Cats (big and small) are the only animals who purr. Contrary to popular belief, purring isn't speaking. The purr comes from two membrane folds, called false vocal cords, that are situated in the larynx behind the actual vocal cords. Cats purr 26 cycles per second, the same as an idling diesel engine

Cats purr both when inhaling and exhaling, keeping the mouth completely closed. Scientists believe purring is produced by blood in a large vein in the chest cavity that vibrates and is then magnified by air in the windpipe. Kittens are born blind and deaf, but the vibrations of their mother's purring is a physical signal that the kittens can feel. It acts as a homing device, signaling them to nurse. Kittens begin purring at about one week old; then it's a signal to the mother cat that they're getting their milk and are content.

Since purring is non-vocal, it doesn't interfere with the suckling. Contrary to another popular belief, cats don't purr purely for pleasure. Be alert: a deep purr can indicate that a cat is in pain or distress. Female cats will purr when in labor. Cats may purr in fear or anxiety. They will also purr in anticipation of being fed or stroked. Big African cats only purr in short bursts, but the house cat can purr for hours. Curiously, scientists tell us that a cat never purrs when alone.

When your cat climbs into your lap, tucks in his paws under himself, and begins to purr, all is right in his world. This is one of the things we love about our cats; that feeling of contentment they share with us. When cats become soft purring bundles of warm reassuring fur, we feel calmer and more peaceful ourselves. We may not always hear the purring - a soft vibrating rumble - but we can feel it. But why do cats purr? And what produces this characteristic sound?

There are many theories to explain how the purr is generated. One study determined that purring involves activation of nerves within the voice box. These nerve signals cause vibration of the vocal cords while the diaphragm serves as a piston pump, pushing air in and out of the vibrating cords, thus creating a musical hum. Veterinarian Neils C. Pederson, author of Feline Husbandry, believes that purring is initiated from within the central nervous system and is a voluntary act. In other words, cats purr only when they want to. The other theory is that the sound comes more from vibrating blood vessels than in the voicebox itself.

The Healing Power of the Purr

Cats are often used as "therapy animals" in convalescent hospitals, or in retirement residences. It is an accepted fact that cat owners have lower blood pressure, especially in older people. The human-feline bond is never quite so close as when a person is holding and petting a purring, vibrating bundle of fur on her lap, and all is well with the world. You may even find yourself purring in response.

Most experienced "cat wranglers" now know that cats don't purr only when they are content and happy. They also purr during tense or traumatic moments. When suddenly and violently injured, even at moments near death, a cat will often purr.

It seems that the measurable Hertz of a cat's purr lies between 25 and 150. Coincidentally (or not) it has been found that sound frequency in this range can stimulate bone growth and healing. That cats have remarkable endurance, and are quite stoic to trauma is well known to veterinary professionals; it is not unlikely that this instinct to purr under duress is directly related.

Purring under stress has more colourfully been described as the cat's mantra, e.g., a self-soothing, self-healing, and relaxing.

The Purr Evolution

Over the course of evolution, purring has probably offered some selective advantage to cats. Most felid species produce a "purr-like" vocalization. In domestic cats, purring is most noticeable when an animal is nursing her kittens or when humans provide social contact via petting, stroking or feeding.

All domestic cats are born with the purring ability. A queen will purr while giving birth. It is unknown whether this is because of happy anticipation, or whether it is a relaxing "mantra." Kittens instinctively purr when nursing, and the momcat purrs right back at them. Your adult cat will purr at the drop of a hat whenever you're near, and especially when you are holding or petting him.

Cat purrs will range from a deep rumble to a raspy, broken sound, to a high-pitched trill, depending on the physiology and/or the mood of a cat. A cat will often "wind-down" when going to sleep, with a long purring sigh that drops melodically from a high to a low pitch. Does your cat's purr sometimes sound like an idling diesel engine? That's because at the lower range of 26 Hertz, the velocities are nearly the same

Scientific Theory

Scientists have demonstrated that cats produce the purr through intermittent signalling of the laryngeal and diaphragmatic muscles. Cats purr during both inhalation and exhalation with a consistent pattern and frequency between 25 and 150 Hertz. Various investigators have shown that sound frequencies in this range can improve bone density and promote healing.

Because cats have adapted to conserve energy via long periods of rest and sleep, it is possible that purring is a low energy mechanism that stimulates muscles and bones without a lot of energy. The durability of the cat has facilitated the notion that cats have "nine lives" and a common veterinary legend holds that cats are able to reassemble their bones when placed in the same room with all their parts. Purring may provide a basis for this feline mythology.

As the cat matures the meaning of the purr changes. Some cats purr to indicate contentment or pleasure, but badly frightened cats and severely ill cats also purr, and so do females while they are delivering their kittens. It is not uncommon for cats to purr when they are close to death. This final purring may indicate a state of anxiety or possibly euphoria, states that have also been described in terminally ill people.