Purring is a non-verbal communication that originates in the laryngeal muscles stimulated by neuron activity, which causes the muscles in the throat to twitch at 25 to 150 vibrations per second. This vibration results in vocal cord separation and purring.
A researcher in bioacoustics believes that a specific frequency of purring, 25 Hz, induces a type of physical therapy, heals wounds, increases bone density and minimizes muscle atrophy, just as it does in humans at that frequency. Cats are known to reduce stress and blood pressure, and purring may help with that. Behaviorists believe that kitten purring tells the mother that "all is well," because meow communication is limited when nursing, but a mother cat can feel and hear purring and respond in kind.
Contentment is only one reason for purring. Older cats approach other cats and purr as an overature to friendship. Purring is also a response to fear, illness and distress. Veterinarians often struggle to hear a heartbeat or examine lungs with this type of purring. A vet may run water during the exam to halt the purring.
Wild cats, hyenas, guinea pigs and raccoons also purr. Mountain lions and bobcats, for instance, can purr, but cannot roar. Wild cats who roar, such as lions and tigers, cannot purr because the structures surrounding their larynx are not strong enough to support it.