Among all the various sounds that the domestic house cat makes, the purring cat has epitomized contentment throughout history. Studies show that being around a purring cat can reduce human pulse and blood pressure and induce feelings of calm and well-being, and even people who don’t like cats often like their purring, or are at least not bothered by it. The gentle hum that cats emit when they are relaxed and comfortable, has been the subject of much speculation and research throughout the long relationship of cats and humans.
This is in part because only certain kinds of cats purr, and sometimes cats of a type that ought to be able to purr for some reason do not. In addition, there is nothing about the anatomy of cats as opposed to other animals that should enable them to make this unique sound, and there are few anatomical differences between cats that purr and cats that do not. The significance of purring in cat behavior, and its role in the evolution of the domestic cats that do it today, has also been the subject of much scientific debate.
How Cats Purr
Purring is a vibrating or buzzing sound made by certain members of all species of felids, the cat family, and is characterized by vibration of the air that is being exhaled and inhaled. The vibrations of purring have a frequency of 20 to 30 per second and vary among the purring feline species and from one cat to another. These vibrations are sometimes accompanied by other sounds that are sometimes referred to as “lurps” or “yowps,” and some cats emit only these sounds rather than the vibrations. It has been suggested that the throbbing sound is produced by the alternate opening and closing of the glottis by the vocal cords and the muscles of the larynx, causing the air passing through them to vibrate.
Some experts in animal behavior have compared purring to opera singing, others to the subtlety and range of human conversation, still others to the interaction between a human mother and child and yet others to smiling, which can sometimes be done in pain and through tears. It is a unique cat trait that serves many functions, not all of them yet understood, and that is a tribute to the complexity of the feline brain.
Why Do Cats Purr?
The reasons that roaring cats roar have been agreed upon for some time: such a loud noise is very effective in defending territory and attracting mates. The reasons for purring are more numerous and less generally agreed upon. Cats usually purr when relaxed and happy, or when eating, and female cats often purr while giving birth. It has been suggested that purring first developed as a means of communication between mothers and nursing kittens, and also that it signals that the animal does not intend to pose a threat.
Cats also purr when injured or in pain, and this has been linked to endorphin release and natural pain relief. A higher-pitched purr, called the “soliciting purr”, has been described in domestic cats trying to get their owners to feed them and suggests that one of the roles of purring is to get food. Purring of similar character has also been associated with fear and being threatened, and may be a communication of accommodation or submission by a cat that feels itself to be in danger.
Cats Can Purr to Help Heal Tissues
Additionally, most purring has a frequency of around 25 cycles per second, and this stimulation at this frequency promotes tissue healing and retards the reduction of bone density in periods of immobility; it has been suggested that this is a protective mechanism against skin ulcers and bone loss in an animal that spends most of its time immobile and asleep.
It has often been said that cats that can roar, such as lions, tigers, leopards and jaguars, cannot purr and purring cats, chiefly the domestic cat but also cheetahs and bobcats, cannot roar. This has been ascribed to differences between roaring cats and purring cats in the nature of the hyoid or lingual bone, a small horseshoe-shaped bone in the neck that supports the tongue, pharynx and larynx and assists with swallowing and tongue movement. The hyoid bone is completely calcified in cats that purr and only partly so in cats that roar, but some cats with incompletely calcified hyoid bones purr rather than roar, such as the snow leopard , so it appears that more than this small bone is required to produce loud vocalization.
Some cat biologists have suggested that it is the much longer vocal tract of the big cats that allows them to roar. In fact, some studies suggest that roaring cats can on occasion purr, but lions, tigers, leopards and jaguars produce purring sounds only when they exhale, while the domestic cat can purr continuously for long periods, both inhaling and exhaling.
It was also thought for some time that other animals besides cats could purr, including the civet and mongoose but not the hyena among the family of viverridae, which are relatives of cats. Rhythmic vibratory breathing called “purring” has also been ascribed to gorillas, raccoons, elephants, lemurs, squirrels guinea pigs, tapirs and rabbits, but these buzzing sounds last for only short periods and do not occur while both breathing in and out, which is now believed to the be the chief characteristic of a purr. The only non-cats shown to purr the way that cats do are two kinds of genet, ancient carnivores that live in Europe and Africa and are more or less intermediate between the cat and the mongoose.
A cat whose larynx becomes paralyzed for whatever reason loses the ability to purr. The pacemaker of purring, however, is not the larynx or the vocal cords but rather a unique “neural oscillator” in the brain, a group of nerve cells that discharge 20 to 150 times per second and cause the rhythmic contraction of the muscles of the larynx, which in turn changes the position of the vocal cords so as to produce air vibration.
The Neurology of Purring
This center is located at the base of the infundibulum, which is the stalk on which the pituitary gland hangs from the hypothalamus, the deep part of the brain that controls hormone secretion and regulates body temperature, appetite, many body rhythms and some basic behaviors. The hypothalamus is important in parenting and attachment behavior, and in interpreting the emotional nature of a stimulus, whether it is pleasant or painful. Pleasant stimuli cause the release of endorphins, which are natural opiates that induce pleasure and stimulate the “purr center”; because these normally block pain, they are also released with injury or pain, and this results in purring by cats who are injured or in pain. The other circumstance in which purring occurs is the mother-kitten relationship, and parenting and attachment behaviors are mediated by the hypothalamus, also.