Jessie Gibb, of Ardgowan School, asks :-

Why do cats purr, and how is the sound made?

Christine Thomson, an anatomist at Massey University's Institute of Veterinary, Animal and Biomedical Sciences, responded.

Domestic cats, cheetahs, pumas, ocelots and tigers purr, both when contented and at other times, including when they are sick, but not when they are asleep.

Purring is due to bursts of sound occurring 25 times per second (40msec/sound), the frequency of sound in each burst is 150-200Hz. The sound bursts occur during inspiration and expiration with brief pauses at the change between the two phases of breathing. Purring involves muscles of the larynx (voicebox), the chest wall (intercostals) and the diaphragm, which is a large sheet of muscle and tendon separating the chest from the abdomen. Within each 40 msec cycle, there are three phases that cause changing air pressure inside the larynx, resulting in vibration and sound production. In the first phase (20-30msec), the laryngeal opening is narrowed causing an increase in air pressure inside the larynx. In phase 2 and 3 the larynx it is actively opened, with the initial change in air pressure causing production of sound (phase 2; 5-10 msec) while the sustained opening during the third phase allows airflow in or out of the trachea. This allows cats to breathe while they purr. Both the heart rate and respiratory rate increase during purring and the latter results in twice the amount of air being shifted during breathing.

The diaphragm contracts alternatively with the laryngeal muscles, contributing to the changing air pressure and results in the purr which can be felt and usually heard.

The frequency of the purr vibration is reported to be similar to sound frequencies that promote bone and muscle healing. This raises the possibility that purring when ill, aids healing in our four-legged feline friends.