Why do cats purr?

Purring is caused by vibrations from within the cat’s larynx and diaphragm muscles. The vibrating motion of the muscles within the larynx dilates and constricts the glottis which is located close to the vocal cords. When air passes through this region on inhaling or exhaling, it causes purring. Cats can sustain purring for long periods because it doesn’t involve holding the breath or just exhaling: the sound is produced both when they breathe in and out.

It is not known how the nervous system communicates to the laryngeal and diaphragm muscles to purr. Recent research suggests that a "neural oscillator", which lies within the brain may be the neurological source but more studies are required to confirm if this is the case.

What kinds of stimuli compel the purring mechanism? The answer is much broader than previously imagined.

Cats will purr if they are content and relaxed. But that is not the only reason. A range of situations, those involving stress and hostile environments also cause cats to purr, suggesting that the behaviour is communicative in nature.

Kittens will begin purring some days after they are born to signal to their mothers that they want milk or to help locate them. Similarly, cats will purr to signal a desire for food, activities, attention or in annoyance. What this means is that purring is a basic communicating cue; cats will purr when being groomed by other cats, perhaps to encourage grooming and pinpoint the spots they like best.

Further, purring can signal anxiety as well: cats are known to purr when they are in distress or anxious, or in hostile or unfamiliar situations. Perhaps this is similar to coping behaviours in humans; often, we will cry in helplessness or tear up during emotional or wrenching moments. The physical reaction defuses the tension or weight of a momentous event. 

Perhaps the phenomenon of purring needs to be approached the way we approach common behaviours in humans. Why do we laugh or cry? It is dependent on the context: there is not usually a clear pathway or single emotion leading to the outburst. Similarly, a wealth of emotions and situations will cause a cat to purr.

As mentioned above, purring can be a natural response to stressful situations as it can relieve anxiety. What has garnered the attention of researchers is the vibrating frequency of purrs. Cats normally purr from 20 Hertz (Hz) to 150 Hz. The vibrations at this frequency are thought to promote bone growth and fortify bone strength as well. The vibrations caused by them are similar to high-intensity exercise as bones react to pressure by becoming stronger.

Perhaps this is why cats purr when they are resting as well: it is a way to repair their limbs after a day spent outside and a method of maintaining bone density and strength. 

Devices that stimulate vibrations at similar frequencies are being considered for therapeutic uses. Some studies have shown that stroking your pet can lower anxiety and lower the risk of stroke: perhaps purring is a contributing factor.