively stable throughout its lifetime (barring injury to the areas involved in creating the sound).
Cats can alter the volume at which they purr, which may indicate the intensity of the emotion causing them to purr. I’ve found that a contented cat being stroked may start with a barely audible purr, but ends up with a loud, steady purr. Some cats habitually purr at barely audible levels while others purr so loudly that they can drown out the TV or keep their owners awake. Personally I have found even a loud purr that vibrates through my body can soothe me to sleep.
The Pedigree Petfoods book "Your Guide to Cats & Kittens" (1973) discussed the familiar purr and noted that cats being restrained for veterinary procedures (blood samples or X-rays) frequently purred. The inference drawn from this behaviour was that the cats were indicating that they were tractable and co-operative and would not need to be forcibly handled. The purr was therefore likened to the obsequious behaviour of a submissive cat when avoiding conflict with a larger, more powerful animal. It also noted that some cats, both male and female, gave low growl-like purrs as a warning when a stranger entered their territory. This is inaccurate, the "growl-purr" is in fact a low growl.
A PURR FOR ALL REASONS
Researchers and owners are putting together information to understand why cats purr and whether purring in different situations means different things.
Kittens are able to purr from a few days after birth. They can purr while suckling from their mother which may communicate contentment or maintain contact with her. Other possible explanation include soliciting care from the mother and encouraging milk flow to the teats (especially as purring and kneading often occur together later in life). Mother cats may also purr while nursing kittens, perhaps to maintain contact with their offspring. Or maybe it’s all down to the hypothalamus detecting a pleasurable sensation and helping to trigger purring.
German ethologist and cat behaviourist Paul Leyhausen interpreted the purr as a signal that the animal is not posing a threat.
The Non-Solicitation Purr (Happy Purr)
As adults, cats purr in a number of situations. They may purr when in physical contact with feline buddies e.g. when resting together, when grooming each other and when rubbing on each other. They also purr during similar positive interactions with people: while being stroked or talked to or while resting on someone’s knee. Many cats also knead while pawing, a behaviour retained from early kittenhood and used in adulthood to communicate contentment. Most owners recognise that a purring, kneading cat is relaxed and happy.
Non-cat owners may not recognise this sort of purr as a friendly signal. One of my houseguests spent a sleepless night with Kitty I lying on his chest “growling” at him. I had to explain to my guest that Kitty I was purring and cuddling. Kitty I’s purr tended to sound slightly gruff when she was deliriously happy.
The Solicitation Purr (Hopeful Purr)
Cats also purr when they want something. Many owners are woken up in the morning by extremely “purry” cats reminding them that it’s breakfast time! This may be reinforced by physical contact such as nudging or head-butting, or may be in the form of purring delivered at point-blank range. To owners well attuned to their cats, this type of purr is audibly different from the happy purr.
In 2009, researchers at the University of Sussex wrote in the journal "Current Biology" that cats use a "soliciting purr" to manipulate their owners. Unlike regular purring, the "soliciting purr" incorporates a "cry" with a similar frequency to a human baby's. Cats produce a low frequency purr by activating the muscles of their vocal folds, causing them to vibrate. Vocalisation is due to the vocal cords held across the air-stream snapping shut at a particular frequency. Purring and vocalising use different mechanisms so it's possible for the cat to embed a high-pitched cry into an otherwise relaxing purr. The more energy that goes into the cry, the more urgent and unpleasant the purr becomes. The cry normally occurs at a low level in normal purring, but cats learn to exaggerate it when it proves effective in getting a response from humans.
Other studies found similarities between a domestic cat's cry and the cry of a human baby - a sound that humans are highly sensitive to. Some people have even mistaken the overheard cry of the Siamese cat (a particularly vocal breed) for that of a baby.
Lead researcher Dr Karen McComb said the research was inspired by the insistent early morning purr of her cat Pepo. Research discovered that the pestering purr was more likely to get owners out of bed to feed the cat while simply meowing got the cat shut out of the bedroom. "Soliciting purrs" sound more urgent and less pleasant than ordinary "non-soliciting" purrs. The relative level of an embedded high-frequency sound could increase the annoyingness of the purr and hence the likelihood of the owner responding. The "soliciting purr" is more common in cats that are highly attached to a single person.
The solicitation purr occurs when cats are anticipating food, hence I long ago nicknamed it the “hopeful purr.” It seems to be found only in a one-on-one relationship between cat and care-giver. It is generally faster than a “happy purr” and is more likely to contain high frequency peaks similar to those of a baby’s cry. Humans are particularly sensitive to those high frequencies and tend to perceive the solicitation purr as more urgent and less pleasant than a non-solicitation purr. It is harder for humans to ignore and probably triggers our innate need to provide care when we hear (consciously or subconsciously) those high frequency ‘cries’.
It seems cats may have evolved or adjusted their purrs to increase their chances of soliciting food and fuss from their humans. Some cats extend the solicitation purr to other situations where they want their owner to do something e.g. to sit down and provide a lap or to open a door.
The Self-Soothing Purr
Cats also purr in response to more painful situations. Their body language conveys that these are not happy purrs or hopeful purrs. An observant owner may be able to tell the difference from the purr alone.
Vets, researchers, rescuers and owners have reported hearing cats purr continuously when they are distressed, chronically ill, in severe pain or when dying. Female cats may purr while giving birth. There is currently no data about how common this sort of purring is.
This sort of purr may aid self-healing and recovery from disease or injury, especially from broken bones. The frequencies in the purr appear to promote bone and tissue healing. It may also help a cat to calm itself in negative situations. Some owners believe it is a request for help.
Because the purr is linked to the release of endorphins in the brain, which are the body’s own pain-killers, the purr may be a side-effect.
The Stress Purr
Owners who are in tune with their cats may also notice what I call the “stress purr” that occurs when the cat is anxious or under stress. I’ve noticed it in timid cats that are settling into a new environment. This is a fast, urgent purr when being stroked. I’ve found that stress purrs eventually give way to more relaxed non-solicitation purrs as the cat becomes more familiar with its new environment. The Stress Purr may be analogous to the nervous smile in humans.
THE HEALING PURR
Since purring uses energy and has been passed on through many generations of cats, it must have some function. One puzzle was why a sick or injured would expend energy on purring, when it needs all its energy for healing? Researchers believed that suggestions that the cat's purr evolved solely to communicate self-contentment goes against evolutionary theory. The fact that cats purr when injured suggested that it had some survival value, for example a healing function. Cats close to death may also purr, suggesting a pain relieving function. Since many cats purr when on their own, the purr cannot merely be a form of communication - why would a cat purr when there is no-one around to communicate with?
Though this sounds far fetched, research in humans has shown certain frequencies of vibration relieved suffering in over three-quarters of test subjects suffering from acute or chronic pain. Ultrasound is often used alongside physiotherapy. Effects include (depending on the patient) generating new tissue growth, augmenting wound tissue strength, improving local circulation and oxygenation, reducing swelling and even inhibiting bacterial growth. Vibration at low frequencies and low intensities can aid bone growth/repair, tendon and muscle strength/repair, joint mobility, reduce inflammation and reduce breathlessness. I have had ultrasound treatment on damaged tissue in a broken foot and one curious effect was a hot feeling at the fracture site!
The soothing effect of a purring cat is well-known to cat lovers. Researchers believed that vibrating (purring) cats were communicating more than just a sense of well-being to their owners. Fauna Communications and ENDVECO initiated a research project recording and analysing the purr to see if it was linked to healing.
Cats are reputed to have nine lives. Their bones tend to heal rapidly and relatively easily. I came across the case of Didi at the Chelmsford CP shelter - Didi's back legs had been so badly broken he should not have been able to walk. He had been found as a stray with his fractures already healed, albeit not entirely straight, showing the amazing self-healing capacity of cats. There are cases of feral cats surviving accidental limb amputations without human intervention. The ability of a large percentage of cats to survive a fall (High Rise Syndrome) is legendary. It is little surprise that a veterinary saying goes "If you put a cat and a sack of broken bones in the same room, the bones will heal." Compared to dogs, cats have fewer orthopaedic problems or muscle injuries and though attributed mainly to their flexible skeleton, self-healing might also play a part. Researchers wondered if the purr provided therapeutic vibrations to speed this healing.
To investigate this theory, scientists recorded and measured the purring of relaxed cats. To measure purr frequency and how purr vibrations spread throughout the cat's body, an ENDEVCO Model 22 accelerometers was used. These are little bigger than a match head and could be fixed to the cats' skin using washable glue and medical tape. About 6-10 minutes of purring were recorded. The cats' purr frequencies were within the therapeutic range of 20 Hz (hertz) to 200 Hz (actual frequencies were 25 Hz, 50 Hz, 100 Hz, 125 Hz, 150 Hz). The most therapeutic frequency ranges are 25-50 Hz and 100-200 Hz which speed bone repair.
There have been studies that indicate that purring can aid in dyspnoea (Cook, 1972; Kidd et al, 2000). In studies of cats and dogs with acute and sub-acute myocardial necrosis (scarred heart muscle), none of the cats in the study had dyspnoea, although all the dogs did. In a study performed by Dr TF Cook, (1973) (“The relief of dyspnoea in cats by purring” in the New Zealand Veterinary Journal) a dying cat that could not breathe and was facing euthanasia, breathed normally once it began purring. The purring opened up the cat's airway, and improvement was "remarkable and the next day commenced to eat...." The purr might decrease breathlessness by vibratory stimulation. Purring may also stimulate tissue repair, hence cats heal fast than dogs after skin grafts.
This healing purr may be used to offset long periods of rest and sleep that would otherwise contribute to a loss of bone density. As all owners know, cats spend a great deal of their time sleeping or dozing. It may be a low energy mechanism that stimulates muscles and bones and actively prevents the sort of muscle and bone abnormalities seen in comparable sized dogs.
Though to humans, the purr is most often considered a sign of contentment or of a cat reassuring itself, the researchers concluded that after a strenuous activity (hunting, defending territory etc), a period of purring could act like a massage session and alleviate sprains and strains as well as speeding the healing of any wounds. The sense of relaxation many owners feel when cuddling a purring cat suggests that the therapeutic function of the purr can extend to humans.
Can the therapeutic qualities of the purr be extended to owners? A few years ago I suffered a rotator cuff muscle injury to one of my shoulders and over-the-counter pain relief was ineffective. Kitty III snuggled as close as she could and purred so hard her body – and my arm – throbbed. I like to think that her purring aided my healing.
REFERENCES AND FURTHER READING